On the range of α-particles in rare gases

In a recent paper R. H. Fowler (1) has calculated the atomic stopping powers of various substances for α-rays on the basis of Henderson’s (2) theory of the loss of energy of an α-ray in passing through matter, and has drawn attention to the desirability of a systematic investigation of the range of α-rays in different media. In particular, the considerable discrepancy between experiment and theory in the case of helium made it desirable to carry out further measurements with this gas for the purpose of confirmation. Also, it was considered that experiments on the range of α-rays in such comparatively simple substances as the rare gases would yield valuable information concerning the relation between the observed values of the stopping power and those calculated by Fowler. The simple monatomic gases have the great advantage, that we are free from the complications of molecular structure and dissociations. The range of the α-particles from polonium was investigated by E. P. Adams (3) for a number of gases by a scintillation method. For convenience, taking the value of the range in air at 15°C. and 760 mm. of mercury to be 3·93 cm., the corresponding values for argon and helium were 4·18 cm. and 13·58 cm. respectively. The ranges of the α-rays from polonium and radium C in helium were found by Taylor (4) to be 17·62 cm. and 32·54 cm. respectively. It will be seen that the results of Adams and Taylor for helium are very different. The preceding values for helium and argon seem to be the only data available in the case of the rare gases.