Published online by Cambridge University Press: 24 October 2008
The number of ions produced by a neutral particle of small mass with a magnetic moment equal to n Bohr magnetons has been calculated (I–III) and found to be 103n2 per km. path in air at N.T.P., being practically independent of the mass and energy of the particle (IV). A comparatively large fraction of the secondary electrons produced have high energy and may therefore be counted if they are produced in the wall of the Geiger counter instead of in the counter itself. This fact should facilitate the experimental detection of the ionization considerably (V).
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† E. Fermi, loc. cit.
‡ The problem is connected with the incoherent scattering of X-rays, since is proportional to the probability that the atom is excited to the state W by an X-ray of wavelength λ which is scattered through an angle θ, where
The total probability for the incoherent scattering through a given angle, i.e. the sum of over all excited states W of the atom, has been calculated by W. Heisenberg (Phys. Zeits. 32 (1931), 735), and by Bewilogua (Ibid. 740), using Fermi's statistical model. The result is
where the function S (υ) has been tabulated by Bewilogua, and
For large values of υ, all the scattering is incoherent, so that S (υ) = 1 (cf. (19)). Therefore we may write
The quantity υ0 defined here is connected with the average excitation potential A by the relation
where I H is the ionization potential of hydrogen. From the Bewilogua table we have calculated υ0 =0·081 and therefore volt.
† The actual calculations were done for Cu. For other substances, the extra path L would be very nearly proportional to 1/Z M.
† If Ra B+C only is used, the extra path for zero neutrins mass is 2·46 cm., whereas for Ra D+E it is 0·70 cm.
‡ Proc. Cambridge Phil. Soc. 31 (1935), 99.Google Scholar