Linear Energy Transfer and Track Structure

Abstract

Publisher Summary This chapter discusses track structures and linear energy transfer. There are two types of energy dissipation of heavy ions—nuclear and electronic stopping. Nuclear stopping has high RBE values and predominates at extremely low specific energies of a few kiloelectronvolts per mass unit, corresponding to the last micrometers of the particle range. At higher energies, nuclear stopping contributes only a few percent to the total stopping; the predominant process is the electronic stopping. The electronic stopping is proportional to the square of the effective projectile charge and increases up to a maximum value. The predominant process of electronic stopping involves the interaction of the projectile with the target electrons. In the collision process, a spectrum of electrons is produced with different kinetic energies ranging from zero up to a maximum energy that depends on the velocity of the primary ion. From the energy loss of the primary ion, two-thirds are transformed into the kinetic energy of the electrons. The residual energy is used to overcome the binding energy of the electrons and for target excitations. In the center of the track, the ionized atoms are pushed out from their original positions by electrostatic repulsion. This process is called Coulomb explosion and is responsible for the formation of the latent track in nuclear track detectors. In DNA experiments, the influence of track structure on the induction of double- and single-strand breaks is observed for LET values greater than 5 keV/μm. In consequence, the ratio of DSB to SSB increases with decreasing track diameters, and the highest values are found at the very end of the tracks. However, if the radiosensitivity is increased by changing the DNA environment or the repair capacity, additional DSB are produced directly in single events.