Impact-parameter-dependent stopping of swift ions
III. Bunching and packing in energy-loss straggling
Department of Physics and Chemistry, University of Southern Denmark, 5230 Odense M, Denmark
2 Institut f. Experimentalphysik, Johannes-Kepler-Universität, 4040 Linz-Auhof, Austria
Revised: 3 February 2010
Published online: 30 March 2010
Bunching denotes a contribution to the fluctuation in electronic energy loss (straggling) of an ion penetrating through a medium composed of multiple-electron atoms. Bunching is neglected in conventional straggling models which assume single-electron excitations. Packing denotes another correction to conventional straggling models which assume the atoms of the stopping medium to be distributed at random. The packing correction is known to be positive for molecular gases but has been asserted to be negative for condensed media. Estimates of the two effects, based on expressions for impact-parameter-dependent energy loss derived in paper 1 of this series, are reported here. Both effects are approximately proportional to the square of the stopping cross section and, hence, large for high-Z ions. Bunching may be the dominating contribution to straggling in gases for high-Z ions. For solids the relative significance of bunching and packing depends on the structure and atomic numbers of the target material. Sizable corrections have been found, which tend to increase as a function of the atomic numbers of ion and target.
© EDP Sciences, Società Italiana di Fisica, Springer-Verlag, 2010