W VALUES OF PROTONS SLOWED DOWN IN MOLECULAR-HYDROGEN

For protons completely slowed down in molecular hydrogen, the mean ene rgy W required for forming an ion pair has been studied in the energy range between 1 keV and 10 MeV, partly by measurements and partly by c alculations. The experimental W values were determined for collimated beams of monoenergetic protons with energies between 1 keV and 100 keV by measuring the positive ion yield they produce in a plane-parallel ionisation chamber filled with hydrogen gas. The calculated W values w ere determined (i) using the Monte Carlo method for energies between 1 keV and 100 keV and (ii) within the framework of an analytical model based on the continuous slowing down approximation in the energy range between 1 keV and 10 MeV, taking into account the proton stopping pow er, the effect of charge exchange, the different ionisation cross sect ions for protons and neutral hydrogen particles, and the contributions of secondary electrons to the total ionisation yield. The complete da ta set for W as a function of energy shows the typical decrease with i ncreasing energy in the region of up to about 5 keV, followed by a smo othly oscillating shape for higher energies. It can be assumed that th e oscillating shape of W as a function of energy is mainly caused by c harge-exchange processes and by the ionisation yield contribution of s econdary electrons.