Calculated neutron air kerma strength conversion factors for a generically encapsulated Cf-252 brachytherapy source

The 252Cf neutron air kerma strength conversion factor (SKN/mCf) is a parameter needed to convert the radionuclide mass (μg) provided by Oak Ridge National Laboratory into neutron air kerma strength required by modern clinical brachytherapy dosimetry formalisms indicated by Task Group No. 43 of the American Association of Physicists in Medicine (AAPM). The impact of currently used or proposed encapsulating materials for 252Cf brachytherapy sources (Pt/Ir-10%, 316L stainless steel, nitinol, and Zircaloy-2) on SKN/mCf was calculated and results were fit to linear equations. Only for substantial encapsulation thicknesses, did SKN/mCf decrease, while the impact of source encapsulation composition is increasingly negligible as Z increases. These findings are explained on the basis of the non-relativistic kinematics governing the majority of 252Cf neutron interactions. Neutron kerma and energy spectra results calculated herein using MCNP were compared with results of Colvett et al. and Rivard et al.