Title of Invention: RADIOACTIVE/NUCLEAR THREAT MONITORING USING LONG DETECTORS
Abstract: 
The present specification discloses a radiological threat monitoring system capable of withstanding harsh environmental conditions. The system has (a) one or more cables for measuring a signal induced by a radiological material emitting ionizing radiation when the radiological material comes within a predefined distance of the cables; (b) one or more stations connected with one or more cables for measuring and recording the induced signal; and (c) a central station in communication with one or more stations for gathering the recorded measurements. Radiological material includes fissile threat material such as a ‘Special Nuclear Material’ (SNM).

Detailed Description: 

CROSS-REFERENCE
The present specification relies on U.S. Patent Provisional Application No. 61/325,783, filed on Apr. 19, 2010, for priority and is hereby incorporated by reference in its entirety.


FIELD OF THE INVENTION
The present specification relates generally to the field of radiological threat detection and more specifically to the use of Long Detector (LD) cables for detecting unshielded and/or lightly shielded radiological and/or Special Nuclear Materials (SNM).
BACKGROUND OF THE INVENTION
Conventional radiological material and Special Nuclear Material (SNM) detection systems are typically designed for use along traditional cargo pathways, i.e. cargo entering a country, state or location through maritime and land points of entry.
What is needed, however, is a system capable of detecting the entry of such materials through terrestrial pathways, and in particular, between land-based border points of entry.
It is probable that radiological materials and SNM carried into countries across borders and other such pathways are lightly shielded or not shielded at all, leading to a detectable radiation signature.
Therefore, what is needed is a radiological threat detection system that can be implemented with minimal operational burden, which has effectiveness at ranges relevant for transport along terrestrial pathways, and is suitable for deployment in harsh land environments, which is often typical of border crossings, and that is operationally feasible and sustainable.
SUMMARY OF THE INVENTION
In one embodiment, the present invention is a radiological threat monitoring system capable of withstanding harsh environmental conditions. In one embodiment, the specification discloses a radiological threat monitoring system, comprising at least one cable having an enclosed, elongated interior volume for carrying an ionization current that is induced within the at least one cable by a radiological material emitting ionizing radiation when the radiological material is within a predefined distance from the at least one cable and at least one detector coupled with the at least one cable for measuring a signal corresponding to the induced ionization current, wherein the at least one detector records the measured signal.
Optionally, the radiological material is a fissile material. The cable is a Panofsky Long Ion Chamber. The system further comprises a second cable having an enclosed, elongated interior volume for carrying an ionization current that is induced within the second cable by a radiological material emitting ionizing radiation when the radiological material is within a predefined distance from the second cable. The first cable is parallel to the second cable. The system further comprises a second detector coupled to the second cable for measuring a signal corresponding to the induced ionization current, wherein the second detector records the measured signal. The monitoring station is in data communication with the at least one detector and second detector.
Optionally, the system further comprises at least one sensor adapted to detect an attempt to tamper with the system. The at least one sensor is adapted to detect a severing of the at least one cable by periodically sending a signal through the at least one cable, to detect when the signal is reflected by an end of the at least one cable; to determine a severing of the at least one cable when the reflected signal is not detected; to calculate a time difference between a time of sending the signal and a time of receiving the reflected signal; and to detect a severing of the at least one cable if the reflected signal is received before a predefined time period.
Optionally, the predefined time period is dependent upon a length of the at least one cable. The at least one sensor periodically sends a predefined status message conveying an operational status of said system to a monitoring station, wherein the monitoring station determines that the system is malfunctioning if the status message is not received at a predefined time interval. The at least one cable comprises a gas-filled coaxial cable adapted to carry a voltage, wherein said coaxial cable comprises at least one inner signal electrode concentrically surrounded by at least one outer electrode, wherein the inner and outer electrodes are separated by a spacer surrounding the inner electrode, and wherein the outer electrode is at a higher voltage compared to the inner electrode.
Optionally, the spacer is a ceramic material resistant to high voltages. The inner electrode comprises a conductive material. The spacer comprises a radiation ionizable dielectric at pressures in the range of 1 to 20 atmospheres before being sealed. The at least one cable comprises a scintillating substance and a plurality of light sensitive detector arranged along a length of the at least one cable. The scintillating substance is a liquid scintillator. The light emitted by the scintillating substance is transmitted to the light sensitive detectors by wavelength shifting fibers. The scintillating substance comprises scintillating fibers.
In another embodiment, the present specification discloses a method of monitoring for radiological material, comprising positioning at least one cable having an enclosed, elongated interior volume for carrying an ionization current that is induced within the at least one cable by a radiological material emitting ionizing radiation when the radiological material is within a predefined distance from the at least one cable; measuring a signal corresponding to the induced ionization current using at least one detector coupled with the at least one cable; and determining, based upon said measurement, whether radiological material is present within the predefined distance from the at least one cable.
Optionally, the at least one cable is positioned proximate to vehicular traffic to passively scan vehicular traffic for radiological material. The at least one cable is positioned by embedding it within a section of paved road. The at least one cable is positioned by laying it on the ground and attached to a plurality of stakes. The at least one cable is positioned by hanging it from a plurality of poles.
Optionally, the method further comprises positioning a second cable having an enclosed, elongated interior volume for carrying an ionization current that is induced within the second cable by a radiological material emitting ionizing radiation when the radiological material is within a predefined distance from the second cable. The first cable is positioned parallel to the second cable. The method further comprises measuring a signal corresponding to the induced ionization current in the second cable using a second detector.
In another embodiment, the present specification discloses a radiological threat monitoring system capable of withstanding harsh environmental conditions, the system comprising: one or more Long Detector (LD) cables for carrying ionization current being induced therein by a radiological material emitting ionizing radiation, the ionizing radiation inducing a signal in a LD when the radiological material comes within a predefined distance of the LD; one or more base stations connected with the one or more LDs for measuring the induced signal, each base station recording the measured signal; and a central station in communication with the one or more base stations for gathering the recorded measurements.
Optionally, the radiological material is a fissile threat material such as a ‘Special Nuclear Material’ (SNM). The system is used for monitoring radiological materials being brought within a predefined distance of an area representing threat targets. The system is used for passive scanning of vehicular traffic. The LDs are deployed in the system by embedding within sections of road. The LDs are deployed in the system by laying the cables on ground, such as by attaching to stakes. The LDs are deployed in the system by hanging from poles. Each of the base stations comprises read-out electronics to measure the induced signal. The central station is in communication with the one or more base stations through a wired network. The central station is in communication with the one or more base stations through a wireless network. The central station is in communication with the one or more base stations through one or more fiber optic pathways. Each base station is powered by a rechargeable battery being recharged during periods of daylight by solar cells. The one or more base station is powered by an electrical grid.
Optionally, the one or more base station is unattended by a human operator, the base station being equipped with one or more infrared cameras for monitoring a predefined area visually. The system further comprises one or more sensors for detecting an attempt to tamper with the system. The system further comprises a sensor deployed in the one or more base stations for detecting a severing of a LD by: periodically sending a signal through the LD; determining if the signal is reflected by an end of the LD; detecting a severing of the LD if the reflected signal is not received at the sensor; determining a time difference between a time of sending the signal and a time of receiving the reflected signal; and detecting a severing of the LD if the reflected signal is received before a predefined time period, the predefined time period being dependent on a known length of the LD.
Optionally, the system further comprises a sensor deployed in each of a first base station and a second base station, the first and second base stations being connected to a first and a second end of a LD respectively, the sensors detecting a severing of the LD by periodically sending a signal from the first base station to the second and determining if the signal is received by the second base station within a predefined time. The base station periodically sends a predefined status message conveying a health status of one or more components of the base station to the central station, the central station determining a malfunctioning base station if the predefined status message is not received at one or more of predefined time intervals.
Optionally, three parallel lines of LDs are used for making the system triply redundant. The LDs are Panofsky Long Ion Chambers (PLICs), a PLIC being a gas-filled coaxial cable supplied with high voltage. The LDs are Panofsky Long Ion Chambers (PLICs), a PLIC being a gas-filled coaxial cable supplied with high voltage comprising at least one inner signal electrode concentrically surrounded by at least one outer high-voltage electrode, the inner and outer electrodes being separated by a spacer surrounding the inner electrode. The spacer is a ceramic material resistant to high voltages. The inner electrode is made of a conductive material. The spacer is filled with a radiation-ionizable dielectric at pressures in the range of 1 to 20 atmospheres, before being sealed. The one or more LDs comprises a scintillating substance and one or more light-sensitive detectors arranged throughout the length of the one or more LDs, the light-sensitive elements being powered through one or more wires connected to at least one base station. The scintillating substance is a liquid scintillator. The light emitted by the scintillating substance is transmitted to the light-sensitive detectors by means of wavelength-shifting fibers. The scintillating substance comprises scintillating fibers.
The aforementioned and other embodiments of the present shall be described in greater depth in the drawings and detailed description provided below.



BRIEF DESCRIPTION OF THE DRAWINGS
These and other features and advantages of the present invention will be appreciated, as they become better understood by reference to the following detailed description when considered in connection with the accompanying drawings:
FIG. 1 shows an embodiment of radiological threat monitoring system of the present invention deployed at a border crossing;
FIG. 2 shows a cross-sectional view of a Long Detector as employed in the present invention, which in one embodiment is a Panofsky Long Ion Chamber (PLIC) coaxial cable being used as a Long Detector; and
FIG. 3 shows a cross-sectional view of a Long Detector comprising a scintillating material, in accordance with an embodiment of the present invention.



DETAILED DESCRIPTION OF THE DRAWINGS
The present invention is directed towards a radiological threat detection system that can be implemented with minimal operational burden and is effective at ranges relevant for transport along terrestrial pathways. The radiological threat detection system of the present invention employs Long Detector (LD) cables and is suitable for deployment in harsh environmental conditions, generally found along state or international borders.
In one embodiment, the radiological threat detection system of the present invention comprises one or more Long Detector cables for carrying ionization current being induced therein by a radiological material emitting ionization radiation; one or more base stations connected with the one or more LDs for measuring the signal produced by the LD, each base station recording the measured signal; and a central station in communication with the one or more base stations for gathering the recorded measurements.
In one embodiment, the radiological material is a fissile threat material such as a ‘Special Nuclear Material’ (SNM).
In one embodiment, the system is used for monitoring radiological materials crossing state or international borders, whereby the LDs are deployed along state or international borders. In another embodiment, the system is used for monitoring radiological materials being brought within a predefined distance of an area representing threat targets, whereas in another embodiment, the system is used for passive scanning of vehicular traffic.
In one embodiment, the LDs are deployed in the system by embedding the LDs within sections of road. In another embodiment, the LDs are deployed in the system by laying on ground, such as by attaching to stakes. In yet another embodiment, the LDs are deployed in the system by hanging from commonly available poles.
In one embodiment, each of the base stations comprises read-out electronics to measure the signal produced by the LD. Also, in one embodiment, the central station is in communication with the one or more base stations through a wired network, whereas in another, the central station is in communication with the one or more base stations through a wireless network. In yet another embodiment, the central station is in communication with the one or more base stations through one or more fiber optic pathways.
In one embodiment, each base station is powered by a rechargeable battery being recharged during periods of daylight by solar cells. In another embodiment, the one or more base station is powered by an electrical grid. Further, in an embodiment, the one or more base station is unattended by a human operator, and is equipped with one or more infrared cameras for monitoring a predefined area visually.
In an embodiment, the system further comprises one or more sensors for detecting an attempt to tamper with the system. Also, in an embodiment, the system comprises a sensor deployed in the one or more base stations for detecting a severing of a LD by: periodically sending a signal through the LD; determining if the signal is reflected by an end of the LD; detecting a severing of the LD if the reflected signal is not received at the sensor; determining a time difference between a time of sending the signal and a time of receiving the reflected signal; and detecting a severing of the LD if the reflected signal is received before a predefined time period, the predefined time period being dependent on a known length of the LD.
In another embodiment, the system comprises a sensor deployed in each of a first base station and a second base station, the first and second base stations being connected to a first and a second end of a LD respectively, the sensors detecting a severing of the LD by periodically sending a signal from the first base station to the second and determining if the signal is received by the second base station within a predefined time.
In an embodiment, a base station periodically sends a predefined status message conveying a health status of one or more components of the base station to the central station, the central station determining a malfunctioning base station if the predefined status message is not received at one or more of predefined time intervals. In an embodiment, three parallel lines of LDs are used for making the system triply redundant.
In an embodiment, the LDs are Panofsky Long Ion Chambers (PLICs), a PLIC being a gas-filled coaxial cable supplied with high voltage comprising at least one inner signal electrode concentrically surrounded by at least one outer high-voltage electrode. The inner and outer electrodes are separated by a spacer surrounding the inner electrode, increasing a number of inner and outer electrodes improving a sensitivity of the PLIC. In an embodiment, the spacer is a ceramic material resistant to high voltages. Also, in an embodiment, the inner electrode is made of a conductive material. In another embodiment, the spacer is filled with a radiation-ionizable dielectric at pressures in the range of 1 to 20 atmospheres, before being sealed. It should be noted however, that other pressure ranges are possible, and that the range of 1 to 20 atmospheres is presented as an exemplary embodiment. This embodiment is described in further detail below with respect to FIG. 2.
In another embodiment, the LDs comprise a scintillating material and wavelength shifting fibers to absorb and transmit the scintillation light to periodic detectors along the length of the detector. This embodiment is described in further detail below with reference to FIG. 3.
The present invention is directed towards multiple embodiments. The following disclosure is provided in order to enable a person having ordinary skill in the art to practice the invention. Language used in this specification should not be interpreted as a general disavowal of any one specific embodiment or used to limit the claims beyond the meaning of the terms used therein. The general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. Also, the terminology and phraseology used is for the purpose of describing exemplary embodiments and should not be considered limiting. Thus, the present invention is to be accorded the widest scope encompassing numerous alternatives, modifications and equivalents consistent with the principles and features disclosed. For purpose of clarity, details relating to technical material that is known in the technical fields related to the invention have not been described in detail so as not to unnecessarily obscure the present invention.
FIG. 1 shows radiological threat monitoring system 100 in accordance with an embodiment of the present invention. The system 100 comprises a plurality of Long Detector cables 105 (hereinafter referred to as ‘LDs’) each of which is connected to base stations 110 at one or both ends, for radiation induced signal readout. In an embodiment, an LD stretching up to a mile with readout at both ends is employed. In various embodiments, the range of a base station may vary between several hundred feet to several miles and the number of base stations employed over a stretch of LD depends upon cost to efficiency ratios. In one embodiment, the system 100 employs redundancy, such as by using more than one line of LDs 105. In one embodiment, the LDs are Panofsky Long Ion Chambers (PLICs), and the base stations 110 comprise read-out electronics to detect and measure ionization current signals induced due to movement of ions or charged carriers within the LDs 105 as a result of an ‘ionization event’. An ‘ionization event’ comprises radiological/fissile threat material, such as radiological or ‘Special Nuclear Material’ (SNM), coming in the vicinity of the LDs 105 and inducing ionization current within the LDs 105 as a result of ionizing radiations from the radiological threat materials. In another embodiment, the LDs contain a scintillating substance, and the base stations 110 comprise read-out electronics receiving signals from light-sensitive elements in the cable, which collect optical photons from the scintillating substance as a result of an ‘ionization event’, as defined herein. In one embodiment, the LDs are elongated cables which house, within their interior, at least one ionization chamber.
According to an aspect of the present invention, the base stations 110 are in communication with a central station or location (not shown) through wired and/or wireless network connectivity, including by cable, wireless, cellular, or satellite transmission. In one embodiment, fiber optic pathways are strung along the LDs 105 thereby connecting the base stations 110 to a central monitoring and/or data gathering station or location.
The radiological threat monitoring system 100 of the present invention is deployable in a plurality of situations to detect and monitor radiological threats. In one embodiment the system 100 is deployed for protection of open borders between countries, such as the entire US-Mexican border where the LDs 105 are embedded along the borders and/or border crossing roads, lengthwise. The system 100 of the present invention advantageously utilizes the ability to construct and deploy sufficiently long LDs 105 at reasonable costs that can withstand harsh environmental conditions.
In an embodiment, when used for open border protection, the base station 110 is unattended and operated from a rechargeable battery, with solar cells for recharging during periods of daylight, and infrared cameras to monitor the area visually. In an alternate embodiment, the base stations 110 are grid-powered. In one embodiment, the system 100 detects radiological threat materials being carried by a person walking across the border or road thereby causing the radiological threat materials to enter into the vicinity of the deployed LDs 105. Due to the nature of radiological and nuclear materials, the ability to detect threat materials increases by a factor of four with each factor two decrease in the distance between the threat materials and the LDs. Hence, it is required that the detector be as close to the threat material as possible, which is achieved by the present invention wherein the detector-to-threat-material distance is unavoidably very small. For example, when carried in a backpack, that distance is, in one embodiment, of the order of three to five feet at the point of closest approach.
The LDs 105 can also be deployed along, or embedded in, sections of roads at international or state borders, at strategic positions at the periphery of or within cities, as well as in or near other locations that may represent targets of opportunity, such as near the White House or state capitols. In one embodiment, the system 100 is used for unobtrusive, passive scanning of vehicular traffic. As an example, if one mile of roadway is equipped with one or more LDs, a vehicle traveling at 60 miles per hour can be passively scanned for a duration of one minute.
In one embodiment, the LDs 105 are laid on the ground, such as by attaching to stakes. In another embodiment, the LDs 105 are hung from poles, such as those used for electrical wiring. In yet other embodiments, the LDs 105 are embedded in covered tracks in the pavement, particularly when deployed in a roadway. LDs can be embedded in the roadway up to a few inches in depth. Embedding them much deeper is possible, but the pavement and overburden constitute a shield for the radiation emanating from the threat materials, and deeper burial will lead to lower sensitivity of the LDs.
According to another aspect of the present invention, the system 100 of the present invention uses sensors to detect tampering attempts, such as attempts to cut the LDs 105. In one embodiment, the sensors (not shown) are located within the base stations. In order to detect cutting of an LD, one can use a plurality of methods. One method is to periodically send a signal through the cable, and wait for the reflected signal to come back, where the time difference from when the signal is sent through the cable to when the reflected signal returns is a measure of the length of the cable. A broken or cut cable leads to the absence of the reflected signal and/or an anomalous time difference. Another method is by connecting the LD to two base stations, one on either end, and to periodically send a signal from one base station to the other. A broken or cut cable results in the signal not arriving at the second base station. Another exemplary method involves monitoring the background radiation levels detected by each LD. If no background signal is detected within a predefined time, the LD may be dysfunctional. Alternatively, if an enormous signal is detected, the LD may be defective, or it may be detecting the presence of a threat. In either case, an alarm is appropriate since further action is required. There are many other methods to detect a breach in the cable, as are known to those skilled in electrical circuit maintenance.
Systems are also present to monitor tampering with base stations 110. In one embodiment, a status message is periodically sent to a central location. In an embodiment, the status message is encoded and/or contains a sequence number. A broken or tampered-with base station will either send a status message stating that one or more of its systems is non-functional or will fail to send a status message entirely, In an embodiment, a broken or tampered-with base station may send an incorrectly encoded message. Either a status message indicating that one or more systems is non-functional or no status message at all is an event that causes an alarm.
In one embodiment, the system 100 of the present invention is redundant and employs a plurality of parallel LDs 105. In one embodiment, the system 100 of the present invention is triply redundant and employs three parallel LDs, 105. In various embodiments, the present invention employs at least three LDs to make the system redundant because if only two LDs are employed and the signals therein differ, it would not be possible to detect which of the two signals is correct. Hence, three or more LDs are used so that it is possible to detect if one of them is reading an anomalous signal not substantiated by the others.
In one embodiment, the LDs 105 are Panofsky Long Ion Chambers (hereinafter referred to as PLICs) which are air or gas-filled coaxial cables supplied with a high voltage. FIG. 2 shows a cross-sectional view of an embodiment of a PLIC comprising coaxial cable 200 with an inner/central signal electrode 205 concentrically surrounded by an outer high-voltage electrode 215. The electrodes 205, 215 are separated from each other by a spacer 210 which surrounds the signal electrode 205, in one embodiment, in spiral or helical form, over the entire length of the coaxial cable 200.
In one embodiment, the spacer 210 is ceramic or synthetic material such as polyethylene, polypropylene, polyamide or any other synthetic material that is sufficiently resistant to high voltages and environmental hazards and evident to those skilled in the art. In an embodiment, the spacer is a helical plastic contraption which is the same length as the LD and occupies a minimum volume. The spacer holds the central conductor in place, but allows free flow of electrons and ions to anode and cathode.
The inner signal electrode 205 is, for example, a copper wire or a copper rod although any other conductive material known to those of ordinary skill in the art can be used. The surrounding high-voltage electrode 215 is a pipe, such as a copper pipe, which, under certain circumstances, can be a corrugated pipe. Persons of ordinary skill in the art would appreciate that in alternate embodiments, the inner electrode 205 is the high-voltage electrode while the outer electrode 215 is the signal electrode. In an embodiment, the diameters of the inner and outer electrodes range from 0.5 inches to 10 inches, the sensitivity of the device increasing with an increase in the diameters.
When used as a PLIC, the annular space between the two electrodes 205, 215 is filled with a radiation-ionizable dielectric, at pressures in the range of 1 to 20 atmospheres in one embodiment, and then sealed. The dielectric could be air or a gaseous mixture such as a mixture of 95% Argon and 5% CO2, although many other gasses that are known to those skilled in the art could be used advantageously. A voltage of between 250V and 1000V is applied to the high-voltage electrode and read-out electronics are installed at one or both ends of the cable.
When a radiological material, such as nuclear or ‘Special Nuclear Material’ (SNM) comes into the vicinity of the PLIC, the radiation emitted from the nuclear or SNM material causes ionization of the dielectric. In the presence of the electrical field, positive ions produced are attracted to, and therefore undergo a pull in the direction of, the cathode (negatively charged conductor), and negative ions and electrons are attracted to, and therefore undergo a pull in the direction of, the anode (positively charged conductor). This ion attraction induces, across the electrodes, an ionization current signal, which is measured by the read-out electronics.
FIG. 3 shows a cross-sectional view of a Long Detector comprising a scintillating material, in accordance with an embodiment of the present invention. As illustrated in FIG. 3, the LD 300 is a scintillation detector filled with a scintillating substance 310, in liquid or in flexible solid form. Scintillating substance 310 emits light when struck by ionizing radiation. Any available liquid or solid plastic fiber scintillating substance may be used. In one embodiment, the scintillating substance 310 is a liquid scintillator, and the emitted light is collected using wavelength-shifting fiber 320 and is transmitted to light sensitive detectors 330. Examples of light sensitive detectors 330 include photodiodes or photomultiplier tubes. In one embodiment, the light sensitive detectors 330 are arranged periodically throughout the length of the LD 300. The light sensitive detectors 330 themselves are powered and communicate through at least one wire 340 running through the length of the LD 300. In another embodiment a string of light sensitive detectors 330, such as photodiodes, is present throughout the length of the LD 300, powered and communicating through at least one wire 340 running through the length of the LD 300. In another embodiment, the scintillating substance 310 comprises one or more scintillating fibers. The light produced inside the fibers is transported to one or more light sensitive detectors 330, positioned at intervals through the length of the LD 300, powered and communicating through at least one wire 340 running through the length of the LD 300. In other embodiments, an optically reflective coating is present on the inner surface of the LD 300 and surrounding the scintillating substance 310 and on any wires and objects that are not light-sensitive elements.
Referring back to FIG. 1, the LDs 105 act as position-sensitive detectors of an “ionization event” or plurality thereof, which are indicative of a subject carrying radiological material in the vicinity of the LDs. In one embodiment, the ‘subject’ is a person, vehicle or receptacle carrying unshielded or lightly shielded radiological/fissile threat material such as SNM. It is possible to shield radiological materials and SNM so that little ionizing radiation escapes the shield. In such a case, a standard LD is not sensitive to the materials as they are carried past the LD by a subject. However, proper shielding of a substantial amount of radiological material or SNM would be quite heavy, and would impractical to carry by one, or several, individuals. Thus, it is unlikely that a substantially shielded material would be carried across an LD by any subject.
In another embodiment, the LD contains the gas Boron-Fluoride (BF3) and is surrounded by a moderator, such as polyethylene, which is a material capable of slowing down neutrons. Some radiological materials, and all SNM, emit neutrons in addition to gamma rays. Neutrons do not ionize gasses very well. After having slowed down in the moderator surrounding the LD, the neutrons react, however, with the nucleus of the Boron-10 atoms in the gas, and each Boron-10 nucleus then decays to a Lithium nucleus and an alpha particle. The Lithium nucleus and alpha particle are emitted at relatively high energy and are capable of ionizing the BF3 gas. This ionization is then detected, as described above, as a current between the electrodes. When radiological materials and SNM are well-shielded for the emission of ionizing radiation such as gamma rays, the emitted neutrons will still easily penetrate such shielding. In another embodiment the inner surface of the outer conductor of the LD is coated with Boron, and filled with a gas such as the Argon/CO2 mixture discussed herein. In this case, after having slowed down in the moderator surrounding the LD, the neutrons react with the nucleus of the Boron-10 atoms in the coating, and each Boron-10 nucleus then decays to a Lithium nucleus and an alpha particle. The Lithium nucleus or the alpha particle are emitted at relatively high energy and are capable of ionizing the gas.
It is also possible to shield threat materials for the emission of neutrons. This is usually accomplished using large amounts of hydrogenous materials, such as polyethylene. When threat materials are well-shielded for both gamma rays and neutrons, they are difficult to detect by passive means, as is known in the art of passive detection. It is, however, unlikely that materials well-shielded for both gammas and neutrons can be hand-carried across terrestrial pathways because of weight and bulk.
When base stations 110 comprising read-out electronics are installed at only one end of the LDs 105, the other end is either left un-terminated or is terminated with a capacitor for the ionization current or signal traveling in that direction, which is subsequently reflected to the base station end. For single ionization events or multiple events that happen very quickly, the difference in arrival time between the direct signal and the reflected signal provides an indication of where, along LD cables 105, the event(s) occurred.
In another embodiment, base stations 110 comprising read-out electronics are installed at both ends of the LD cables 105. This configuration is used to detect the presence of radioactive materials that are carried by one or more persons walking in a roughly perpendicular direction to the LDs 105 resulting in a series of ‘ionization events’—first increasing and then decreasing in frequency and/or intensity. In such a scenario, it is possible that during the maximum intensity, individual signals are inseparable, which might lead to problems in determining the location of the signal along the LD cables using reflection alone. To overcome this challenge, read-out sub stations 110 are provided at both ends of the LD cables 105 that utilize the timing results of ionization current signals reaching both ends to determine the location of the ‘ionization events’.
In another embodiment, the base stations 110 collect signals from light-sensitive elements embedded in the cable. The light-sensitive elements can be constructed to transmit position information, by the addition of suitable electronics, as will be obvious to those of ordinary skill in the art.
It should be noted that a LD will also be subject to radiation that is normally occurring in the background, either from naturally radioactive materials present in the environment, or from cosmic radiation, or from ionizing radiation produced by lightning. These events, however, occur at random locations along the length of the LD, whereas the passage of radiological material or SNM happens at one or more particular locations with a predictable time signature, allowing the system to differentiate between natural background and the passage of radiological materials or SNM.
LDs are, by their nature, not the most sensitive radiation detectors, and a significant amount of radioactive material must be used in order to provide a detectable signal. The sensitivity of LDs can be improved, as is known to those skilled in the art, by a number of means, including using a higher voltage across the electrodes, or a higher pressure of the gas, or the use of other gasses.
In one embodiment, the LD is operated at very high voltage, as more of a Geiger tube, where an ionizing event leads to the breakdown of the gas as a spark. It is then necessary to protect the LD from damage, by ensuring that the voltage across the electrodes drops quickly so that the spark is not unduly sustained. This can be achieved by putting a resistor in line with the high-voltage supply: as soon as an appreciable current flows between the electrodes, that current also drops the voltage across the electrodes. If the current is due to a spark, the reduced voltage leads to the termination of the spark.
In another embodiment, the LD is operated at moderately high voltage, but the inner (positively charged) conductor (the anode) is modified in that it has sharp protrusions along its entire length. Such a configuration is known in the art as a “drift chamber”. In one embodiment, the protrusions are created by wrapping a very thin conductive wire around the main inner conductor, the thin wire providing the protrusion. In another embodiment, the inner conductor has multiple tiny spikes along its entire surface. These protrusions serve to increase the gradient of the electric field locally. Electrons drifting through the gas to the anode will accelerate in the areas of high electric field gradient, and cause surrounding gas molecules to ionize, in a process known to the art as “gas amplification”. This gas amplification serves to increase the sensitivity of the device.
The above examples are merely illustrative of the many applications of the system of present invention. Although only a few embodiments of the present invention have been described herein, it should be understood that the present invention might be embodied in many other specific forms without departing from the spirit or scope of the invention. Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive, and the invention may be modified within the scope of the appended claims.



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