CUORE: how 988 cold tellurium crystals are listening for a process that may not exist

In a vast underground cavern about 1.4 kilometers beneath Italy’s Gran Sasso massif, 988 small cubes of tellurium dioxide sit in a stack about a meter tall. Each cube is about five centimeters on a side and weighs almost a kilogram. The entire array — about 750 kilograms of tellurium and oxygen — is enclosed in a series of nested copper and lead shields, suspended inside a multi-stage cryostat that holds the crystals at a temperature of about 10 millikelvin, just one-hundredth of a degree above absolute zero.

This is CUORE — the Cryogenic Underground Observatory for Rare Events — and it is one of the most sensitive neutrinoless double-beta decay experiments in the world. Its goal is to detect the radioactive decay of tellurium-130 in the rare hypothetical channel where two beta-decay events occur simultaneously without emitting any neutrinos.

If that decay exists at all, it would prove that neutrinos are Majorana particles — their own antiparticles — and reshape the picture of fundamental physics. After many years of running, CUORE has not yet found the decay. It has set increasingly stringent upper limits on the rate, and its successor experiment is being built to push further.

What a cryogenic bolometer is

The CUORE detector works through a deceptively simple principle. Each tellurium dioxide crystal is a bolometer — a thermal detector that measures the temperature rise produced when energy is deposited in it.

For ordinary materials at room temperature, the heat capacity is enormous, and the temperature rise produced by even a high-energy particle is unmeasurably small. But at cryogenic temperatures, the heat capacity drops dramatically. At 10 millikelvin, the heat capacity of a tellurium dioxide crystal is reduced by a factor of about 10¹² compared to room temperature.

At those temperatures, the deposition of about 1 MeV of energy raises the crystal’s temperature by a few hundred microkelvin — a tiny but easily measurable amount. A precision thermistor attached to each crystal records the temperature pulse as a voltage signal. The amplitude of the signal is proportional to the energy deposited. The shape of the pulse depends on the location and nature of the event.

For 0νββ searches, the relevant energy is the Q-value of the decay — about 2.5 MeV for tellurium-130. A 0νββ event would deposit essentially all of that energy in a single crystal, producing a single-crystal pulse at exactly the Q-value. The signal would appear as a sharp spectral line at 2,527 keV.

Why “bolometer” is special

Compared to other 0νββ detection techniques — like LEGEND’s germanium semiconductor approach or KamLAND-Zen’s xenon-in-scintillator approach — the cryogenic bolometer has several distinctive properties.

Excellent energy resolution. Bolometers achieve energy resolution of about 0.1-0.5% at the relevant energies, comparable to germanium and better than liquid scintillator. This sharpens the search for the narrow 0νββ peak above any continuous background.

Crystal-by-crystal granularity. Because each crystal is read out independently, events depositing energy in multiple crystals (gamma rays, alpha-induced cascades) can be distinguished from single-crystal events. This is a powerful background-rejection tool.

Compact, modular design. The CUORE crystals stack into a compact array. New crystals can be added or replaced with modest effort.

Versatile isotope choice. The same bolometer technique can be applied to many candidate isotopes. CUORE uses tellurium-130; the successor experiment, CUPID, uses molybdenum-100; other groups are exploring calcium-48, cadmium-116, and others.

The cryostat that makes it possible

Keeping 988 crystals at 10 millikelvin in a stable, well-shielded environment is itself a significant engineering achievement.

CUORE’s cryostat is a six-stage dilution refrigerator — a system that uses the quantum-mechanical properties of helium-3 and helium-4 mixtures to produce continuous cooling at sub-Kelvin temperatures. The outermost stage is at room temperature; subsequent stages step down through 40 K, 4 K, 600 mK, 50 mK, and finally about 10 mK in the innermost volume that holds the detector array.

The system is housed inside a vacuum chamber with multiple layers of thermal and radiation shielding. The crystals themselves are protected by an inner copper shield, an inner lead shield, and an outer layer of polyethylene to attenuate neutron backgrounds. Outside the main cryostat is a larger external shielding structure including ancient Roman lead (chosen because it has been underground long enough that any traces of radioactive lead-210 have decayed away to negligible levels).

At full operation, the entire underground volume holding CUORE is one of the coldest, most radioactively quiet places anywhere known in the universe. Background event rates in the signal region are on the order of one count per kilogram per year — a remarkably low number for a detector of this size and complexity.

What CUORE has found (so far)

Through many years of operation, CUORE has accumulated approximately one ton-year of exposure on tellurium-130. The collaboration’s published analyses have set successively tighter upper limits on the 0νββ half-life of tellurium-130, with the latest result giving:

T₁/₂(0νββ) > 2.2 × 10²⁵ years (at 90% confidence)

This translates, depending on which nuclear matrix element calculation you trust, to an upper limit on the effective Majorana neutrino mass of roughly 75–255 meV. The wide range reflects the substantial uncertainty in the nuclear matrix element calculations themselves.

CUORE’s results are competitive with the leading germanium experiments (LEGEND-200) and the leading xenon experiments (KamLAND-Zen 800). Each experiment uses a different isotope with different nuclear physics, so a discovery in any one would need cross-confirmation in the others — a redundancy that the field considers important.

The transition to CUPID

CUORE’s main limitation in pushing further is alpha-particle background. Trace radioactivity in detector materials — particularly polonium-210 and its progeny — can produce alpha particles whose energy is deposited in the crystals. Some of these events produce pulses in the same energy range as a 0νββ signal would.

The successor experiment, CUPID (CUORE Upgrade with Particle IDentification), addresses this by replacing the tellurium dioxide crystals with lithium molybdate crystals enriched in molybdenum-100, and by adding light detection to each crystal. Lithium molybdate emits a small amount of scintillation light when a charged particle deposits energy in it, with different light yields for alpha particles than for electrons. By measuring both the temperature pulse (the calorimetric energy) and the light pulse for each event, CUPID can distinguish alpha-induced backgrounds from genuine beta-decay events on an event-by-event basis.

CUPID is currently being built. The initial CUPID configuration will reuse much of the CUORE cryogenic infrastructure but with the new crystals and light-detection scheme. Operations are expected to begin in the late 2020s, with sensitivity to half-lives roughly an order of magnitude longer than CUORE’s current limit.

Why running this long matters

The expected event rate for 0νββ — even at the maximum rate allowed by current upper limits — is extraordinarily small. A detector with one kilogram of source isotope expects to see, at most, a few events per year in the relevant energy window. The 0νββ search is therefore a multi-decade endeavor by nature.

CUORE’s contribution is to drive down the upper limits steadily over time. Each year of additional running pushes the half-life sensitivity higher. CUPID will continue this trajectory with better backgrounds. LEGEND-1000 (the next-generation germanium experiment) and KamLAND2-Zen (the next-generation xenon experiment) will do the same in different isotopes.

By the early 2030s, all three programs together should be sensitive to the inverted-ordering region of effective Majorana mass — the regime where, if neutrinos are Majorana and the mass ordering is inverted, a signal is essentially guaranteed. If no signal is found across all three isotopes by then, the inverted-ordering-Majorana scenario will be excluded, and the simplest theoretical pictures will need substantial revision.

A cold, quiet experiment

There is a particular discipline to operating an experiment like CUORE. The signal you are looking for may not exist. The decay channel may be forbidden by nature in the simplest way. Even if it does exist, you may not find it within your detector’s lifetime. The reward for years of careful operation is a single number — an upper limit, or, with luck, a discovery — that takes a sentence to state.

But each upper limit constrains the underlying theory. Each new generation of detector pushes the sensitivity to a smaller and smaller fraction of nuclear lifetimes. The accumulated knowledge of 0νββ over the past three decades has narrowed the possibilities considerably, and the next decade will narrow them further.

The 988 cold tellurium crystals beneath Gran Sasso are listening for one of the rarest processes that the laws of physics may allow. If they hear it, the implications will reshape particle physics. If they do not, by 2030 or so, the question itself may have an answer that — whatever it is — comes from CUORE, CUPID, and their counterparts working together.

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For the broader 0νββ search, see The hunt for neutrinoless double-beta decay. For why Majorana fermions matter, see Majorana or Dirac?. For Ettore Majorana himself, see The physicist who vanished. For the connection to the mass ordering, see Normal or inverted.

Neutrino Times Editorial Team. (2025, November 19). CUORE: how 988 cold tellurium crystals are listening for a process that may not exist. Neutrino Times. https://neutrino-times.com/articles/cuore-cryogenic-bolometers-tellurium-130/

Neutrino Times Editorial Team. "CUORE: how 988 cold tellurium crystals are listening for a process that may not exist." Neutrino Times, November 19, 2025. https://neutrino-times.com/articles/cuore-cryogenic-bolometers-tellurium-130/.

Neutrino Times Editorial Team. "CUORE: how 988 cold tellurium crystals are listening for a process that may not exist." Neutrino Times, 19 Nov. 2025, https://neutrino-times.com/articles/cuore-cryogenic-bolometers-tellurium-130/.

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  note         = {Accessed: 2025-11-19}
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