SNO+ Collaboration Overview

• SNO+ Collaboration is an international partnership operating a versatile liquid scintillator neutrino detector at SNOLAB for rare-event searches.
• Detector upgrades include advanced purification, calibration, and PMT configurations, delivering ultra-low backgrounds and high energy resolution.
• Its phased approach—water, scintillator, and tellurium loading—enables precision studies of solar, reactor, geoneutrinos, and neutrinoless double beta decay.

The SNO+ Collaboration is an international scientific partnership operating a large-scale liquid scintillator neutrino detector at SNOLAB, Sudbury, Canada. Its primary objectives are the search for neutrinoless double beta decay (0νββ), precision studies of solar, reactor, and geoneutrinos, and the investigation of rare processes in particle and astrophysics. SNO+ builds upon the legacy infrastructure of the Sudbury Neutrino Observatory (SNO) but with extensive upgrades to material, purification, calibration, and analysis techniques, enabling a diverse and sensitive neutrino program across multiple detection phases.

1. Detector Concept, Design, and Upgrades

The SNO+ detector utilizes a 12-meter diameter spherical acrylic vessel (AV) as its primary target region, surrounded by approximately 9500 PMTs mounted on a geodesic support structure, and immersed in 7000 tonnes of ultra-pure water shielding for background suppression (Collaboration et al., 2021). Key components include:

• Acrylic Vessel & Hold-Down System: The AV is filled with 780 tonnes of detection medium—ultrapure water (water phase), liquid scintillator (LAB/PPO, scintillator phase), or LAB-based scintillator loaded with natural tellurium (tellurium phase). Buoyancy of the less-dense scintillator is counteracted by a hold-down rope-net constructed of low-radioactivity materials.
• Photomultiplier Tubes (PMTs): The PMT system provides 54% optical coverage, supporting high light yield collection and fine energy resolution, with upgrades to crate controllers and trigger electronics to handle increased event rates in scintillator mode.
• Purification & Calibration Plants: Multi-stage distillation, solvent extraction, metal scavenging, and cover-gas systems (with advanced radon traps) are integral for radiopurity (Fatemighomi et al., 24 Jan 2025). In situ calibration employs optical fiber systems, a deployable laserball, and multiple radioactive sources (Hunt-Stokes, 28 Mar 2024).
• Data Acquisition & Analysis: The DAQ and offline pipeline (ZDAB/ROOT), MC generation (GEANT4/GLG4sim), and real-time event monitoring were adapted for the high light yield and timing precision required in the scintillator and 0νββ phases.

These technical measures enable SNO+ to achieve ultra-low backgrounds, high light yields, and the flexibility required for multipurpose rare-event searches.

2. Experimental Program Phases and Scientific Objectives

SNO+'s program is structured into sequential phases, each expanding the detector’s capabilities and physics reach (Inácio et al., 28 Mar 2024, Tam, 2022, Caden, 2017):

• Water Phase: Commissioning and background studies with 905 tonnes of ultrapure water (2017–2019), focused on solar ^8B neutrinos, reactor antineutrino searches via inverse beta decay (IBD), and nucleon decay to invisible final states (Collaboration et al., 2022).
• Scintillator Phase: Operation with LAB-based scintillator (780 tonnes, 2022–), increasing light yield by ~50x compared to water. Enabled precision studies of solar ν fluxes, reactor anti-ν oscillations, event directionality, and geoneutrinos (Collaboration et al., 30 May 2024, Collaboration et al., 28 Aug 2025).
• Tellurium Phase (0νββ Search): Gradual introduction of natural Te (up to 3.9 tonnes, corresponding to ~1.3 tonnes ^130Te at 0.5% by mass) for the neutrinoless double beta decay program, with plans for future increases up to 24 tonnes Te (Inácio et al., 28 Mar 2024, Collaboration et al., 2021). The metal loading uses diolisation chemistry (TeBD in LAB), maintaining optical clarity and radiopurity at target loadings (Biller, 2014).

This phased strategy allows incremental commissioning, background modeling, and calibration ahead of rare-event data taking. Each phase yields valuable physics results and validates operational methods for subsequent, more sensitive configurations.

3. Neutrinoless Double Beta Decay: Techniques, Sensitivities, and Theoretical Framework

The primary scientific goal of SNO+ is the search for 0νββ in ^130Te, a probe for the Majorana nature of neutrinos and lepton number violation (Inácio et al., 28 Mar 2024, Collaboration et al., 2021, Fischer, 2018). Key aspects include:

• Isotope Selection: ^130Te is chosen for its high natural abundance (~34%, reducing enrichment needs), high Q-value (2.54 MeV, above much of the natural radioactive background), and compatibility with the LAB/PPO-based scintillator.
• Tellurium Loading & Purification: Purification proceeds in two stages—surface (nitric acid recrystallization, metal scavenger columns) and underground (thermal crystallization)—with cosmogenic activation controlled by additional underground storage (“cool-down”) (Biller, 2014). The TeBD loading technique achieves target transparency and radiopurity in the host LAB.
• Sensitivity: The target for initial tellurium loading is 0.5% by mass (1.3 tonnes ^130Te), reaching a projected 0νββ half-life sensitivity of $S_{1/2}^{0\nu} = 2 \times 10^{26}$ yr (90% CL) in 3 years, and $>10^{27}$ yr for higher loadings (~3%) (Inácio et al., 28 Mar 2024). The governing relation,

$$(T_{1/2}^{0\nu})^{-1} = G_{0\nu} |M_{0\nu}|^2 \left(\frac{\langle m_{\beta\beta} \rangle}{m_e}\right)^2,$$

relates measured half-life limits to the effective Majorana mass ⟨mββ⟩, with nuclear matrix elements and phase space factors drawn from up-to-date theory (Fang et al., 2010, Fang et al., 2011).

• Background Control: Achieved through massive water shielding, self-shielding in the scintillator, advanced purification, and time/direction discrimination techniques. Pulse shape and timing-based methods differentiate multi-site backgrounds (e.g., β–γ from ^208Tl) from single-site β events (Hunt-Stokes, 28 Mar 2024).
• Comparative Analysis: The SNO+ measurement complements and cross-validates other 0νββ searches by utilizing a different isotope, detection medium, and deployment strategy (Biller, 2014).

4. Versatile Neutrino Physics: Solar, Reactor, and Geoneutrinos

SNO+ is uniquely capable as a multipurpose neutrino laboratory, yielding advances in multiple areas:

• Solar Neutrinos: By exploiting the high light yield and purity of LAB, SNO+ measures the pp, pep, CNO, and ^8B solar neutrino fluxes. Notably, event-by-event directional reconstruction of solar ^8B neutrinos using Cherenkov–scintillation time separation has been demonstrated for the first time in high light-yield liquid scintillator (Allega et al., 2023). The collaboration also achieved the first real-time observation of ^8B solar neutrino charged-current interactions on ^13C via delayed coincidence, marking a direct cross section measurement (Collaboration et al., 28 Aug 2025).
• Reactor Antineutrino Oscillation Measurements: SNO+ has produced spectral analyses of long-baseline reactor antineutrinos (dominated by CANDU reactors), measuring Δm₂₁² through oscillatory distortions in the prompt energy spectrum (Collaboration et al., 30 May 2024, Collaboration et al., 7 May 2025). The current best fit is

$$\Delta m_{21}^2 = 7.96^{+0.48}_{-0.42}\times10^{-5}~\text{eV}^2$$

(unconstrained), and $7.58^{+0.18}_{-0.17}\times10^{-5}~\text{eV}^2$ (combined constraint), with $\sin^2\theta_{12} = 0.308 \pm 0.013$ and a geoneutrino flux of $73^{+47}_{-43}$ TNU—the first such measurement in the Western Hemisphere.

• Geoneutrinos and Earth's Radiogenic Heat: SNO+ observes geoneutrinos from local crust (notably the Huronian Supergroup-Sudbury Basin) and mantle. High uncertainty in local Th/U content is identified as a limiting factor for mantle signal extraction (Baldoncini et al., 2016, Strati et al., 2018). The precision of geoneutrino and reactor antineutrino measurements is enhanced by sophisticated geological models and signal subtraction strategies, with further improvements expected via increased exposure and background constraints.

5. Calibration, Background Management, and Analysis Innovations

• Scintillation Timing Calibration: The SNO+ optical model is rapidly tuned using in-situ ^214Bi–^214Po (BiPo214) coincident backgrounds, providing high-statistics, radiopure calibration samples for β and α time response. The emission-time model is a multi-component exponential with a common rise time, calibrated by matching time-residual distributions in data and simulation (Hunt-Stokes, 28 Mar 2024).
• Time-Based Particle Discrimination: Utilizing differences in time residual PDFs between single-site (signal-like) and multi-site (background) events, a log-likelihood ratio classifier is constructed:

$$\Delta \log(\mathcal{L}) = \frac{1}{N_{\text{hit}}}\sum_{i} \log \left(\frac{P_S(t_{\text{res}}^i)}{P_B(t_{\text{res}}^i)}\right)$$

enhancing background rejection in the 0νββ region and for solar neutrino studies.

• Radon Background and Cover Gas: The SNO+ collaboration developed a sensitive activated charcoal trap for radon assay in N₂ cover gas systems, reaching sub-0.1 mBq/m³ sensitivity (Fatemighomi et al., 24 Jan 2025). Purification approaches for all detector materials (including tellurium and surfactants) achieve stringent radiopurity and control rare-event backgrounds.

6. SNO+ as a Development Platform for Detector Concepts and Future Prospects

SNO+ is a vital platform for the development, demonstration, and scaling of new detector technologies, including water-based liquid scintillator (WbLS) for the Advanced Scintillator Detector Concept (ASDC). Pioneering chemistry for hydrophilic isotope loading, separation of Cherenkov and scintillation light by advanced timing, and large-scale purification approaches have been validated for potential scale-up to 30–100 kiloton detectors (Alonso et al., 2014).

Future SNO+ phases envisage higher Te loadings, upgrades to light-detection hardware, and the continuation of a multiphysics program encompassing dark matter searches and particle astrophysics. Enhanced directionality and improved pulse shape discrimination are expected to further broaden the physics reach and the ability to suppress backgrounds in rare-event searches.

7. Scientific Impact and Outlook

SNO+ has achieved several milestones:

• Definitive observation of reactor antineutrinos in a water Cherenkov detector (3.5σ) at baselines of 240 km (Collaboration et al., 2022).
• Demonstration of high-sensitivity Δm₂₁² measurements with liquid scintillator, validating and complementing KamLAND and solar neutrino results (Collaboration et al., 30 May 2024, Collaboration et al., 7 May 2025).
• First evidence for real-time solar neutrino interactions on ^13C (4.2σ), extending low-energy neutrino-nucleus interaction measurements (Collaboration et al., 28 Aug 2025).
• Establishment of robust methodologies for radiopurity, calibration, and rare-event data analysis, directly transferrable to future large-scale low-background experiments.

The SNO+ Collaboration continues to produce world-leading results in neutrino physics, with a program designed for systematic advances in background control, event reconstruction, and scientific scope. Future upgrades—both in isotope loading and photodetector technology—will extend sensitivity to lower neutrino mass regimes and enhance the breadth of scientific discovery related to fundamental symmetries, neutrino properties, and geophysics.