Electromagnetic interactions of high-energy cosmic-ray muons

Abstract

The production of knock-on electrons and electron pairs by cosmic-ray s has been measured from cloud-chamber photographs of soft showers produced in lead and iron plates. The cloud chamber was located in a mine at a depth of 2.74 × 10 4 g/cm 2. Eighteen thousand muon traversals were recorded, about half in the lead plates and the remainder in the iron plates. The energies of the electrons that initiated the showers were obtained from experimental and theoretical shower curves. The observed number of events in 14 intervals of transferred energy, from 200 MeV to greater than 10 GeV, is compared with the theoretical predictions for knock-on electrons and direct pair production of electrons. In general, good agreement between theory and experiment is obtained for the sum of the direct pair and knock-on results in both lead and iron. From 1–5 GeV transferred energy, a small excess of events is observed for both the lead and iron data. From the dependence of the theoretical cross sections upon Z and A, this excess number of events appears to be due to the knock-on process.