Low-Energy Electrons Emitted from Solid-State Targets in Ion-Atom Collisions

We have continued our studies of the energy and angular dependence of low-energy electron emission pertinent to theneedsoftheradiobiology. Inourpreviousreport[1], wedescribedbrieflyourmethodofaccountingfortheproperties of the solid-state targets, both bulk and surface: We use dedicated TRAX Monte Carlo simulations to analyze the measured angular and energydistributions of electrons emitted after the impact of 1 keV and 500 eV electrons on thin carbon targets. Such analysis allows us to account quantitativelyfortheinhomogeneityofthetargetthickness and for its roughness. The TRAX code [2] treats each basic interaction of an electron or a heavy ion separately and does not require models for multiple scattering, angular straggling, etc. as an input. The basic processes taken intoaccountforelectronsare: elasticscattering,excitation, and ionization. This bottom-upapproach in simulating the emission and transport phenomena of electrons makes the TRAX code very well-suited for our purposes. An elaborated treatment of the problems, the descriptions of our experimental set-up and of the performedexperiments can be foundin Ref. [3]. Based on the findings for the electron beam, we analyzed the spectra of electrons emitted in collisions of 3.6 and11.4MeV/ucarbonionsimpingingonthesamecarbon target. Additionally, we bombarded Ni, Ag, and Au targets. In Fig. 1, we compare our results with common simple conventional theory and with the results of our TRAX simulations. We used the modified Rutherford formula as described in Ref. [4], which according to the authors ’worksremarkablywellforbothsoft-collisionsandbinaryencounter electrons.’ As usual, we took solely the ’active’ electrons (those with velocities smaller than the projectile velocity) into account. We calculated the evolution of projectile chargestates withinthetargetswiththeEtachacode [5], and we used the numbers to weigh the square of the chargestateneededtocalculatethecrosssections. Weused the average charge state after the target to obtain the number of projectiles from the measured Faraday cup current. We would like to emphasize that the effective charge state can vary quite strongly with the fluctuations of the target thickness. The measured and calculated energy distributions are displayed in the left part of Fig. 1. The discrepancies between the theory and experimentare large. Even more important, the measured and calculated cross sections show differentsequences, i.e. a differentscaling with the atomic number of the target. Apparently, the electron transport phenomena play an important role. In the right part of the figure we compare the measured data for the carbon tar