Target-thickness-dependent electron emission from carbon foils bombarded with swift highly charged heavy ions

Abstract

We have measured electron yields from the beam entrance and exit surfaces of thin carbon foils (d\ensuremath{\approxeq}4--700 \ensuremath{\mu}g/${\mathrm{cm}}^{2}$) bombarded with swift (13.6 MeV/u) highly charged (q=16--18) argon ions. The dependence of the electron yields on target thickness and charge state of the ions is analyzed within the framework of an extended semiempirical model. Due to the high velocity of the ions, it is possible to distinguish electron production in primary ionization (related to the stopping power and the effective charge of the ions) from secondary electron production due to the transport of so-called \ensuremath{\delta} electrons (cascade multiplication). By combining the experimental results with numerical simulations of electron transport in matter by a Monte Carlo method, we have obtained electron transport lengths of high energy (E\ensuremath{\gg}100 eV) \ensuremath{\delta} electrons parallel and perpendicular to the ion trajectory, as well as diffusion lengths of slow electrons (E\ensuremath{\ll}100 eV). In order to study the velocity dependence of these transport lengths, we have not only investigated 13.6 MeV/u Ar ions, but also 1 MeV/u C and 3.9 MeV/u S, for which experimental results are available [Koschar et al., Phys. Rev. A 40, 3632 (1989)]. We discuss the origin of electron yield reductions (compared to a simple scaling with the square of the nuclear charge) with heavy ions and present measurements of double differential energy and angular electron distributions of 13.6 MeV/u ${\mathrm{Ar}}^{17+}$ ions.