The α Rays


THE α rays from radium appear to start life without electric charge, and subsequently become charged owing to collisions with the gas molecules they strike in their path. It seems, therefore, worth while inquiring what their behaviour would be if they were liable to become discharged again at a later collision, and to go on repeating this cycle during the ionising portion of their path. Very possibly the particle is capable of losing more than one electron, in which case it would seem certain that it will have a greater charge at some portions of its path than at others. Looked at in this way the problem is a statistical one of considerable complexity, but my point of view will be sufficiently well illustrated by considering the average α particle to behave as if it had the following constitution. For a distance x of its path it possesses an electric charge e. This is succeeded by a distance x′, during which its electric charge is e′. This is followed by a distance x with charge e, then a distance, x′ with charge e′, and so on, repeating indefinitely. Let the particle have a mass m and initial velocity v0, then, confining our attention to a portion of the path so small that v0 is not appreciably diminished by the collisions which occur, it is easy to show that the quantity measured by the electrostatic deflection as mv02/e would really be mv02(x + x′)/ex + e′x′, whilst the quantity measured by the electromagnetic deflection as mv0/e would be mv0(x + x′)/ex + e′x′. Thus the measurements would give v0 correctly, but the quantity denoted by e/m would ex + e′x′/m(x + x′) be. It is evident that the apparent value of e/m would be independent of the pressure at which the measurements were made, since change of pressure changes both x and x′ in the same ratio.

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RICHARDSON, O. The α Rays. Nature 75, 223 (1907) doi:10.1038/075223b0

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