THE ENERGY-DEPENDENCE OF W-VALUES FOR PROTONS OF ENERGIES UP TO 10 MEV

The behaviour of the mean energy W required to produce an ion pair in gases as a function of the primary energy of charged particles is stil l not completely understood, in particular for protons and heavier par ticles. As a contribution to solving this problem, an analytical model based on the continuous slowing down approximation has been applied t o determine the energy dependence of W for protons slowed down in mole cular nitrogen, methane, carbon dioxide, water vapour and methane-base d tissue-equivalent gas in the energy range up to 10 MeV. Besides the proton stopping powers for calculating the energy degradation, the eff ect of charge exchange, different ionisation cross sections for proton s and neutral hydrogen particles, and the contributions of secondary e lectrons were taken into account in the determination of the ionisatio n yield. This procedure led to a qualitatively satisfactory agreement between the calculated W values and those determined experimentally in the whole energy region.