SUBJECT: M.S. Thesis Presentation
   
BY: Robert Kelm
   
TIME: Monday, March 9, 2009, 12:00 p.m.
   
PLACE: Neely Building, 118
   
TITLE: In-Water Neutron and Gamma Dose Determination for a New Cf-252 Brachytherapy Source
   
COMMITTEE: Dr. Chris Wang, Chair (NRE)
Dr. Nolan Hertel (NRE)
Dr. Sang Hyun Cho (NRE)
 

SUMMARY

Recently, the Oak Ridge National Laboratory (ORNL) successfully encapsulated a new generation medical grade Cf-252 sources having intensities and size comparable to that of the widely used high-dose-rate (HDR) Ir-192 brachytherapy sources. Advent of the new sources, therefore, marked a new era for Cf-252-based neutron brachytherapy (NBT). A recent study has been conducted at Georgia Tech on determining the neutron and gamma dose rates in water surrounding the new source. A Lucite-walled water phantom was built for this study. The neutron and gamma dose rates were determined both by ion chamber measurements and by Monte Carlo code MCNP. The results show that the measured neutron absorbed dose rates were approximately 25% lower than that predicted by MCNP for all dose positions in water, suggesting that the Cf-252 content of the new source is actually 25% lower than the ORNLís estimate. The measured gamma absorbed dose rates in water, on the contrary, are higher than that predicted by MCNP. The differences between the measured and MCNP-predicted gamma doses are not uniform for all dose positions; they are most pronounced (~a factor of two) at the distance of 1 cm, and fall to approximately 30% at distances 2 cm and beyond. These results suggest that the spectrum of prompt gamma rays emitted from the new Cf-252 source may contain significantly more low-energy gamma rays than the previously published spectrum used in MCNP.