Observed Trends in Values of Target Mean Excitation Energy and Barkas-Effect Parameter Extracted from Stopping Power Measurements

摘要

In the course of analyzing stopping power measurements in terms of modified Bethe-Bloch theory, results for a number of experiments with proton and alpha particle projectiles revealed consistent trends in extracted values of mean excitation energies and Barkas-effect parameters. For a given target the value of mean excitation energy determined with proton data exceeds that determined with alpha particle data, whereas the opposite trend prevails for the values of Barkas-effect parameter. One series of experiments carried out by the same group, utilizing four organic (co)polymers as targets, manifests the above trends with the added feature that the difference in parameter values increases monotonically with increasing target (average) atomic number. A conjectureis advanced in an attempt to explain the observed trends in parameter values. An investigation of further measurements with the same targets and Li ion projectiles reveals that the original trends are generally followed in the analyses featuring projectiles of higher atomic number than the two higher projectiles initially included.