In response to a request for a 237U(n, f) cross section evaluation up to E{sub n} = 20 MeV, they have married a data set from the previous reliable estimate [1-3] of the cross section up to E{sub n} = 2.5 MeV, to an estimate of the remaining cross section up to E{sub n} = 20 MeV, deduced from simple physics arguments. This straw-man, work-in-progress estimate of the 237U(n, f) cross section is intended to be used in sensitivity-test comparisons to other evaluations of the cross section (e.g., ENDF/B-VI [4] and ENDL [5]). The simple approach used in this work to generate a consistent cross section up to E{sub n} = 20 MeV is validated using the well-known 235U(n, f) cross section as a test case (see Fig. 1). The corresponding estimate of the 237U(n, f) cross section is plotted in Fig. 2 and listed in Table I.
The authors have deduced the {sup 237}U(n, f) cross-section over an equivalent neutron energy range of 0 to 20 MeV using the Surrogate Ratio method. A 55 MeV {sup 4}He{sup 2+} beam from the 88 Inch Cyclotron at Lawrence Berkeley National Laboratory was used to induce fission in the following reactions {sup 238}U({alpha}, {alpha}'f) and {sup 236}U({alpha}, {alpha}'f). The {sup 238}U reaction was a surrogate for {sup 237}U(n, f) and the {sup 236}U reaction was used as a surrogate for {sup 235}U(n, f). The energies of the scattered alpha particles were detected in a fully depleted segmented silicon telescope array (STARS) over an angle range of 35{sup o} to 60{sup o} with respect to the beam axis. The fission fragments were detected in a third independent silicon detector located at backward angles between 106{sup o} to 131{sup o}.
The surrogate reaction 238U(3He, tf) is used to determine the 237Np(n, f) cross section indirectly over an equivalent neutron energy range from 10 to 20 MeV. A self-supporting ~;;761 mu g/cm2 metallic 238U foil was bombarded with a 42 MeV 3He2+ beam from the 88-Inch Cyclotron at Lawrence Berkeley National Laboratory (LBNL). Outgoing charged particles and fission fragments were identified using the Silicon Telescope Array for Reaction Studies (STARS), consists of two 140 mu m and one 1000 mu m Micron S2 type silicon detectors. The 237Np(n, f) cross sections, determined indirectly, were compared with the 237Np(n, f) cross section data from direct measurements, the Evaluated Nuclear Data File (ENDF/B-VII.0), and the Japanese Evaluated Nuclear Data Library (JENDL 3.3) and found to closely follow those datasets. Use of the (3He, tf) reaction as a surrogate to extract (n, f) cross section in the 10 to 20 MeV equivalent neutron energy is found to be suitable.
The purpose of this note is to combine existing information on the 237U(n, f) cross section to determine if some consistency can be obtained for the neutron induced fission excitation of 237U. The neutron induced fission cross section of the 6.8 day 237U was measured directly by McNally et al. in 1968 using the Pommard nuclear device test. At the same time critical assembly measurements were done at Los Alamos using the Flattop assembly. A previous measurement was also made at LASL in 1954 with two different neutron sources, each peaked near 200 keV. The results were 0.66 " 0.10 b and 0.70 " 0.07 b for the (n, f) cross section. More recently Younes and Britt have reanalyzed direct reaction charged particle data of Cramer and Britt that had determined the fission probability of the 238U compound nucleus as a function of nuclear excitation energy. They have combined fission probabilities with calculated neutron absorption cross sections, including corrections for the differences in angular momentum between the direct and neutron induced reactions. From this analysis they have extracted equivalent 237U(n, f) cross sections. The technique for extracting surrogate (n, f) cross sections from (t, pf) data has been demonstrated in a recent publication for the test case 235U(n, f). In addition to this experimental information, Lynn and Hayes have recently done a new theoretical study of the fission cross sections for a series of isotopes in this region. A summary plot of the data is shown in Fig. 1. Below 0.5 MeV the McNally, Cowan, and Younes-Britt results are in reasonable agreement. The average cross section in the Younes-Britt results, for En = 0.1 to 0.4 MeV, is 0.80 times the McNally values which is well within the errors of the McNally experiment. Above 0.5 MeV the McNally results diverge toward higher values. It should be noted that this divergence begins approximately at the 237Np threshold and that 237Np is the daughter of the 6.8 day 237U decay.
Using existing experimental data for neutron-induced total, elastic, inelastic, reaction and fission cross sections, as well as results from nuclear model calculations and evaluations from nuclear reaction data libraries, we derived an estimate for the cross sections for the 235U(n,2n) and 239Pu(n,2n) reactions for the neutron energy range from threshold to approximately 12 MeV. In effect, our approach is based on subtracting the fission and inelastic cross sections from the total reaction cross section where the difference is expected to yield the (n,2n) cross section. In addition to this subtraction approach, a ratio method and a differential method have also been explored. For 235U(n,2n), as a test case, we arrive at a cross section consistent with previous measurements, and for 239Pu(n,2n) we obtain a peak value of 400 " 60 mb for the incident neutron energy range of 10 (less-than or equal to) E{sub n} (less-than or equal to) 12 MeV.
Using both the absolute and relative surrogate techniques, the 236U(n, f) cross section was deduced over an equivalent neutron energy range of 0 to 20 MeV. A 42 MeV 3He beam from the 88-Inch Cyclotron at Lawrence Berkeley National Laboratory was used to perform a (3He, [alpha]) pickup reaction on targets of 235U (J{sup {pi}}=7/2−) and 238U (J{sup {pi}}=0) and the fission decay probabilities were determined. The 235U(3He, [alpha]f) and 238U(3He, [alpha]f) were surrogates for 233U(n, f) and 236U(n, f), respectively. The cross sections extracted using the Surrogate Method were compared to directly measured cross sections. The sensitivity of these cross sections to the J{sup {pi}}-population distributions was explored.
The Surrogate Ratio Method (SRM) was employed in the first experimental determination of the 231Th(n, f) cross section, relative to the 235U(n, f) cross section, over an equivalent neutron energy range of 360 keV to 10 MeV. The 230Th(n, f) cross section was also deduced using the SRM, relative to the 234U(n, f) cross section, over an equivalent neutron energy range of 220 keV to 25 MeV. The desired compound nuclei were populated using (3He,3He) and (3He) reactions on targets of 232Th and 236U and relative fission decay probabilities were measured. The surrogate 230,231Th(n, f) cross sections were compared to cross section evaluations and directly-measured experimental data, where available.
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