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.