Disintegration of Light Atomic Nuclei by the Capture of Fast Neutrons

W. D. Harkins et al.

Editor’s Note

Observations of nuclear disintegration induced by neutrons provoked suspicions that such nuclear processes play a role in the Sun and other stars. Here William Draper Harkins and colleagues report experiments on the disintegration of nitrogen and several other nuclei by high-energy neutrons. They noticed that the incident neutrons must possess a certain threshold energy, which is converted to a mass increase in the product particles. For nitrogen this threshold is equivalent to the energy of particles in a gas with average temperature of about 1010 K. A small fraction of particles in a gas at typical solar temperatures would also have such velocities, suggesting that light atoms may undergo such disintegrations in stars.　　中文

ABOUT thirteen disintegrations of neon nuclei have been obtained in 3,200 pairs, and approximately 100 disintegrations of nitrogen nuclei in 7,600 pairs of photographs of a Wilson chamber through which neutrons were passing. The source of the neutrons consisted of beryllium powder intimately ground with a mixture of mesothorium and thorium-X. The neutron source used was on the average more powerful in the experiments with neon than with nitrogen. If all the factors are taken into account, it is found that with identical atomic concentrations of neon and of nitrogen in the chamber, the neon nuclei are disintegrated much less often than those of nitrogen.　　中文

The average energies of the neutrons which have been found to disintegrate light nuclei are, in millions of electron volts, 5.8 for nitrogen, 7.0 for oxygen, and 11.6 for neon. Here the value for oxygen is taken from the work of Feather. The mass data indicate that the energy needed to supply mass increases in just this order, and is respectively –1.4×106, 0 and +2×106 electron volts, if the mass of the neutron is assumed to be that given by Chadwick, 1.0067, which is probably too high. Obviously the value assumed does not affect the differences between the energy values.　　中文

In a gas, ethylene, which consists of hydrogen and carbon, three disintegrations were obtained in 3,200 pairs of photographs. If carbon (12) is disintegrated by capture of the neutron the reaction is

C12 + n1 → C13 → Be9 + He4

12.0036+1.0067→9.0155+4.00216

or, if the mass assumed for the neutron is correct, Δm = 0.0074, which is equivalent to 6.9×106 electron volts.　　中文

This corresponds to a velocity of 3.6 × 109 cm. per sec., so only neutrons of a velocity higher than this should be effective in disintegrating carbon of mass 12. The smallness of the yield of disintegrations which we have obtained with carbon is thus to be expected, especially since probably less than one-fifth of the neutrons have velocities higher than this.　　中文

A remarkable relation which has been found to hold without exception is: in disintegrations by capture of a neutron the kinetic energy almost always decreases, is sometimes conserved, but in no case increases.　　中文

It has been pointed out previously by Harkins that the values for the energy which disappears suggest definite energy values for the γ-rays into which this energy is converted, but the accuracy of the work is not yet sufficient to prove that this is true.　　中文

It may be assumed that the neutrons in the stars are scattered by the atomic nuclei and thus take part in the temperature distribution of velocities of the atoms. If the neutrons of higher velocity are captured much more often than those of lower velocity, the distribution will be affected. Our experiments give no information concerning the capture of neutrons without disintegration, but only for those cases in which the capture is revealed by the accompanying disintegration.　　中文

It is of interest in this connexion to consider the minimum energy of the neutron which has been found to give a disintegration. The values, in millions of electron volts, are 1.9 for nitrogen, and 7.8 for neon. The corresponding maximum values are 16.0 and 14.5, the lower maximum for neon being due to the smallness of the number of disintegrations which have been obtained in this gas. An energy of 1.9×106 corresponds to a mean temperature of the order of 1010 degrees, but at 108 degrees a considerable number of neutrons should have this energy, and a moderate number even at 107, so it is not unreasonable to suppose that nitrogen nuclei are disintegrated by this process in the stars.　　中文

A part of this work was presented by Harkins on June 23 at a symposium on nuclear disintegration under the auspices of the Century of Progress Exposition, Chicago. Other papers were presented by Cockcroft, Lawrence and Tuve, and a general discussion of the theory was given by Bohr.　　中文

(132, 358; 1933)

William D. Harkins, David M. Gans and Henry W. Newson