TECHNICAL FIELD 
       [0001]    The present invention relates to a sample measurement device, and in particular, to a sample measurement device which measures a radioactive substance in a sample using a liquid scintillator. 
       BACKGROUND ART 
       [0002]    A sample measurement device is a device that individually measures a plurality of samples. Representative sample measurement devices include a liquid scintillation counter. The liquid scintillation counter has a rack-transporting mechanism which transports a rack holding a plurality of sample containers, a measurement unit which measures light caused by a radioactive substance contained in each sample container, a container transporting mechanism which transports the sample container between the rack and a measurement chamber unit, or the like. In the sample container, in addition to liquid sample, liquid scintillator is included. When a radiation (for example, a β ray) is emitted from the radioactive substance in the sample, light emission in the liquid scintillator is caused by the radiation. The light is detected by a pair of photomultiplier tubes forming the measurement unit. 
         [0003]    In general, a shielding structure must be provided in order to block an extrinsic radiation at a periphery of a measurement chamber which stores the sample container to be measured. Such a shielding structure is formed from a metal which is very heavy. Therefore, it is more preferable that the shielding structure be placed below a rack placement surface rather than being placed above the rack placement surface. In addition, when a system is employed in which the sample container is transported upward from the rack, because a shaft which supports the sample container would exist within a lifting/lowering path during the measurement, there is a problem in that light shielding in the lifting/lowing path is difficult. Therefore, it is desired that the measurement chamber be provided below the rack placement surface, and also to withdraw the sample container downward from the rack. 
         [0004]    Patent Document 1 discloses a liquid scintillation counter of the related art. In the liquid scintillation counter described therein, the measurement chamber is provided above a transport table for transporting the rack. Patent Document 2 discloses a rack transporting device which can be used for the liquid scintillation counter or the like. 
       CITATION LIST 
     Patent Literature 
       [0005]    [Patent Document 1] JP 2007-278969 A 
         [0006]    [Patent Document 2] JP 2007-176666 A 
       SUMMARY 
     Technical Problem 
       [0007]    If a complex mechanism is provided in order to withdraw the sample container downward from the rack, a problem arises in that the cost is increased. While many drive sources are provided for the sample measurement device, it is desired to reduce the number of drive sources. 
         [0008]    An advantage of the present invention is in the enablement of withdrawal of the sample container downward from the rack with a simple mechanism. Another advantage of the present invention is that the sample container can be held and released without the use of a dedicated drive source and a dedicated controller. 
       Solution to Problem 
       [0009]    (1) According to one aspect of the present invention, there is provided a sample measurement device, comprising: a rack that has a sample storage unit having an open/close mechanism; a rack-transporting mechanism that transports the rack on a transport surface; a guide block provided on the transport surface and that enters a lower part of the rack during transport of the rack to open or close the open/close mechanism; 
         [0010]    and a lifting/lowering mechanism that transports a sample container between the sample storage unit and a sample measurement chamber provided below the transport surface in an open state of the open/close mechanism. 
         [0011]    According to the above-described configuration, the guide block enters the lower part of the rack by the transport of the rack, and the open/close mechanism provided on the sample storage unit is operated by the guide block. In an open state of the open/close mechanism, a sample container before measurement is sent from the sample storage unit to the lifting/lowering mechanism, and a sample container after the measurement is returned from the lifting/lowering mechanism to the sample storage unit. In the above-described configuration, because the sample measurement chamber is provided below the transport surface, it becomes unnecessary to place the shielding member, which is very heavy, at a high position. In addition, the light shielding of the lifting/lowering path for lifting/lowering the sample is also facilitated. Because the guide block causes the open/close mechanism to operate at a lower part of the rack, it is not necessary to provide a member which transmits an open/close drive force from an outer side of the rack to the inside of the rack. 
         [0012]    According to another aspect of the present invention, preferably, in the sample measurement device, the open and close operations of the open/close mechanism is caused by a relative movement of the rack with respect to the guide block. According to such a configuration, a part of a rack-transporting force produced by the rack-transporting mechanism can be used to open the open/close mechanism. Therefore, it is not necessary to provide a dedicated drive source and a dedicated controller for an opening operation of the open/close mechanism. With such a configuration, the structure can be simplified. In order to close the open/close mechanism, it is desirable to use an elastic force accumulated by the opening operation of the open/close mechanism. Alternatively, a part of the rack-transporting force may be utilized for the closing operation. 
         [0013]    According to another aspect of the present invention, preferably, in the sample measurement device, the guide block has: a front-side form that contacts the open/close mechanism during the relative movement to transition the open/close mechanism from a closed state to an open state; and a rear-side form that contacts the open/close mechanism during the relative movement to recover the open/close mechanism from the open state to the closed state; holding of the sample container is executed in the closed state; and holding of the sample container is released in the open state. According to such a configuration, the open/close mechanism is operated by the open/close mechanism contacting the guide block. An overall shape of the guide block is determined such that a desired opening operation and a desired closing operation are executed at desired timings. The front-side form is the form of a portion that first enters the lower part of the rack during the transport of the rack, and the rear-side form is a form on the side opposite the front-side form. 
         [0014]    According to another aspect of the present invention, preferably, in the sample measurement device, the front-side form has a pair of front-side inclined surfaces that apply a press-spreading force on the open/close mechanism. According to such a configuration, when the open/close mechanism moves forward while contacting the pair of front-side inclined surfaces, the open/close mechanism is gradually changed from the closed state to the open state. The pair of front-side inclined surfaces have a function to convert a force in a longitudinal direction into a force on both sides in a short-side direction. By suitably setting the angle of the pair of the front-side inclined surfaces, it is possible to adjust an opening rate. Preferably, the pair of the front-side inclined surfaces has a line-symmetrical shape. Preferably, in the front-side form, a width in the short-side direction is gradually increased from a downstream side in the longitudinal direction to an upstream side. 
         [0015]    According to another aspect of the present invention, preferably, in the sample measurement device, the rear-side form has a pair of rear-side inclined surfaces that allows a recovery movement of the open/close mechanism Specifically, when the open/close mechanism moves forward while contacting the pair of the rear-side inclined surfaces, the open/close mechanism is gradually changed from the open state to the closed state. In this case, it is preferable to provide, in order to maintain a contact state of the open/close mechanism with respect to the pair of inclined surfaces, a means that produces an elastic recovery force, in the open/close mechanism itself. In this case, the pair of the rear-side inclined surfaces has a function to restrict or adjust the rate of the closing operation to the short-side direction according to the position in the longitudinal direction. Preferably, the pair of the rear-side inclined surfaces has a line-symmetrical shape. Preferably, in the rear-side form, a width in the short-side direction is gradually decreased from a downstream side in the longitudinal direction toward an upstream side. 
         [0016]    According to another aspect of the present invention, preferably, in the sample measurement device, the guide block has an intermediate form provided between the front-side form and the rear-side form that maintains the open state of the open/close mechanism, and the intermediate form has an opening that forms an upper end portion of a lifting/lowering path of the sample container. An open width of the open/close mechanism in the open state is set such that the sample container can be lowered into the opening and can be received from the opening. In the open state of the open/close mechanism, if the open/close mechanism protrudes in both outer sides from the lateral width of the rack (width in the short-side direction), it is preferable to provide a passage or the like in the rack to allow the protrusion. 
         [0017]    According to another aspect of the present invention, preferably, in the sample measurement device, the guide block has: a front-side protrusion provided at a front side of the opening and that temporarily supports a lower surface of the sample container from below, after holding of the sample container by the open/close mechanism disappears; and a rear-side protrusion provided at a rear side of the opening that temporarily supports the lower surface of the sample container from below, until the holding of the sample container by the open/close mechanism is resumed. According to such a configuration, fall-off or unnecessary change in orientation of the sample container before the opening can be prevented, and fall-off or unnecessary change in orientation of the sample container at a position deeper than the opening can prevented. Preferably, a front edge portion (and a rear side portion) on the upper surface of each protrusion is formed as a tapered surface. 
         [0018]    According to another aspect of the present invention, preferably, in the sample measurement device, the lifting/lowering mechanism has a head that has a placement surface on which the sample container is placed, and, in a state where the head is inserted into the opening, the placement surface is positioned at substantially the same height as an upper surface level of the front-side protrusion and the rear-side protrusion. The height and the form are preferably determined such that the protrusions and the head are not obstructed by the lower part of the sample container. 
         [0019]    According to another aspect of the present invention, preferably, in the sample measurement device, the rack has a longitudinal direction and a short-side direction orthogonal to the longitudinal direction, the rack has a pair of legs distanced from each other in the short-side direction, and a width of the guide block in the short-side direction is substantially equal to a gap between the pair of the legs. According to such a configuration, when the guide block is inserted between the pair of the legs, the guide block restricts a change of orientation of the rack. In other words, the orientation and the position of the rack are determined 
         [0020]    According to another aspect of the present invention, preferably, in the sample measurement device, the guide block has a pair of tapered surfaces for centering in the short-side direction upon entrance to the pair of the legs. According to such a configuration, even when there is a deviation in a direction of travel of the rack, the orientation thereof can be forcefully changed to an appropriate orientation. With such a configuration, the operation of the open/close mechanism can be set appropriate, and it becomes easy to always position the sample container at an appropriate position. 
         [0021]    According to another aspect of the present invention, preferably, in the sample measurement device, the rack has: a plurality of sample storage units arranged along a longitudinal direction; and a pair of legs distanced from each other in a short-side direction orthogonal to the longitudinal direction, each sample storage unit respectively has the open/close mechanism, and the guide block causes the plurality of open/close mechanisms to be sequentially opened and closed while passing a region between the pair of the legs during a transport process of the rack in the longitudinal direction. According to such a configuration, the open/close mechanisms are sequentially opened and closed at appropriate timings by the transport of the rack. 
         [0022]    (2) According to another aspect of the present invention, there is provided a sample measurement device, comprising: a rack serving as a member that holds a plurality of sample containers arranged along a longitudinal direction and that has a pair of legs distanced from each other in a short-side direction orthogonal to the longitudinal direction; a transporting mechanism that transports, on a transport path extending in an X direction, the rack while coinciding the longitudinal direction with the X direction; a guide block which is a block provided on the transport path extending in the X direction, that enters between the pair of the legs, and that has a reference surface parallel to the X direction; and a pressing unit provided on one side of the guide block, that applies a pressing force on an outer surface of one leg to cause an inner surface of the one leg to closely contact the reference surface and form a state of an appropriate position and an appropriate orientation of the rack. 
         [0023]    According to the above-described configuration, when the pressing unit applies the pressing force on the outer surface of one leg, the inner surface of the one leg closely contacts the reference surface. Because the reference surface is a surface parallel to the X direction, with the close contact, the longitudinal direction of the rack can be precisely adapted to the X direction. In addition, in the state where the inner surface of one leg is in close contact with the reference surface, the rack position in the Y direction is also naturally set appropriate. In other words, with the above-described configuration, the position and the orientation of the rack can be easily set appropriate basically by simply adjusting the position and the orientation of the guide block. 
         [0024]    According to another aspect of the present invention, preferably, in the sample measurement device, the guide block is placed at a location for introducing or taking out a sample container to be measured, and the sample container to be measured is introduced or taken out in a state where the position and the orientation of the rack are set appropriate. 
         [0025]    According to another aspect of the present invention, preferably, in the sample measurement device, the sample container to be measured is positioned at a reference position in the X direction, the reference surface is a surface spreading in an upstream side and a downstream side in the X direction from the reference position in the X direction, and the pressing unit includes a first contact member that applies a first pressing force on the outer surface of the one leg in an upstream side in the X direction relative to the reference position in the X direction, and a second contact member that applies a second pressing force on the outer surface of the one leg at a downstream side in the X direction relative to the reference position in the X direction. According to such a configuration, pressing forces may be produced on the rack in front and rear of the reference position in the X direction, to reliably cause the inner surface of the one leg to closely contact the reference surface. The first contact member and the second contact member may be configured to continuously contact the rack or intermittently contact the rack. In the former case, the contact members are preferably configured to reduce a sliding resistance caused in the movement of the rack in the X direction. In the latter case, it is preferable to provide a mechanism that moves the two contact members forward when the rack is stopped. 
         [0026]    According to another aspect of the present invention, preferably, in the sample measurement device, the first contact member is a first roller, and the second contact member is a second roller. According to such a configuration, it is possible to set the sliding resistance during the movement of the rack in the X direction very low while forming the close contact state. The reference surface is preferably a completely flat surface parallel to the X direction. Preferably, a tapered surface is formed on the front end side and the rear end side thereof. 
         [0027]    According to another aspect of the present invention, preferably, in the sample measurement device, the pressing unit comprises a first movable member having a first front-end section rotatably holding the first roller and a first rear-end section at a side opposite the first front-end section, a second movable member having a second front-end section rotatably holding the second roller and a second rear-end section on a side opposite the second front-end section, and an elastic member that applies an elastic force on the first movable member and the second movable member. 
         [0028]    According to another aspect of the present invention, preferably, in the sample measurement device, the pressing unit has a common rotational axis forming a rotational axis of the first movable member and a rotational axis of the second movable member, and the elastic member is provided between the first rear-end section and the second rear-end section. According to such a configuration, an elastic force produced by a single elastic member is transmitted to two rollers through two movable members. With such a configuration, the mechanism can be simplified. In addition, with such a configuration, it is easy to set the pressing forces by two rollers equal to each other. 
         [0029]    According to another aspect of the present invention, preferably, in the sample measurement device, a center of rotation of the first roller and a center of rotation of the second roller are set at a position farther away from the guide block relative to the common rotational axis in a Y direction orthogonal to the X direction. According to such a configuration, resistance when the front-end section of the one leg is received between the first roller and the reference surface can be reduced. In preparation of the case of return-transporting the rack and in order to produce equal pressing forces, it is preferable to set a negative offset also for the center of rotation of the second roller. 
         [0030]    According to another aspect of the present invention, preferably, in the sample measurement device, a lifting/lowering path for lifting/lowering the sample to be measured is provided between the rack and a sample measurement chamber, and the guide block has an opening forming an upper end of the lifting/lowering path. According to such a configuration, the lifting/lowering path and the guide block can be practically physically integrated with each other, thereby facilitating appropriate positioning of the rack with respect to the lifting/lowering path. According to such a positioning of the rack, an advantage can be obtained of reducing influences on the transport surface by a machining error, an assembly error, or the like of a wall member, a separating member, or the like. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0031]      FIG. 1  is a diagram showing an overview of a sample measurement device according to a preferred embodiment of the present invention. 
           [0032]      FIG. 2  is a top view of a sample measurement device. 
           [0033]      FIG. 3  is a perspective view of a sample measurement device. 
           [0034]      FIG. 4  is a first perspective view of a rack. 
           [0035]      FIG. 5  is a second perspective view of a rack. 
           [0036]      FIG. 6  is a first perspective view of a rack body. 
           [0037]      FIG. 7  is a second perspective view of a rack body. 
           [0038]      FIG. 8  is a first perspective view of an adapter. 
           [0039]      FIG. 9  is a second perspective view of an adapter. 
           [0040]      FIG. 10  is a first perspective view of an attachment. 
           [0041]      FIG. 11  is a second perspective view of an attachment. 
           [0042]      FIG. 12  is an explanatory diagram of an operation of an attachment. 
           [0043]      FIG. 13  is a cross sectional diagram of a rack. 
           [0044]      FIG. 14  is a diagram showing a non-operation state of a stopper. 
           [0045]      FIG. 15  is a diagram showing an operation state of a stopper. 
           [0046]      FIG. 16  is an explanatory diagram of an operation of a lower end unit. 
           [0047]      FIG. 17  is a diagram showing an open state of an arm. 
           [0048]      FIG. 18  is an enlarged top view of a guide block. 
           [0049]      FIG. 19  is a cross sectional diagram of a guide block. 
           [0050]      FIG. 20  is a perspective view showing a rack introduction state to an X transport path. 
           [0051]      FIG. 21  is a perspective view showing a handing state of a sample container. 
           [0052]      FIG. 22  is an enlarged perspective view showing a handing state of a sample container. 
           [0053]      FIG. 23  is a cross sectional diagram showing a handing state of a sample container. 
           [0054]      FIG. 24  is a diagram showing an operation state of a pressing unit. 
           [0055]      FIG. 25  is a first enlarged perspective view of a pressing unit. 
           [0056]      FIG. 26  is a second enlarged perspective view of a pressing unit. 
           [0057]      FIG. 27  is a diagram for explaining a function of a pressing unit. 
           [0058]      FIG. 28  is an XZ cross sectional diagram showing a head lifted state. 
           [0059]      FIG. 29  is an XZ cross sectional diagram showing a handing state of a sample container. 
           [0060]      FIG. 30  is an XZ cross sectional diagram showing a sample measurement state. 
           [0061]      FIG. 31  is a YZ cross sectional diagram showing a head lifted state. 
           [0062]      FIG. 32  is a YZ cross sectional diagram showing a shutter operation state. 
           [0063]      FIG. 33  is an enlarged cross sectional diagram showing a light-shielding structure at a lower part of a sample measurement chamber. 
           [0064]      FIG. 34  is a first perspective view of a shutter mechanism. 
           [0065]      FIG. 35  is a second perspective view of a shutter mechanism. 
           [0066]      FIG. 36  is a cross sectional diagram of a shutter mechanism. 
           [0067]      FIG. 37  is a first perspective view showing another adapter. 
           [0068]      FIG. 38  is a second perspective view showing another adapter. 
           [0069]      FIG. 39  is a schematic view explaining an operation of another adapter. 
           [0070]      FIG. 40  is a cross sectional diagram for explaining an operation of another adapter. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0071]    Preferred embodiments of the present invention will now be described with reference to the drawings. 
       (A) Overview of Sample Measurement Device (FIGS.  1 - 3 ) 
       [0072]      FIG. 1  shows a sample measurement device according to a preferred embodiment of the present invention. In the present embodiment, the sample measurement device shown in  FIG. 1  is a scintillation counter. The scintillation counter measures a radioactive substance contained in a sample using a liquid scintillator. Alternatively, the present invention can be applied to other sample measurement devices. 
         [0073]      FIG. 1  is a schematic diagram showing an overall structure of a sample measurement device  10 . An X direction is a first horizontal direction, a Y direction is a second horizontal direction, and a Z direction is a vertical direction. The sample measurement device  10  has a transport table  12  having a transport surface extending in the X direction and the Y direction. A plurality of racks  14  are transported on the transport table  12 . In the present embodiment, a Y transport path  16 , an X transport path  20 , a Y transport path  18 , and another X transport path are provided on the transport table  12 . A transporting mechanism  16 A is a mechanism for transporting the rack  14  in a forward direction in the Y direction on the Y transport path  16 . A transporting mechanism  18 A is a mechanism for transporting the rack  14  in a reverse direction in the Y direction on the Y transport path  18 . A mechanism for transporting the rack  14  in the forward direction in the X direction on the X transport path  20  is not shown in  FIG. 1 . Similarly, a mechanism for transporting the rack  14  in the reverse direction in the X direction on the other X transport path is not shown in  FIG. 1 . 
         [0074]    The rack  14  has a longitudinal direction and a short-side direction. The longitudinal direction is a direction of arrangement of a plurality of storage units  24 . The short-side direction is a direction orthogonal to the longitudinal direction. In each storage section or part  24 , a sample container  22  is held. The sample container  22  is, for example, a vial, a test tube, or the like. The sample container  22  includes a body  22 A and a cap  22 B. In the body  22 A, a liquid sample  22 C is stored. In the body  22 A, a liquid scintillator for measuring the radioactive substance in the liquid sample is also stored. In general, the liquid scintillator is a substance that emits light upon reception of a radioactive ray (β ray in the present embodiment). 
         [0075]    In the present embodiment, as will be described later in detail, an adaptor is mounted for each storage unit  24  on the rack body. As a function of the adapter, an open/close mechanism  26  is realized. That is, the rack  14  has a plurality of open/close mechanisms  26  corresponding to the plurality of storage units  24 . Each open/close mechanism  26  takes a closed state and an open state. In the closed state, the open/close mechanism  26  holds the sample container  22 , and in the open state, the holding of the sample container  22  is released and the sample container  22  is freed. In the present embodiment, the sample container  22  which is set as the measurement target is withdrawn downward from the rack  14 . 
         [0076]    A guide block  30  is fixedly placed at a location where a target container is introduced or extracted on the X transport path  20  on the transport table  12 . The guide block  30  is a member that enters a lower part of the rack  14 . More specifically, the guide block  30  is a member that enters between a pair of legs of the rack  14 , to apply an opening force on each open/close mechanism  26 . In addition, the guide block  30  is a member that cooperates with a pressing unit  32  to be described later, to achieve an appropriate position and an appropriate orientation of the rack. The guide block  30  has an opening in its center, penetrating in the vertical direction. The opening corresponds to an upper end of a lifting/lowering path  28 . The lifting/lowering path  28  is a passage for the sample container which is set as the measurement target to be lifted or lowered between the rack  14  and a sample measurement chamber  34 . In the present embodiment, the guide block  30  is positioned with high precision with respect to the lifting/lowering path  28 . In other words, as will be described later, the guide block  30  is physically integrated to a structure that forms the lifting/lowering path  28 . 
         [0077]    In the present embodiment, the pressing unit  32  is provided in order to achieve an appropriate position and an appropriate orientation of the rack  14  during the introduction or extraction of an individual sample container. The pressing unit  32  applies a pressing force on an outer surface of one leg of a pair of legs of the rack  14 , to thereby cause an inner surface of the one leg to closely contact a reference surface of the guide block  30 . With the formation of such a close contact state, the position and orientation of the rack  14  can be set appropriate. 
         [0078]    During the sample measurement, a sample container  36  which is set as the measurement target is stored in the sample measurement chamber  34 . A lifting/lowering mechanism  40  is provided for lifting and lowering the sample container  36 . With the lifting/lowering mechanism  40 , the sample container  36  can be moved in the up-and-down direction in the lifting/lowering path  28 . 
         [0079]    In the present embodiment, the lifting/lowering mechanism  40  has a shaft  43 , a head  44  provided at an upper end of the shaft  43 , a slide mechanism  46  which drives the shaft  43 , or the like. As will be described later, in the handing of the sample container between the rack  14  and the head  44 , the head  44  is inserted into an opening formed in the guide block  30 . In this state, the open/close mechanism  26  provided on the storage unit  24  positioned immediately above the opening is set in the open state. More specifically, in the transport process of the rack  14 , the open/close mechanism  26  contacts the guide block  30 , and the open/close mechanism  26  receives an opening force in the horizontal direction from the guide block  30  so that the open state of the open/close mechanism  26  is formed. 
         [0080]    The sample measurement chamber  34  is mounted on a base  48 . When light emission is caused in the sample container  36  stored in the sample measurement chamber  34 , the light is detected by a pair of photomultiplier tubes  38 . The pair of photomultiplier tubes  38  are provided for executing a coincidence counting process. In the present embodiment, a special light-shielding structure  50  is provided at a lower part of the sample measurement chamber  34 . With the light-shielding structure  50 , intrusion of extrinsic light through the surface of the shaft  43  into the sample measurement chamber  34  is prevented. The light-shielding structure  50  is provided over a lower surface of the head  44  and an upper surface of the base  48  to which the lower surface of the head  44  contacts. In the head  44  also, a predetermined light-shielding structure is provided. These elements will be described later in detail. 
         [0081]    In the present embodiment, the sample measurement chamber  34  is provided below the transport table  12 . Therefore, an advantage can be obtained in that a very heavy shielding member provided at the periphery of the sample measurement chamber  34  can be placed at a lower side of the transport table  12 . In addition, as will be described below, during the sample measurement, when extrinsic radiation is blocked and extrinsic light is blocked at a predetermined location on the lifting/lowering path  28 , because there is no shaft  43  in this location, the extrinsic radiation and extrinsic light can be easily and reliably blocked. 
         [0082]    Specifically, a shutter mechanism  42  is provided on the lifting/lowering path  28 , in a manner to extend across the path. The shutter mechanism  42  in the present embodiment includes an upper shutter mechanism and a lower shutter mechanism. That is, a double shutter mechanism is realized. The upper shutter mechanism is a mechanism which inserts a radiation-shielding member at an upper side of the sample measurement chamber  34  to block the extrinsic radiation, and the lower shutter mechanism is a mechanism which inserts a light-shielding plate across the lifting/lowering path  28  to block intrusion of extrinsic light from above. 
         [0083]    In the sample measurement device  10  of the present embodiment, the rack  14  is intermittently sent in the X direction on the X transport path  20  and in an orientation where the longitudinal direction is coincided with the X direction. In this case, the guide block  30  enters a lower part of the rack  14 , and the open/close mechanisms  26  provided on the storage units  24  are sequentially activated. The rack  14  stops in a state where a center line of each storage unit  24  and a center line of the lifting/lowering path  28  are matched. In this state, a sample container before measurement is sent from the rack  14  into the sample measurement chamber  34 . After the measurement is completed, the sample container  36  after measurement is returned to the original sample container storage unit. Then, with the transport of the rack  14 , the open/close mechanism  26  is returned from the open state to the closed state in the storage unit  24  receiving the sample container. This sequence of processes is repeatedly executed for each storage unit  24 . 
         [0084]      FIG. 2  is a top view of the sample measurement device shown in  FIG. 1 . As already described, on the transport table  12 , the rack  14  is transported in the horizontal direction. As the transport path of the rack  14 , in the present embodiment, the Y transport path  16 , the X transport path  20 , the Y transport path  18 , and the X transport path  51  are provided. Normally, many racks  14  are placed on the transport table  12 , and each rack  14  is sequentially transported on each transport path. 
         [0085]    A center position on the X transport path  20  is a reference position for introducing and extracting the sample container. The guide block  30  is provided in a manner such that a center of the guide block  30  matches the reference position. Near the guide block  30 , the pressing unit  32  which realizes a pressing function to the rack  14  is provided. On the X transport path  20 , the rack  14  is transported in the X direction by the transporting mechanism  52  in a manner such that the longitudinal direction of the rack  14  is parallel to the X direction. The transporting mechanism  52  has a hook member  54 . As will be described later, while a tip of the hook member  54  is hooked with respect to the protruding portion of the rack  14 , the hook member  54  is moved in the X direction. With this process, the rack  14  is transported in the X direction. The hook member  54  is configured so as not to obstruct operations of the open/close mechanisms even in a state where the tip portion thereof is engaged with respect to the rack  14 . 
         [0086]    In the X transport path  50  at the opposite side also, a transporting mechanism  56  for transporting the rack  14  in the X direction is provided. The transporting mechanism  56  basically has the same structure as the transporting mechanism  52 . On the X transport path  50 , no member corresponding to the guide block  30  is provided, but a pressing unit having the same structure as the pressing unit  32  is placed. 
         [0087]      FIG. 3  is a perspective view showing a part of the transport table. As described above, on the Y transport path  16 , the rack  14  is transported in the Y direction. In this case, the rack  14  is translated in a manner such that the short-side direction of the rack  14  is directed to the Y direction. On the X transport path  20 , the rack  14  is transported in a manner such that the longitudinal direction of the rack  14  is directed to the X direction. In order to strictly match the longitudinal direction of the rack  14  with the X direction during introduction and extraction of the sample container, the pressing unit  32  described above presses one leg of the rack  14  toward the side of the guide block  30 . 
         [0088]    The guide block  30  is provided on the reference position on the X transport path  20 . In  FIG. 3 , a head which is a part of the lifting/lowering mechanism is entered in the opening of the guide block  30 . That is, in  FIG. 3 , the head is in a lifted state. As described above, the transporting mechanism  52  has the hook member  54 . 
       (B) Rack and Adapter (FIGS.  4 - 17 ) 
       [0089]    Next, the rack and the adapter will be described in detail. 
         [0090]      FIG. 4  is a perspective view showing the rack  14  viewed from diagonally above. The rack  14  holds a plurality of sample containers arranged in the longitudinal direction (X direction in  FIG. 4 ). The rack  14  includes a rack body  58 , and a plurality of adapters  60  detachably attached to the rack body  58 . A plurality of storage units  24  are provided in the rack  14 , and the adapter  60  is provided for each storage unit  24 . As will be described later in detail, the adapter  60  includes a ring-shaped frame  66 , and a pair of arms  70  and  72  extending from the ring-shaped frame  66  in the downward direction. The arm  70  and the arm  72  as a whole form the open/close mechanism described above. 
         [0091]    The rack  14  has a pair of legs  64 A and  64 B distanced from each other in the short-side direction. Each of the legs  64 A and  64 B extends in the longitudinal direction. A region between the pair of legs  64 A and  64 B forms a cavity section for allowing the guide block to pass. A front side and a rear side of the cavity section are both openings. On a front end of the rack  14 , a protrusion portion  58 A is provided. The tip of the hook member is inserted into the rack opening of the protrusion portion  58 A. Alternatively, a rack may be used in which, in addition to the longitudinal direction, a plurality of storage units are arranged in the short-side direction. 
         [0092]      FIG. 5  is a perspective view of the rack  14 , viewed from diagonally below. As described above, the rack body has the pair of legs  64 A and  64 B. The adapter  60  has the ring-shaped frame  66  and the pair of arms  70  and  72  placed in a distanced manner from each other in the short-side direction. The pair of arms  70  and  72  have a pair of lower end structures  74  and  76 , which form a primary portion of the open/close mechanism  26 . 
         [0093]      FIG. 6  shows the rack body  58  viewed from diagonally above. The rack body  58  has a plurality of storage holes  24 A arranged in the longitudinal direction. In each storage hole  24 A, a one-side surface structure and an other-side surface structure are provided on both sides in the short-side direction. Because these structures have a symmetric shape, the one-side surface structure will be described as a representative of these two structures. A side plate  78  extending in the vertical direction is provided on one side of the storage hole  24 A. A rib formed as a thick portion is provided between two adjacent side plates  78  or over the two adjacent side plates  78 . A substantial portion of the side plate  78  is formed as a thin portion. At an upper part of the side plate  78 , an upper opening  82  is provided, and, at a lower part of the side plate  78 , a lower opening  84  is formed. The upper opening  82  and the lower opening  84  are each an opening penetrating in the short-side direction.  FIG. 7  shows the rack body  58  viewed from diagonally below. 
         [0094]    Next, a structure and a function of the adapter will be described with reference to  FIGS. 8-17 . 
         [0095]      FIG. 8  shows the adapter  60  viewed from diagonally above. As described above, the adapter  60  has the ring-shaped frame  66  and the pair of arms  70  and  72  extending from the ring-shaped frame  66  in the downward direction. The ring-shaped frame  66  has a ring shape corresponding to the storage hole, and has a rib or the like. The pair of arms  70  and  72  are distanced from each other in the short-side direction (Y direction in  FIG. 8 ), and the sample container is maintained between the arms. The arm  70  and the arm  72  have a form symmetric with each other. Here, the arm  72  will be described. The arm  72  has an upper portion  86  connected to the ring-shaped frame  66 , a wave-shaped portion  88  serving as a bending portion provided at a lower side of the upper end portion  86 , and a lower end portion  90  provided at a lower side of the wave-shaped portion  88 . The lower end portion  90  has a hook portion  92  which is bent in the inner side. An attachment to be described later is attached on each hook portion  92  of the two arms  70  and  72 , and two lower end structures  74  and  76  to be described later in detail are thus formed. The wave-shaped portion  88  has a bellows form. When an opening force to an outer side in the horizontal direction is applied on the hook portion  92 , the arm  70  is bent about the wave-shaped portion  88  by this force. At the same time, the arm  72  is similarly deformed. With this process, the two arms  70  and  72  are set in a state where the arms are opened in the short-side direction. The wave-shaped portion  88  is formed as an elastic portion. Because an elastic recovery force is achieved by the elastic portion, when the opening force in the horizontal direction stops acting on the two arms  70  and  72 , the two arms  70  and  72  are returned to their original shapes by the above-described elastic recovery force. In other words, the two arms  70  and  72  are set in the closed state. 
         [0096]      FIG. 9  shows the adapter viewed from diagonally below. As described above, the adapter  60  has the two arms  70  and  72 , each of which has the hook portion  92 . In the present embodiment, the hook portion  92  has a hook base  92 A which is a bent portion and which forms a base surface. The hook base  92 A has a U shape when viewed from the above. In the hook base  92 A, a contact member  92 D extending in the vertical direction is provided. The contact member  92 D has a semi-cylindrical shape or a D shape when viewed from above. When the contact member  92 D contacts an inclined surface formed in the guide block during a movement of the rack, an opening force is generated toward the outer side in the horizontal direction. A connector  92 E is provided below the hook base  92 A, and is fixed on the contact member  92 D. The connecter  92 E has a rectangular shape, and an attachment is detachably attached using the connector  92 E. On lower end portions of the arms  70  and  72 , an L-shaped channel  92 F having an L shape when viewed in the X direction is formed. 
         [0097]      FIG. 10  is a first perspective view of the attachment attached to the body of the adapter.  FIG. 10  shows the attachment  96  viewed from diagonally above. 
         [0098]    As shown in  FIG. 10 , the attachment  96  has a connection structure  98  which is connected to the hook portion shown in  FIGS. 8 and 9 . The connection structure  98  forms a connection end, and more specifically has a lower plate  106  and an upper plate  108 . The region between the plates is a slit  110 , and the connector shown in  FIG. 9  is inserted into the slit  110 . In  FIG. 10 , in the slit  110 , a protrusion (not shown) is provided, and the protrusion is fitted into an opening formed in the connector. With this process, the attachment  96  is mounted on the body of the adapter. 
         [0099]    A seat plate  102  serving as a movable piece is provided above the connection structure  98 . The seat plate  102  forms a contact end, and an upper surface thereof functions as a seat surface. That is, a lower surface of the sample container is placed on the seat plate  102 . 
         [0100]    A C-shaped arm  100  having a C shape is provided between the connection structure  98  and the seat plate  102 . The C-shaped arm  100  functions as an elastic deformation section. In a natural state, the seat plate  102  is in an inclined orientation. 
         [0101]    When a pressing force is applied on the seat plate  102  from above through the sample container, the C-shaped arm is elastically deformed to absorb the pressing force. In this state, the seat plate  102  is set in a horizontal orientation. 
         [0102]    On a right end and a left end of the seat plate  102 , a pair of stopper pieces  104 A and  104 B extending downward are provided. With an up-and-down movement of the seat plate  102 , the pair of stopper pieces  104 A and  104 B also move in the up-and-down direction. Thus, when the seat plate  102  is lowered in the downward direction, the stopper pieces  104 A and  104 B also are moved in the downward direction, and the lower end positions thereof are further lowered. As a result, as will be described later, even if the lower end structure attempts to move toward the outer side in the horizontal direction, the stopper pieces  104 A and  104 B collide with the adapter body, and the opening movement of the lower end structure is blocked. 
         [0103]      FIG. 11  is a second perspective view of the attachment  96 . Specifically,  FIG. 11  shows the attachment  96  viewed from diagonally below. As described above, the attachment  96  has the connection structure  98 , the C-shaped arm  100 , and the seat plate  102 . On the right end and the left end of the seat plate  102 , the pair of stopper pieces  104 A and  104 B are provided. Alternatively, the body of the adapter and the attachment may be integrally formed. 
         [0104]    The function of the adapter will now be described with reference to  FIGS. 12-17 .  FIG. 12  is an explanatory diagram for explaining an operation of the adapter. The adapter is attached to the rack body. Arms  70  and  72  have lower end structures  74  and  76 . Each of the lower end structures  74  and  76  has the attachment  96 . In  FIG. 12 , an attachment in an inclined orientation before deformation is shown with a reference numeral  96 A, and an attachment in a horizontal orientation after the deformation is shown with a reference numeral  96 B. 
         [0105]    In a state where the sample container is lowered into the storage unit, the seat plates of the pair of attachments contact the lower surface of the sample container. With this configuration, the sample container is supported from below. In this case, as shown with the reference numeral  96 A, the seat plates of the attachments are in a lifted state, and two arms are in the closed state. 
         [0106]    In the closed state, when an excessive pressing force  111  is applied from above toward below with respect to the sample container, the attachment is deformed as shown with the reference numeral  96 B, by the pressing force  111 . Specifically, the seat plate in each attachment is lowered downward and is set in the horizontal orientation. At the same time, the stopper pieces  104 A and  104 B attached to the seat plate are moved downward. On the other hand, due to the pressing force from above, the two arms  70  and  72  attempt to move in a direction away from each other; that is, the opening direction (refer to reference numeral  112 ). However, because the stopper pieces  104 A and  104 B are lowered downward along with the seat plate, even when the lower end structures  74  and  76  attempt to pass the lower opening formed in the rack body and move to the outside, the stoppers  104 A and  104 B collide with the inner surfaces of the legs  64 A and  64 B, blocking such an opening movement  112 . In other words, in the opening movement by the pressing force, the lower end structures  74  and  76  do not protrude to the outside of the rack body through the lower opening, and the holding of the sample container is maintained. While the sample container stores the liquid sample including a radioactive substance, falling of such a sample container from the rack can be reliably prevented. 
         [0107]    On the other hand, when the pressing force  111  is not present, only the force due to the weight of the sample container is applied to the attachment. In this case, as shown with the reference numeral  96 A, in each attachment, the seat plate maintains the lifted orientation. In such a case, the lower end positions of the stopper pieces  104 A and  104 B are at the lifted end, and thus, when an opening force to the outer side in the horizontal direction is applied to the lower end structures  74  and  76 , protrusion of the lower end structures  74  and  76  to the outside of the rack body through the lower opening formed in the rack body is permitted. In other words, in the pair of the arms, deformation from the closed state to the open state is permitted. 
         [0108]    As described, according to the present embodiment, the protrusion of the lower end structures  74  and  76  to the outside of the rack body is permitted only when an appropriate force to the outer side in the horizontal direction is applied from the guide block. When an abnormal force is caused in the vertical direction, the change of the pair of the arms from the closed state to the open state can be prevented by the functions of the stopper pieces  104 A and  104 B. 
         [0109]      FIG. 13  is a cross sectional diagram of the rack. Specifically,  FIG. 13  shows a state where the adapter is attached to the rack body. In the arms  70  and  72 , a part of the wave-shaped portion  88  is stored in the upper opening shown with R1. The entirety of the arms  70  and  72  is substantially stored in a thick thickness D1 of the rack body, except for the lower end structures  74  and  76 . Thus, in the state before the deformation of the arms  70  and  72 , no portion in the arms  70  and  72  protrudes from the rack body toward the outer side. With this configuration, even when a specific rack is moved in the longitudinal direction in an aligned state of a plurality of the racks, no hooking occurs that would block the movement. 
         [0110]    In a normal closed state where no pressing force from above is applied, when an opening force shown with a reference numeral  114  is applied to the lower end structures  74  and  76  by the contact with the guide block, the lower end structures  74  and  76  move in a direction away from each other, and protrude toward the outer side of the rack through the pair of lower openings. 
         [0111]    In  FIG. 13 , R3 shows a size in the vertical direction of the lower opening  84 . R2 shows a size in the vertical direction of a main region of the lower opening  84 , and R4 shows a size in the vertical direction of a sub area of the lower opening. When the opening force  114  is applied in the closed state, the lower end structures  74  and  76  pass through the pair of lower openings. On the other hand, when the pressing force from above is caused, the plurality of stopper pieces are lowered downward, and, even when the lower end structures  74  and  76  attempt to move in a direction away from each other, the plurality of stopper pieces collide with the inner surfaces of the pair of legs, and such an opening movement is blocked. This operation will now be further described with reference to  FIGS. 14 and 15 . 
         [0112]      FIG. 14  shows a state where the stopper piece is not operating. In such a case, in the lower end structure  76 , the stopper pieces  104 A and  104 B are in the lifted position, and there is a gap hl between a lower side level of the main area in the lower opening  84  and the lower end levels of the stopper pieces  104 A and  104 B. Therefore, the lower end structure  76  can protrude to the outside through the lower opening  84 . 
         [0113]    In the contrary,  FIG. 15  shows an operation state of the stopper piece. Specifically, when the pressing force  114  is applied to the lower end structure  76 , the stopper pieces  104 A and  104 B are lowered downward, and the lower end levels thereof become further lower than the lower side level of the main area of the lower opening  84 . An overlapped portion in this process is shown in  FIG. 15  with Ah. In this state, even when the lower end structure  76  attempts to protrude to the outside through the lower opening  84 , the stopper pieces  104 A and  104 B collide with the inner sides of the leg  64 B, and the movement is reliably prevented. 
         [0114]      FIG. 16  shows a state where the sample container  22  is stored in the rack. Part (A) shows a normal state and part (B) shows a state where the pressing force is caused. Reference numerals  74 A and  76 A show the lower end structures before deformation and in the lifted state, and reference numerals  74 B and  76 B show the lower end structures after deformation and in the lowered state. 
         [0115]      FIG. 17  shows an open state of the arm  72 . With the movement of the rack in the X direction and contact with the guide block, an opening force  116  in the horizontal direction is applied on the lower end structure. With this process, as described above, the entirety of the arm  72  including the lower end structure is moved in an open movement. The lower end structure in the open state is shown with a reference numeral  76 C. In this case, the arm  72  is bent and deformed about the wave-shaped portion in the arm  72 . An amount of protrusion of the overall arm  72  from the side surface of the rack is shown by a reference numeral  118 . In such an open state, because the supporting function with respect to the lower surface of the sample container  22  disappears, if there is no member below the sample container  22 , the sample container  22  naturally falls below as shown by a reference numeral  120 . In the present embodiment, in such an open state, the sample container  22  is placed on the head. 
         [0116]    When the sample container is moved from the rack to the head, the pair of lower end structures is moved from a position immediately below the sample container to a retracted position in an outer side in the horizontal direction. Then, after the sample container after the sample measurement is returned to the inside of the sample storage unit, the pair of the arms is returned to the original shape. That is, the pair of lower end structures enter the lower side of the sample container. With this configuration, the sample container  22  is supported by the pair of the lower end structures, and is held. 
         [0117]    As described, according to the present embodiment, in each storage unit, the open/close mechanism can be transitioned from the closed state to the open state at an appropriate timing. In addition, when an abnormal pressing force in the vertical direction is caused in place of the appropriate opening force, unnecessary opening movement is reliably prevented by the function of the plurality of stopper pieces as described above. With such a configuration, falling of the sample container can be prevented beforehand. Further, an elastic deformation section is provided in each arm, and, with the elastic recovery force achieved thereby, the arm can be returned to its original shape. Therefore, the transition from the open state to the closed state can be achieved by the function of the arm itself. According to the present embodiment, no dedicated drive source and no dedicated controller are necessary for the opening operation of the pair of the arms. In addition, no dedicated drive source and no dedicated controller are necessary for the closing operation of the pair of the arms. The pair of the arms can be opened and closed using a part of the transporting force of the rack and in synchronization with the transportation of the rack. 
       (C) Guide Block (FIGS.  18 - 23 ) 
       [0118]      FIG. 18  shows a part of the transport table as an enlarged top view. Specifically,  FIG. 18  shows the guide block  30  and the pressing unit  32 . 
         [0119]    The guide block  30  is fixedly placed on a top frame  122 . The top frame  122  corresponds to a top plate of a structure including the lifting/lowering path. The guide block  30  has a lower layer  124 , an upper layer  126 , a front-side support plate  130 , and a rear-side support plate  132 . These elements are integrated. Each of the lower layer  124  and the upper layer  126  has a flat plate shape spreading in the horizontal direction. The front-side support plate  130  and the rear-side support plate  132  have a form standing upwards. At the center of the guide block  30 , an opening  128  is formed. The opening  128  has a circular or elliptical shape. The opening  128  forms an upper end of the lifting/lowering path, and, in  FIG. 18 , the head  44  is inserted into the opening  128 . A width W1 in the Y direction of the lower layer  124  is approximately the same as a gap between the pair of legs of the rack. Strictly speaking, the width W1 is slightly smaller than the gap. On each of a front end portion (left end portion in  FIG. 18 ) and a rear end portion (right end portion in  FIG. 18 ) of the lower layer  124 , a pair of inclined surfaces  134  are formed. In other words, the lower layer  124  has a shape tapered in both directions. A width in the Y direction of a front surface and a rear surface of the lower layer  124  is W2, and W1&gt;W2. Because the pair of inclined surfaces  134  are formed on the front end portion in this manner, even if there is a position deviation in the Y direction for the rack side in the process of entry of the guide block  30  into a region between the pair of legs, the position deviation can be resolved. In addition, a pair of inclined surfaces are also formed on the rear end portion of the lower layer  124 . With such a configuration, even if it becomes necessary to return-transport the rack, the guide block  30  can be smoothly inserted into the lower part of the rack from the rear side of the rack. 
         [0120]    The upper layer  126  is a portion layered on top of the lower layer  124 , and a pair of inclined surfaces  136  are formed with a relatively long distance at a front-side portion of the upper layer  126 . A width in the Y direction of the front end surface in the upper layer  126  is W3. Here, W1&gt;W2&gt;W3. The pair of inclined surfaces  136  are provided such that the width in the Y direction of the upper layer  126  continuously changes from W3 to W1 along the X direction. 
         [0121]    When the pair of contact members of the pair of open/close mechanisms contacts the pair of inclined surfaces  136 , and the rack is moved forward while the contact state is maintained, an opening force to the outer side in the horizontal direction is applied on the pair of contact members by the function of the pair of inclined surfaces  136 . With this process, the adapter is changed from the closed state to the open state. In the present embodiment, a pair of inclined surfaces are also formed on the rear-side portion of the upper layer  126 . With the pair of the inclined surfaces on the rear side, the recovery from the open state to the closed state can be gradually achieved, thus preventing a rapid change of the open/close mechanism. 
         [0122]    In the present embodiment, the pair of inclined surfaces  136  on the front side and the pair of inclined surfaces on the rear side formed on the upper layer  126  have a symmetrical shape, but alternatively, these surfaces may have an asymmetrical shape. 
         [0123]    The guide block  30  can be roughly divided from the upstream side to the downstream side of the X direction, into a front-side form, an intermediate form, and a rear-side form. Looking into the upper layer  126 , the front-side form of the upper layer  126  realizes the function to change the open/close mechanism from the closed state to the open state. The intermediate form of the upper layer  126  realizes a function to maintain the open state. The rear-side form of the upper layer  126  realizes a function to return the open/close mechanism from the open state to the closed state. The guide block  30  as a whole has a symmetrical shape with reference to a center line passing through a center position in the Y direction and parallel to the X direction. In a state where the guide block  30  enters a region between the pair of legs of the rack, centering (positioning in the Y direction) of the rack is executed. With this process, the center position of the opening  128  and the center position of the sample container to be measured or a sample storage unit that stores the sample container can be easily matched in the Y direction. 
         [0124]    In the present embodiment, as described above, the guide block  30  is fixed on the top frame  122  of the structure including the sample measurement chamber and the lifting/lowering mechanism. The guide block  30  is placed with a certain degree of freedom in the horizontal direction with respect to the transport table. In other words, so long as the guide block  30  is appropriately positioned, it becomes unnecessary to strictly position the transport table itself. For example, even if there is a machining error or an assembly error in the transport table, if the error is within an allowable range, such an error would not cause a problem in the handing of the sample container between the rack and the lifting/lowering mechanism. 
         [0125]    In the process of change of the open/close mechanism from the closed state to the open state with the function of the pair of inclined surfaces  136  formed on the front side of the guide block, the supporting function of the sample container by the open/close mechanism disappears. If the support function disappears before the bottom surface of the sample container is sufficiently placed on the head  44 , there is a possibility of fall-off or change of orientation of the sample container. In consideration of this, in the guide block  30  according to the present embodiment, the front-side support plate  130  is provided. The front-side support plate  130  has a form protruding upward from the upper surface of the upper layer  126 . The front-side support plate  130  realizes a function to support the sample container temporarily and in an auxiliary manner in a state where the supporting function of the sample container by the open/close mechanism has disappeared. A front end and a rear end of the front-side support plate have tapered surfaces of a shoulder shape, such that hooking of a corner portion of the sample container on the front-side support plate is prevented. By preparing an auxiliary support on the front side of the opening  128  in this manner, it becomes possible to prevent fall-off or orientation change of the sample container, and to smoothly transfer the sample container from the rack to the upper surface of the head. 
         [0126]    In the present embodiment, the rear-side support plate  132  is provided on the rear side of the opening  128 . The rear-side support plate  132  has a form similar to that of the front-side support plate  130 . With the rear-side support plate  132 , in a case where, after the sample container after the measurement is returned to the storage unit and in a period until the open/close mechanism returns from the open state to the closed state, if the support function by the upper surface of the head  44  partially disappears, the sample container can be temporarily supported in an auxiliary manner by the upper surface of the rear-side support plate  132 , to prevent fall-off and orientation change of the sample container. 
         [0127]    As described, according to the guide block  30  of the present embodiment, an opening force to the outer side in the horizontal direction can be applied to each open/close mechanism using a part of the rack transporting force. Therefore, because it is not necessary to provide a dedicated drive source or a dedicated drive mechanism for producing such an opening force, there is an advantage that the device structure can be simplified. In addition, the operation timing of each open/close mechanism can be naturally matched with respect to a reference position, and there is an advantage that it is not necessary to control the opening/closing by a controller. Furthermore, because the guide block  30  is placed with reference to the reference position itself, the rack can be centered merely by inserting the guide block  30  between the pair of the legs of the rack. In other words, the sample container or the storage unit having the sample container can be appropriately positioned with respect to the reference position by merely inserting the guide block  30  to the lower part of the rack. 
         [0128]      FIG. 18  also shows the pressing unit  32  in addition to the guide block  30 . A structure of the pressing unit  32  will now be described. The structure and an operation of the pressing unit  32  will again described later with reference to  FIGS. 24-26 . 
         [0129]    In  FIG. 18 , the pressing unit  32  is a unit which applies a pressing force on the rack from the outside of the rack, to realize appropriate position and orientation of the rack. In the present embodiment, the pressing unit  32  has a pair of rollers  138  and  140  which contact the rack and apply the pressing force. A rotational center of the roller  138  is set at a point distanced from the reference position  143  by a certain distance  144  in the upstream side; that is, the front side, in the X direction. A rotational center of the roller  140  is set at a point distanced from the reference position  143  by the certain distance  144  in the X direction. 
         [0130]    As will be described later, the roller  138  is rotatably attached on an end of one movable plate, and the roller  140  is attached to an end of the other movable plate. A common rotational axis of the movable plates is shown with a reference numeral  142 . In the Y direction, the rotational centers of the rollers  138  and  140  are set farther away from the guide block  30  with respect to a rotational center of the common rotational axis  142 . In other words, a negative offset is given to the rotational centers. 
         [0131]    Reference surfaces are formed in the guide block  30  for cooperation with the pressing unit  32  to achieve appropriate position and orientation of the rack. Specifically, reference surfaces  124 A and  126 A are formed. The reference surface  124 A is one side surface of the lower layer  124  and the reference surface  126 A is one side surface of the upper layer  126 . In the present embodiment, the reference surfaces  124 A and  126 A are surfaces parallel in the X direction, and are vertical surfaces. Of the two reference surfaces  124 A and  126 A, the reference surface  124 A spreads wider in the X direction. When the guide block enters a region between the pair of legs of the rack, the pair of rollers  138  and  140  are pressed against the outer surface of one leg (leg on the side of the pressing unit  32 ) of the pair of legs. With this process, the inner surface of the one leg closely contacts the reference surfaces  124 A and  126 A. The reference surfaces are vertical surfaces parallel in the X direction, and the inner surfaces of the pair of legs are vertical surfaces parallel in the longitudinal direction. Thus, in a state where the inner surface is in close contact with the reference surface, the X direction and the longitudinal direction of the rack become parallel to each other. At the same time, the rack is positioned at a predetermined position in the Y direction. As a result, appropriate position and orientation of the rack can be realized. 
         [0132]    In the present embodiment, in an initial state of the rollers  138  and  140 , there is a certain gap AW between the respective roller and the reference surfaced  124 A. Such a gap AW is provided as necessary. In the present embodiment, with the gap AW and the negative offset of the roller  138 , an angle θ1 for receiving a tip portion of one leg is increased, or a resistance when the tip portion is received is reduced. With the function of the inclined surface  134  and such an open angle θ1, even if there is a position deviation in the Y direction in the rack, the one leg can be smoothly inserted between the reference surface  124 A and the roller  138 . 
         [0133]    In a state where the guide block  30  is inserted between the pair of legs as described above, a pressing force is applied from the rollers  138  and  140  on the outer surface of the one leg. With this process, the inner surface of the one leg closely contacts the reference surfaces  124 A and  126 A. Therefore, an appropriate position (in particular, a position in the Y direction) and an appropriate orientation of the rack can be achieved with a simple mechanism. In the present embodiment, because the rollers  138  and  140  are provided as the pressing member, even when the rack is sent toward the front side, a sliding resistance can be reduced. 
         [0134]    In the present embodiment, the pressing unit  32  presses the rack with respect to the guide block  30  placed at the appropriate position and orientation. Thus, even if there is a machining error or an assembly error in the transport table, the rack can be accurately positioned with respect to the reference position. In addition, in the present embodiment, the rollers  138  and  140  are provided with an equal spacing in the front-and-rear direction from the reference position in the X direction. Therefore, a force can be equally applied on both sides of the reference position on the rack. Even if there is a deflection in the rack, in the present embodiment, the pressing is applied in a range in the X direction where the guide block  30  exists, and thus, even with the deflection, the sample container to be measured can be appropriately positioned with respect to the reference position. 
         [0135]    A description of a function of the guide block will be continued with reference to  FIGS. 19-23 . 
         [0136]      FIG. 19  shows a cross sectional diagram of the guide block  30 . As described above, the guide block  30  has the upper layer  124 , the lower layer  126 , the front-side support plate  130 , and the rear-side support plate  132 . In  FIG. 19 , each member is shown as a separate member, but in the present embodiment, the members are integrated. The front-side support plate  130  has an upper surface  130 B, and inclined surfaces  130 A and  130 C provided in front of and to the rear of the upper surface  130 B. The rear-side support plate  132  has a structure similar to that of the front-side support plate  130 . 
         [0137]    The opening  128  penetrating through the guide block  30  in the up-and-down direction is formed at a center of the guide block  30 . In  FIG. 19 , the head  44  is inserted into the opening  128 . The head  44  has a placement surface  44 A serving as an upper surface, and the sample container  22  is placed on the placement surface  44 A. As shown in  FIG. 19 , in the process of transition from the closed state to the open state of the open/close mechanism, the sample container  22  is supported in an auxiliary manner by the upper surface  130 B of the front-side support plate  130 . As a result, the sample container  22  can be smoothly transferred onto the placement surface  44 A. During the transfer, it is desirable that a level of the placement surface  44 A of the head  44  and a level of the upper surface  130 A are substantially matched. Alternatively, one of these surfaces may be slightly above or below the other surface. 
         [0138]      FIG. 20  shows a state where the rack  14  is introduced to the X transport path. 
         [0139]    As described above, the rack  14  has the protrusion  58 A, and a part of the hook member  54  of the transporting mechanism  52  is inserted to the protrusion  58 A. The guide block  30  is provided at the center of the X transport path, and the pressing unit  32  is provided nearby. 
         [0140]      FIG. 21  shows a state where the rack  14  is progressed on the X transport path. Specifically,  FIG. 21  shows a state where the front-most sample container is positioned with respect to the opening of the guide block  30 . In this state, the pressing unit  32  functions, and an appropriate position and an appropriate orientation of the rack  14  are realized. 
         [0141]      FIG. 22  shows a part of the contents shown in  FIG. 21  as an enlarged view. The rack  14  has a plurality of storage units, and the adapter  60  is attached in each storage unit. The adapter  60  has the open/close mechanism  26 . In  FIG. 22 , only one open/close mechanism  26  is set in the open state. The guide block  30  functions to form such an open state. As described above, the guide block  30  has the lower layer  124 , the upper layer  126 , etc. The guide block  30  is inserted between the pair of legs in the rack  14 . In  FIG. 22 , only the leg  64 B on the other side is shown. On the rear side of the lower layer  124 , the pair of inclined surfaces  134  is formed, and on the rear side of the upper layer  126 , the pair of inclined surfaces  136  is formed. In the front-side portions of the lower layer  124  and the upper layer  126  also, the pair of inclined surfaces are formed. In the state shown in  FIG. 22 , the pressing force in the Y direction is applied on the rack by the pressing unit  34 , so that the appropriate position and orientation of the rack  14  are achieved and stabilized. 
         [0142]      FIG. 23  is a YZ cross sectional diagram of a portion shown in  FIG. 22 .  FIG. 23  includes a cross section of the guide block  30 . As described above, in the rack  14 , the adapter  60  is provided in each storage unit, and the sample container  22  is stored inside the storage unit. In the state shown in  FIG. 23 , the open/close mechanism  26  is in the open state, and thus, the lower end structures  74  and  76  protrude from the rack body in both sides in the horizontal direction. Such a function is caused by the pair of contact members of the lower end structures  74  and  76  contacting the pair of inclined surfaces of the guide block  30 . 
         [0143]    In  FIG. 23 , a pressing force is applied from the pressing unit  32  to the rack  14  such that the position and orientation of the rack  14  are stabilized. The lifting/lowering path  28  is formed on the lower side of the guide block  30 . As already described, the lifting/lowering mechanism has the shaft  43  and the head  44 . These elements move in the up-and-down direction inside the lifting/lowering path  28 . 
         [0144]    As shown in  FIG. 23 , the shutter mechanism  42  is provided below the guide block  30  across the lifting/lowering path  28 . The shutter mechanism  42  will now be briefly described. The shutter mechanism will be again described later with reference to  FIGS. 34-36 . 
         [0145]    The shutter mechanism  42  includes an upper shutter mechanism  42 A and a lower shutter mechanism  42 B. The upper shutter mechanism  42 A is a mechanism for blocking extrinsic radiation from above, and the lower shutter mechanism  42 B is a mechanism for blocking extrinsic light entering through the lifting/lowering path  28 . 
         [0146]    Specifically, the upper shutter mechanism  42 A has a lead block  148 . With the lead block  148  covering the upper side of the sample measurement chamber across the lifting/lowering path  28 , the extrinsic radiation (in particular, cosmic rays or the like) directed toward the sample measurement chamber through the lifting/lowering path  28  is blocked. Portions of the sample measurement chamber other than the lifting/lowering path  28  are basically covered with a shielding member. Alternatively, when the blockage of the space ray is the objective, the placement of the shielding member on the lower side of the sample measurement chamber may be omitted. 
         [0147]    The upper shutter mechanism  42 A has a tube guide  146 . The tube guide  146  is positioned on the lifting/lowering path  28  when the lead block  148  is in the retracted position, and realizes a guide function with respect to the head  44  and the sample container  22  in this state. In the state where the lead block  148  is moved forward, the tube guide  146  moves to a position retracted from the lifting/lowering path  28 . The tube guide  146  is a hollow member similar to a sleeve, and has an alignment function as will be described later. 
         [0148]    The lower shutter mechanism  42 B has a light-shielding plate  150  and a slit structure  230 . When the light-shielding plate  150  is moved forward, a part of the light-shielding plate  150  is inserted into a slit of the slit structure  230 , and, with this process, the light-shielding plate  150  is placed across the lifting/lowering path  28 . In this state, the extrinsic light from above is blocked by the light-shielding plate  150 . 
         [0149]    As described, according to the guide block, the open/close mechanism can be opened using a part of the drive force for transporting the rack in the X direction. Therefore, there is an advantage that it is not necessary to provide a dedicated drive source for such an opening operation. In addition, when the guide block is inserted between the pair of legs, an appropriate position of the rack in the Y direction can be achieved. That is, the centering can be realized naturally. In addition, because the guide block of the present embodiment has a member which supports in an auxiliary manner the lower side of the sample container in a halfway state between the open state and the closed state, it is possible to prevent disturbance in the orientation or the like of the sample container during the operation of the open/close mechanism. Further, the guide block  30  of the present embodiment has the reference surface which functions with the pressing unit, and, with the cooperation of these elements, the appropriate position and orientation of the rack can be easily realized. 
       (D) Pressing Unit (FIGS.  24 - 27 ) 
       [0150]      FIG. 24  shows an operation state of the pressing unit  32  as a perspective view. When the rack  14  is transported in the X direction, the pressing unit  32  realizes its function on the left side in the direction of movement of the rack  14 . As described above, the pressing unit  32  has the pair of rollers  138  and  140 . In order to give an elastic urging force to the pair of rollers, a first movable plate  154  and a second movable plate  156  are provided. 
         [0151]      FIG. 25  is a first perspective view of the pressing unit. The first movable plate  154  is an upper plate, and the second movable plate  156  is a lower plate. The plates  154  and  156  rotate about the common rotational axis  142 . The first movable plate  154  has a crank shape and the second movable plate  156  also has a crank shape. 
         [0152]    The first movable plate  154  has a front-side bent portion  154   a,  an intermediate portion  154   b,  and a rear-side bent portion  154   c.  The roller  138  is rotatably attached to an end of the front-side bent portion  154   a.  The second movable plate  156  has a front-side bent portion  156   a,  an intermediate portion  156   b,  and a rear-side bent portion  156   c.  The roller  140  is rotatably provided on an end of the front-side bent portion  156   a.  A pin  160  is a restriction member for preventing unnecessary excessive rotation of the first movable plate  154  in a counterclockwise direction as viewed from above. Similarly, a pin  158  is a restriction member for preventing unnecessary excessive rotation of the second movable plate  156  in the clockwise direction as viewed from above. 
         [0153]      FIG. 26  is a second perspective view of the pressing unit. As described above, the first movable plate  154  has the rear-side bent portion  154   c,  and a movement axis  164  is provided at an end thereof. The second movable plate  156  has the rear-side bent portion  156   c,  and a movement axis  166  is provided at an end thereof. A spring  162  is provided in a state of being extending more than in the natural state, between the movement axis  164  and the movement axis  166 . In other words, an elastic recovery force is caused at all times in the spring  162 , which is transmitted to the pair of rollers  138  and  140  through the first movable plate  154  and the second movable plate  156 . With this configuration, the pressing force to the rack is produced. However, as described above, the rotational angles of the plates  154  and  156  in the initial state are restricted by the pair of restriction pins. With this configuration, the gap AW shown in  FIG. 18  is set. 
         [0154]      FIG. 27  is a diagram showing a function of the pressing unit. A negative offset  168  is set for the centers of rotation of the rollers  138  and  140  with respect to the center of the common rotational axis  142 . That is, the centers of rotation of the rollers  138  and  140  are shifted in the Y direction in a direction farther away from the guide block, relative to the center of the rotational axis  142 . The spring  162  is placed between the first movable plate  154  and the second movable plate  156 , and, when the rollers  138  and  140  move in a direction away from the guide block, the spring is extended, and a stronger elastic urging force is produced as a reaction force. With this configuration, the pressing force  172  which presses the rollers  138  and  140  toward the side of the rack is produced. 
         [0155]    In the present embodiment, because the spring  162  is placed between the two movable plates, in the state where the leg is entered only in the region between the roller  138  and the reference surface, a weak pressing force F1 can be produced, and when the legs are entered in the regions between the two rollers  138  and  140  and the reference surface, a strong pressing force F2 which is a sum of the forces for the two rollers  138  and  140  can be produced. In other words, a gradational force can be realized according to the situation of the entrance. In the initial state where the leg is not entered between the roller  138  and the reference surface, a gap AW is formed in the region, and thus, the force applied on the leg when the leg enters the region (reaction force, impact force) can be reduced as compared to a case where the gap is not formed. 
         [0156]    According to the pressing unit as described above, as described with reference to  FIG. 18 , the pressing force can be applied to the outer surface of one leg so that the inner surface of the one leg is in close contact with the reference surface of the guide block. In addition, in this case, because the rollers  138  and  140  are placed to have the negative offset  168 , as shown in  FIG. 18 , an advantage can be obtained in which the open angle for receiving the one leg can be set relatively large. In other words, the rotational movement of the first movable plate can be executed smoothly. Moreover, because in the present embodiment the spring is placed between the two movable plates, the pressing force can be gradationally increased according to the state of sandwiching. 
       (E) Structure of Lower Side of Transport Surface (FIGS.  28 - 33 ) 
       [0157]    Next, a structure of a lower side of the transport surface will be described.  FIG. 28  shows a state where the rack  14  is introduced in the X transport path. In this state, the head  44  is inserted into the opening of the guide block  30 . That is, the head  44  is at the uppermost position. The sample measurement chamber  34  is provided immediately below the opening, and a pair of photomultiplier tubes  174  and  176  forming the measurement unit are provided on respective sides of the sample measurement chamber  34 . A shielding structure  178  is provided in a manner to wrap the entirety of the sample measurement chamber  34  and the pair of photomultiplier tubes  174  and  176 . The shielding structure  178  is formed from lead or the like, and blocks radiation reaching from the outside. However, shielding in the lifting/lowering path is executed by the shutter mechanism to be described later. 
         [0158]      FIG. 29  shows a state where a front-most sample container  36  is moved onto the head  44 . In this state, the open/close mechanism described above is in the open state by the function of the guide block. 
         [0159]      FIG. 30  shows a sample measurement state. The sample container  36  which is set as a measurement target is placed in the sample measurement chamber  34  by the function of the lifting/lowering mechanism  40 . In other words, the sample container  36  is placed between the pair of photomultiplier tubes  174  and  176  in a non-contact state with the photomultiplier tubes. In the sample measurement state, the shutter mechanism is operated across the lifting/lowering path  28 . In  FIG. 30 , the lead block  148  is inserted across the lifting/lowering path  28 . Along with the lead block  148 , a light-shielding plate to be described later is inserted across the lifting/lowering path  28 . 
         [0160]      FIG. 31  shows a YZ cross section of the sample measurement device.  FIG. 31  shows a non-measurement state. In  FIG. 31 , the head  44  is at a position of a lifted end. The lifting/lowering mechanism  40  has a vertical plate  182 , and a rail  184  is attached on the vertical plate  182 . A slide block  186  is provided to be movable in the up-and-down direction with respect to the rail  184 . A lower end of the shaft  43  is attached to the slide block  186 . The head  44  is attached to an upper end of the shaft  43 . A drive force of a motor  188  is transmitted to the slide block  186 , which is then driven in the up-and-down direction. With this movement, the shaft  43  and the head  44  also move in the up-and-down direction. The vertical plate  182  is connected to a base frame  181  and a base plate. 
         [0161]    A case to be described later is fixed on the base frame  181 . The shielding structure  178  is provided on the base plate  180  in a manner to wrap the sample measurement chamber  34  and the case. As described above, the shielding structure  178  is formed from lead or the like. 
         [0162]    The shutter mechanism  42  includes the upper shutter mechanism  42 A and the lower shutter mechanism  42 B. The upper shutter mechanism  42 A has the lead block  148  that blocks extrinsic radiation, and the tube guide  146 . The lower shutter mechanism  42 B has the shielding plate to be described later in detail. 
         [0163]      FIG. 32  shows the YZ cross section of the sample measurement device as an enlarged view.  FIG. 32  shows the sample measurement state. The sample container  36  is placed in the sample measurement chamber  34 . As described above, the shutter mechanism  42  has the upper shutter mechanism  42 A and the lower shutter mechanism  42 B. The upper shutter mechanism  42 A has the lead block  148  and the tube guide  146 . In  FIG. 32 , the lead block  148  is placed across the lifting/lowering path. In addition, the light-shielding plate  150  is inserted across the lifting/lowering path. In this state, the extrinsic radiation from above is blocked, and, at the same time, the extrinsic light from above is blocked. With this configuration, a sample measurement of high precision can be realized. 
         [0164]      FIG. 33  shows a lower part of the sample measurement chamber as an enlarged view.  FIG. 33  shows the sample measurement state. The upper surface of the base frame  181  forms a reference level  206 . That is, in the movement control of the sample container  36  in the up-and-down direction, the reference level  206  forms an origin point in the Z direction. 
         [0165]    A case  190  having a shape to wrap the sample container  36  is provided on the base frame  181 . On the outer side of the case  190 , the shielding structure  178  is provided. An opening is formed at the center of the base frame  181 , and the shaft  43  passes through the opening. The body of the head  44  is attached to the upper end of the shaft  43 . Specifically, a well  192  is formed in the body of the head  44 , and the upper end  43 A of the shaft  43  is inserted into the well  192 . A ring-shaped stopper  193  is provided on the upper end  43 A, and a spring  196  is placed between the stopper  193  and a bottom surface  192 A of the well  192 . 
         [0166]    A top plate  194  is provided in a manner to cover the well  192 . The top plate  194  is formed as a layered structure in the present embodiment, and includes at least an elastic sheet at a lower side and a metal reflective layer at an upper side. Alternatively, optical reflection may be produced by painting. The lower elastic member sheet functions as a light-shielding sheet. For example, as shown in  FIG. 18  which has already been described, two screw members are used to attach the top plate  194  on the body of the head  44 . With such a configuration, a superior light-shielding state is formed. 
         [0167]    Because the spring  196  is provided on the upper end  43 A of the shaft  43 , even if the shaft  43  is lowered slightly excessively, the excess is absorbed by the spring  196 . Therefore, the head  44  can be easily lowered to a point where the lower surface of the head  44  closely contacts the upper surface of the base frame  181 ; that is, the reference surface. In addition, in such a close contact state, as will be described below, a superior light-shielding state can be formed. The stopper  206  is fixedly placed below the head  44  in the shaft  43 , and even when the spring  196  is changed from a compressed state to an elongated state, the amount of change is restricted by the stopper  206 . In other words, the elongation of the spring  196  is allowed until the stopper  206  contacts the lower surface of the head  44 . 
         [0168]    Next, the light-shielding structure (inner light-shielding structure) will be described. The light-shielding structure is a structure constructed over the upper surface of the base frame  181  and the lower surface of the head  44 . 
         [0169]    A first ring groove  198  is formed on the upper surface of the base frame  181  in a manner surrounding the opening through which the shaft  43  passes. In correspondence to this structure, a first ring protrusion  202  is formed on the lower surface of the head  44 . In a state where the head  44  is positioned at the lowermost position; that is, the defined position, the first ring protrusion  202  enters the first ring grove  198 , and a state is formed in which the protrusion and the groove are fitted. In this case, the shaft  43  is slightly lowered exceeding the lowermost position of the head by a controller (not shown). With this process, an elastic action by the spring  196  is realized. That is, a force pressing the head  44  downward is produced by the spring  196 . As a result, a superior close contact state is formed between the first ring groove  198  and the first ring protrusion  202 . That is, a superior light-shielding state can be formed. Thus, it becomes possible to reliably block extrinsic light entering the inside of the sample measurement chamber along the outer surface of the shaft  43  and the upper surface of the base frame  181 . In addition, with the fitting between the first ring groove  198  and the first ring protrusion  202 , a positioning function in the horizontal direction can be obtained for the head  44 , with which the sample container  36  can be appropriately positioned in the sample measurement chamber. 
         [0170]    In the present embodiment, an outer light-shielding structure is constructed at an outer side of the inner-light shielding structure described above. Specifically, a second ring groove  200  is formed on the upper surface of the base frame  181  in a manner to surround the first ring groove  198 . On the other hand, a second ring protrusion  204  is formed on the lower surface of the leg of the case  190  in a manner to surround the first ring protrusion  202 . The second ring protrusion  204  enters the inside of the second ring groove  200  in the assembly state, to form a fitted state between the protrusion and the groove. With this configuration, it becomes possible to reliably block the extrinsic light attempting to enter the inside of the sample measurement chamber from the periphery of the base frame  181  along the upper surface of the base frame  181 . 
         [0171]    The extrinsic light entering the head  44  along the outer surface of the shaft  43  is blocked at the inside of the head  44 . That is, the top plate  194  has the light-shielding sheet, which is fixed in a closely contacted manner with respect to the body of the head  44 , and the extrinsic light is confined in the head  44  by the light-shielding sheet. In other words, intrusion of the extrinsic light through a gap between the top plate  194  and the body of the head  44  is prevented. In order to achieve sufficient light shielding in the head  44 , desirably, a sheet made of a black elastic material is used as the light-shielding sheet. The body of the head  44  is formed from, for example, a hard resin or the like. The base frame  181  is formed from a metal or the like. The case  190  is formed from a metal. 
       (F) Light-Shielding Unit (FIGS.  34 - 36 ) 
       [0172]      FIG. 34  shows a first perspective view of the shutter mechanism  42 . Specifically,  FIG. 34  shows the shutter mechanism  42  as viewed from diagonally above. 
         [0173]    As already described, the shutter mechanism  42  has the upper shutter mechanism  42 A and the lower shutter mechanism  42 B. The upper and lower shutter mechanisms are attached to a fixed structure. Specifically, fixed blocks  208  and  210  are provided in parallel with each other, and a shaft  212  and a guide member  214  are placed between the fixed blocks. A block is attached in a slidable manner on the shaft  212 , and forms a part of a movable member  220 . The guide member  214  forms a guide rail, and a roller  224  of the movable member  220  rotationally moves on the guide member. 
         [0174]    A block base  218  spreading in the horizontal direction is provided over the fixed block  208  and the fixed block  210 . The block base  218  is a member corresponding to the top frame  122  shown in  FIG. 13 . That is, the guide block  30  is fixed on the block base  218 . A motor  226  forms a single drive source in the slide mechanism  182 , and a feed screw  216  is driven by the motor  226 . In the present embodiment, the feed screw  216  is formed by a trapezoidal screw. A block (not shown) is connected to the trapezoidal screw  216 , and, with the rotation of the feed screw  216 , the movable member  220  having the block moves in the horizontal direction. The upper shutter mechanism  42 A is attached to a movable frame of the movable member  220 , and, similarly, the lower shutter mechanism  42 B is also attached to a movable frame  222 . 
         [0175]      FIG. 35  shows a second perspective view of the shutter mechanism  42 . Specifically,  FIG. 35  shows the shutter mechanism  42  as viewed from diagonally below. The movable member  220  includes the upper shutter mechanism  42 A and the lower shutter mechanism  42 B. The lower shutter mechanism  42 B has the light-shielding plate  150 . The light-shielding plate  150  is formed from, for example, a thin metal plate, and examples of the materials forming such a metal plate include zinc, copper, and the like. Preferably, the light-shielding plate  150  is formed from a metal having a light-shielding function and also a function to block a braking radiation. A braking radiation is a radiation secondarily produced when the extrinsic radiation is blocked by the lead block. 
         [0176]    The light-shielding plate  150  has a body  150 A forming a horizontal portion, and a bent portion  150 B continuous from the body  150 A, and the bent portion  150 B forms a vertical portion. Further, the bent portion  150 B is connected to an attachment portion  150 C, and the attachment portion  150 C is fixed on the movable frame described above. On a base end side of the body  150 A of the light-shielding plate  150 , an elastic member block  228  is provided. Specifically, the elastic member block  228  has a slit  228 A formed therethrough in the horizontal direction, and a part of the body  150 A; in particular, a part of the base end, is inserted into the slit  228 A. The slit  228 A and the body  150 A are not fixed with respect to each other, and a relative horizontal movement is allowed. The elastic member block  228  may be fixed on the movable frame or simply attached to the body  150 A. The elastic member block  228  is formed from, for example, a rubber member or the like. In  FIG. 35 , a slit structure (fixed structure) which receives the body  150 A in the light-shielding plate  150  is omitted in the drawing. 
         [0177]      FIG. 36  shows a cross sectional diagram of the shutter mechanism  42 . As described above, the upper shutter mechanism  42 A has the lead block and the tube guide  146  arranged in the horizontal direction. When the lead block is at the retracted position, the tube guide  146  is positioned on the lifting/lowering path, and the centering function (alignment function) with respect to the sample container or the like is realized. 
         [0178]    Specifically, on the inner surface of the tube guide  146 , a tapered surface  146 A is employed at the upper end portion, and a tapered surface  146 B is employed at the lower end portion. With these inclined surfaces, when the sample container moves from the upper part toward the lower part or when the sample container moves from the lower part toward the upper part, even if there is a position deviation in the horizontal direction, the sample container can be positioned at an appropriate position in the horizontal direction by the function of the inclined surfaces. Alternatively, the alignment function with respect to the head may be realized. 
         [0179]    Next, the lower shutter mechanism  42 B will be described.  FIG. 36  shows a slit structure  230 . The slit structure  230  forms a part of the lower shutter mechanism  42 B, and is a fixed structure. The slit structure  230  can be roughly divided into an upper plate  232  and a lower plate  234 , and a slit is formed between the plates. The periphery of the slit is sealed except for the entrance which accepts the light-shielding plate  150 . That is, entrance of light into the slit from the outside is blocked. 
         [0180]    The light-shielding plate  150  is in the retracted position in  FIG. 36 , and, in this case, only the tip of the light-shielding plate  150  enters the slit structure  230 . The light-shielding plate  150  is at a position completely deviated from the lifting/lowering path. 
         [0181]    The elastic member block  228  is attached to the base end side of the light-shielding plate  150 . The elastic member block  228  has the horizontal slit  228   a,  and the light-shielding plate  150  penetrates therethrough. When the shutter mechanism  42  executes the shutter operation, the movable portion in the upper shutter mechanism  42 A and the movable portion in the lower shutter mechanism  42 B move from the retracted position toward a front position. With this configuration, the lead block is inserted at the upper side on the lifting/lowering path, and the light-shielding plate  150  is inserted to the lower side. With such a double shutter state, the extrinsic radiation is blocked, and, at the same time, the extrinsic light is blocked. The slit structure  230  is fixed at the upper end of the case  190  described above. 
         [0182]    When the movable portion in the lower shutter mechanism  42 B reaches the forward position, the end of the slit structure  230  is inserted in a recess  228 B serving as a depression formed in the elastic member block  228 , and an end surface of the end strongly contacts a back surface of the recess  228 B. In other words, the slit structure and the recess are strongly and closely contacted with each other. With this process, the intrusion of the extrinsic light into the inside of the slit through the slit entrance of the slit structure  230  can be reliably blocked. On the attachment end side of the light-shielding plate  150 , a certain deflection portion such as the vertical portion is present, and, when the light-shielding plate  150  is pressed to the front end, the reaction thereof can be absorbed by the base end side of the light-shielding plate  150 . 
         [0183]    According to the shutter mechanism of the present embodiment, shielding of the radiation and shielding of the light can be simultaneously executed with a single drive source and a single slide mechanism, and, thus, it is possible to simplify the mechanism, and at the same time, the control. In addition, because the light-shielding plate is formed from a member having a function to block or attenuate the braking radiation, an advantage can be obtained in that, even if the braking radiation is produced at the lead block, reaching of the radiation to the sample measurement chamber can be effectively reduced. 
         [0184]    Further, because the elastic block is provided at a root side of the light-shielding plate and the intrusion of the extrinsic light into the inside of the slit structure is blocked by close contact of the slit structure to which the light-shielding plate is inserted and the elastic member block, an advantage can be obtained in that the extrinsic light entering from the periphery can be effectively blocked in addition to the extrinsic light coming in from above and through the lifting/lowering path. In the present embodiment, because the sample measurement chamber is provided on a lower side of the transport table; that is, because there is no shaft in the lifting/lowering path when the shutter mechanism is operated, an advantage can also be obtained in that the structure of the shutter mechanism can be simplified. 
         [0000]    (G) Alternative Configuration of Adapter ( FIGS. 37-40 ) Next, an alternative configuration of the adapter will be described with reference to  FIGS. 37-40 . 
         [0185]      FIG. 37  shows a first perspective view of an adapter  236  according to a second preferred embodiment of the present invention. The adapter  236  has an annular frame  238  and a pair of arms  240  and  242  connected thereto. The arm  240  and the arm  242  have shapes symmetric from each other. The arm  242  will be described as a representative of the two arms. The arm  242  has an upper portion  242 A, a wave-shaped portion  242 B, and a lower portion  242 C. A lower end structure  246  is formed in the lower portion  242 C. Similarly, the arm  240  has a lower end structure  244 . 
         [0186]    The lower end structure  244  and the lower end structure  246  has a structure symmetric from each other. The lower end structure  244  has a rib  248  protruding in the horizontal direction and a support plate  250  fixed thereto. An upper end  250   a  of the support plate  250  has an inclined portion inclined in the inner side. A reinforcement plate  252  extending from the arm body is connected to an intermediate position of the support plate  250 . 
         [0187]      FIG. 38  shows a second perspective view of the adapter  236 . As described above, in the arms, the lower end structures  244  and  246  are formed. 
         [0188]    Functions of the lower end structures  244  and  246  will now be described with reference to  FIG. 39 . When an opening force  258  is applied to the outer side in the horizontal direction by the contact with the guide block, the lower end structures  244  and  246  move toward the outer side in the horizontal direction. This movement is shown by a reference numeral  260 . On the other hand, when an excessive pressing force  254  is applied from above and through the sample container in a state where the two arms  240  and  242  are in the closed state, the upper end portion of the support plate is deformed in a manner to fall toward the inside as shown by a reference numeral  256 . With such a deformation, all or a primary portion of the pressing force  254  from above is absorbed. In other words, because the pressing force is concentrated in the deformation portions of the lower end structures  244  and  246 , no opening movement of the two arms  240  and  242  as described above is caused. 
         [0189]    In the first preferred embodiment of the present invention described above with reference to  FIG. 12  or the like, the stopper connected to the deformation portion is used. In contrast, in the present embodiment, erroneous operation by the pressing force from above is prevented using the deformation portion which moves to fall toward the inside. In both preferred embodiments, deformation is effectively used. 
         [0190]      FIG. 40  shows a cross sectional diagram of the rack. The adapter  236  described above is attached on the rack body  262 . Reference numerals  240 A and  242 A show a state where the arms are opened, and reference numerals  240 B and  242 B show a state where the arms are closed. As described above, even when the pressing force is applied from above in the closed state, the pressing force is absorbed by the elastic deformation of a part of the structure, and the opening movement of the pair of arms is effectively prevented. In the case where the adapter according to the second preferred embodiment of the present invention is used also, the guide block shown in  FIG. 18  or the like is used.
Each adapter disposed on a rack includes a pair of arms that configure an opening/closing mechanism. During rack conveyance, a guiding block is slotted between a pair of legs contained in the rack. The opening/closing mechanism being abutted against the guiding block causes the opening/closing mechanism to perform an opening/closing operation. When the state of the opening/closing mechanism is changed from closed to open, a pre-measurement sample container is passed from a sample storage unit to a lifting mechanism. Subsequent to the post-measurement sample container being returned to the sample storage unit, the state of the opening/closing mechanism is changed from open to closed.