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

Abstract

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,133,135 Xe directly. The ICRUaccepted KERMA to activity concentration conversion factor for 133 Xe, for a semi-infinite cloud measured 1 m off the ground, is 9.68 pGy/hr per Bq/m 3 [3]. However, on various dates, the two detectors in Ottawa reported a conversion value of 2.6 ± 0.2 pGy/hr per Bq/m 3 ; 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 131m Xe, 133m Xe, 85 Kr, 85m Kr, 135 Xe.

Examination of local atmospheric transport of radioxenon in the Ottawa River Valley

“…2 the 135 Xe/ 133 Xe ratio for the 12 h period release from the medical isotope production facility stack is shown with the 135 Xe/ 133 Xe ratio as measured by a NaI detector located at the Sheenboro measurement site. This information was provided in dose rate and was converted to activity concentrations using conversion factors of 2.6 pGy/h per Bq/m 3 for 133 Xe and 55.10 pGy/h per Bq/m 3 for 135 Xe [23]. Ratios were only calculated when the activity concentration of both isotopes exceeded their respective detection limits of 3.85 Bq/m 3 for 133 Xe and 0.45 Bq/m 3 for 135 Xe [24].…”

Simulation code for estimating external gamma-ray doses from a radioactive plume and contaminated ground using a local-scale atmospheric dispersion model

“…We assumed the condition of just after radionuclide descent from the atmosphere onto the ground and did not consider ground roughness and initial migration into the ground. The cloud sources, whose volume and activity were 1 × 1 × 1 m 3 and 1 kBq/m 3 , respectively, were located in the air at altitudes of 1, 5, 10, 20, 30, 40, 50, 60, 80, 100, 125, 150, 200, 350, 500, 750, and 1000 m. We used the reciprocal transform method with the receptor and source [ 22 , 23 ] in this process to obtain the results for all sources at once. A previous study [ 24 ] reported that the size of the dose-response function was sufficient to consider dose contributions from distant sources of the radionuclides.…”

North Korean nuclear test of October 9th, 2006: The utilization of health Canada’s radionuclide monitoring network and environment Canada’s atmospheric transport and dispersion modelling

“…These detectors are set up to report air KERMA for health physics concerns, but after careful study, we can convert these values to activity concentration [8,9]. This careful study was a result of understanding the radioxenon transport down the Ottawa Valley [1].…”