The effect of target absorption on the attenuation characteristics of bremsstrahlung generated at constant medium potentials

Abstract

General expressions for the spectral distribution and the attenuation of the intensity and exposure-rate of bremsstrahlung generated in a thick plane absorbing target by electrons of constant medium energy are derived in terms of the intensity-photon energy distribution of the radiation at its point of origin, on the assumption that electron penetration of the target is given by the Thomson-Whiddington relation. Comparison of computed exposure-rate attenuation curves with published experimental results shows that agreement is sensitive to the form assumed for the variation with bremsstrahlung energy of the coefficient B of the differential radiative cross section of the target nucleus, and that significantly better agreement is obtained when this is assumed to decrease with increasing photon energy, rather than remain constant, in general agreement with the predictions of Bethe and Heitler (1934). It is concluded that the reasonable agreement shown with the results of computations based upon the Kulenkampff-Kramers expression for radiation intensity leaving the target is fortuitous, being the result of the near-cancellation of errors arising from (i) the neglect of target absorption, and (ii) the assumption of a constant value for B.