Comparison of the Bragg-gray Theory Corollaries to Fast-neutron Cavity Ionization Measurements at 14.8 MeV

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Abstract
Spherical cavity ionization chambers having equilibrium thickness walls of tissue-equivalent (TE) type A-150 plastic, graphite or Mg were exposed to equal fluences of 14.8 MeV neutrons from a D-T generator. The chambers were filled with methane-based TE gas, CO2 or Ar at pressures adjusted to give approximately the same energy loss by charged particles crossing any chamber cavity filled with any gas. A new method for unfolding the ionization measurements into parts due to wall interactions vs. gas interactions was developed. This method requires as input the values of these ionization components for a single case of 1 chamber containing 1 gas as calculated by Rubach, based on the charged-particle spectral calculations of Caswell and Coyne. The other 8 cases can thence be derived. Possible difficulties with the validity of the 2nd Bragg-Gray corollary (describing the ratio of ionization between 2 different chambers containing the same gas) apparently exist.

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