The Relative Stopping Powers of Carbon and Lead for Slow Mesons

An experimental determination of the relative stopping powers of carbon and lead for slow mesons has been made with an anticoincidence arrangement of G-M counters. For mesons having an average energy of approximately 4×${10}^7$ ev, the experiments revealed that 28.5 g/${\mathrm{cm}}^2$ of carbon is equivalent in stopping power to 24±5 g/${\mathrm{cm}}^2$ of lead. This gives a value for the ratio of the stopping powers of equal masses of carbon and lead, $\frac{S_{\mathrm{C}}}{S_{\mathrm{Pb}}}=0.84\mathrm{}\pm 0.18$, to be compared with the theoretical value of $\frac{S_{\mathrm{C}}}{S_{\mathrm{Pb}}}=1.82\mathrm{}$ calculated with the ionization theory. However, it is not necessary to invoke a hitherto unknown absorption process to account for this discrepancy, since it may be explained as arising from the scattering and transition effects. The data corrected for these effects show that 28.5 g/${\mathrm{cm}}^2$ of carbon is equivalent in stopping power to 45±7 g/${\mathrm{cm}}^2$ of lead, and $\frac{S_{\mathrm{C}}}{S_{\mathrm{Pb}}}=1.6\mathrm{}\pm 0.3$, in agreement with the theoretical value within the experimental uncertainty. These experiments indicate that in a dense absorber, any additional absorption process is unimportant compared with ionization for mesons having energies of about 4×${10}^7$ ev, or higher.