Energy-loss straggling caused by the inhomogeneity of target material

Abstract

Rutherford backscattering spectroscopy (RBS) accompanying with the sharp 4.808-MeV resonance of proton beams in carbon has been applied to examine the energy-loss straggling in detail, which causes the broadening of proton beam energy in the penetrating path. In the present measurements, RBS peak profiles of homogeneous and inhomogeneous carbon materials have been obtained with an incident energy of 5.5MeV. Careful analyses of those profiles have revealed that the energy-loss straggling can be separated into two parts; one is collision straggling and the other is density straggling. The collision straggling is caused by the statistical fluctuation in collisions of proton with target atoms, which has been intensively studied since the theoretical work by Bohr [Philos. Mag. 30, 581 (1915)]. The density straggling is caused by the statistical fluctuation in local density of target material, which has been discussed in the present work. The random inhomogeneity as a measure of the fluctuation in the local density is introduced into the existing theory of the energy-loss straggling. Following the theoretical treatment, we have successfully deduced the random inhomogeneity of various carbon materials and the spatial spread of 4.808-MeV resonance protons in the materials. Some applications of the present method for the RBS with the sharp resonance are also presented.