Angular distribution of X-radiation following electron bombardment of free atoms

Measurements of the angular distribution of X-radiation produced by electron bombardment of free atoms (argon, krypton, xenon) at incident electron energies between 3 keV and 15 keV are presented. The observed anisotropy or polarisation of characteristic X-radiation (L alpha 1,2 transition in xenon) is small (a few per cent) and near threshold approaches the expected value for Q 0 >>Q 1 . The measured anisotropies of bremsstrahlung radiation may be reproduced reasonably well by a modified Sommerfeld formula if the velocity beta is chosen to be half of the incident electron velocity beta 0 . The measured anisotropy may be described by a parameter P, which is compared with the degree of polarisation obtained from theoretical calculations.