radguide

17 Radiation for Radionuclide Users is particularly sensitive to medium-to-high energy beta particles (e.g., as from 32P) yielding counting efficiencies of around 20 percent. The GM meter is also useful in detecting radiation levels near relatively large (e.g., at least µCi quantity) sources of medium-to-high energy gamma or x-rays. The GM detector, however, is not particularly sensitive to low energy beta particles (such as those from 35S and 14C) yielding efficiencies of no more than 5 percent, nor is it very sensitive to low energy gamma and x-rays (such as from 125I). In addition, neither the GM nor any other portable radiation meter is capable of detecting the very low energy beta particles from 3H. Unlike some other types of portable survey instruments, the GM detector does not actually “measure” exposure or dose rate. It instead “detects” the number of particles interacting in its sensitive volume per unit time. The GM should thus most appropriately be read-out in counts per minute (cpm), although it can be calibrated to approximate mrem/hr for certain situations. Scintillation Detectors Scintillation detectors are based upon the use of various phosphors (scintillators) which emit light in proportion to the quantity and energy of the radiation they absorb. The light flashes are converted to photoelectrons which are multiplied in a series of dynodes (i.e., a photomultiplier) to produce a large electrical pulse (See Figure 4). Because the light output and resultant electrical pulse is proportionate to the amount of energy deposited by the radiation, scintillators can be used in systems designed to identify the amount of specific radionuclides present. Solid scintillation detectors are particularly useful in identifying and quantifying gamma and x-ray emitting radionuclides. The common gamma well-counter employs a large (e.g., 2”x2”) crystal of Sodium Iodide (NaI) within a lead shielded well. The sample vial is lowered directly into a hollowed chamber within the crystal for counting. Such systems are extremely sensitive, but do not have the resolution of more recently developed semiconductor counting systems. Portable solid scintillation detectors are also widely used for conducting various types of radiation surveys. Of particular use to researchers working with radioiodines is the thin crystal (NaI) detector which is capable of detecting the emissions from 125I with efficiencies nearing 20 percent (a GM detector is less than one percent efficient for 125I). The most common means of quantifying the presence of beta particle emitting radionuclides is through the use of liquid scintillation counting. In these systems, the sample and phosphor are combined in a solvent within the counting vial. The vial is then lowered into a well between two photomultiplier tubes for