3D-Deuteron Track Recoils Produced by Neutron Capture in Hydrogen Measured by MIMAC-35 cm

Abstract

The neutron capture is a process that concerns most of the nuclei used to build our detectors. This process produces protons, alpha particles, and gamma rays which generate background signals. Characterizing this background is important for rare event searches, such as dark matter detection or Coherent Elastic Neutrino-Nucleus Scattering (CEvNS). This paper presents the result of the direct measurement of thermal neutron captures in hydrogen using a new MIcro-TPC MAtrix Chamber (MIMAC-35 cm) detector with a sensitive volume of 35 x 35 x 29 cm3. Data were collected over more than 5 days (443519 sec) with a gas mixture at 30 mbar of 70% isobutane (C4H10) and 30% trifluoromethane (CHF3). Our discrimination method is based on using 3D tracks and released ionization energy, in order to discriminate nuclear recoils (NR) from the dominant electron recoil (ER) background. This method enables the clear identification of 1.3 keV deuteron tracks resulting of nuclear capture reaction 1H(n, γ)2H. We observed 51 neutron capture events among more than 11 million total events mainly produced by muons in the experimental room of our ground laboratory. In parallel we have measured the thermal neutron flux just below the chamber with a BF3 detector and a simulation has been performed to estimate the number of captures expected. This work shows the discrimination power of MIMAC search for low-energy (E < 1 keV) rare event with a huge background without any shielding.

Figures (16)

• Figure 1: The strategy principle of MIMAC detector. beaufort_2021
• Figure 2: Energy calibration of the MIMAC detector. Left: The energy spectrum showing the fluorescence peaks from Al, Cd, and Cu. Right: The resulting linear calibration curve.
• Figure 3: Thermal neutron capture spectrum in the BF$_3$ detector. Left (a): Measured data over 86.15 hours. Right (b): PHITS simulation result. The peaks at 2.31 MeV (94% branch) and 2.79 MeV (6% branch) are clearly visible in both.
• Figure 4: The local differential neutron flux $\phi(E)$ (red dashed line, right axis) used for the analytical model, shown alongside the capture cross-sections $\sigma(E)$ for $^{10}$B (blue) and $^{1}$H (green) (left axis). The thermal neutron peak overlaps with the region where cross-sections are highest, illustrating that the measured capture rate is dominated by thermalized neutrons. The $^{10}$B cross-section is approximately four orders of magnitude larger than that of $^{1}$H, which accounts for the high statistics obtained in the BF$_3$ reference measurement compared to the MIMAC detector.
• Figure 5: PHITS simulation result of the thermal neutron capture spectrum in the MIMAC detector, with a peak at 1.3 keV.