Angular dependence of probability for proton-induced Cu K-shell ionization at large scattering angles

The probability of proton-induced Cu K-shell ionization has been investigated for small impact parameters by measuring coincidences between K x-rays and 0.5 to 2.0 MeV protons scattered through large angles. In such measurements, corrections for random coincidences are important, and it is shown that high-frequency time structure of the beam current can lead to large systematic errors. After elimination of these problems, the measured ionization probability for 2 MeV protons was nearly constant at large scattering angles, in agreement with predictions from semiclassical approximation (SCA) calculations with straight-line trajectories. At lower energies, however, a significant increase in the ionization probability has been found for the largest scattering angles. It is shown that these results may be explained in the SCA model when the deflection of the projectile is taken into account. Interference between the dipole ionization amplitudes corresponding to the projectile trajectory before and after the deflection, respectively, leads to a term proportional to the cosine of the scattering angle in the expression for the ionization probability. (author)