FISSION NEUTRON SPECTRUM OF Pu$^{239}$

In the paper under review$^{1}$, the fission neutron spectrum of Pu$^{239}$ was investigated by the photoplate method. The source of thermal neutrons was a thermal column of a boiler with heavy water. The arrangement of the sample and detector was the same as in paper$^{2}$ (see also$^{3}$). The detector consisted of Ilford C2 photoplates. The plutonium sample was deposited on a nickel disk 0.5 inch in diameter and 0.125 inch thick. “Good geometry” was ensured.

A total of 5500 recoil-proton tracks were obtained after 35 hours of irradiation at a boiler power of 5.5 kW. Measurements were made by two microscopists independently of one another. Corrections were introduced for the change in the neutron scattering cross section on protons and for protons that did not stop in the emulsion (for details see$^{2}$). The results are shown in the graph. On the same graph the fission neutron spectrum of U$^{235}$ and the semiempirical Watt curve$^{4}$,

\[ N(E_n)=\mathrm{const}\cdot e^{-E_n}\,\sinh\sqrt{2E}, \]

are plotted. The data for U$^{235}$ and the Watt curve are normalized to the data for Pu$^{239}$ at 1.5 MeV. At this energy the error is the smallest, since the recoil-proton tracks have sufficient length for accurate measurements, while the corrections for protons emerging from the emulsion are still small.

The results show that the fission neutron spectra of Pu$^{239}$ and U$^{235}$ are identical within the limits of experimental error. Both spectra have a maximum in the region 0.6–0.8 MeV and fall exponentially above 2 MeV. The slope of the spectrum for Pu$^{239}$ is $4.3 \pm 0.2$ MeV per decade of intensity, and for U$^{235}$ it is $3.9 \pm 0.8$ MeV. Both values lie within the limits of experimental error. The mean neutron energy

\[ \overline{E}_n=\frac{\int N(E_n)\cdot E\cdot dE}{\int N(E_n)\cdot dE} \]

is equal to 2.0 MeV. The point at 0.4 MeV for Pu$^{239}$ is very low for this energy. This can be explained by the low sensitivity of the photoplates in this energy region.

Both spectra have a larger number of neutrons in the region of high energies than follows from Watt’s formula. But the statistics in this region are too poor to draw any conclusions. In the remaining region, Watt’s semiempirical formula describes the experimental data very well.

L. K.

References

1. N. Nereson, Phys. Rev. 88, 823 (1952).
2. N. Nereson, Phys. Rev. 85, 600 (1952).
3. UFN, vol. XLVIII, 585 (1952).
4. B. E. Watt, Phys. Rev. 87, 1037 (1952).