title: W value measurements for protons in tissue-equivalent gas and its constituent gases.
abstract: Values of W (the mean energy required to create an ion pair when a charged particle is stopped in a gas) are needed in radiation dosimetry to convert ionisation chamber current readings to absorbed dose. For neutron dosimetry the most important W values are those for protons, and this paper describes absolute measurements for protons with energies between 1.2 and 3.5 MeV for methane-based tissue-equivalent gas and its constituent gases, i.e. methane, carbon dioxide and nitrogen. An ionisation chamber with a parallel-plate arrangement was used to determine the integrated charge created by a measured number of protons, and W values were determined with an estimated uncertainty of about +/- 0.5%. The results are presented and compared with previous measurements. In general the present data tend to be somewhat lower than previously reported values. Finally the accuracy of a formula for predicting W values for gas mixtures from the values for the individual constituents is investigated.
mesh_terms: D000818:Animals; D005740:Gases; D006801:Humans; D011827:Radiation / Q000379:methods*; D011878:Radiotherapy / Q000379:methods*; D011879:Radiotherapy Dosage; D013816:Thermodynamics
chemical_list: D005740:Gases