Some Energy-Loss and Channeling Phenomena for GeV Particles

Abstract

Energy-loss measurements for 2-15 GeV positive and negative protons, kaons, pions, and electrons transmitted through thin Si and Ge crystals acting as intrinsic detectors are discussed. For the random direction, the results are compared with the relativistic Bethe-Bloch stopping theory with and without restrictions. The strong influence of density effect is included through the Sternheimer calculations. The most probable energy loss clearly saturates at the "Fermi plateau", which is completely constant up to the maximum γ value (2 × 104) investigated here. The spectral distribution of energy losses is considerably wider than the Landau distributions, especially for very thin (∼ 100 μm) targets but is in good agreement with straggling calculations based on photoabsorption cross sections. For axial and planar directions in the crystals, positive particles show considerably reduced energy loss as compared to random, and the results are in good agreement with the Esbensen and Golovchenko calculations. Axially channeled negative particles show an increase in energy loss by as much as 20%, as compared to random. The influence of relativistic effects on channeling phenomena is introduced through a short discussion of the first electron/positron experiments, where the basic features of the effects were explored, and the results are compared to yields from close-encounter processes for heavy GeV particles. The experimental results have been compared to calculated-yield curves based on the Lindhard channeling model, and very good agreement is found over the enormous range of five to six orders of magnitude in momenta. The strong influence of channeling on small-angle scattering for GeV particles is considered through a discussion of multiple scattering, dechanneling, and the so-called "doughnut effect". The latter, in which the correlated scattering from atomic rows results in ring-shaped intensity distributions on the exit side, is found to exist for angles of incidence far beyond the critical angle, ψ1. This shows that the continuum description in the GeV region can be used for angles of incidence large compared to ψ1. Radiation from 2-55 GeV/c planar-channeled electrons and positrons has been measured and compared to classical calculations based on harmonic and more realistic anharmonic potentials and for cases, where the dipolarity condition is not satisfied. For positrons, it is found that for certain incident momenta between 1 and 10 GeV/c, the channeling radiation is practically independent of transverse energy, resulting in very sharp peaks in the spectra. This offers the possibility of making strong, nearly monoenergetic γ sources in the energy range of 1-100 MeV. Finally, a few applications of high-energy channeling in investigations of inner-shell excitations and δ-ray yields are discussed, together with some recent results on the use of channeling for bending of GeV beams.