Determination of the Ranges and Straggling of Low-Energy Alpha Particles in a Cloud Chamber

Ranges of 105 alpha particles with energies of 0.48, 0.545, and 0.615 Mev as fixed by a velocity selector have been measured by the use of a low-pressure cloud chamber. From the results of these measurements the average ranges $R_{\mathrm{Av}}$ and the straggling coefficients in air at 15° and 760 mm were calculated for these three energies. The values of $R_{\mathrm{Av}}$ were found to be respectively 0.299, 0.327 and 0.354 cm; and the values of the standard deviation were respectively 0.011, 0.0135, and 0.010 cm. After a correction to take into account the difference in definition, the $R_{\mathrm{Av}}\text{'}\mathrm{s}$ are two to three percent higher than those following from a range-energy curve given by Bethe.