SU-GG-T-92: Gamma Dose Near a New Miniature Cf-252 Brachytherapy Source

Abstract

Purpose: A new generation of high‐activity miniature 252Cf sources for interstitial brachytherapy has recently been encapsulated at the Oak Ridge National Laboratory (ORNL). The neutron and gamma doses in water near the source were measured. The measured gamma doses were found to be slightly greater than the neutrondoses, contradicting the established rule of thumb that neutron‐to‐gamma dose ratio is approximately 2:1. The current study was conducted to resolve this discrepancy. Method & Materials: Previous report suggests that the source gamma spectrum used by the Monte Carlo (MCNP code) calculations may have underestimated the intensity of low‐energy photons. These photons can be direct emissions from prompt fissions or fission product decays. They can also be produced indirectly via bremsstrahlung from slowing down of the beta particles emitted from fission products. To thoroughly account for the low‐energy photons, we updated the ORIGEN‐S (an ORNL code) data library to include the experimentally measured 252Cf prompt fission gamma spectrum and explicitly simulated the production and decay of fission products using spontaneous 252Cf fission yield data. We also employed the code BETA‐S in conjunction with ORIGEN‐S to generate a beta spectrum from the fission products. This beta spectrum was then used as an input to MCNP to calculate the bremsstrahlung photons and the corresponding doses in water. Results: The inclusion of beta emissions in MCNP calculations doubles the gamma doses in water, and therefore, the MCNP‐predicted gamma doses now closely match the measured results. Conclusions: The aforementioned discrepancy is now resolved. The gamma doses near the new 252Cf source are indeed slightly greater than the neutrondoses. This is because the new source capsule is so thin that many bremsstrahlung photons are able escape the capsule.