Energy of Neutrons Liberated in the Nuclear Fission of Uranium Induced by Thermal Neutrons

H. von Halban, jun. et al.

Editor’s Note

The fissioning of a uranium nucleus, physicists now knew, releases further neutrons, which might in principle trigger splitting in other nuclei. But researchers knew little about the energy of the released neutrons. Here Hans von Halban and colleagues in Paris clarify this matter. In a series of experiments, they used an ionization chamber to probe the distribution of energies of “fast” neutrons—that is, those having energy above 1.5 million electronvolts (1.5 MeV). The number of neutrons declines at higher energies, but some neutrons can carry away as much as 11 MeV. Knowledge of the distribution of neutron energies played an important role in the engineering of nuclear reactors, as slow neutrons trigger fission events more effectively than fast ones.　　中文

IT has been shown that fast neutrons are liberated in the process of nuclear fission induced in uranium by primary thermal neutrons. Two different methods of detection have been used: in the first method1, the primary and (if any) secondary neutrons are absorbed in a medium in which an endo-energetic reaction can take place, leading to the formation of an easily detectable radioactive nucleus. If the energy threshold is situated above the maximum energy of the primary neutrons, any positive results observed must be ascribed to the secondary neutrons. In the second method2, elastic collisions of fast neutrons with heavier nuclei are observed by means of an ionization chamber filled with a gas at atmospheric pressure and connected to a linear amplifier. In order to study separately the effect due to the primary thermal neutrons, the experiment is performed with, and without, a cadmium shield between the source and the uranium mass.　　中文

The first method having shown us that fast secondary neutrons are produced with energies of at least 2 Mev. (sufficient to transform 32S into radioactive 32P in detectable quantities), we sought to ascertain, by the second method, whether neutrons of energy notably higher than 2 Mev. are also present in the secondary radiation. In our experiment, the oxygen-filled ionization chamber was placed in a nearly cubical box (9 cm. × 9 cm. × 8 cm.) containing uranium oxide and surrounded by a thick layer of paraffin wax. The source (300 mgm. Raγ + Be), surrounded by a lead shield (5 cm. in the direction of the chamber) was buried in the wax. In order to absorb thermal neutrons, the uranium box could be screened on all sides with a cadmium foil. The pulses were recorded either in the presence or in the absence of this foil and the part of the effect (projection of oxygen nuclei by fast neutrons liberated in the uranium) due to thermal neutrons could thus be evaluated.　　中文

In view of the large number of of accidental pulses due to the strong γ-radiation emitted by the source, only nuclei recoiling with at least 1.5 Mev. could be taken into consideration. The distribution curve shows that the frequency of pulses observed falls off rapidly between 1.5 Mev. and 2.5 Mev.; between 2.5 Mev. and 3.7 Mev. the frequency decreases much more slowly, pulses observed in this second region being, however, very rare. The total number of pulses recorded is small (with cadmium: 84 pulses in90 minutes; without cadmium: 161 pulses in 90 minutes); but it appears clearly that recoils with energy of about 2.5 Mev. are notably more frequent in the absence of cadmium and, therefore, that neutrons possessing an energy of at least 11 Mev. are liberated in uranium irradiated with thermal neutrons.　　中文

The high energy of these fast neutrons shows that their parent nuclei are in a highly excited state at the moment of their liberation, which is probably simultaneous with the fission. In this way a non-negligible fraction of the fission energy is disposed of; a further fraction is carried off by the β- and γ-rays afterwards emitted by the nuclei produced in the fission. The remainder available as kinetic energy for these recoiling nuclei is therefore considerably smaller than the total amount of energy liberated in the fission process (about 200 Mev.).　　中文

(143, 939; 1939)

H. von Halban, jun., F. Joliot and L.Kowarski: Laboratoire de Chimie Nucléaire, Collège de France, Paris, May 20.

References:

1. Dodé, M., von Halban, jun., H., Joliot, F., and Kowarski, L., C.R., 208, 995 (1939).

2. Szilard, L., and Zinn, W., Phys. Rev., 55, 799 (1939).