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.