Monte Carlo simulations of semi-infinite clouds of radioactive noble gases

Radiation Protection Bureau, Health Canada, 775 Brookfield Rd, AL6302D1, ON, K1A 1C1, Ottawa, Canada

Abstract

Health Canada maintains detector networks across Canada. One of these networks consists of NaI(Tl) detectors that measure air KERMA [1]. Located beside the NaI(Tl) detector in Ottawa is a radioxenon analyzer [2] that measures the activity concentration of ¹³¹m, 133m, 133, 135Xe directly. The ICRU-accepted KERMA to activity concentration conversion factor for ¹³³Xe, for a semi-infinite cloud measured 1 m off the ground, is 9.68 pGy/hr per Bq/m³ [3]. However, on various dates, the two detectors in Ottawa reported a conversion value of 2.6 ± 0.2 pGy/hr per Bq/m³; we have resolved this discrepancy [4] and have expanded on the study of other isotopes by focusing on the NaI(Tl) detector. Greater accuracy in the conversion value between the air KERMA and activity concentration will assist meteorological modellers in verifying their models [1]. Two Monte Carlo methods were used in this investigation. The first is the analogue geometry where the detector is immersed in a semi-infinite radioactive source. A second method is to apply a reciprocal transform of the analogue geometry. This expedited the calculation of larger clouds. By using these two methods together, we have calculated new values for KERMA rate to activity concentration for 4 of isotopes of noble gases, namely ^131mXe, ^133mXe, ⁸⁵Kr, ^85mKr, ¹³⁵Xe.