Electron scattering from silicon semi-conductor detectors

Abstract

A quantitative investigation of electron scatter from Si semiconductor detectors over the energy range 50–625 keV has been made. All detectors used has depletion depths greater than the electron range in Si. The scatter coefficient, p, was determined in the usual way from the areas under the line and the so-called “low energy tail” which constitute the observed spectrum when mono-energetic electrons fall on a detector. Experimental results are given for the dependence of p on primary electron energy. E 0, on the angle subtended by the detector at the source and on the size and material of any collimating aperture. Measurements of the energy distributions of the scattered electrons are also given. The measured value of p for E 0=625 keV and a detector of area 100 mm 2 is shown to vary from 0.15 ± 0.02 for perpendicular electron incidence to 0.42 ± 0.02 for 35% collection efficiency. No differences in scattering properties were observed between lithium-ion-drift, and surface-barrier detectors and the spread in measurements between detectors was within that due to the experimental method. All experimental results given here, as well as the apparent inconsistencies of those in recent publications 2, 3) are satisfactorily explained on the basis that the value of p measured in this way is not identical with the saturation backscatter coefficient, p b, for Si, as had been assumed hitherto. Instead, p is shown to be a “total scatter coefficient”, p t, incorporating 3 groups of electrons: (a). Those backscattered from the Si; (b). Those which have lost some energy before reaching the detector by being scattered from other parts of the apparatus, and if no aperture is used; (c). Those which are scattered out of the sides of the sensitive volume of the detector. It is shown how the relative contribution of the 3 groups to p t varies with E 0 and the geometry of the experiment and how a knowledge of these factors and of p b can be used to calculate p t for any geometry to agree to within ±0.03 with the measured value.