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

TL;DR

This study measures the $^{238}$U/$^{235}$U fission cross section ratio up to about 1 GeV at n_TOF, confirming existing evaluations and theoretical models, and addressing discrepancies in high-energy data.

Contribution

The paper provides the first high-precision measurement of the fission cross section ratio up to 1 GeV, combining multiple datasets for consistency and comparison with models.

Findings

Data are consistent within 3-4% systematic uncertainty.

Results agree with current evaluations up to 200 MeV.

Extends the energy range for fission cross section ratio measurements to ~1 GeV.

Abstract

The $^{238}$U to $^{235}$U fission cross section ratio has been determined at n_TOF up to $\sim$1 GeV, with two different detection systems, in different geometrical configurations. A total of four datasets have 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 the neutron energy. The result confirms current evaluations up to 200 MeV. A good agreement is also observed with theoretical calculations based on the INCL++/Gemini++ combination up to the highest measured energy. The n_TOF results may help solving a long-standing discrepancy between the two most important experimental dataset available so far above 20 MeV, while extending the neutron energy range for the first time up to $\sim$1 GeV.