to produce a dose reading. This modifies the tube characteristic so each count resulting from a particular radiation type is equivalent to a specific quantity of deposited dose. More can be found on radiation dose and dose rate at absorbed dose and equivalent dose. Count rates versus disintegration rates Disintegrations per minute (dpm) and disintegrations per second (dps) are measures of the activity of the source of radioactivity. The SI unit of radioactivity, the becquerel (Bq), is equivalent to one disintegration per second. This unit should not be confused with cps, which is the number of counts received by an instrument from the source. The quantity dps (dpm) is the number of atoms that have decayed in one second (one minute), not the number of atoms that have been measured as decayed. The efficiency of the radiation detector and its relative position to the source of radiation must be accounted for when relating cpm to dpm. This is known as the counting efficiency. The factors affecting counting efficiency are shown in the accompanying diagram. Surface emission rate The surface emission rate (SER) is used as a measure of the rate of particles emitted from a radioactive source which is being used as a calibration standard. When the source is of plate or planar construction and the radiation of interest is emitting from one face, it is known as " emission". When the emissions are from a "point source" and the radiation of interest is emitting from all faces, it is known as " emission". These terms correspond to the spherical geometry over which the emissions are being measured. The SER is the measured emission rate from the source and is related to, but different from, the source activity. This relationship is affected by the type of radiation being emitted and the physical nature of the radioactive source. Sources with emissions will nearly always have a lower SER than the Bq activity due to self-shielding within the active layer of the source. Sources with emissions suffer from self-shielding or backscatter, so the SER is variable, and individually can be greater than or less than 50% of the Bq activity, depending on construction and the particle types being measured. Backscatter will reflect particles off the backing plate of the active layer and will increase the rate; beta particle plate sources usually have a significant backscatter, whereas alpha plate sources usually have no