Monte Carlo simulation of environmental airborne gamma-spectrometry

Monte Carlo codes are described which have been written from first principles in order to account for and investigate a wide range of problems encountered in making environmental γ-ray measurements. The simulations are complete in concept and combine both γ-ray transportation processes and detector characteristics within a single simulation package. Results are presented for 7.62 × 7.62 cm NaI(Tl) detectors for which cross comparisons with experimental results are most easily made, and with large 16 l NaI(Tl) arrays predominantly used for airborne surveys. Radiation transport, gamma-ray interaction, detector angular response and detection are dealt with at extended (100 m) altitudes for 137Cs, 134Cs and 131I. A complete solution is proposed that could, in principle, form the basis of calibrating airborne detectors by theoretical means, especially where field calibration factors are difficult to obtain in the case of recent nuclear fallout and a large number of radionuclides. Estimates of calibration factors for 137Cs, 40K, 214Bi and 208Tl are presented.