Emission of Energetic Secondary Electrons Produced by 1.3-Mev Electron Bombardment

The emission of energetic secondary electrons (delta rays or knock-on electrons) produced by 1.3-Mev primaries which penetrate thin targets with negligible mean energy loss has been investigated. These have previously not been observable in the customary thick-target experiments because of the inherent impossibility of distinguishing between true secondaries and reflected primaries which have suffered large energy losses. The present arrangement does not record back-scattered primaries, but only emitted electrons. The delta ray yields from various targets (Al, Ni, Au, Ag, Cu, C) ranging in thickness from 5×{}10^-6 to 10^-2 in. have been measured. The determinations of I_δIP vary between 0.3 percent and 2.1 percent independent of surface treatment, and there is only a small dependence of the yield upon target material for an equivalent thickness expressed in mg cm^-2. For 45.8 mg cm^-2 Ni, the delta ray yield is 2.08±{}0.15 percent. This may be compared with 1.48±{}0.25 percent, the yield of low-energy secondaries as measured with the cleanest surface attainable in the demountable experimental tube.