V. GAMMA-RAYS

1. NATURE OF GAMMA-RAYS

The y-rays, like the X-rays, are similar in nature to rays of light. Recent investigations show, in fact, that some of the y-rays are merely X-rays produced in the radioactive atoms. While experiments on the reflection of X-rays and y-rays from crystals leave no doubt that the wave theory of light is here applicable, yet there are still to be solved some of the problems pointed out by Bragg when he proposed the corpuscular theory of X-rays which served so well to stimulate research in this field. The same difficulties exist in the case of visible radiation. Theoretical investigations on the structure of y-rays by Sommerfeld and Kunz, based on the electromagnetic theory of light, lead to conclusions which are not very different from those of a corpuscular theory. In Kunz' theory, in fact, if the electromagnetic field is endowed not only with energy but also with mass and momentum, then this electromagnetic mass and momentum concentrated in a comparatively small space is not very different from the notion of a "light particle" in the old emission theory. Such "corpuscular" ideas are also consistent with the very recent theories of A. H. Compton and Debye on the scattering of y-ray quanta. Bothe-Geiger have suggested a method of getting an experimental check on these theories by using two counting chambers, one recording the scattering electrons and the other the scattered y-rays. The terminology of both wave and corpuscular theories will here be used indiscriminately. Very recently Bohr-Kramers-Slater have made suggestions which may solve the apparent dilemma for all ranges of frequency.

2. EMISSION OF GAMMA-RAYS

Counting methods have been applied to the study of y-rays (see page 38). Hess-Lawson, using the electrical method of Rutherford-Geiger, determined the number of y-rays emitted per sec by quantities of RaB (82RaI) and RaC (83RaI) in equilibrium with 1 gm of Ra (88Ra), to be 1.43 × 1010 and 1.49 x 1010, respectively. They used a spherical counting chamber of copper, and for the absorption coefficient in copper of the B-rays excited therein by the y-rays, assumed values obtained by other observers who used ionization methods. The problem has also been investigated by Kovarik, who used Geiger's point-discharge counting method, passed the y-rays through Al, Cu, Sn, Pt, and Pb, and deter

mined by the same method the coefficients of absorption for the y-rays and for the B-rays they produce. The mean value so obtained for the number of y-rays emitted per sec by RaB+C (82RaI+83RaI) in equilibrium with 1 gm of Ra (88Ra) was 7.28 × 101o, which is nearly (within 2%) one y-ray per atom disintegrating.

The random emission in time of penetrating y-rays from radium, i. e., from RaC (83RaI), was proved by Hess-Lawson, who tested Bateman's formula. Experiments which have some bearing on the spatial distribution of y-rays have been performed by Kovarik. Two similar counting chambers, each connected to a separate indicating instrument, were placed at the same (varied) distance from a single source of y-rays (radium in equilibrium), but in different directions from it. The average number of events (i. e., y-rays) counted per unit time in the two chambers was the same, but the events did not take place simultaneously in the two chambers; in fact, each apparatus registered events as if these were caused by corpuscular radiation emitted by the central source at random in direction and in time. The recorded events were not rendered simultaneous by placing one of the chambers behind the other, or by placing them side by side. Of course, the nature of the y-ray is not the only factor here involved. Not only must a y-ray traverse both counting chambers, but it must ionize atoms in each, in order that its passage may be registered by both at once. Nevertheless, the number of atoms in the material of each chamber is so great that one would expect frequent coincidences if the events depend upon energy abstracted from a spreading wave.

3. ENERGY AND WAVE-LENGTH OF GAMMA-RAYS

The energies and wave-lengths of the y-rays have been obtained in a number of ways, yet a very great deal of further research is required. in this particular field. The data and tentative conclusions are given. in Tables V-A to V-D, pages 114 to 123 inclusive. The direct experimental determination of y-ray wave lengths by the reflection from a crystal was first accomplished by Rutherford-Andrade for the y-rays of RaB (82RaI) and RaC (83RaI), using rock-salt for the crystal and photographing the reflected lines. Both surface planes and planes requiring for their use transmission through the crystal were utilized. They showed that certain strong lines of the RaB (82RaI) y-ray spectrum are identical with characteristic X-rays (L-series) of non-radioactive lead. The shortest wave length measured was that of a y-ray of RaC (83RaI) reflected at a grazing angle of 44′ from rock-salt and therefore having a wave length of about 70 X. U. (1 X. U.=10-11 cm=10-3 Å. U.). The counting method was applied by Kovarik to the study of the high frequency y-rays of RaC

(83 RaI) reflected from calcite. Besides checking some of the principal. lines reported by Rutherford-Andrade he obtained several of shorter wave length, the shortest measured being reflected at 16' from calcite so that its wave length is about 27 X. U. (see Table V-C, page 116). In connection with the crystal reflection methods, Perkins showed that by measuring the difference in ionization due to y-rays passing through a crystal and through an equal amount of amorphous material having the same chemical composition, it is possible to estimate the amount of reflection of the y-rays by the crystal in any chosen direction.

=

The determination of y-ray wave lengths from mass absorption coefficients is made on the supposition that the relation between mass absorption coefficient and wave length found for X-rays may be applied, with suitable extrapolation, to the case of y-rays. For X-rays, outside regions of selective absorption, p/p kλ" where A is the wave length and n has a value between 2.5 and 3. Rutherford found that as the mass absorption coefficient, u/p, of y-rays approaches the order of magnitude of the mass scattering coefficient 7/p, it varies more slowly with λ, probably as the first power, and from his X-ray data he concluded that the very penetrating y-rays have most probably a wave length between 20 and 7 X. U. Minna Lang applied her extensive work on the absorption of hard X-rays to the y-rays of all known radio-elements and showed that many of these are probably characteristic X-rays (K, L, and M-series). Kohlrausch, using his data on the absorption and scattering of the hard y-rays of RaC (83 RaI), concludes that these rays have a between 60 and 170 X. U. Prelinger gets from 30 to 80 X. U. Ahmad from scattering and absorption of hard y-rays gets as low as 17 X. U. Owen-Fleming-Fage get a slightly greater result from similar experiments. A. H. Compton, by considering diffraction in the structure of individual atoms, decides that the hard y-rays of RaC (83RaI) have λ between 25 and 30 X. U. (see Table V-B, page 115).

The energies of y-rays have also been arrived at by measuring the energy of B-rays "excited" by them in various elements, including particularly the abundant isotope of the radioactive atom emitting the y-rays. The velocity of the B-particles caused to be emitted by the y-rays from the atom of any element depends upon the frequency of the y-rays and upon the work necessary to separate the emitted electron from the rest of the atom. The photoelectric equation E=hv-W, is applicable. In this equation E is the energy of the "excited" B-ray as measured outside the atom, v is the frequency of the exciting y-rays and W is the work of separation. The energy E is obtained from the value of Hr in magnetic deflection experiments, the work W is the energy corresponding to the appropriate absorption edge in the X-ray spectrum of the atom in

the electronic structure of which the B-ray arises. The work of separation W will have different values for different energy levels in the same atom and different values for the same energy level in different atoms. (See Table IV-C, page 98.) By trial, a value of the y-ray energy hv can usually be obtained to fit the experimental data. Since h is Planck's universal constant, v (or λ) can also be obtained if desired. It was in this way that Rutherford-Andrade showed that the soft y-rays of RaB (82RaI) are the L-series X-rays of lead (82). In a similar way Ellis,' Ellis-Skinner, Meitner,1 de Broglie-Cabrera and Thibaud have shown that some of the y-rays of radio-elements belong to the K, L, M, or other series of X-rays of the atoms concerned in the B-ray disintegration considered, and have calculated their wave lengths. It is evident, therefore, that some of the y-rays are of extra-nuclear source. The most penetrating y-rays cannot be so accounted for and must therefore be of nuclear origin. Swinne was among the first to make this clear.

3a. CONNECTION BETWEEN GAMMA-RAYS AND BETA-RAYS (OR ALPHA-RAYS)

1

Rutherford first showed that a relation exists between B-ray and y-ray spectra, and pointed out a possible explanation for the origin of B-rays and y-rays from the same radio-elements. From the more recent work of Ellis,1 Meitner 1 and others it has been definitely established: (1) That some of the B-rays are of photoelectric origin from the extra-nuclear part of the atom, being "excited" by the y-rays; (2) that some of the y-rays have their origin in rearrangements of electrons in the same part of the atom, i. e., that they belong to the ordinary X-ray types; (3) that the change in nuclear charge during the disintegration requires some B-rays in each case to be of nuclear origin; (4) that some of the y-rays, including all of the very penetrating rays, are of nuclear origin. The principal point in dispute, at present, is whether emission of nuclear B-rays precedes or follows the emission of nuclear y-rays. Lise Meitner 2 adopts the former view while Ellis 2 adopts the latter.

2

Ellis and Ellis-Skinner in their analysis of nuclear y-rays show evidence for energy levels in the nucleus analogous to those in the extranuclear structure as found by X-ray analysis. Ellis 2 supposes that when a condition of instability arises in the nucleus an electron occupying a level of higher energy falls to a level of lower energy, the excess energy being emitted as a y-ray of corresponding frequency, and that since several changes of this kind are possible, y-rays of several different but definite frequencies may be emitted from one nucleus, and that different groups of frequencies may be emitted from different individual nuclei of the same radio-element. Some of the y-rays cause (photoelectric) emission