Gamma Dose Rate Near a New ²⁵²Cf Brachytherapy Source

A new generation of medical grade ²⁵²Cf sources was developed in 2002 at the Oak Ridge National Laboratory. The combination of small size and large activity of ²⁵²Cf makes the new source suitable to be used with the conventional high-dose-rate remote afterloading system 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 MCNP-predicted gamma dose rate is a factor of two lower 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 the source gamma spectrum by including in the ORIGEN-S data library the experimentally measured ²⁵²Cf prompt gamma spectrum as well as the true ²⁵²Cf spontaneous fission yield data to explicitly model delayed gamma emissions from fission products. We also investigated the bremsstrahlung X-rays 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 X-rays 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.