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Better knowledge of the neutron population in reactors is crucial to improve the accuracy of neutronics simulations of present day or future reactor cores. This population is partially driven by (n, xn) reactions. However, the cross sections of these reactions are not precisely known. This is particularly true for the 238U(n, n’γ) cross section, which is on the High Priority Request List. One method to measure this cross section is to use the prompt γ-ray spectroscopy coupled to time-of-flight measurements. This allows the total (n, n’) cross section to be inferred from the measured (n, xnγ) cross sections and the level scheme information. However, the knowledge of the 238U level scheme is still very incomplete, so an initiative to experimentally revisit the 238U structure has been launched using γ-γ coincidences spectroscopy. The v-Ball γ-spectrometer was coupled to the LICORNE directional neutron source of the ALTO facility, allowing study of inelastic scattering on 238U via γ-γ coincidences. The analysis of data obtained during the first v-Ball campaign was performed using the Radware escl8r software. At the present time, 73 γ-transitions and 50 levels registered in ENSDF have been confirmed and 120 new γ-transitions and 50 new levels have been found.

The design of new generation fast nuclear reactors requires highly accurate cross-section measurements in the MeV energy region. The 242Pu fission cross section is of particular interest for Pu incineration and nuclear waste production. There are discrepancies around 1 MeV incident neutron energy between libraries and among experimental data. Some data suggest the presence of a strong structure between 1 and 1.2 MeV whereas it is barely visible on some other data and its shape is very different among evaluations. The large majority of the 242Pu(n,f) measurements have been carried out with respect to the 235U(n,f) secondary-standard cross section. This introduces a strong correlation between measurements from different research teams. Moreover, this reference cross section exhibits structures, in particular a steep increase of +10% at 1 MeV. Therefore, we aim to re-measure the 242Pu(n,f) cross section relative to the primary-standard 1H(n,n)p cross section, by using a proton recoil detector. This standard has a very high accuracy (0.4%), is not used for other 242Pu measurements, and is structureless. An experiment has been carried out in October 2022 at the MONNET facility in JRC Geel, with incident neutron energies from 0.9 MeV to 2.0 MeV. The experimental setup will be presented, and the analysis procedure will be detailed.

Accurate actinides fission cross sections around 1 MeV are of primary importance for the safety of generation IV reactors. To have accurate measurements, the neutron flux must also be accurately estimated. This is usually done with respect to the 235U(n,f) cross section. It is however possible to measure the neutron flux with respect to the 1H(n,n)p cross section which is a primary standard, providing an independent and precise measurement. Typically, the usual proton recoil technique uses a silicon detector for neutrons of energy between 1 and 70 MeV. However, the high electron and gamma background due to neutron production under irradiation makes the use of this or any other detector not suitable for an accurate measurement below 1 MeV. To this end, the Gaseous Proton Recoil Telescope is developed and characterized. The goal is to provide quasi-absolute neutron flux measurements with an accuracy better than 3%. It consists of a double ionization chamber with a Micromegas segmented detection plane and the gaseous pressure can be adjusted to protons – and hence neutron – energy. The sensitivity to gamma and electrons background, the intrinsic efficiency as well as the resolution of this detector have been investigated.

The study of the 242Pu fission cross section has become more important since this isotope is present in Pressurized Water Reactor Uranium Oxide and Mixed Oxide fuel, as well as in fast neutron reactors fuel. This nucleus has been also selected for monitoring the high neutron flux RJH reactor (CEA-Cadarache). For all these reasons, the fission cross section structure of this fertile isotope, and especially its class-II states resonances were investigated in this work. Two class-II states have been identified in the Resolved Resonance Range (RRR) and their parameters have been supplied to the CONRAD code. 68 class- II states have been identified in the Unresolved Resonant Range (URR) and their parameters have been added in the TALYS code data library to reproduce as well as possible the experimental fission cross section. The methodology to identify and insert the class-II states in RRR and URR is described here.

To accurately measure neutron-induced fission cross sections, to characterize neutron-beam lines or to make dosimetric investigations, it is necessary to have high accuracy measurements of neutron fluence. It is possible to perform independent and precise neutron flux measurements with respect to the 1H(n,n)p elastic scattering cross section. The use of a silicon detector is recommended from 1 to 70 MeV neutron energy. However, it has been observed that a high electrons background forbids its use below 1 MeV. Hence, a new gaseous proton-recoil telescope is developed and characterized to overcome this limit. It should provide quasi-absolute neutron flux measurements with an accuracy around 3% and is not sensible to gamma and electrons background. It consists in two ionization chambers read by a segmented micromegas technology detection plane. The gas pressure inside is adjustable to the proton range in the detector and therefore to the neutron energy. This detector is described in details below and the newest results of its characterization are presented. A special attention is paid to detection efficiency measurements.