K-shell radiative electron capture with bare 60-MeV/u Kr ions channeled in a Si crystal: Experiments and simulations

We have performed K-shell radiative electron capture (K-REC) measurements with bare 60.1-MeV/u incident krypton ions, both in channeling conditions and for random orientation of a 37-mu m silicon crystal. The sampled electron densities are quite different in each case, which has an influence both on the shape and on the amplitude of the K-REC photon peak. We have developed simulations of the K-REC photon lines: for this we have determined the impact parameter distribution at statistical equilibrium for various beam incidence conditions (direction and angular spread) using the continuum potential model for channeled ions. Multiple scattering effects were included. The K-REC photon peak was calculated within thc nonrelativistic dipole approximation, K-REC being assumed to be a purely local process. Solid state electron densities were used, and impact parameter dependent electron momentum distributions (Compton profiles) were calculated for 2s and 2p silicon electrons. A remarkable agreement is found between the spectra measured with very high statistics, and the calculated ones, which leads to the following results: (i) The dependence of the K-REC yield on the beam incidence angle is obtained separately for silicon core and valence electrons, which was never observed before. We find that the core electron contribution to REC is still significant for axial alignment, whereas it is generally neglected in the literature. (ii) Electron Compton profiles are found to vary significantly with impact parameter. (iii) The free electron gas model represents a fair approximation for the description of valence electron Compton profiles. (iv) The K-REC cross section is measured with an absolute accuracy better than 20%, and found to be close to the value calculated within the nonrelativistic dipole approximation.