K-shell ionization of Cu by positron impact

Complete text of publication follows. Ionization by positron impact has been extensively studied both theoretically and experimentally in recent decades. In most cases noble gas atoms were used as the target. For designing new experiments, such as production of antimatter, ionization cross sections for many other atoms are also necessary. Recently, improvements in experimental techniques have enabled the determination of inner shell ionization cross sections by positron impact. The target was a Cu film deposited on a carbon thin film. Since the total thickness of the target was thin enough to neglect the positron energy loss, the positrons were treated as individual particles interacting with individual Cu atoms. The classical treatment of various collision problems has been quite successful for the calculation of these ionization cross sections. In particular the classical trajectory Monte Carlo (CTMC) method is widely used. It is a nonperturbative method and hence all the interactions between colliding particles can be taken into account exactly within the framework of the classical dynamics. It has been shown that the CTMC method can be also applied for light projectile impacts. In this work, Cu K-shell ionization cross sections have been studied theoretically and compared with the experimental results. We also applied the binary encounter approximation (BEA). In both cases we used the frozen charge state model and the Slater-rule for the determination of the static nuclear charge of 1s electron bound to the Cu nucleus. The present calculations were performed solving the non-relativistic equations. Fig. 1 shows the K-shell ionization cross sections of Cu as a function of the positron impact from the threshold energy up to 30 keV. While the CTMC data is close to the experimental data, the BEA data underestimates it. This fact suggests that the Gryzinski-Kowalski model used in our BEA calculations, is not suitable to calculate K-shell ionization cross sections of incident positrons in the energy region near to the ionization threshold. We hope that the modification of the simple BEA model for positron impact ionization, in order to take into account the Coulomb-deflection effect and to use more realistic electron velocity distributions, will improve our recent results. Work of this kind is in progress and will be published soon. Acknowledgements This work was supported by the Hungarian Scientific Research Fund OTKA No. K72172 and by the Bilateral Japan-Hungary Cooperative Research Program. We would like to express our personal thanks to Andrew Bergman for the critical reading of the manuscript.