Measurement of the Statistics of Secondary Electron Emission

This paper describes apparatus for the direct measurement of the statistics of secondary electron emission, i.e. the frequency distribution in the number of secondaries produced by individual primary particles. The method is to accelerate the secondary electrons to about 50 keV and to focus them on to a cooled silicon semiconductor detector. Unless a correction is made for the adverse effect of backscattering of electrons from the detector (up to 37%) the resolution of the system does not reach the expected value. A method whereby these scattered electrons are returned to the detector by a magnetic field is described. The resolution of the system is then sufficient to distinguish clearly between the groups even when they contain as many as 15 electrons. Results and comparison with theory for secondary electrons produced by an alpha particle emerging from an aluminum foil are presented. Preliminary results with an image intensifier transmission dynode are also shown.