# First tests of Li_2W_4 bolometric detectors using MMC sensors for the detection of CEνNS

B. Mauri, Martin Loidl, C. Nones, Matias Rodrigues, M. Vivier

► To cite this version:

B. Mauri, Martin Loidl, C. Nones, Matias Rodrigues, M. Vivier. First tests of Li_2W_4 bolometric detectors using MMC sensors for the detection of CEνNS. Journal of Low Temperature Physics, 2023, 211, pp.220-226. 10.1007/s10909-022-02911-9. hal-04090288

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# First Tests of Li₂WO₄ Bolometric Detectors Using MMC Sensors for the Detection of CEvNS

B. Mauri¹, M. Loidl², C. Nones¹, M. Rodrigues², M. Vivier¹

¹ IRFU, CEA, Université Paris-Saclay, 91191 Gif-sur-Yvette, France (contact beatrice.mauri@cea.fr)

² CEA, LIST, Laboratoire National Henri Becquerel, 91191 Gif-sur-Yvette, France

## Abstract

Coherent elastic neutrino-nucleus scattering (CEvNS), detected for the first time more than 40 years after its prediction, represents an experimental challenge because of its signature: a single nuclear recoil with energy in the range of 10 eV to a few 10's keV on average. One of the main features of CEvNS is its cross section, around two orders of magnitude higher compared with the inverse β decay. BASKET (Bolometers At Sub keV Energy Threshold) project aims at investigating cryogenic calorimeters for the CEvNS detection, able to maintain the background level below the expected signature while being operated in above ground conditions in close vicinity to a nuclear reactor. The main requirements for such detectors are a low enough energy threshold of O(10 eV), and a fast signal rise time in the range of 0.1–1 ms to achieve good timing resolution and hence good mitigation of pile-up events. In this article, we report on the first tests of a prototype detector coupling a Li₂WO₄ crystal to a magnetic metallic calorimeter (MMC) thermal sensor.

## Keywords

Neutrino physics; Coherent elastic neutrino-nucleus scattering; Low temperature detectors; Scintillating bolometers; Magnetic metallic calorimeters; Cryogenics

## 1 Introduction

The CEvNS cross section presents a very interesting advantage compared to the other standard detection channels. Provided that a suitable target material is chosen, the CEvNS cross section can be up to a couple of orders of magnitude greater than the common detection processes such as the inverse beta decay (IBD). As shown in Eq. (1), the cross section is strongly dependent on the target nucleus since the weak charge $Q_W$ is proportional to the neutron number $N$ (Eq. 2) and the contribution due to the term $(1 - 4 \sin^2 \theta_W)Z$ (where $\theta_W$ is the Weinberg angle and $Z$ the number of protons in the nucleus) is negligible. As a consequence, we can replace it with $Q_W^2$ in Eq. (1). Other parameters playing a role in the equation are the Fermi's constant $G_F$, the form factor $F(q)$, the mass of the target nucleus $m_N$, and the recoil energy $E_R$. This gives the possibility to miniaturize neutrino detection set-ups, which would open very interesting perspectives for the long-range detection of neutrinos. Moreover, CEvNS can occur without a neutrino energy threshold.

However, the detection of CEvNS represents one of the modern particle physics challenges since its standalone signature is a nuclear recoil characterized by very small energies [1], typically in the range of 10's of eV to a few 10's of keV depending on the target nuclei.

$$ \frac{d\sigma}{dE_R} = \frac{G_F^2}{4\pi} Q_W^2 F^2(q^2) m_N \left(1 - \frac{E_R}{E_R^{\text{max}}}\right) \quad (1) $$

$$ Q_W = N - Z(1 - 4 \sin^2 \theta_W) \quad (2) $$

This process, where the neutrino scatters off the target nucleus as a whole (i.e. the sum of nucleons) assuming a sufficiently low momentum transfer, was observed for the first time in 2017 by the
COHERENT collaboration using a CsI[Na] scintillating detector exposed to the neutrino flux from the Spallation Neutron Source at the Tennessee Oak Ridge National Laboratory [2] and a second detection followed in 2020 using a liquid argon detector [3].

Still largely unexplored until today, this process would offer a new way to test the Standard Model (SM) at very low energies, and also probe new physics. Indeed, it is sensitive to the Weinberg angle [4] at low momentum transfer (see Eq. 1) and to the neutrino electromagnetic properties [5, 6]. Furthermore, it could also give an input to sterile neutrino [7], supernovae neutrino [8], axion [9], and dark matter searches [10].

The BASKET (Bolometers at Sub-keV Energy Threshold) R&D project started in 2017 with the purpose to study and develop gram-scale cryogenic detector prototypes based on scintillating crystals equipped with innovative thermal sensors for the detection and measurement of CEvNS at reactor experiments.

The main candidate as BASKET scintillating target material is molybdenum doped lithium tungstate (Li₂Mo₀.₀₅W₀.₉₅O₄), a very promising compound for the following reasons:

* Since the CEvNS cross-section scales with $N^2$, the tungsten ($N$ around 110) strongly enhances the CEvNS rate;

* As proved in past studies, the natural content of ⁶Li (8 % in the crystal used for the tests presented in this article) reveals excellent particle identification capabilities allowing the tagging and the characterization of the neutron background thanks to the ⁶Li(n,t)α reaction.

This compound requires special care in its storage as it is a hygroscopic crystal.

The BASKET final goal is to achieve very sensitive cryogenic detectors with an ultra low energy threshold of O(10 eV) and a rise time of the order of O(100 µs) so that they can easily be operated in above-ground conditions, i. e. with a rather high background rate, such as those encountered in a reactor CEvNS experiment.

## 2 Detector Assembly and Low-Temperature Tests

The present article reports on two cryogenic tests conducted above ground at the LNHB (Laboratoire National Henri Becquerel).

The detector absorber (labelled *l* in Fig. 1) is a 1 cm³ Li₂WO₄(Mo) crystal [11]. It was produced at the Nikolaev Institute of Inorganic Chemistry (Novosibirsk, Russia) through the Czochralski method with Mo doping at 5%. Mo doping is mandatory for the crystal growth but it does not affect the detector performance because of its chemical affinity with W as observed in many studies dedicated to the Li₂MoO₄ compound [12, 13].
<figure><img src="image_2.png" /><figcaption>**Fig. 1 Left:** view of the detector set-up. The labelled items are detailed in the text. **Right:** Schematic cross sectional view of the detector</figcaption></figure>

<figure><img src="image_3.png" /></figure>

A particle interaction within the crystal releases energy through ionization and molecular excitation which is converted into athermal and eventually thermal phonons causing a temperature rise ΔT. A gold film (2) with dimension 5 mm × 5 mm and thickness 200 nm is evaporated on a surface of the crystal to collect the athermal phonons. Since Li₂WO₄ crystals are hygroscopic, the assembly process should minimize their exposition to ambient air. Moreover, a periodical surface mechanical cleaning was necessary in order not to degrade the collection of athermal phonons into the gold film. The absorber is thermally linked to a magnetic metallic calorimeter (MMC) using two gold bonding wires (3) running from the gold film on the crystal surface to the Ag:Er MMC sensor (4) [14]. The MMC optimized for an absorber heat capacity of 28 pJ/K at 20 mK was produced at the Kirchhoff Institute of Physics of Heidelberg University.

The MMC was read out by a single stage dc-SQUID (Superconducting QUantum Interference Device, 6). The produced heat flows from the MMC sensor through a weak thermal link to the heat sink, consisting of an on-chip heat bath strongly thermally coupled to the detector holder via several gold bonding wires (5). A $^{55}$Fe–$^{109}$Cd source was fixed in front of the Li₂WO₄(Mo) crystal, and the detector was shielded all around with a cylindrical lead cap.

The calorimeter was cooled down to 13 mK in a dilution refrigerator. The signal was recorded as a continuous data stream at a sampling rate of 105 Hz and analysed offline. The measurement was divided into two data sets in which the only difference was the dynamic range of the acquisition to have a partition including the characteristic neutron capture on lithium peak and a partition focused on the lower energies to probe the performances of the set-up in this region.

In Fig. 2, the energy spectrum obtained after the offline analysis is presented. We can clearly appreciate the main background gamma lines and we can start identifying the $^{6}$Li(n,t)$\alpha$ peak that is one
of the main interesting features of this crystal compound since it could be used for neutron monitoring
at a nuclear power plant.

<figure><img src="image_4.png" /></figure>

**Fig. 2** Energy spectrum obtained after the off-line analysis. It presents some of the characteristic background gamma lines and the neutron reaction on lithium typical of this compound

Figure 3 displays the energy spectrum of the selected events below 100 keV, showing the X-ray and gamma lines of the <sup>55</sup>Fe and <sup>109</sup>Cd in the source.

<figure><img src="image_5.png" /></figure>

**Fig. 3** Energy spectrum of the acquired data with a shorter dynamic range compared to the one of Fig. 2. In the inset, the histogram of the baseline level (in arbitrary units) that was determined from sections of the continuous data stream without pulses

By means of this energy spectrum, we extracted the detector features reported in Table 1.
<table><caption>**Table 1** Main detector features evaluated using the smaller energy range</caption><tr><td>Energy resolution (FWHM at 5.895 keV) [eV]</td><td>331 ± 6</td></tr><tr><td>Baseline resolution (FWHM) (eV)</td><td>194.1</td></tr><tr><td>Sensitivity (μΦ₀/keV)</td><td>170</td></tr><tr><td>Rise-time (10–90%) (ms)</td><td>1.6</td></tr><tr><td>Decay-time (90–30%) (ms)</td><td>5.9</td></tr></table>

The pulse rise time was much longer than expected from a similar experiment, i. e. O(100 µs) [15]; this was attributed to a very weak bonding of the gold film—despite a thin (25 Å) Ti sticking layer— to the crystal. This was manifest when trying to bond gold wires to the film: instead of sticking to the film, the bonding process resulted in a hole in the Au film at every attempt, irrespective of the bonding machine parameters (e.g. applied force). To circumvent this issue, thin gold foils (3 µm) could be wedge bonded to the Au film (circled areas in Fig. 1); subsequently, 10-µm-thick Au wires were bonded to the foils and the MMC sensor. The weak bond of the gold film to the crystal results in an extremely low efficiency in phonon collection, we suppose that practically no athermal phonons can cross the crystal-film interface and the film is only slowly heated up via thermal phonons leaking from the crystal into the film, thus explaining the slow signal rise. In a second configuration of the detector, a new gold film was evaporated to the freshly polished crystal; a 3-µm-thick gold foil strip was bonded to the film and to the MMC sensor to replace the bonding wires as a thermal link. Nevertheless, the detector performance did not improve, and the characteristics were close to those in Table 1.

### 3 Perspectives and Next Steps

These tests demonstrate the necessity to further modify the detector set-up to address the hygroscopic nature of the Li₂WO₄ crystal. Indeed, it is likely to be responsible for a bad attachment of the gold pad and, as a consequence, it can explain the slow collection of the athermal phonons observed in the measurement here reported [16].

In the next run, we use a freshly polished Li₂WO₄ crystal in order to be sure to not be penalized by the hygroscopic nature of this compound. With the goal to speed up the detector response, we have removed the gold film from the crystal surface and replaced it with a gold foil (3 mm × 3 mm, 3 µm thick) glued to the crystal with a two-component epoxy glue (Araldite® Rapid). This new configuration will be tested in the next future.

**Acknowledgements** We acknowledge the financial support of the Cross-Disciplinary Program on Instrumentation and Detection of CEA, the French Alternative Energies and Atomic Energy Commission, France. This work has been partially funded by the P2IO LabEx (ANR-10-LABX-0038) in the framework of “Investissements d’Avenir” (ANR-11-IDEX-0003-01) managed by the French National Research Agency (ANR), France.

**Data Availability** The data sets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.

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