Secondary electron emission from ionization of water vapor by 0.3- to 2.0-MeV He+ and He2+ ions

Yields of secondary electrons (delta rays), differential in ejected electron energy and emission angle, are reported for 0.075 to 0.5 MeV/amu helium ions, both singly and doubly charged (α particles) arising from single collisions with water molecules in vapor phase. Results are compared with similar data reported previously for protons with the same velocity. We found that the bound electron of the He+ ion effectively screens the nuclear charge for collisions at large impact distances, thereby suppressing the yield of low-energy secondaries while contributing directly to the yield of high-energy secondaries. This results in a secondary electron spectrum of higher mean energy and lower total yield for He+ than the bare He2+ produces. Comparison of the yields for He2+ with similar data for protons indicates deviations from Z2 scaling which occur primarily for secondary electrons ejected at small angles, and hence, close to the path of the primary ion and for electron velocities similar to the ion velocity. These results imply that the detailed structure of helium ion tracks is somewhat different from what would be predicted from a simple Z2 scaling of proton track structure