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) measureme nts with bare 60.1-MeV/u incident krypton ions, both in channeling con ditions and for random orientation of a 37-mu m silicon crystal. The s ampled electron densities are quite different in each case, which has an influence both on the shape and on the amplitude of the K-REC photo n peak. We have developed simulations of the K-REC photon lines: for t his we have determined the impact parameter distribution at statistica l equilibrium for various beam incidence conditions (direction and ang ular spread) using the continuum potential model for channeled ions. M ultiple scattering effects were included. The K-REC photon peak was ca lculated within thc nonrelativistic dipole approximation, K-REC being assumed to be a purely local process. Solid state electron densities w ere used, and impact parameter dependent electron momentum distributio ns (Compton profiles) were calculated for 2s and 2p silicon electrons. A remarkable agreement is found between the spectra measured with ver y high statistics, and the calculated ones, which leads to the followi ng results: (i) The dependence of the K-REC yield on the beam incidenc e angle is obtained separately for silicon core and valence electrons, which was never observed before. We find that the core electron contr ibution 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 descriptio n of valence electron Compton profiles. (iv) The K-REC cross section i s measured with an absolute accuracy better than 20%, and found to be close to the value calculated within the nonrelativistic dipole approx imation.