NEUTRON W VALUES IN METHANE-BASED TISSUE-EQUIVALENT GAS UP TO 60 MEV

The determination of reliable neutron W values for a dosimetrically im portant medium such as methane-based tissue-equivalent gas relies on k nowing the W values of the charged particles produced by the neutron i nteractions in that medium. Following the recent evaluation of proton W values in methane-based tissue-equivalent gas by Siebert et al, it w as thought appropriate to perform a similar re-evaluation for other si gnificant secondary charged particles, namely He-4, C-12, N-14 and O-1 6. This would allow the calculation of neutron W values with a complet ely revised set of data. The re-evaluation shows evidence of a 'bump' in the He-4 W value curve at similar to 200 keV .amu(-1) similar to th at reported for protons, but not for C-12, N-14 or O-16 ions. It also highlights the almost complete absence of experimental data above 60 k eV .amu(-1) for heavy recoils. Determination of neutron W values has s hown a small increase below similar to 1 MeV, and a small decrease abo ve similar to 8 MeV, when compared to the values reported by Siebert e t al. Furthermore, neutron W values have been calculated for energies up to 60 MeV for the first time.