A New Beam Monitor at NFS/SPIRAL2 Based on Position-Sensitive PPACs Detecting Fission Fragments from ${}^{238}$U$(n,f)$

TL;DR

The paper presents a novel beam-monitoring approach for the NFS ToF area based on position-sensitive PPACs detecting coincident $^{238}$U$(n,f)$ fission fragments. By determining neutron energy event-by-event via Time-of-Flight and reconstructing the fission point, the setup delivers neutron flux and beam-profile measurements with minimal interference to other experiments. The results show a maximum flux of $(17.3\pm0.5)\times10^9$ n$\,/\mu$C/sr/1-MeV at $E_n\approx17$ MeV, a beam diameter of $44.4\pm0.4$ mm, and a thermal-background upper limit of $n_{th}/n_{fast} \leq (3.85\pm0.43)\times10^{-5}$, validating the method against standard data libraries. These findings provide a practical, high-precision in-beam neutron-monitoring technique suitable for parallel operation with other experiments at NFS.

Abstract

A new experimental setup has been installed at the Time-Of-Flight area of the Neutrons For Science facility (NFS) at GANIL/SPIRAL2 for neutron beam monitoring. This setup consists of an array of Position-Sensitive Parallel-Plate Avalanche Counters (PS-PPACs) that detects both fission fragments in coincidence from secondary neutron-induced fission reactions in several ${}^{238}$U targets. The neutron energy is determined on an event-by-event basis using the Time-of-Flight method, and the reaction point within the U targets is reconstructed, enabling the measurement of the neutron beam flux and beam profile. The high transparency of the setup allows it to operate in parallel with other experiments running at NFS, thus providing an in-beam monitor of the neutron intensity. In this work, we report on the characteristics of this new setup, its operating principle, and the first results obtained using the high-intensity white-spectrum neutron beam at NFS. This beam is produced via reactions between a primary 40-MeV deuteron beam, accelerated in the SPIRAL2 LINAC, and a 8 mm-thick rotating beryllium converter target.

Figures (11)

• Figure 1: Schematic view of the experimental setup. The Uranium target is surrounded by two PS-PPACs, meant for fission-fragments coincidence detection.
• Figure 2: Schematic layout of the detection system (left) and picture of the reaction chamber (right) allocating three PS-PPACs and two $^{238}$U targets.
• Figure 3: Selection of fission fragments from anode signals. a) Energy loss correlation detected in the anodes of two PS-PPACs surrounding the target 1. Fission fragments (FF) are well separated from alpha particles. Fission fragments from target 2, crossing PS-PPAC 2 and arriving to PS-PPAC 1 are also detected. b) Energy loss correlation of fission fragments between PS-PPAC 1 and PS-PPAC 3 in order to separate fission fragments coming from the target 1 and from the target 2. c) and d) Final identification of fission fragments from corresponding targets. The red lines represents the discrimination between alpha particles and fission fragments applied in the analysis.
• Figure 4: Rate of detected fission fragments as a function of the neutron Time of Flight. Vertical lines indicates the $ToF_n$ corresponding to neutron energies of 1.5 MeV and 40 MeV.
• Figure 5: Neutron energy calibration from Time-of-Flight. (Top) $^{238}$U(n,f) fission fragments production as a function of the neutron energy from a neutron beam impinging directly into the U target (green) and from an attenuated neutron beam passing through 4-cm Carbon target before reaching the U target (blue). (Bottom) Total absorption cross section of $^{12}$C(n,tot) as a function of the neutron energy from present data (black) compared with reference data from ENDF/B-VIII.0 data base (red).