⚛️ nuclear experiments

Test of $^{116}$ CdWO $_4$ and Li $_2$ MoO $_4$ scintillating bolometers in the CROSS underground facility with upgraded detector suspension

The CROSS underground facility successfully tested upgraded detector suspensions with $^{116}$ CdWO $_4$ and Li $_2$ MoO $_4$ scintillating bolometers, achieving high energy resolution and radiopurity levels suitable for next-generation double-beta decay experiments while identifying specific noise sources for future mitigation.

Original authors: A. Ahmine, I. C. Bandac, A. S. Barabash, V. Berest, L. Bergé, J. M. Calvo-Mozota, P. Carniti, M. Chapellier, I. Dafinei, F. A. Danevich, T. Dixon, L. Dumoulin, F. Ferri, A. Giuliani, C. Gotti, P. Gras

Published 2026-07-28

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CC BY 4.0

Original authors: A. Ahmine, I. C. Bandac, A. S. Barabash, V. Berest, L. Bergé, J. M. Calvo-Mozota, P. Carniti, M. Chapellier, I. Dafinei, F. A. Danevich, T. Dixon, L. Dumoulin, F. Ferri, A. Giuliani, C. Gotti, P. Gras, D. L. Helis, A. Ianni, L. Imbert, H. Khalife, V. V. Kobychev, S. I. Konovalov, P. Loaiza, P. de Marcillac, S. Marnieros, C. A. Marrache-Kikuchi, M. Martinez, C. Nones, E. Olivieri, A. Ortiz de Solórzano, Y. Peinaud, G. Pessina, D. V. Poda, Th. Redon, Ph. Rosier, J. A. Scarpaci, V. I. Tretyak, V. I. Umatov, M. Velazquez, M. M. Zarytskyy, A. Zolotarova

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Silent Hunt for the Universe's Ghosts

Imagine the universe is a giant, noisy party. Most of the guests are loud and easy to spot: stars exploding, particles colliding, and atoms decaying in predictable ways. But physicists suspect there are a few "ghosts" at this party—particles so elusive and rare that they barely interact with anything else. One of the most famous ghosts is the "neutrinoless double-beta decay." In simple terms, this is a super-rare event where two neutrons in an atom turn into two protons and two electrons, but without releasing the usual ghostly neutrinos. If this happens, it would prove that neutrinos are their own antiparticles and could help explain why the universe is made of matter instead of just empty space.

To catch these ghosts, scientists need detectors that are incredibly sensitive and incredibly quiet. Think of it like trying to hear a pin drop in a hurricane. The detectors must be cooled down to temperatures colder than deep space (near absolute zero) so that the heat of the atoms doesn't drown out the tiny signal of a decay. They also need to be shielded from cosmic rays and background radiation, which is why these experiments are often buried deep underground in mines or under mountains. The goal is to build a "super-microphone" that can distinguish between a real ghost signal and the background noise of the universe.

The Paper: Tuning the Super-Microphone

This paper describes a team of scientists who went into a deep underground laboratory in Spain (the Canfranc Underground Laboratory) to test and upgrade their "super-microphones." These microphones are special devices called scintillating bolometers. To understand how they work, imagine a crystal that acts like a tiny, super-sensitive scale. When a particle hits it, the crystal gets slightly warmer (heat) and also flashes a tiny bit of light (scintillation). By measuring both the heat and the light at the same time, the scientists can tell exactly what kind of particle hit them. This is crucial because it helps them filter out the "noise" (like background radiation) and focus on the rare signals they are hunting for.

The team tested two different types of crystals in their upgraded setup:

Cadmium Tungstate ( $^{116}\text{CdWO}_4$ ): A heavy, dense crystal that acts like a reliable reference. It's been used before, so the scientists knew how it should behave.
Lithium Molybdate ( $\text{Li}_2\text{MoO}_4$ ): A newer, advanced crystal developed specifically for future, larger experiments. This one is a bit trickier because it doesn't flash as brightly as the other one, making it harder to distinguish signals.

The Big Upgrade: The Magnetic Shock Absorber
The biggest change in this experiment was the "suspension system." The detectors are inside a machine called a pulse-tube cryostat, which uses a mechanical pump to get things cold. Unfortunately, that pump vibrates, like a washing machine on spin cycle. These vibrations can shake the sensitive detectors and create false signals. In the past, the team used simple springs to hang the detectors, but the vibrations still got through.

For this test, they installed a new system with magnetic dampers. Imagine hanging a heavy weight from a rope inside a tube. As the weight swings, it moves through a magnetic field, which creates a "magnetic drag" (like moving your hand through thick honey) that stops the swinging almost instantly. This new system successfully stopped most of the shaking, allowing the detectors to sit much more still.

The Results: Sharper Senses and New Clues
With the new suspension, the detectors performed beautifully:

Crystal Clear Resolution: Both crystals could measure energy with incredible precision. For high-energy gamma rays (a type of radiation), they achieved an energy resolution of about 6 keV (kilo-electronvolts). This is among the best results ever recorded for these types of crystals. The older Cadmium Tungstate detector improved its "noise floor" (the quietness of the background) by about 2 keV compared to previous tests.
The Light Detector Boost: The team also tested a special light detector that uses a trick called the Neganov-Trofimov-Luke effect. By applying a voltage of 60 V, they amplified the tiny light signals. This turned a standard noise level of about 100 eV down to a whisper-quiet 16–18 eV. This is a massive improvement, making it much easier to spot the faint flashes of light from rare events.
Particle ID: Even though the Lithium Molybdate crystal is dim, the upgraded light detector was so sensitive that the team could still clearly tell the difference between different types of particles (like alpha particles vs. gamma rays). This is vital for rejecting background noise.

What They Found (and Didn't Find)
The team also checked the "radiopurity" of the new Lithium Molybdate crystal—essentially checking if it was contaminated with radioactive dust from the manufacturing process.

They found tiny traces of $^{210}\text{Po}$ and $^{226}\text{Ra}$ , each at a level of about 0.1 mBq/kg (millibecquerels per kilogram).
They did not find a dangerous amount of $^{40}\text{K}$ (Potassium-40), setting an upper limit of 6 mBq/kg.
The $^{228}\text{Th}$ (Thorium) activity was expected to be at least ten times lower than the other contaminants.

The One Catch: The Hum
Despite the amazing success of the magnetic dampers, the scientists noticed something strange. Even with the new suspension, there were still tiny, high-frequency "humming" vibrations in the data. They suspect this isn't coming from the pump shaking the detector directly, but rather from the wires picking up the noise like an antenna. It's a reminder that even when you fix the big problems, the tiny details (like how you route your cables) still matter.

The Bottom Line
This paper doesn't claim to have found the neutrinoless double-beta decay yet. Instead, it proves that the new "magnetic shock absorber" suspension works, and that the new Lithium Molybdate crystals are ready for the next generation of giant experiments. The detectors are now quieter, sharper, and better at telling the difference between a ghost and a background noise, bringing the scientific community one step closer to solving the mystery of the universe's missing matter.

Technical Summary: Test of 116CdWO4 and Li2MoO4 Scintillating Bolometers in the CROSS Underground Facility

Problem and Context
The search for neutrinoless double-beta decay ( $0\nu\beta\beta$ ) requires next-generation experiments capable of operating tonne-scale masses of isotopes with ultra-low background and high energy resolution. Scintillating bolometers, which simultaneously measure heat and scintillation light, offer a critical advantage through particle identification (PID) to reject background events. The CROSS project aims to develop metal-coated bolometer technology for background reduction, while the CLYMENE project focuses on optimizing the growth of large-volume Lithium Molybdate ( $Li_2MoO_4$ ) crystals for future experiments like CUPID. A significant challenge in operating these detectors is the vibrational noise introduced by pulse-tube cryostats, which are preferred for their efficiency but can degrade detector performance if not mechanically decoupled effectively.

Methodology
The authors tested two scintillating bolometers within the CROSS underground facility at the Canfranc Underground Laboratory (LSC), Spain. The experiment utilized a dedicated low-background cryostat equipped with a new detector suspension system featuring magnetic dumpers to mitigate pulse-tube induced vibrations.

Detector Configuration:
- Reference Detector: A $^{116}$ CdWO $_4$ scintillating bolometer (582 g), previously tested in the facility, served as a reference to benchmark the new suspension and noise conditions.
- Test Detector: A $Li_2MoO_4$ scintillating bolometer (245 g) grown via the CLYMENE project using an optimized Czochralski technique. This detector was coupled to a Germanium (Ge) light detector featuring concentric Aluminum electrodes to exploit the Neganov-Trofimov-Luke (NTL) effect for signal amplification.
- Supporting Hardware: Both modules were equipped with Neutron Transmutation Doped (NTD) Germanium thermistors for heat readout. The Ge light detectors were used to measure scintillation light for PID.
Experimental Setup:
- The detectors were cooled to a base temperature of 15 mK.
- A new suspension system was installed, consisting of a vessel suspended by Kevlar ropes connected to ring-shaped NdFeB magnets moving within fixed Copper tubes at the 1 K stage. This design utilizes eddy currents to provide efficient damping of oscillations.
- Data acquisition involved continuous digitization at 2 kS/s with Bessel filtering.
Analysis:
- Data processing employed the Gatti-Manfredi optimum filter to extract signal amplitude and pulse-shape parameters (rise time, decay time).
- Particle identification was achieved by correlating heat and light signals.
- Radiopurity was assessed by analyzing $\alpha$ and $\gamma(\beta)$ event spectra from background data and calibration runs with a $^{232}$ Th source.

Key Results

Noise and Energy Resolution:
- The upgraded suspension significantly improved the baseline noise resolution of the $^{116}$ CdWO $_4$ detector by $\sim$ 2 keV compared to previous measurements in the same facility.
- Both detectors achieved an energy resolution of $\sim$ 6 keV FWHM for 2.6 MeV $\gamma$ -quanta (from the 2615 keV peak of $^{208}$ Tl). This represents the best reported resolution for a massive CdWO $_4$ thermal detector and is among the best for $Li_2MoO_4$ thermal detectors.
- The Ge-based optical bolometer noise was improved by a factor of 2 to $\sim$ 100 eV FWHM.
- By applying a 60 V bias to the $Li_2MoO_4$ light detector to utilize the NTL effect, the noise resolution was further reduced to $\sim$ 20 eV FWHM (specifically 16–18 eV FWHM depending on the working point).
Particle Identification:
- High-efficiency particle identification was achieved for both detectors.
- The $^{116}$ CdWO $_4$ detector demonstrated efficient separation of $\alpha$ and $\gamma(\beta)$ events, consistent with previous results.
- Despite the low scintillation efficiency of $Li_2MoO_4$ (approx. 0.6 keV/MeV), the enhanced sensitivity of the NTL-amplified light detector allowed for clear separation of $\alpha$ and $\gamma(\beta)$ populations.
Radiopurity of CLYMENE Crystal:
- Analysis of $\alpha$ events revealed traces of $^{210}$ Po and $^{226}$ Ra, with activities of $\sim$ 0.09 mBq/kg and 0.12 mBq/kg, respectively.
- No signature of $^{228}$ Th was detected; its activity is expected to be at least an order of magnitude lower than the detection limit.
- Analysis of the $\gamma(\beta)$ spectrum yielded an upper limit for bulk $^{40}$ K contamination of $\le$ 6 mBq/kg (90% C.L.), excluding dangerous levels of potassium contamination.
Noise Artifacts:
- Despite the suspension upgrade, high-frequency harmonics of pulse-tube induced noise remained in the data, suggesting noise pick-up by the cabling rather than mechanical vibration.

Significance and Claims
The paper claims that the new suspension system with magnetic dumpers successfully improved the noise conditions for the $^{116}$ CdWO $_4$ reference detector, validating the design for future large-scale experiments. The study demonstrates that the advanced $Li_2MoO_4$ crystal grown under the CLYMENE project, despite being produced without the additional purification steps typically reserved for rare-event searches, possesses a radiopurity level suitable for next-generation experiments like CUPID.

Furthermore, the work highlights the viability of the Neganov-Trofimov-Luke effect in bolometric light detectors, showing it can drastically improve signal-to-noise ratios and particle identification capabilities even for materials with low scintillation efficiency. The authors conclude that while mechanical decoupling was improved, future iterations must address electromagnetic noise pick-up in the cabling to fully realize the potential of pulse-tube cryostats for ultra-low background physics.

The Silent Hunt for the Universe's Ghosts

The Paper: Tuning the Super-Microphone

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