ANGULAR DISTRIBUTION IN DEUTERON PHOTODISINTEGRATION*)

The deuteron plays, in nuclear physics, in a certain sense the same role that the hydrogen atom plays in atomic physics. Theoretical results concerning the deuteron do not contain the approximations that must be made in solving problems for more complex nuclei, and therefore they are most convenient for quantitative comparison with experiment.

The theory of deuteron photodisintegration, i.e., the splitting of the deuteron nucleus by γ-rays, developed by Bethe for central forces, is valid for [[unclear: word begins “vy-”]]

*) W. M. Woodward and I. Halpern, Phys. Rev. 76, 108 (1949).

...producing the fission of γ-quanta with an energy not much exceeding the fission threshold. This corresponds to the region of γ-ray energies of the order of \(1\,M_{\mathrm{ev}}\) above the fission threshold, with which the authors worked. For unpolarized γ-rays the theory gives for the angular distribution of the fission protons (neutrons) in the system of the center of gravity a law \(a+b\sin^2\theta\), where \(\theta\) is the angle between the fission proton (neutron) and the incident γ-quantum. The spherically symmetric term \(a\) is due to the interaction of the γ-quantum with the magnetic moment of the proton or neutron, and the term \(b\sin^2\theta\) to the interaction with the charge of the proton.

The experimental arrangement is shown schematically in Fig. 1. The source of γ-rays with an energy \(\sim 3\,M_{\mathrm{ev}}\) was a graphite target in which electrons obtained in an electrostatic generator were slowed down. To reduce the background of soft γ-rays the target was surrounded by a layer of lead several inches thick. The intensity of the γ-rays was monitored by registering photoneutrons from heavy water, a vessel with which was placed behind the chamber.

The protons of photodisintegration were registered by three proportional plane-parallel counters mounted as a telescope in order to select fission protons leaving the target at definite angles. Each counter consisted of three wire grids, to the middle one of which a high voltage was applied. The first and second counters were connected in coincidence, and the third in anticoincidence with the first two. Such a connection of the counters with the radio circuit makes it possible to select protons which pass through the first two counters and do not reach the third. The telescope was placed in a chamber filled with deuterium, which slowed the fission protons, served as the filling gas for the counters, and was the target in the proton path before the first counter. By changing the pressure in the chamber it was possible, by means of the telescope of counters, to select protons of a given range and from them to determine the energy of the γ-quantum which had caused the disintegration. By rotating the telescope about an axis passing through the center of the target, observations could be made at various \(\theta\).

Fig. 1.

In the results of the measurements obtained, corrections were introduced for the incomplete transparency of the counters, for the inhomogeneity of the γ-ray flux through the gas target at different \(\theta\), and for the fact that the measurements were carried out in the laboratory coordinate system. In measuring the ranges of protons and the energies of γ-quanta determined from them, the geometry of the apparatus was taken into account. The total errors did not exceed 10%.

The angular dependence was checked for one energy and several angles of observation. The results of the experiment showed good agreement with the theory.

From observations at angles of 0 and 90° one can draw a conclusion about the ratio \(\tau\) of the cross sections of the photomagnetic and photoelectric disintegration, or about the ratio of the probability that the disintegration will occur through the magnetic or electric interaction of the \(\gamma\)-quantum with the deuteron nucleus. With certain assumptions about the form of the potential of the nuclear forces, it is possible theoretically to predict the dependence of \(\tau\) on the energy. The authors measured this dependence. As can be seen from Fig. 2, satisfactory agreement with theory was obtained.

Fig. 2

The work is of interest, since in it the dependence of \(\tau\) on the energy was measured for the first time and the validity of the theoretical conclusions concerning the angular distribution for \(\gamma\)-quanta of the given energies was clearly demonstrated.

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