High-accuracy determination of the ${}^{238}\mathrm{U}/{}^{235}\mathrm{U}$ fission cross section ratio up to $\approx 1$ GeV at n_TOF at CERN

The ${}^{238}\mathrm{U}$ to ${}^{235}\mathrm{U}$ fission cross section ratio has been determined at n_TOF up to $\approx 1$ GeV, with two different detection systems, in different geometrical configurations. A total of four datasets has been collected and compared. They are all consistent to each other within the relative systematic uncertainty of 3–4%. The data collected at n_TOF have been suitably combined to yield a unique fission cross section ratio as a function of neutron energy. The result confirms current evaluations up to 200 MeV. Good agreement is also observed with theoretical calculations based on the $\mathrm{INCL}++/$Gemini$++$ combination up to the highest measured energy. The n_TOF results may help solve a long-standing discrepancy between the two most important experimental datasets available so far above 20 MeV, while extending the neutron energy range for the first time up to $\approx 1$ GeV.

Figure 1

Anisotropy in the emission angle of FF in the neutron-induced fission of ${}^{238}\mathrm{U}$ (upper panel) and ${}^{235}\mathrm{U}$ (lower panel), as a function of the neutron energy. The curves represent the results of a fit of the various experimental data with Legendre polynomials.

Figure 2

Efficiency for detecting FFs in coincidence with the PPAC setup, in the two geometrical configurations. In the tilted setup, the gain of efficiency at large emission angles is compensated by a corresponding loss at all other angles.

Figure 3

Ratio between the efficiencies for detecting a fission event from the ${}^{235}\mathrm{U}$ and ${}^{238}\mathrm{U}$ samples respectively. The various structures in the efficiency ratio corresponds to the structures in the angular anisotropy of FF emission in the two reactions.

Figure 4

The ${}^{238}\mathrm{U}/{}^{235}\mathrm{U}$ fission cross section ratio measured at n_TOF with the different setups, in the neutron energy range 500 keV - 20 MeV. The error bars represent only the statistical uncertainties. For comparison, the ratio calculated on the basis of the evaluated cross sections from ENDF/B-VII.1 is also shown in the figure (solid line).

Figure 5

The ${}^{238}\mathrm{U}/{}^{235}\mathrm{U}$ fission cross section ratio from the different n_TOF measurements, in the neutron energy range 500 keV–1 GeV.

Figure 6

Ratio between the various datasets collected in the four measurements at n_TOF and their weighted average. The dashed lines represent the $\pm 3$% uncertainty limits.

Figure 7

${}^{238}\mathrm{U}/{}^{235}\mathrm{U}$ (n,f) cross section ratio obtained from the weighted average of the four n_TOF datasets compared with previous results from Refs. [4] and [6], with the ratio extracted from the ENDF/B-VII.1 library and with the results of model calculations from Ref. [24].

Figure 8

Deviation of the average n_TOF ${}^{238}\mathrm{U}/{}^{235}\mathrm{U}$ (n,f) cross section ratio from the predictions of the major evaluated nuclear data libraries and from the INCL++/GEMINI++ model calculations. The horizontal lines represent the limit of $\pm 2$% deviation.