$\gamma$ Rays Excited by Neutrons

Recently it was reported in the pages of this journal[^1] that, when neutrons act on a substance, $\gamma$ radiation appears (Li’s experiments). Recently a large number of works have appeared devoted to the study of this effect and clarifying certain points in Li’s work, in particular eliminating the objection, raised by Goldhaber’s experiments on the nuclear photoeffect. These works[^2][^3][^4] established first of all that practically only slow neutrons are responsible for the appearance of $\gamma$ rays. Fast neutrons produce a very small effect. There is only one case (Au),[^4] for which an appreciable slowing effect is observed even for slow neutrons arising from the collision of a deuteron with a deuteron. In this case, however, the experiments were not carried out with sufficient accuracy.

Since $\gamma$ rays arise mainly from slow neutrons, it becomes clear why Li’s experiments showed a larger effect for paraffin than for carbon. This ex-

is explained by the fact that the effect for carbon was observed for fast neutrons, whereas for paraffin it was observed for slow neutrons, since the neutrons entering the paraffin were first slowed down and only after that, once they had hit beryllium, produced γ-radiation.

Another essential circumstance that it has been possible to clarify consists in the fact that the magnitude of the observed effect, and sometimes its presence or absence, depends on the material of the indicator (in recent works a Geiger–Müller counter has usually been used) employed for detecting γ-radiation. This can be explained by the fact that neutrons, falling on the walls of the indicator, produce γ-radiation in them, and for a number of substances also artificial radioactivity, as a result of which an additional number of impulses appears in the counter, changing when a substance being investigated for the presence of the effect in it is placed in front of it. This unaccounted change in the number of impulses in the counter may distort the magnitude of the observed effect.

Fleischmann\(^5\) showed, for example, that if an iron counter is used as the γ-ray indicator, then the effect for Fe, Cu, and Pb could hardly be detected, whereas with a graphite counter one can prove the presence of a large effect for Fe and Cu and a noticeable one for Pb.

Let us turn to the presentation of some results of the latest works. The most detailed question of γ-radiation from neutrons was examined in the work of Fleischmann\(^2,5\). This author used as an indicator a graphite counter, since, according to his experiments, neutrons in graphite do not produce a noticeable effect. First of all it was shown that γ-rays from slow neutrons arise in paraffin, Fe, Cu, Cd, and Pb and are almost absent for C and B\(_2\)O\(_3\); here slow neutrons are understood as neutrons absorbed by a layer of B\(_2\)O\(_3\) with a thickness of 3 g/cm\(^2\) (from the experiments of Rasetti\(^6\) it follows that γ-rays also arise in Cl, Co, Y, Ag, Ir, Hg). Combining, then, lead filters for absorbing γ-rays and filters of B\(_2\)O\(_3\) for absorbing the slow neutrons arising upon slowing down in paraffin, Fleischmann investigated in considerable detail the dependence of the magnitude of the effect under study on the thickness of the layer of the substance in which γ-radiation is excited. This dependence was determined for paraffin, Fe, Cu, Cd, Pb, and in so doing two different cases were investigated for each substance. In the first of them, layers of the substance under study of various thicknesses were placed in front of the counter, i.e. the γ-radiation emitted in the direction of the incident neutron flux was measured; in the second, the substance under study was placed behind the counter, i.e. the γ-radiation emitted in the direction opposite to the direction of motion of the neutron beam was measured. In the first case, for all the substances studied, the curves of variation of γ-radiation with the thickness of the layer increase from zero to a certain maximum value and then gradually fall to zero. Direct measurements of the absorption coefficients of slow neutrons and of the γ-radiation arising in the given substance showed that the rise of these curves is due to absorption of slow neutrons (summation of γ-radiation over the increasing thickness of the layer), while the fall at large thicknesses is due to absorption of the emerging γ-radiation. Some peculiarities are present for the case of paraffin, for which the experiments were performed with fast neutrons that gradually slowed down in the paraffin itself before producing γ-radiation in it. For this reason the rise of the curves for paraffin is at first somewhat delayed and then proceeds more rapidly, in contrast to the other elements, for which it is almost linear.

For the second case—that of an increase in the intensity of γ-radiation in the direction opposite to the incident neutrons—curves analogous to one another are obtained for all the substances investigated, increasing from zero to a certain constant value with increasing thickness of the irradiated layer.

ABSTRACTS

Both these and other curves give an idea of the absorption of neutrons in the given substance.

In Li’s experiments the energy of the γ-quanta arising from the action of neutrons on a substance was roughly determined. Fleischmann determined this energy more carefully for the cases H, Fe, Cd, Cu, Pb and obtained different results. To determine the energy of the γ-radiation arising in a given element, he measured its absorption coefficients in a series of elements and then compared them with the absorption coefficients of γ-radiation of known energy (γ-radiation of Ra, filtered through 10 cm Pb, and γ-radiation of ThC″, filtered through 5 cm Pb). The results obtained by comparing the absorption coefficients in aluminum are given in Table 1.

Table 1

Let us note that Rasetti determined the energy of the γ-radiation arising upon irradiation with slow neutrons of Cl, Co, Y, Ag, Cd, Ir, Hg, and found for it values oscillating between 4 and \(5 \cdot 10^6\) eV. For Cd, for example, he found \(4.5 \cdot 10^6\) eV. It is true that one must bear in mind that in his method for determining the energy of γ-radiation (from the absorption produced by it of electrons) there are not entirely well-founded assumptions.

If one also recalls that Li found, for the case of hydrogen, an energy equal to \(3—4 \cdot 10^6\) eV, and for iron and lead \(1.5 \cdot 10^6\) eV, then one must conclude that the question of the energy of the arising γ-radiation has not yet been definitively resolved.

For comparing the magnitude of the effect for various substances there is as yet also an insufficient amount of data. In this respect it should first of all be noted that Fleischmann’s rough calculations⁷ showed that for Cd, Cu, Fe, Pb and paraffin, for each absorbed slow neutron there arises approximately one γ-quantum. The absolute magnitude of the effect being investigated may be said to be as follows. In his first paper Li indicated, for the magnitude of the effective cross section for the production of γ-radiation by neutrons, in the case of hydrogen, the value \(10^{-25}—10^{-26}\ \text{cm}^2\). Artsimovich, Kurchatov, Latyshev and Khromov,³ investigating the absorption of slow neutrons in paraffin, water and carbon according to the γ-radiation arising in them, found that the effective cross section for H, C, O is approximately the same and is equal to \(2—3 \cdot 10^{-25}\ \text{cm}^2\) (the latter circumstance to some extent contradicts Fleischmann’s data, who found that graphite gives only an insignificant effect both for fast and for slow neutrons). Kikuchi and collaborators⁴ estimate the effective cross section as being of the order of \(10^{-25}\ \text{cm}^2\) for Cu, Ni, Fe and about one hundred times greater for cadmium, which strongly absorbs slow neutrons (\(10^{-21}—10^{-22}\ \text{cm}^2\)).

Concerning the mechanism of origin of the γ-radiation under consideration, one may conclude the following. Since γ-radiation arises when slow neutrons interact with matter, its occurrence can only be explained by absorption of neutrons by nuclei. The energy thereby released gives the binding energy of the neutron in the newly formed nucleus. For the case of aluminum it was shown by direct measurements that the γ-radiation is emitted immediately at the moment the neutron joins the nucleus. It must be thought that the same also takes place for all other substances.

In the case of hydrogen, γ-radiation appears when a deuton is formed from a proton and a neutron. There are now no objections to this explanation. The previously mentioned¹ contradiction

the experiments of Li and the experiments of Chadwick and Goldhaber can now be explained as follows. From the experiments on the nuclear photoeffect (Chadwick and Goldhaber) and from thermodynamic considerations it follows that, for neutrons with energies of several hundred thousand volts, the effective cross section for the formation of a deiton from a proton and a neutron has a value of the order of \(10^{-29}\ \mathrm{cm}^2\), i.e. it is smaller than that for the nuclear photoeffect. The experiments of Li, however, as well as the subsequent experiments of other authors, show that the formation of a \(\gamma\)-ray by a neutron proceeds with a greater probability than the splitting of a deiton by a \(\gamma\)-quantum. Here, however, it must be borne in mind that the first process occurs for slow neutrons, whereas in the second comparatively fast ones are formed. On the other hand, theory shows that the effective cross section for the first of the processes mentioned rapidly increases as the energy of the neutrons decreases, and for slow neutrons can attain a value of \(10^{-25}\)—\(10^{-26}\ \mathrm{cm}^2\), which agrees in order of magnitude with the values observed experimentally. Thus the contradictions between the experiments of Li and those of Chadwick and Goldhaber are only apparent and are explained by the fact that in both cases neutrons of different velocities are involved.

In conclusion it may be noted that, although the question of the excitation of \(\gamma\)-radiation by neutrons has not yet been studied sufficiently fully, it is nevertheless possible to state that there are no contradictions with other experimental facts, and also, in all probability, with the data of theory.

L. Groshev, Moscow

LITERATURE

1. Uspekhi fizicheskikh nauk, 15, No. 8, 1935.
2. Fleischmann. Z. Physik, 97, 242, 1935.
3. Arzimowitsch, Kurtschatow, Latyschew, Chromow, Phys. Z. Sow. Union, 8, 472, 1935.
4. Kikuchi. Aoki, Hustmi, Proc. Phys. Math. Soc. Jap., 17, 369, 1935.
5. Fleischmann, Z. Physik, 97, 265, 1935.
6. Rasetti, Z. Physik, 97, 64, 1935.
7. Fleischmann, Z. techn. Phys., 16, 412, 1935; Physik. Z., 36, 806, 1935.