Determination of detector response to ℓ/ℓ sources using the ITS Monte Carlo Code System

An area of concern for the radiological survey community is the variation of detector response to specific conditions. Several types of portable detectors used for frisking and routine field surveys are modelled using the ITS Monte Carlo Code System. The detectors modelled include Pancake Geiger-Mueller (PGM) counters, organic and inorganic scintillators. The efficiency of a particular detector is highly dependent on several source parameters such as radius, energy spectrum, and backing material. Detector efficiency is slightly affected by composition of the surrounding air (dependent on temperature, relative humidity and barometric pressure). The Monte Carlo model of each detector is benchmarked against published data of detector efficiency for standard reference sources. In order to validate each code application, the experimental setup is modelled by Monte Carlo methods which show excellent agreement with the experimental values for standard sources. Due to this agreement, the Bicron PGM detector is modelled for "non-standard sources" such as 100-cm² area sources and 4π point isotropic sources located at various radial distances from the center of the detector face. The spectrum of beta particle energy is the major factor that affects efficiency since beta particles with energy less than 45 keV cannot traverse a 1 cm air column and the mica detector window. For one-inch standard sources, the low energy β source ¹⁴C has an efficiency of 3.6%, while the high energy β source ²¹⁰Bi has an efficiency of 24.3%. The effect of source radius on efficiency is solely due to the decreased solid angle subtended by the detector with increasing source radius. For the mixed isotope ⁹⁰Sr-⁹⁰Y deposited on stainless steel, a point source has an efficiency of 27.0% while the same isotope and backing material has only a 6.34% efficiency for a 100-cm² source. The backscatter effect is shown that, for a point ¹⁴C source, the efficiency is 8.4% for stainless steel backing and only 5.8% for a low-Z backing material such as linen. The least pronounced effect is that of moist air composition. The effects of temperature, pressure and humidity often compete since the first two affect density and the latter affects composition. As one would expect, efficiency increases as density decreases, and efficiency decreases as the weight fraction of hydrogen increases. These parameters may change simultaneously, so each case must be examined individually. By examining several cases of moist air composition, it is shown that detector efficiency for a point ¹⁴C source deposited on stainless steel can vary by as much as +/- 0.3%.