Letter | Published:

The Passage of α-Rays and, β-Rays through Matter


THE loss of energy suffered by a fast-moving electrified particle passing through matter, and the ionisation produced by the moving particle, are phenomena which have, so far, not received accurate quantitative explanation. There are two main theories of the stopping-power due respectively to Bohr (Phil. Mag., 25, 10; 1913; and 30, 581; 1915) and Henderson (Phil. Mag., 44, 680; 1922). The theory of the primary ionisation due to a fast-moving particle is as it was left by Thomson in 1912 (Phil. Mag., 23, 449). In all these theories, classical mechanics is used to calculate the possible energy transfers during encounters between the moving particle and the atomic electrons, and Fowler (Camb. Phil. Soc., 21, 521; 1923) has suggested that the discrepancy which exists between theory and experiment may be due to the inaccuracy of classical mechanics in this field. Whether this is so or not, it may be of interest that a fair proportion of the discrepancy between classical calculations and experimental results disappears when the motion of the atomic electrons is allowed for. The effect of this motion may be considerable, though the velocity of the electron be small compared with that of the moving electrified particle.

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