The straggling of α particles from radium C

Abstract

It is well known that a pencil of homogeneous α particles in passing through matter decrease in energy and become increasingly heterogeneous with the amount of matter traversed. It can be shown that this straggling of the α particles, so named by Darwin, is a necessary consequence of any theory of absorption of α particles in which the loss of energy depends on the transfer of energy to the electrons or nuclei of the atoms which it encounters. The wellknown fact that the ranges of individual α particles, whether measured by the scintillation, photographic, or Wilson cloud method, are subject to fluctuations, is an illustration of this phenomenon. This straggling is most clearly shown by observing the broadening in a magnetic field of a narrow pencil of homogeneous α rays after their passage through a definite thickness of absorbing matter. No quantitative measurements have hitherto been made by this method, but this broadening was clearly shown by Rutherford in experiments on the capture and loss of electrons by α particles. It is also seen in the photographs obtained by Henderson in experiments on the same subject. The first experiments on the straggling of α particles were made in 1910 by Geiger, who measured the variation in range of α particles by the scintillation method. This method was also used by other observers, but the values obtained were all several times too great, as was shown by the experiments of Makower, who detected the α particles by their photographic action and pointed out that the failure of the scintillation method is probably to be ascribed to the difficulty of counting extremely feeble and comparatively bright scintillations simultaneously. Recently the straggling of particles has been measured by the Wilson cloud method by I. Curie, Meitner and Freitag and I. Curie and Mercier. The results of these experiments will be discussed in 10.