Electromagnetic Interactions of High-Energy Muons

The production of knock-on electrons and of electron pairs by muons of mean energy >50 BeV has been measured from cloud-chamber photographs of soft showers produced in lead plates. The cloud chamber was operated at a depth of 8.42×${10}^4$ g/${\mathrm{cm}}^2$ underground. About half the data were taken while the cloud chamber was tilted 66° in order to favor observation of the high-average-energy muons that come in at large zenith angle. The energies of the electrons that initiated the showers was obtained from an experimental calibration (to be published). The shower energies that were studied extended from 85 MeV to about ${10}^4$ MeV. The observed frequency is compared with the expected frequency calculated from the results of Bhabha for knock-on electrons and those of Zapolski and of Murota, Ueda, and Tanaka for electron pairs. The agreement is satisfactory except for knock-on electrons in the region of ${10}^9$ eV, where the predicted frequency appears to be significantly lower than the observed frequency.