PROPORTIONALITY BETWEEN THE SECONDARY-ELECTRON YIELD AND THE ELECTRONIC STOPPING POWER FOR PROTON IMPACT ON ALUMINUM

Many experiments have shown that the backward yield of secondary electrons induced by protons in solid targets is proportional to the electronic stopping power S(e): gamma(B) = LAMBDA(B)S(e). The parameter AB can be considered as a ''material parameter'' because it is constant over a wide proton energy range. The forward yield, which can be measured with very thin targets, seems to exhibit the same property. Using a Monte Carlo model for the electron transport in aluminium, we have shown that the anisotropy of the angular distribution of the electrons excited by the proton plays a dominant role together with the number of elastic collisions suffered by an electron before it escapes. When this number is low (forward emission at low proton energy), the yield is approximately proportional to the total excitation cross section and, when it is large (backward emission), the yield is proportional to the stopping power.