Measurement of MeV Ion Track Structure in an Organic Solid.

Ion tracks, formed by the interaction of energetic charged particles with matter, have played an important role in the development of modern physics. More recently, MeV-GeV ion track production has been used to modify materials in a controlled manner [1,2], being a potential tool to engineer material properties in the nanometer to micrometer scale. MeV-GeV ions are finding recently wider use in radiation therapy for, e.g., treatment of different forms of cancer. In spite of this, the understanding of the evolution of energy deposited around the path of an ion has not progressed markedly since early works in the sixties and seventies [3–5]. Ions with velocities higher than the Bohr velocity syB ­ 0.22 cmynsd deposit their energy in electronic excitation and ionizations in a cylindrically symmetric region around the path of the impacting ion, forming the track core. The incident ions cause ejection of energetic secondary electrons that transport part of this energy out of the core [3,4]. The mean deposited energy density, esrd, after the ion passage is approximated by a esrd ~ r22 dependence, where r is the radial distance from the ion path. Such a dependence is obtained from theory employing the continuous slowing down approximation [4,6] for the secondary electrons, Monte Carlo simulations [5–7], as well as experiments performed in gases [8,9]. Several energy-transport mechanisms [10,11] have been advanced to describe the subsequent energy propagation and dissipation. However, no direct experimental determination of the initial or evolved energy distribution exists for targets in the condensed phase. In this work a first estimate of esrd, the radial profile of the mean deposited energy density, is made for a solid material. When fast ions interact with dielectric materials, a trail of radiation damage in the bulk (damage tracks) [3] as well as material erosion of the surface (electronic sputtering) are observed [12,13]. Whereas the characteristics of latent damage tracks have been determined by various physical methods ex situ, long after the impacting event [14], the study of the sputtered material can in principle provide “real time,” in situ information on the complex physical and chemical processes occurring in ion tracks