Observation of a Nuclear Recoil Peak at the 100 eV Scale Induced by Neutron Capture

Coherent elastic neutrino-nucleus scattering and low-mass dark matter detectors rely crucially on the understanding of their response to nuclear recoils. We report the first observation of a nuclear recoil peak at around 112 eV induced by neutron capture. The measurement was performed with a ${\mathrm{CaWO}}_4$ cryogenic detector from the NUCLEUS experiment exposed to a ${}^{252}\mathrm{Cf}$ source placed in a compact moderator. We identify the expected peak structure from the single-$\gamma$ de-excitation of ${}^{183}\mathrm{W}$ with $3\sigma$ and its origin by neutron capture with $6\sigma$ significance. This result demonstrates a new method for precise, in situ, and nonintrusive calibration of low-threshold experiments.

Figure 1

Overview of the cryostat with the detector setup (a) and the neutron source configuration (b). The detector setup consists of a ${\mathrm{CaWO}}_4$ cube equipped with a W thin-film TES (a.1) held by bronze clamps (a.2) inside a copper housing (a.3). The copper support box is mechanically decoupled via a two-stage spiral spring (a.4) from the mixing chamber (a.5) of the dry dilution refrigerator (a.6). The ${}^{252}\mathrm{Cf}$ neutron source is inserted in a central PE cube (b.1) surrounded by several layers of graphite (b.2), PE (b.3), lead (b.4) and borated PE (b.5).

Figure 2

Energy spectrum recorded during the source measurement. The two peaks from the ${}^{55}\mathrm{Fe}$ source are used to set the energy scale of the detector (see text). The inset shows the distribution of filtered baselines after quality cuts.

Figure 3

Energy spectra (from 60 to 300 eV) measured by the NUCLEUS detector for the source and background (scaled to source exposure) measurements. The error bars represent the Poissonian uncertainties. The red solid and blue dashed lines illustrate the best fit with and without the Gaussian contribution, respectively.

Figure 4

Comparison between the recoil spectrum measured with the neutron source in place (black points) and the expected spectrum built from the measured ambient background and the simulation with (red line curve) and without (blue curve) the predicted recoils induced by neutron capture. Error bars are statistical only.