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 t he mean energy W expended to produce an ion pair upon complete slow-do wn for heavy particles in gases. This model is applied to the calculat ion 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 theo retical results for argon are about 15% higher than the experimental v alues 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 reache s the maximal value. Such a phenomenon was also observed in the case o f the W values for protons, which show a clear minimum at the energy o f the maximal cross-section for charge exchange cycles.