Determination of W values for tissue-equivalent gas and its constituents using alpha particles in the range 1.1-3.9 MeV

Experimental W values of low-energy protons in the alkane series from methane to pentane

Accurate measurements of the mean energy expended in a gas per ion pair formed (W value) for protons slowed down in alkanes are needed (i) in the fields of neutron metrology and microdosimetry, and (ii) in the field of basic radiation research to check the validity of theoretical models describing the degradation of protons in matter. Here, data for protons at energies of less than 100 keV are of particular interest because W increases strongly with decreasing proton energy in the very low-energy region and shows some structure between 10 keV and 100 keV. For these reasons, the energy dependence of W was studied for completely slowed down monoenergetic protons in the alkane series from methane to pentane at energies 1 keV < or = T < or = 100 keV, by measuring the positive ionization yield produced in a plane-parallel ionization chamber. The final results, which have uncertainties of less than 1.5%, are given in tabular form and also in an analytical representation. They were used to determine the differential value omega of the mean energy expended in a gas per ion pair formed and to analyse differences in proton degradation in the alkanes as a function of the number of carbon atoms.

W values for helium particles in nitrogen and argon in the energy range from 10 keV to 10 MeV

An analytical model is presented that facilitates the calculation of the mean energy W expended to produce an ion pair upon complete slow-down for heavy particles in gases. This model is applied to the calculation of the W values for helium beams of energies from 10 keV to 10 MeV in argon and nitrogen gas. Good agreement of the theoretical W values with the experimental ones is found in the case of nitrogen. The theoretical results for argon are about 15% higher than the experimental values throughout the entire energy range. However, they show a similar energy dependence. The theoretical and the experimental W values for nitrogen exhibit an extraordinary energy dependence at energies around 300 keV, at which the cross-section for charge exchange cycles reaches the maximal value. Such a phenomenon was also observed in the case of the W values for protons, which show a clear minimum at the energy of the maximal cross-section for charge exchange cycles.