The Relative Specific Ionization of Fast Mesons

Abstract

An experiment has been performed with cosmic-ray mesons to study the specific ionization in the relativistic region. A cloud chamber was used to measure the rate of droplet production for two groups of mesons, one with momentum $p$ between 70 and 250 $\frac{\mathrm{Mev}}{c}$ and the other with $p>\mathrm{}1500\mathrm{} \frac{\mathrm{Mev}}{c}$. These two groups of momenta were selected by the appropriate arrangement of Geiger counters. The cloud chamber used to measure the ionization was also used to measure the magnetic rigidity of the particles in the lower group of momenta. The particles in the other group produced tracks which were essentially straight in the magnetic field of 4800 gauss. The group of mesons with lower momenta ionized at or near the minimum rate which was determined to be 14.7±0.35 droplets/mm on the photographic film. The other group of mesons gave an experimental average of 17.9±0.25 droplets/mm, in good agreement with the value of 18.5 droplets/mm predicted on the basis of the Bethe-Bloch ionization theory and the accepted momentum spectrum for sea-level cosmic-ray mesons. The good agreement obtained between the experimental and theoretical distributions shows that the present ionization theory with its rise in the relativistic region adequately describes the observed rate of energy loss for energetic mesons with values of $\frac{p}{\mu }$ up to approximately 100. Too few mesons were available with momenta large enough to check the effect of polarization in helium.