In Part I, p. 669, a technique has been described for determining the ratio of the ionization in a light gas (hydrogen or helium) to that in air when ionized by the same X-ray beam, homogeneous rays of medium wave-length and soft heterogeneous rays being available for the measurement. The ionization ratio can be converted into the ratio of the energies absorbed by the two gases by making use of the known value of the ratio of the energies required to form a pair of ions in the two gases; and since the ionization in air is due almost entirely to photoelectrons and the absorption coefficient is known, the energy in the incident beam can be obtained from the energy absorbed by air; thus the energy absorbed by the light gas can be correlated with the energy in the incident beam. The radiation of medium wave-length (about ½ A.) ionizes the light gas chiefly through the agency of recoil electrons, so that after applying a correction (obtained from the soft ray ratio) for the ionization due to photoelectrons, the fraction of the energy in the incident beam converted into recoil electron energy by the gas may be obtained, and compared with the predictions of the quantum theory of recoil scattering. In this paper the comparison is carried out with measurements on hydrogen, and for convenience it will be made between the experimental and calculated values of the ionization ratios. Excluding the early work of Shearer already mentioned in I no experimental determination of the total energy associated with recoil electrons has hitherto been made by any method as direct as the present one, though less direct methods have been employed. In general, however, the experimental technique was open to criticism, and the interpretation of the measurements uncertain, so that it is not surprising that the results were inconsistent either with one another or with theory. It is also possible to calculate the total energy associated with recoil electrons from other experimental facts concerning recoil scattering, but the experimental errors involved combine to make the final result very unreliable. The energy associated with recoil electrons is, however, not only of theoretical interest but also of great practical importance, since all effects arising from scattering, in the scattering substance itself, are due to recoil electron emission.
The Total Energy in the Interaction of X-Ray Photons with Capacitors
