SUMMARY

A generation of medical grade 252Cf sources was developed in 2002 at the Oak Ridge National Laboratory (ORNL). The combination of small size and large activity of these 252Cf sources makes them suitable for interstitial brachytherapy. A recent in-water calibration experiment showed that the measured gamma dose rates near the new source are slightly greater than the neutron dose rates; contradicting the well established neutron-to-gamma dose ratio of approximately 2:1 at locations near a 252Cf brachytherapy source. Specifically, the MCNPpredicted gamma dose rate is a factor of two higher than the measured gamma dose rate at the distance of 1 cm, and the differences between the two results gradually diminish at distances farther away from the source. To resolve this discrepancy, we updated both the prompt and delayed source gamma spectrum used by ORIGEN-S to more accurately model 252Cf spontaneous fission. We also investigated the bremsstrahlung xrays produced by the beta particles emitted from fission-product decays. The results show that the discrepancy of gamma dose rates is mainly caused by the omission of the bremsstrahlung xrays in the MCNP runs. By including the bremsstrahlung x-rays, the MCNP results show that the gamma dose rates near a new 252Cf source agree well with the measured results and that the gamma dose rates are indeed greater than the neutron dose rates. The calibration experiment also showed discrepancies between the experimental and computational neutron dose profiles obtained. Specifically the MCNPpredicted neutron dose rates were ~25% higher than the measured neutron dose rates at all distances. In attempting to resolve this discrepancy the neutron emission rate was verified by NIST and an experiment was performed to explore the effects of bias voltage on ion chamber charge collection. Further study is needed to completely resolve the neutron dose profile discrepancy and some suggestions to resolve this issue are discussed.