W n and Neutron Kerma for Methane-Based Tissue-Equivalent Gas

Abstract

Homogeneous tissue-equivalent ionization chambers containing a methane-based gas mixture are widely used to determine the absorbed dose of neutrons employed in radiobiology and radiotherapy. Conversion of the measured ionization charge to the absorbed dose requires knowledge of Wn, the mean energy expended to form an ion pair in the gas by the initial spectra of secondary charged particles produced by the neutrons. The computed charged particle spectra in the gas and the relative kermas contributed by the various types of charged particles are discussed. These spectra are combined with an evaluation of W for the secondary particles to compute Wn as a function of neutron energy. Over the energy range of 0.1-20 MeV, Wn varied from 32.8-31.0 eV, respectively, including sharp changes in Wn due to large resonances in the energy transferred to C and O. These data are recommended for use in evaluating Wn for each neutron spectrum needing accurate dosimetry. A single value of 31.9 .+-. 0.9 eV is recommended for less demanding applications or when neutron spectra are poorly known.