ENERGY DISTRIBUTION OF FISSION FRAGMENTS OF URANIUM-235 AND URANIUM-233

In the paper under review¹ an investigation was made of the energy distribution of the fission fragments of U²³³ and U²³⁵. For U²³⁵ such measurements were carried out—

Fig. 1. Graph of the energy distribution of U²³⁵ fragments. The solid curves show the relative frequency of occurrence of a given combination of energies. The solid straight lines are lines of constant total (kinetic) energy; the dashed lines are lines of constant ratio of fragment masses.

FROM CURRENT LITERATURE

...were already carried out more than once, so that the experimental results can be compared with earlier measurements.

In the experiments carried out by the pulse-coincidence method2,3, a double ionization chamber was used. On the cathode common to both parts of the chamber, a thin film containing \(3—5\ \mu\mathrm{g}/\mathrm{cm}^{2}\) of uranium was deposited. The uranium was irradiated with neutrons from a uranium boiler. In one part of the chamber only fragments having energies in a specified narrow interval \(5\ \mathrm{MeV}\) wide were recorded. The mean position of the interval of recorded energies was varied every \(5\ \mathrm{MeV}\), thus covering the entire spectrum of fragment energies. Simultaneously, in the other part of the chamber the second fission fragment was recorded and its energy was determined with the aid of a thirty-channel pulse analyzer4. Unlike the previously described multichannel analyzers5, the instrument used could record pulses of any shape. This was achieved by sending the registered pulse not immediately into the channel recording the energy in a given range, but through a “pulse converter”—an electronic circuit transforming a pulse of arbitrary shape into a rectangular pulse of constant duration, with an amplitude equal to the maximum amplitude of the original pulse.

The calibration of the pulse magnitudes was carried out with the aid of a pulse generator, which in turn was calibrated with the aid of natural \(\alpha\)-particle sources (it was assumed that the mean ionization losses in argon for \(\alpha\)-particles and fission fragments are the same).

Fig. 2. Distribution graph of \(U^{233}\) fragments by energy.

The results of the experiments for \(U^{235}\) and \(U^{233}\) are shown in Figs. 1 and 2. From the graphs presented, certain interesting features of the energy distribution of the fragments are evident. The form of the energy distribution of the light fragments, for a given energy of the heavy fragment, depends only very weakly on the latter, and conversely. Thus, when the energy of the heavy ...

fragment by 25 MeV, the average energy of the light fragments changes only by 4 MeV.

It is also interesting that both changes occur in the same direction. Therefore the total “double-humped” curve of the distribution of fragments by energies cannot be interpreted as a simple inversion of the mass-distribution curve.

From the measurement data one can construct the dependence of the total kinetic energy of the fragments on the mass ratio of the fragments. It turns out that for both \(U^{235}\) and \(U^{233}\) the maximum kinetic energy corresponds to a mass ratio of about 1.25. Since the total energy of the fragments (kinetic + excitation energy) should, according to theoretical calculations, have a maximum at a mass ratio equal to unity, from the experimental results one may conclude that in the region of nearly equal masses the excitation energy increases strongly. In this sense the experiments on the study of the energy of plutonium fission fragments were also interpreted,\(^{6}\) showing a decrease of the total kinetic energy in the region of a mass ratio equal to 1.2.

B.

CITED LITERATURE

1. D. C. Brinton and G. N. Hanna, Phys. Rev. 75, 990 (1949).
2. W. Jentschke, Zeits. f. Physik 120, 165 (1943).
3. A. Flammersfeld, P. Jensen und W. Gentner, Zeits. f. Physik 120, 450 (1943).
4. C. H. Westcott and G. C. Hanna, Rev. Sci. Instr. 20, 181 (1949).
5. H. F. Freundlich, E. P. Hincks and W. J. Ozeroff, Rev. Sci. Instr. 18, 90 (1947).
6. S. Katcoff, J. A. Miskel and C. W. Stanley, Phys. Rev. 74, 631 (1948).