Particle discrimination in a NaI crystal using the COSINUS remote TES design

The COSINUS direct dark matter experiment situated at Laboratori Nazionali del Gran Sasso in Italy is set to investigate the nature of the annually modulating signal detected by the $\mathrm{DAMA}/\mathrm{LIBRA}$ experiment. COSINUS has already demonstrated that sodium iodide crystals can be operated at mK temperature as cryogenic scintillating calorimeters using transition edge sensors, despite the complication of handling a hygroscopic and low melting point material. With results from a new COSINUS prototype, we show that particle discrimination on an event-by-event basis in NaI is feasible using the dual-channel readout of both phonons and scintillation light. The detector was mounted in the novel remoTES design and operated in an above-ground facility for $9.06\mathrm{g}·\mathrm{d}$ of exposure. With a 3.7 g NaI crystal, $e^{-}/\gamma$ events could be clearly distinguished from nuclear recoils down to the nuclear recoil energy threshold of 20 keV.

Figure 1

Schematic of remoTES and light detector. In the remoTES, the TES is deposited on a wafer, which is separated from the absorber crystal. The coupling between the absorber and the TES consists of an Au pad glued on the absorber surface and connected to the TES by two Au-wire bonds [12]. The TES of the light detector is deposited directly on the light detector crystal surface.

Figure 2

(a) The remoTES wafer mounted in the copper holder. The sapphire wafer is placed on sapphire balls and held by a bronze clamp. A ${}^{55}\mathrm{Fe}$ source is taped on a copper piece facing the absorber for the purpose of energy calibration. (b) Microscopic view of the Au pad glued on the NaI crystal and the wire bonding to the remoTES.

Figure 3

(a) SOS light detector mounted in a copper holder. (b) Microscopic view of the electrical connections of the light detector TES and of its separate ohmic heater.

Figure 4

(a) Normalized averaged pulses in the light detector (dashed, dark-blue curve) and in the NaI-remoTES detector (solid, water-green curve). A cleaned sample of nonsaturated events from the Co57 dataset was used for averaging. (b) Deviation of reconstructed energy from nominal energy for simulated events, obtained by superimposing scaled averaged signal pulses onto empty traces for the phonon channel.

Figure 5

Survival probability for simulated events in the analysis chain, estimated using the background dataset. An exponentially decaying energy spectrum was simulated to allow for better statistics at low energies. Statistical uncertainties are indicated for each energy bin. The analysis threshold at an efficiency of 20% is marked by a dashed blue line.

Figure 6

Energy spectrum from the ${}^{57}\mathrm{Co}$ $\gamma$ calibration measurement. A constant conversion factor from pulse amplitude to energy was extracted by fitting a gaussian function to the right shoulder of the 122 keV, and used to calibrate the spectrum. The additional ${}^{57}\mathrm{Co}$ 136 keV peak and two $\gamma$ escape peaks due to I K-$\alpha$, expected at around 88.8 keV and 102.8 keV, are used to cross check the result. The latter deviate by about 2% from their nominal value.

Figure 7

Left: 2D histogram of LY versus phonon channel energy for the background dataset with color-coded number of entries. Right: $e^{-}/\gamma$ energy spectrum of the background dataset. Different contributions are highlighted.

Figure 8

Left: 2D histogram of LY versus phonon channel energy for the neutron calibration dataset with color-coded number of entries. Right: combined energy spectrum of the neutron calibration dataset. Different contributions from the recoil bands are indicated with their respective color. Additionally, the compton contribution to the $e^{-}/\gamma$ spectrum, determined from the background dataset, is highlighted in orange.