Secondary Electron Emission from Ionization of Water Vapor by 0.3- to 2.0-MeV He + and He 2+ Ions

Abstract

Yields of secondary electrons (.delta. rays), differential in ejected electron energy and emission angle, are reported for 0.075-0.5 MeV/amu [atomic mass unit] He ions, singly and doubly charged (.alpha. particles) arising from single collisions with water molecules in vapor phase. A comparison with similar protons with the same velocity is provided. 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 produces a secondary electron spectrum of higher mean energy and lower total yield for He+ than the bare He2+. 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 close to the path of the primary ion and for electron velocities similar to the ion velocity. The detailed structure of He ion tracks is somewhat different from the prediction for a simple Z2 scaling of proton track structure.