A measurement of WN/e for methane-based tissue-equivalent gas in a d(4)-Be neutron field

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Abstract
Measurements of absorbed dose were carried out in a d(4)-Be neutron field using tissue-equivalent (TE) plastic ionisation chambers and a TE plastic calorimeter in a TE plastic phantom. The gamma -ray dose fraction at the same position was measured using two types of Geiger-Muller (GM) dosemeters. Good agreement was found in the absorbed dose as determined by the use of the two types of ionisation chambers: spherical and thimble-shaped. Bragg-Gray theory was employed, along with measurements using the ionisation chambers and the calorimeter to compute a value for the ratio of absorbed dose to specific charge, which is proportional to the product of the stopping power ratio for secondary charged particles in the chamber wall and gas (Sw,g)N, and the average energy expended in the gas per ion pair formed (WN/e). The value of WN/e for the d(4)-Be field studied was found to be 31.0+or-1.8 JC-1 for methane-based TE gas, assuming the stopping power ratio to be 1.000+or-0.044 for neutrons.