Deep penetration radiation transport calculations have been performed to determine the dose rates at a positron emission tomography (PET) facility in China. The radiation source is a cyclotron that is used for the production of the PET isotopes, with a labyrinthine concrete wall arrangement to minimize dose to operation personnel. Since directionally biased Monte Carlo calculations did not reach an acceptably low standard error after a week of CPU time, approximate adjoint fluxes were calculated for each of four detector locations using coarse-mesh adjoint discrete ordinates calculations and were used to bias the Monte Carlo calculations. This biasing resulted in improved computational efficiency of a factor of one to three orders of magnitude. This result shows the value of even approximate adjoint flux biasing to speed up Monte Carlo calculations.

Dose calculations using the respiratory tract model presented in Publication 66 of the International Commission on Radiological Protection (ICRP) frequently predict that the basal cells of the anterior portion of the nose, the extrathoracic region ET1 of the model, are the most highly irradiated tissue of the body. The dose to the basal cells is averaged over a layer of tissue 10 microm thick located at a depth of 40 microm into the airway. Reported here are the results of a series of absorbed fraction calculations undertaken to compare with values tabulated in ICRP Publication 66. The Monte Carlo code MCNP4B and the geometric model of the ET1 region specified in Publication 66 were used in the calculations. Although some calculated differences are evident between the two sets of absorbed fractions, typically less than 20%, the calculations confirm that the electron absorbed fractions tabulated in Publication 66 are not responsible for the high estimates of the ET1 dose.