Text: This interaction is of basic importance to nuclear reactions induced by neutrons. Because the deutron consists of a neutron bound to a proton by nuclear forces, it offers a natural system for the investigation of the mutual interaction between the neutron and the proton. The manner in which the proton and the neutron split apart in the disintegration of the deutron is particularly significant. The binding energy of this combination is known from a variety of observations to be approximately 2.23 Mev. Any reaction which can introduce an amount of energy greater than 2.23 Mev offers the possibility of disintegrating the deutron. The fact that the spin of the deutron is equal to the numerical sum of the spin of the proton plus that of the neutron also shows that the spins of the proton and the neutron must be aligned parallel to each other in the deutron. Of the possible ways of splitting the deutron, that offered by the (y,n) reaction, sometimes called the photo effect, is most attractive. The only particles involved are the proton and the neutron. Consequently the disintegration of the deutron has been investigated extensively both in theory and in experiment. This disintegration is, in principle, a dual process in which both the electric and magnetic fields of the photons play a role. The electric field reacts with the instantaneous dipole moment to split the neutron from the proton, leaving the spins parallel. At photon energies a few Mev above the threshold, the magnetic field of the photon radiation reacts with the magnetic dipole moments of the neutron and the proton to produce anti parallel spins. This spin-flop, originally pointed out by Fermi, is called the photomagnetic process. It becomes important at photon energies in excess of 20 Mev. The electric separation of the proton and neutron involves properties of the neutron-proton potential in the triplet state (parallel spins) and the photomagnetic process involves the singlet state (antiparallel spins). (67) The radioactive capture of neutrons by protons is the inverse process to the photodisintegration of the deutron, and theory applicable to one also fits the other. Because the theory predicts a vanishingly small cross section for the photoelectric process of slow neutrons, some other explanation is required for the observations which proved that neutrons have a measurable cross section for capture by hydrogen. It was the search for the explanation which led Fermi to the development of the theory of the photomagnetic process. With only S states (l = 0) involved, instead of P state (l = 1), infrequent for slow neutrons, Fermi could show that the cross section should follow the 1/v law. Furthermore the photomagnetic process was adequate to explain the observed cross section of hydrogen for thermal neutrons. In spite of the theoretical as well as practical interest in the value of the absorption cross section of hydrogen for thermal neutrons, no very precise values have become available. The absence of information arises mainly from the difficulties in measuring such a small effect in a nuclear reaction which leads to a nonradioactive product. The best current values have been obtained by using pile oscillators in which the absorption of neutrons is detected by a decrease in the reactivity of the reactor and by measurement of the diffusion length of thermal neutrons in water.

See Also:

Source: 67