SNO+: Tellurium-130 in Scintillator at SNOLAB

The Sudbury Neutrino Observatory was a singular success of late-twentieth-century particle physics. The 1,000 tonnes of heavy water (D${}_2$O) at the centre of an acrylic vessel deep in the Creighton mine resolved the solar neutrino problem in 2001-2002 by detecting all three flavors of solar neutrino simultaneously, proving that the missing solar electron neutrinos had oscillated into the other flavors rather than disappearing entirely. The SNO results, combined with Super-Kamiokande’s atmospheric oscillation measurement, established the modern picture of neutrino oscillation and earned Arthur McDonald the share of the 2015 Nobel Prize.

When SNO’s solar-neutrino programme finished in 2006, the question was what to do with the now-empty acrylic vessel and its surrounding 9,500-photomultiplier-tube array — 250 million dollars of detector infrastructure with no particle-physics target. The answer was SNO+: drain the heavy water, refill the vessel with 780 tonnes of liquid scintillator, and turn the detector into a 0νββ search.

The conversion took several years and involved one of the more delicate engineering operations in modern neutrino physics — the heavy water had to be carefully extracted and stored as it remains valuable scientific material, and the acrylic vessel had to be re-cleaned to scintillator-purity standards while keeping it intact under buoyancy stresses. SNO+ began full operation with pure scintillator in 2017 and began the headline 0νββ phase with tellurium-130 loading in 2024.

This post is about SNO+‘s tellurium-130 search, how the scintillator approach to tellurium compares with CUORE’s bolometric approach, and where SNO+ fits in the broader 0νββ landscape.

The SNO+ apparatus

SNO+ inherits the SNO apparatus: a 12-metre-diameter spherical acrylic vessel, made from 5-cm-thick ultra-pure UV-transparent acrylic, suspended in a water shield within a 22-metre-diameter cavern. The acrylic vessel is viewed by 9,500 8-inch photomultiplier tubes mounted on a geodesic support structure at a radius of 8.5 metres. Light-concentrators improve the effective photocathode coverage to about 55%. The water shield, the cavern dimensions, and the 6,000-metres-water-equivalent overburden of the SNOLAB site provide background suppression from cosmic-ray muons, neutrons, and external gammas at world-leading levels.

The conversion to SNO+ replaced the D${}_2$O with linear alkylbenzene (LAB) scintillator doped with 2 g/L of PPO (2,5-diphenyloxazole) primary fluor and 15 mg/L of bis-MSB secondary wavelength shifter. The total fill is approximately 780 tonnes of LAB-based scintillator, providing about 11,000 scintillation photons per MeV of energy deposit — a substantially brighter signal than the Cherenkov light SNO measured.

The transition from a Cherenkov-dominated to a scintillation-dominated readout changes the detector’s character significantly. Cherenkov light is emitted in a directional cone and only above the Cherenkov threshold (about 0.78 MeV for electrons in heavy water). Scintillation light is isotropic and has no kinematic threshold, so SNO+ can in principle observe lower-energy events than SNO could. The total photon yield is also much higher, improving the energy resolution from SNO’s ~30% at MeV scales to SNO+‘s ~5% at MeV scales.

The tellurium loading

The 0νββ phase loads tellurium-130 into the scintillator at 0.5 per cent by mass. Tellurium is normally insoluble in organic solvents like LAB, but SNO+‘s chemistry uses a butanediol complex that holds the tellurium in solution while preserving the scintillator’s optical transparency.

The chemistry works as follows. Tellurium is supplied as telluric acid (Te(OH)${}_6$), which is dissolved in 1,2-butanediol to form a stable complex. The complex is then mixed with the LAB scintillator at the required mass loading. The resulting solution maintains the scintillation light yield, with only a modest reduction relative to pure scintillator, and preserves the optical transparency at meter scales.

At 0.5 per cent loading, the total tellurium content in the 780-tonne scintillator volume is approximately 3.9 tonnes of natural tellurium. Natural tellurium contains 33.7 per cent ${}^{130}$Te by isotopic abundance, so the active 0νββ isotope mass is about 1,300 kg of natural Te giving 440 kg of ${}^{130}$Te.

This is roughly twice the LEGEND-200 isotope mass and roughly half the projected nEXO isotope mass. Among the next-generation 0νββ experiments, SNO+‘s tellurium-130 mass sits in the competitive range — neither the highest nor the lowest, but substantial.

The 0νββ signature

The signature of neutrinoless double beta decay in SNO+ is the same as in any scintillator experiment: a sharp energy peak at the ${}^{130}$Te Q-value of $Q_{\beta \beta }=2,527$ keV, sitting above the broad continuum from the two-neutrino mode and any backgrounds.

Energy resolution is the central detector parameter. SNO+ achieves approximately 5 per cent FWHM at the Q-value — meaning the signal peak is smeared over a region of about $\pm 60$ keV around 2.527 MeV. Backgrounds in this window must be controlled at the level of perhaps a few counts per kiloton-year of exposure for the 0νββ search to reach competitive sensitivity.

The most significant backgrounds in the signal region come from several sources:

The two-neutrino mode $2\nu \beta \beta$ of the same ${}^{130}$Te isotope, which has a half-life of about $7\times 10^{20}$ years. The endpoint of the two-neutrino spectrum is also at 2.527 MeV, but the bulk of the spectrum sits at lower energies. Only the tail of the two-neutrino distribution overlaps with the 0νββ peak; with SNO+‘s 5% resolution, this contributes roughly 5-10 counts per kt-year in the signal window.

External gamma rays from the photomultiplier-tube glass, support structures, and surrounding rock. These are suppressed by the fiducialisation cut that restricts the analysis to a central spherical volume of about 6-metre radius — a fiducial mass of about 1,300 tonnes of scintillator containing roughly 4.4 kg of ${}^{130}$Te in the inner sphere.

Cosmogenic backgrounds from muon-induced reactions producing radioactive isotopes in the scintillator. The deep SNOLAB site suppresses these to manageable levels but does not eliminate them entirely.

Solar neutrino backgrounds from ${}^8$B electron-elastic scattering, which produces electrons in the same energy range. The single-event nature of solar-neutrino events and the directional information from the Cherenkov contribution help distinguish them from $\beta \beta$ events, but the suppression is not complete.

The combined background budget in the signal window is targeted at approximately 5-10 counts per kt-year, requiring careful purification of every component of the scintillator and tellurium chemistry.

The phase plan

SNO+‘s 0νββ programme is being conducted in stages.

Phase I (2017-2024) ran with pure scintillator (no tellurium loading), establishing the detector performance, measuring solar neutrinos at sub-MeV energies, characterising the background environment, and providing first results on geoneutrinos. The scintillator phase also produced a competitive limit on the ${}^{130}$Te 0νββ half-life from natural-abundance tellurium that diffuses through the scintillator, plus measurements of the antineutrino flux from the Bruce nuclear power plant 240 km away.

Phase II (2024 onward) loads tellurium at 0.5% by mass. The expected exposure of 5 years at this loading gives projected sensitivity of approximately $T_{1/2}^{0\nu }>2\times 10^{26}$ years at 90% confidence, corresponding to effective Majorana mass approximately 40-150 meV depending on the nuclear matrix element. This is competitive with the current world-best limits from KamLAND-Zen and LEGEND-200 but with completely different systematic uncertainties.

Phase III is being designed with higher tellurium loading, potentially up to 3%. The chemistry of the higher loading is more challenging — keeping the tellurium in stable solution and maintaining adequate scintillator transparency at the higher loading require additional R&D — but if successful, would scale the isotope mass by a factor of 6 and the sensitivity by approximately the square root of that (since SNO+ is still background-limited). Phase III projected sensitivity reaches $T_{1/2}^{0\nu }>10^{27}$ years, into the inverted-mass-ordering region for $|m_{\beta \beta }|$.

The cross-check with CUORE/CUPID

SNO+ and CUORE/CUPID both pursue ${}^{130}$Te 0νββ but with very different detector technologies. A signal observed in one and not the other would be a major scientific event with several possible interpretations.

The two experiments differ in dominant systematics:

SNO+: limited by 2νββ pile-up at the tail of the spectrum, external gamma backgrounds, solar neutrino backgrounds, and the modest energy resolution (5%). The scintillator approach has well-understood detection physics but limited spectral resolution.

CUORE/CUPID: limited by surface alpha backgrounds and the limited isotope mass (a few hundred kg). The bolometric approach has excellent energy resolution (0.2%) but much more complex infrastructure.

The two technologies are sensitive to different parts of the signal spectrum. SNO+ integrates over the full peak with a smooth-background fit; CUORE/CUPID identifies the peak as a narrow line above a much smaller background. A consistent signal in both would be highly compelling; a signal in only one would indicate either a problem with the detector that did not see it or a non-trivial nuclear-matrix-element effect that we don’t currently understand.

The same logic applies to the multi-isotope comparison across the broader 0νββ portfolio. Signals in ${}^{76}$Ge (LEGEND), ${}^{136}$Xe (KamLAND-Zen or nEXO), and ${}^{130}$Te (SNO+ or CUORE/CUPID) at consistent effective Majorana mass values would essentially prove the Majorana nature of neutrinos. Inconsistencies between isotopes would point to nuclear matrix element systematics, requiring more refined nuclear physics input.

SNO+ as a multipurpose detector

The tellurium loading is the headline programme, but SNO+ has a broader physics agenda. Even during the 0νββ phase, the detector continues to produce results on:

Solar neutrinos at sub-MeV energies, particularly the ${}^7$Be and pep solar neutrinos. These probe the structure of the proton-proton chain in the Sun and complement Borexino’s earlier sub-MeV measurements with completely independent systematics.

Geoneutrinos from ${}^{232}$Th and ${}^{238}$U decay chains in the Earth’s mantle and crust. SNO+‘s location at the SNOLAB site provides geographic complementarity to KamLAND in Japan and Borexino at LNGS in Italy, allowing the geoneutrino flux to be mapped across different parts of the Earth’s interior.

Reactor antineutrinos from the Bruce Power nuclear complex 240 km away. The baseline is matched to the solar mass splitting $\Delta m_{21}^2$ at the relevant energies, providing constraints on the mixing parameters complementary to KamLAND.

Supernova neutrino burst detection through inverse beta decay on free protons in the scintillator. SNO+ is a contributing member of the SNEWS network.

The combination of 0νββ, solar, geo, reactor, and supernova capabilities in a single detector makes SNO+ one of the most versatile low-energy neutrino experiments currently operating.

Summary

SNO+ is the successor experiment to the Sudbury Neutrino Observatory at SNOLAB in Canada, reusing the 12-metre acrylic vessel and 9,500-photomultiplier array originally built for SNO’s solar neutrino programme. The detector was converted to a liquid scintillator filling in 2017 and entered its tellurium-loaded phase in 2024 with 0.5 per cent of natural tellurium dissolved in 780 tonnes of LAB-based scintillator. The active ${}^{130}$Te isotope mass is approximately 440 kg, with the search for neutrinoless double beta decay targeting a sharp energy peak at $Q_{\beta \beta }=2.527$ MeV. Phase II projected sensitivity is $T_{1/2}^{0\nu }>2\times 10^{26}$ years, corresponding to effective Majorana mass approximately 40-150 meV. Phase III with higher tellurium loading aims at $10^{27}$ years and into the inverted-mass-ordering region. As the fifth distinct detector technology in the contemporary 0νββ search, SNO+ provides a critical cross-check on signals from CUORE/CUPID (the bolometric tellurium approach) and from LEGEND/KamLAND-Zen/nEXO (the germanium and xenon approaches). The broader SNO+ physics programme also includes sub-MeV solar neutrinos, geoneutrinos, reactor neutrinos from Bruce Power, and supernova-burst detection — making the converted SNO infrastructure one of the most versatile low-energy neutrino observatories currently operating.