Landau Distribution and Density Effect at High Gas Pressures

Studies of the ionization energy losses of fast $\mu$ mesons have been carried out in a specially constructed proportional counter with argon gas at pressures up to 40 atmospheres. With increasing pressure, a strong reduction in the relativistic rise was observed; the extent of the reduction being at least as great as that expected from Sternheimer's calculation of the density effect. Furthermore, the distribution in energy losses, which at low pressures is much wider than expected from theoretical considerations, was observed to become narrower with increasing pressure. At our highest pressure, however, the energy loss distribution was still considerably wider than predicted by the Landau theory.