Transmission of Positrons and Electrons

The transmissions of monoenergetic beams of positrons and electrons with energies up to 960 kev have been measured in aluminum, brass, silver, tin, lead, and gold. The absorber forms the window of a $2\pi$ counter whose counting efficiency is better than 99% down to a few hundred electron volts. Particles from a radio-active source, focused into a beam by a 90-degree magnetic analyzer impinge perpendicularly on the absorber window of the $2\pi$ counter. The total transmission is therefore measured independently of forward angle of emergence or of partial energy loss. Positrons are found to be transmitted to a greater extent than electrons except at low energies in aluminum. These results are correlated with previous backscattering experiments and are in qualitative agreement with theoretical calculations of Rohrlich and Carlson. The shapes of the transmission curves are compared semiquantitatively with predictions of the Spencer theory of electron penetration.