Transmission and absorption measurements of high energy electrons with metallic and insulating foils.

摘要

The possible dependence of the extrapolated ranges of electrons on the detector characteristics has been investigated by two methods. These experiments were performed under well standardised geometrical conditions, employing electron beams (0.25 - 0.75 MeV) derived from a Van de Graaff electrostatic generator. The possible effects of poor geometry and small bias voltages on range values have also been investigated, but the results show that these effects could be kept very small in normal transmission measurements. The range values obtained in five metals (Be, Al, Cu Ag and Au) and in four dielectrics (glass, mica, sulphur and polystyrene) by direct current collection method and in A1, Ag and mica by an ionisation method, show that the latter method yields larger range values. A comparison with other results showed that the range values obtained with the Geiger counter are also greater than those obtained by the current collection method, The ionisation-in-depth results in Al showed good agreement with the other results, and with Spencer's theory for equivalent source geometry. A universal transmission curve was obtained for each element using the true range values calculated from the Bethe-Bloch stopping power formula, and its intercept was found to be a smooth function of the atomic number for both metals and insulators. Electron bombardment conductivity has been studied primarily in sulphur and in selenium for comparison. Large current gains were observed in sulphur in confirmation of von Hippel's theory of the conductivity of the elements of Group VIb of the periodic table. The single crystals of sulphur showed as much bombardment conductivity as selenium, but much greater conductivity than the polycrystalline specimen, as was noted by Joffe (1928) and as would be expected for a more perfect crystal lattice. The dependence of electron bombardment conductivity and the current distribution in bombarded dielectrics on the depth of penetration has been studied and the effects of space- charge on those phenomena investigated and discussed.