Deducing the ${}^{237}\mathrm{U}$($n,f$) cross section using the surrogate ratio method

We have deduced the cross section for ${}^{237}\mathrm{U}$($n,f$) over an equivalent neutron energy range from 0 to 20 MeV using the surrogate ratio method. A 55 MeV ${}^4\mathrm{He}$ beam from the 88 inch cyclotron at Lawrence Berkeley National Laboratory was used to induce fission in the following reactions: ${}^{238}\mathrm{U}$$(\alpha ,{\alpha }^{\prime }f)$ and ${}^{236}\mathrm{U}$$(\alpha ,{\alpha }^{\prime }f)$. The ${}^{238}\mathrm{U}$ reaction was a surrogate for ${}^{237}\mathrm{U}$$(n,f)$, and the ${}^{236}\mathrm{U}$ reaction was used as a surrogate for ${}^{235}\mathrm{U}$$(n,f)$. Scattered α particles were detected in a fully depleted segmented silicon telescope array over an angle range of ${35}^{°}$ to ${60}^{°}$ with respect to the beam axis. The fission fragments were detected in a third independent silicon detector located at backward angles between ${106}^{°}$ and ${131}^{°}$.

Figure 1

STARS detector setup with silicon telescope downstream from target and fission detector 10 mm upstream of target. The $\Delta e^{-}$ shield located in front of the $\Delta E$ detector ranged out the fission fragments.

Figure 2

Typical ${}^{226}\mathrm{Ra}$ calibration spectrum for an individual ring. Data show the five dominant α particles present in the ${}^{226}\mathrm{Ra}$ decay chain vs channel number in the SILENA ADC.

Figure 3

$\Delta E$ vs $(\Delta E+E)$ plot obtained from nominally 55 MeV α particle incident on ${}^{238}\mathrm{U}$ in coincidence with fission events. Starting in the lower left corner, we see protons, deuterons, and tritons. The large band in the middle of the plot represents α particles.

Figure 4

Effective thickness energy curve. Particles bands are designated as in Fig. 3.

Figure 5

Typical fission spectrum from a ring in the fission detector. Centroids of the light and heavy fission fragments are indicated with arrows.

Figure 6

Number of $(\alpha ,{\alpha }^{\prime }f)$ events as a function of excitation energy. These data are used to calculate relative probability of fission for these nuclei. ${}^{238}\mathrm{U}$ (triangles) compared to ${}^{236}\mathrm{U}$ (circles).

Figure 7

In-plane fission fragment anisotropies as a function of excitation energy of the nucleus for ${}^{238}\mathrm{U}$ (red triangles) and ${}^{236}\mathrm{U}$ (blue circles). Anisotropy is most pronounced at the fission barrier $E_{\mathrm{ex}}$ of approximately 6 MeV and decreases toward unity thereafter. Fission anisotropies are equal within the experimental uncertainties over the excitation energy range of interest.

Figure 8

Ratio of ${}^{238}\mathrm{U}$$(\alpha ,{\alpha }^{\prime }f)$ to ${}^{236}\mathrm{U}$$(\alpha ,{\alpha }^{\prime }f)$ events as a function of excitation energy of the corresponding nucleus.

Figure 9

Upper panel shows ${}^{235}\mathrm{U}$$(n,f)$ cross section from ENDF/B-VII [19] used to determine ${}^{237}\mathrm{U}$$(n,f)$ cross section. Lower panel compares our ${}^{237}\mathrm{U}$$(n,f)$ cross section (red triangles) to results from earlier work by Younes and Britt [20] (blue squares, no error bars).