Measurement of the normalized $^{238}$U(n,f)/$^{235}$U(n,f) cross section ratio from threshold to 30 MeV with the fission Time Projection Chamber

TL;DR

This study reports the first measurement of the $^{238}$U(n,f)/$^{235}$U(n,f) cross section ratio from threshold to 30 MeV using a novel fission Time Projection Chamber, providing valuable data for neutron flux standards.

Contribution

It introduces a new experimental measurement of the cross section ratio using the fissionTPC, enhancing the data available for nuclear reaction evaluations.

Findings

First cross section ratio measurement with fissionTPC

Normalized ratio provided from threshold to 30 MeV

Data normalized to ENDF/B-VIII.0 at 14.5 MeV

Abstract

The normalized $^{238}$U(n,f)/$^{235}$U(n,f) cross section ratio has been measured using the NIFFTE fission Time Projection Chamber from the reaction threshold to $30$~MeV. The fissionTPC is a two-volume MICROMEGAS time projection chamber that allows for full three-dimensional reconstruction of fission-fragment ionization profiles from neutron-induced fission. The measurement was performed at the Los Alamos Neutron Science Center, where the neutron energy is determined from neutron time-of-flight. The $^{238}$U(n,f)/$^{235}$U(n,f) ratio reported here is the first cross section measurement made with the fissionTPC, and will provide new experimental data for evaluation of the $^{238}$U(n,f) cross section, an important standard used in neutron-flux measurements. Use of a development target in this work prevented the determination of an absolute normalization, to be addressed in future measurements. Instead, the measured cross section ratio has been normalized to ENDF/B-VIII.$\beta$5 at 14.5 MeV.