The absorption of penetrating radiation

Abstract

A great many careful experiments in different parts of the world have established the widespread existence of a radiation which appears to be travelling downwards through the earth’s atmosphere, and produces one or two ions per cubic centimetre per second in air at the surface of the earth. The nature and origin of the radiation is still uncertain, but since it is able to produce a measurable ionisation after passing through 50 metres of water it is referred to as “penetrating radiation.” It has been maintained that there is nothing inconsistent with experiment in the view that penetrating radiation is in fact a beam of β-particles of great energy, and C. T. R. Wilson has pointed out that the enormous electrical fields accompanying thunderstorms would be expected to give rise to particles having energy perhaps as great as 10 9 volts. On the other hand, the hypothesis that the new radiation is an ultra-γ-radiation has been widely adopted, and interesting numerical relations concerning the wave-lengths present in the beam, as deduced from the absorption curve of the radiation, have recently been revealed. In this paper -the significance of an experimental absorption curve is investigated, more particularly with the object of arriving at a valid method of inferring from it the spectral distribution of the primary radiation. § 2. The Scattering Formulæ We can distinguish two processes which lead to a reduction in intensity of a beam of γ-rays in passing through matter, namely, photo-electric absorption, and the Compton scattering process which gives rise to a deflected quantum and a recoil electron. When the absorbing medium is lead and the source of γ-radiation is radium C, these two processes are of about equal importance. The former, however, varying roughly as the cube of the wave-length, decreases very much more rapidly than the latter as the hardness of the beam increases and may be assumed to be negligible for the radiation under consideration, which is at least twenty times as penetrating as the radium C γ-rays. We do not know that these are the only ways in which γ-ray energy may be absorbed. Indeed, absorption coefficients determined experimentally are in general more than 20 per cent, greater than estimates deduced with the aid of the Compton formulæ from independent evidence as to the spectral distribution of the radiation. I t does not appear likely, however, that a new absorbing mechanism is indicated, but rather that the Compton formulæ are an inadequate description of the scattering process; for the interaction of radiation and free electrons has quite recently been investigated anew on the basis of a completely relativistic wave mechanic, developed from the relativity quantum mechanics of Dirac, by Klein and Nishina, who arrive at formulæ which approach asymptotically to those of Dirac for long wave-lengths, but which differ from them already by 40 per cent, for radium C γ-rays. The old and the new formulæ diverge as the wave-length of the radiation becomes shorter, so that from the standpoint of this paper it is of the greatest importance to discriminate, if possible, between them. The points of contact of theory and experiment are discussed in § 7 and conclusions are reached which are definitely favourable to the new formulæ. All numerical computations have therefore been made in duplicate and the results to which the two sets of formulæ lead are contrasted.