Periodic Reporting for period 5 - CROSS (Cryogenic Rare-event Observatory with Surface Sensitivity)
Période du rapport: 2024-01-01 au 2025-03-31
Atoms are made of charged fermions: electrons and quarks. Their behavior is described by the Dirac equation, which also makes a profound prediction: for every particle there exists an antiparticle with the same mass and opposite charge. Neutrinos, however, stand out as a fascinating exception. They are fermions too, yet they carry no electric charge. In the standard picture, they are treated like Dirac particles, much like electrons, but with the “lepton number” playing the role normally occupied by electric charge. There is, however, an intriguing alternative. Neutrinos might instead be Majorana particles: fermions that are completely neutral and identical to their own antiparticles. Confirming this possibility is one of the most important and far-reaching goals in modern particle physics and cosmology.
The only practical avenue to determine whether neutrinos are Majorana particles is to observe neutrinoless double beta decay (0νDBD). This is a hypothetical nuclear process in which an even–even nucleus transforms into a lighter isobar containing two more protons and emits only two electrons: no neutrinos, no other particles. Current experiments have established lower limits on the 0νDBD half-life in the range of 10²⁴ to 10²⁶ years. Yet none of them can reach the extraordinary sensitivity of 10²⁷ to 10²⁸ years, where the probability of a definitive discovery becomes compelling. The CROSS technology aims to bridge exactly this gap, enabling a dramatic leap forward into this unexplored region of sensitivity.
Beyond neutrino physics, the methods pioneered by CROSS promise major advances in radiation detection and in the identification of minute traces of radioactive contaminants. Detectors based on CROSS techniques can measure extremely low neutron fluxes with exceptional spectroscopic precision, opening new possibilities for environmental monitoring, nuclear non-proliferation verification, and homeland security.
CROSS is built around arrays of bolometers whose active materials contain two of the most promising isotopes for 0νDBD searches: ¹⁰⁰Mo and ¹³⁰Te, embedded respectively in crystals of lithium molybdate (Li2MoO₄) and tellurium dioxide (TeO2). Through a series of ambitious innovations, CROSS is developing an approach capable of achieving zero background at exposures of one tonne of isotope per year, an essential condition given the extreme rarity of the process we seek.
The central challenge in 0νDBD searches is the suppression of background events. In bolometric detectors, the most problematic contributions arise from energy depositions near the crystal surfaces. CROSS is developing a technique to distinguish these surface events from those occurring in the bulk, where a true 0νDBD signal would originate. This is accomplished through pulse-shape discrimination enabled by superconducting films deposited on the crystal surfaces. An additional major innovation is a new generation of highly sensitive bolometric light detectors whose response is enhanced by an electric field applied to the light absorber. This allows for powerful rejection of additional backgrounds, including alpha particles and random coincidences of unrelated events. The core ideas of CROSS are being validated in aboveground tests and in a large pilot experiment known as CROSS demonstrator, placed underground in the Canfranc laboratory to be shielded against the cosmic radiation. The entire program is naturally intertwined with CUPID, one of the most promising next-generation bolometric experiments.
The only practical avenue to determine whether neutrinos are Majorana particles is to observe neutrinoless double beta decay (0νDBD). This is a hypothetical nuclear process in which an even–even nucleus transforms into a lighter isobar containing two more protons and emits only two electrons: no neutrinos, no other particles. Current experiments have established lower limits on the 0νDBD half-life in the range of 10²⁴ to 10²⁶ years. Yet none of them can reach the extraordinary sensitivity of 10²⁷ to 10²⁸ years, where the probability of a definitive discovery becomes compelling. The CROSS technology aims to bridge exactly this gap, enabling a dramatic leap forward into this unexplored region of sensitivity.
Beyond neutrino physics, the methods pioneered by CROSS promise major advances in radiation detection and in the identification of minute traces of radioactive contaminants. Detectors based on CROSS techniques can measure extremely low neutron fluxes with exceptional spectroscopic precision, opening new possibilities for environmental monitoring, nuclear non-proliferation verification, and homeland security.
CROSS is built around arrays of bolometers whose active materials contain two of the most promising isotopes for 0νDBD searches: ¹⁰⁰Mo and ¹³⁰Te, embedded respectively in crystals of lithium molybdate (Li2MoO₄) and tellurium dioxide (TeO2). Through a series of ambitious innovations, CROSS is developing an approach capable of achieving zero background at exposures of one tonne of isotope per year, an essential condition given the extreme rarity of the process we seek.
The central challenge in 0νDBD searches is the suppression of background events. In bolometric detectors, the most problematic contributions arise from energy depositions near the crystal surfaces. CROSS is developing a technique to distinguish these surface events from those occurring in the bulk, where a true 0νDBD signal would originate. This is accomplished through pulse-shape discrimination enabled by superconducting films deposited on the crystal surfaces. An additional major innovation is a new generation of highly sensitive bolometric light detectors whose response is enhanced by an electric field applied to the light absorber. This allows for powerful rejection of additional backgrounds, including alpha particles and random coincidences of unrelated events. The core ideas of CROSS are being validated in aboveground tests and in a large pilot experiment known as CROSS demonstrator, placed underground in the Canfranc laboratory to be shielded against the cosmic radiation. The entire program is naturally intertwined with CUPID, one of the most promising next-generation bolometric experiments.
1. Surface-event sensitivity with thin-film coatings
CROSS achieved clear α and β surface-event discrimination in small Li2MoO₄ and TeO2 bolometers using Pd and Al–Pd films: a significant advance for bolometric 0νββ searches. Coatings preserved small-crystal performance and enabled millimetre-scale depth tagging. Scaling to full-size crystals proved unsuccessful: all coatings degraded signal amplitudes and pulse shapes, preventing use in the demonstrator. Nonetheless, CROSS produced the most comprehensive dataset on metallic-film impacts in macro-bolometers, guiding future R&D.
2. Phonon-sensor optimization
Although fast NbSi sensors were planned, tests showed that simpler NTD Ge thermistors already provide adequate pulse-shape discrimination. The demonstrator therefore uses NTDs exclusively.
3. Li2MoO₄ production and the ¹⁰⁰Mo demonstrator
CROSS produced 32 enriched Li2MoO₄ crystals (~5 kg of ¹⁰⁰Mo) using new purification and crystallization routes. They show excellent purity and bolometric performance. The demonstrator has begun data taking, targeting a 0νββ sensitivity of ~3.5×10²⁴ yr by 2026 and providing a key validation step for CUPID.
4. TeO2 program and new ¹³⁰Te supply chain
After SICCAS halted large-crystal growth, CROSS established a new ¹³⁰Te path with US partners. Although a large array could not be produced, six enriched 45 mm TeO2 bolometers (91% ¹³⁰Te) were successfully fabricated and are operating in the demonstrator.
5. NTL-amplified light detection
CROSS developed Ge light detectors with NTL gain (~10 eV RMS noise, up to 100 V bias, ~0.5 ms rise). These devices significantly improve scintillation readout and pile-up rejection and are now integrated in the demonstrator.
6. Low-background mechanical structures
Two copper-frame designs (Thick and Slim) for mounting 45 mm bolometers were developed; the Slim version reduces copper mass from ~15% to ~6%. Both achieved 5–7 keV FWHM at 2615 keV and provide effective α/γ(β) separation.
7. CROSS cryostat and facility
A full 8 mK cryogenic facility with extensive shielding, muon veto, anti-radon enclosure, and readout for 84 bolometers was completed at LSC, enabling rare-event searches and array-scale testing.
8. Large-crystal runs and CUPID collaboration
Underground campaigns confirmed high performance of full-size LMO and TeO2 bolometers and informed demonstrator assembly. A joint program with CUPID uses the facility to refine CUPID’s detector baseline.
9. Custom electronics and DAQ
CROSS developed and validated dedicated low-noise electronics and DAQ boards optimized for multi-channel bolometric readout.
CROSS achieved clear α and β surface-event discrimination in small Li2MoO₄ and TeO2 bolometers using Pd and Al–Pd films: a significant advance for bolometric 0νββ searches. Coatings preserved small-crystal performance and enabled millimetre-scale depth tagging. Scaling to full-size crystals proved unsuccessful: all coatings degraded signal amplitudes and pulse shapes, preventing use in the demonstrator. Nonetheless, CROSS produced the most comprehensive dataset on metallic-film impacts in macro-bolometers, guiding future R&D.
2. Phonon-sensor optimization
Although fast NbSi sensors were planned, tests showed that simpler NTD Ge thermistors already provide adequate pulse-shape discrimination. The demonstrator therefore uses NTDs exclusively.
3. Li2MoO₄ production and the ¹⁰⁰Mo demonstrator
CROSS produced 32 enriched Li2MoO₄ crystals (~5 kg of ¹⁰⁰Mo) using new purification and crystallization routes. They show excellent purity and bolometric performance. The demonstrator has begun data taking, targeting a 0νββ sensitivity of ~3.5×10²⁴ yr by 2026 and providing a key validation step for CUPID.
4. TeO2 program and new ¹³⁰Te supply chain
After SICCAS halted large-crystal growth, CROSS established a new ¹³⁰Te path with US partners. Although a large array could not be produced, six enriched 45 mm TeO2 bolometers (91% ¹³⁰Te) were successfully fabricated and are operating in the demonstrator.
5. NTL-amplified light detection
CROSS developed Ge light detectors with NTL gain (~10 eV RMS noise, up to 100 V bias, ~0.5 ms rise). These devices significantly improve scintillation readout and pile-up rejection and are now integrated in the demonstrator.
6. Low-background mechanical structures
Two copper-frame designs (Thick and Slim) for mounting 45 mm bolometers were developed; the Slim version reduces copper mass from ~15% to ~6%. Both achieved 5–7 keV FWHM at 2615 keV and provide effective α/γ(β) separation.
7. CROSS cryostat and facility
A full 8 mK cryogenic facility with extensive shielding, muon veto, anti-radon enclosure, and readout for 84 bolometers was completed at LSC, enabling rare-event searches and array-scale testing.
8. Large-crystal runs and CUPID collaboration
Underground campaigns confirmed high performance of full-size LMO and TeO2 bolometers and informed demonstrator assembly. A joint program with CUPID uses the facility to refine CUPID’s detector baseline.
9. Custom electronics and DAQ
CROSS developed and validated dedicated low-noise electronics and DAQ boards optimized for multi-channel bolometric readout.
Current bolometric experiments, notably CUORE, show that the dominant background comes from energy-degraded alpha particles emitted by surface contamination. The classical solution proposed for the next-generation CUPID experiment is to use luminescent bolometers, which reject alpha events through simultaneous heat-and-light detection, exploiting their lower light yield compared to beta events. Over the course of the project, CROSS has developed three innovative strategies to address the most challenging background sources:
- Making detectors sensitive to surface events, allowing rejection of both alpha and beta interactions occurring on detector surfaces.
- Improving light detectors used in scintillating bolometers.
- Substantially reducing the passive materials surrounding the active crystals.
- Making detectors sensitive to surface events, allowing rejection of both alpha and beta interactions occurring on detector surfaces.
- Improving light detectors used in scintillating bolometers.
- Substantially reducing the passive materials surrounding the active crystals.