Ionization Loss byμMesons in Helium

The ionization loss by cosmic-ray $\mu$ mesons in helium gas has been measured as a function of momentum. The ionization loss was determined with proportional counters and the momenta were measured by a magnetic spectrometer which resolved particles in the momentum region from 3.3 Bev/c to 140 Bev/c. It was found that helium gas at 2.7-atmospheres pressure exhibits a density-effect saturation of the most probable ionization loss and that this saturation is complete at a $\frac{p}{\mu c}$ value of about 200. Under the conditions of normalization used in this experiment, the value of the ionization loss at which the Fermi plateau occurs is 1.28±0.04 times the value at the minimum. This is in agreement with calculations based on the Sternheimer theory for the particular counter filling used in this experiment.