Ionization Loss and Straggling of Fast Electrons

The most probable energy loss of 9.6- and 15.7-Mev electrons in samples of about one gram per cm² of beryllium, polystyrene, aluminum, copper, and gold has been measured. The losses measured were of the order of one Mev, and the resolution of the apparatus made possible an accuracy of 20 kev. The observed distributions of energy losses are found to be in good agreement with the Landau straggling calculations for the light elements. For the heavier elements there is a spreading of the distribution introduced by radiation and K electron effects. Calculations made by Yang and Kennedy for gold, including these effects, check well with the experimental data.Applying Fermi's correction for the polarization effect at extreme relativistic velocities to Landau's result for the most probable energy loss, one obtains for the predicted loss in Mev Δpc=0.1537D(ΣZΣA)×[19.43+ln(Dρ)], where D is the absorber thickness in g/cm² and ρ is the absorber density in g/cm³. Experimental results for the light elements are in excellent agreement with this theory. The heavier elements show losses somewhat smaller than those calculated.