Scattering and loss of energy of fast electrons and positrons in lead

Abstract

A cloud chamber investigation has been made of the scattering and loss of energy in a thin lead lamina (0.13 mm) of fast electrons and positrons (5.0 to 17.0 Mev) produced as secondaries by the gamma-radiation from Li7+H1. A comparison of the experimental scattering versus angle in the plane of the chamber with the Mott-Rutherford theory of single scattering shows good agreement above θ=13°. Below this angle the experimental points behave in a manner reasonably consistent with multiple scattering. The dependence of the scattering on energy agrees with the relativistic term (m0c2/W)2 in the theoretical scattering expression. Some evidence for an excess scattering at large angles of electrons over positrons has been found. The average loss of energy for two groups of tracks of mean energy 9.0 Mev and 13.5 Mev was found to be 35 Mev/cm and 54 Mev/cm, respectively. These values are roughly 1.5 times the theoretical values, a result in agreement with the findings of Crane and coworkers at Ann Arbor. From the observed scattering it does not seem possible to account completely for this excessive loss of energy on the basis of a longer effective path in the lamina. Two out of 97 tracks in the 9.0 Mev group and nine out of 179 tracks in the 13.5 Mev group lost more than 200 Mev/cm. The small excess over the large radiative losses to be expected theoretically is not statistically significant. No difference in the loss of energy of electrons and positrons was found.