Energy Straggling Parameters from Single Event Spectra

The variance of an energy straggling distribution for thin absorbers is determined by the cross section for energy loss in single collisions. The ratio, δ 2, of this variance and the mean energy loss is independent of the thickness of the absorber; it is also numerically equal to the quotient of the second by the first moment of the single collision spectra. Measured doubly differential cross sections for electron emission by energetic charged particles from low-Z molecular targets provide a major part of the primary data required for calculating δ 2. The relationship between δ 2 and experimentally measurable cross sections for single collision energy loss is developed. Values of δ 2 calculated from measured single collision cross sections are compared with values obtained from the free electron model and experiment.