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This paper reports measurements of neutron induced fission cross sections for Pa-231 and Pu-239 using an electrostatic accelerator, with Pa-231 studied over 140 to 1740 keV and Pu-239 over 1.5 to 1250 keV. Motivated by earlier observations that some fissioning nuclei show threshold region irregularities while odd-even nuclei such as Np-237 and Am-241 appear smoother, the study tests whether this smooth behavior is general for odd-even nuclei. The Pu-239 results agree with prior data within experimental errors, while Pa-231 shows three previously unobserved maxima at neutron energies near 330, 550, and 880 keV. The authors suggest that subsequent decreases in the Pa-231 cross section may result from competition with newly opened inelastic scattering channels, indicating that irregular fission cross section behavior can also occur in odd-even nuclei.
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PHYSICS
S. M. DUBROVINA, V. A. SHIGIN
FISSION CROSS SECTIONS OF $\mathrm{Pa}^{231}$ AND $\mathrm{Pu}^{239}$ BY NEUTRONS IN THE ENERGY INTERVAL $1.5 \div 1500$ keV
(Presented by Academician A. P. Aleksandrov, April 7, 1964)
For a number of fissioning nuclei (such as $\mathrm{Th}^{230}$, $\mathrm{Th}^{232}$, $\mathrm{U}^{234}$) ($^1$), significant irregularities were found in the behavior of the fission cross section as a function of neutron energy near the fission threshold. For odd-even nuclei $\mathrm{Np}^{237}$, $\mathrm{Am}^{241}$, a smooth increase of the fission cross section is observed as the neutron energy increases, and no appreciable irregularities are present. It seemed of interest to us to determine whether the absence of irregularities is characteristic of all odd-even nuclei. To this end we studied the behavior of the fission cross section of the odd-even nucleus $\mathrm{Pa}^{231}$. Measurements of the fission cross section of $\mathrm{Pa}^{231}$ were carried out by us in the neutron-energy interval 140–1740 keV. Along with these measurements, we also carried out measurements of the fission cross section of $\mathrm{Pu}^{239}$ in the neutron-energy interval $1.5 \div 1250$ keV.
The measurements were performed on an electrostatic accelerator. The experimental method is analogous to that described earlier ($^2$).
Fig. 1
Figures 1 and 2 show the fission cross sections of $\mathrm{Pa}^{231}$ and $\mathrm{Pu}^{239}$ obtained by us. The errors in determining the relative behavior of the fission cross section are also shown there. The absolute value of the fission cross section was determined, in the case of $\mathrm{Pa}^{231}$, with an accuracy of 15%, and in the case of $\mathrm{Pu}^{239}$, 10%.
The neutron-energy dispersion in measuring the fission cross section of $\mathrm{Pa}^{231}$ was $\pm 20$ keV; in measuring the fission cross section of $\mathrm{Pu}^{239}$ it was $\pm 20 \div 30$ keV at neutron energies $\geq 200$ keV, while at neutron energies of 100, 30, 6, and 1.6 keV it was respectively 10, 8, 3, and 0.8 keV.
The results we obtained for Pu\(^{239}\), within the limits of measurement errors, agree with the results of other authors \((^3)\).
The measurements we carried out indicate a substantial difference between the behavior of the fission cross section of Pa\(^{231}\) and the behavior of the cross sections of Np\(^{237}\) and Am\(^{241}\).
In the behavior of the fission cross section of Pa\(^{231}\), three maxima were found at neutron energies of 330, 550, and 880 keV (which had not been observed by other authors \((^3)\)).
Fig. 2
The considerable decreases in the fission cross section of Pa\(^{231}\) following these maxima are apparently caused by competition from newly opening channels of inelastic neutron scattering. However, identification of these channels proves difficult, since the positions and characteristics of the corresponding excitation levels of Pa\(^{231}\) have not been studied sufficiently.
Thus, the results of our measurements show that irregularities in the behavior of the fission cross section are also observed in odd-even nuclei.
Received
6 I 1964
CITED LITERATURE
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- G. V. Gorlov, B. M. Gokhberg et al., Atomic Energy, 6, issue 4, 453 (1959); B. M. Gokhberg, G. A. Otroshchenko, V. A. Shigin, DAN, 128, No. 5, 911 (1959).
- D. J. Hughes, R. B. Schwartz, Neutron Cross Sections, N. Y.—London, 1958.