Simulations of semi-infinite clouds of 133Xe

Health Canada maintains a number of detector networks across Canada. One of these networks consists of NaI(Tl) detectors that measure air Kinetic Energy Release in a MediA (KERMA). Located beside the NaI(Tl) detector in Ottawa is a radioxenon analyzer, that measures the activity concentration of 131m,133m,133,135Xe directly. The International Commission on Radiological Units (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/h perBq/m3. However, after averaging many observations, the two detectors in Ottawa reported a conversion value of 2.6±0.2 pGy/h perBq/m3. This study focuses on the interpretation of the NaI(Tl) detector. thods employing the Monte Carlo simulation were used in this investigation. The first is a brute force method in which simulations are used to model a detector immersed in a semi-infinite radioactive source. No Monte Carlo variance reduction techniques other than photon forcing, are performed. The second method applies a reciprocal transformation of the geometry. This transformation constructs a condensed elongated source surrounded by a ring detector. The reciprocal transform method proved to be an acceptable estimate of the total KERMA but its greatest advantage is that it decreases the computation time from months to hours. Therefore, the reciprocal transform method may be used to estimate the total KERMA at much greater distances than the brute force method can achieve. By a thorough study of the methodology, an improved air KERMA to activity concentration factor has been obtained for 133Xe.