Sudbury Neutrino Observatory

Objective

Resolve the solar neutrino problem by independently measuring the total flavor-summed flux and the electron-neutrino fraction of solar ⁸B neutrinos.

Method

A 1-kiloton spherical acrylic vessel filled with heavy water (D₂O), surrounded by a geodesic array of 9,456 photomultipliers and further surrounded by an outer water shield. Deuterium enables three distinct detection channels: charged current (νe-only), neutral current (flavor-blind), and elastic scattering (νe-weighted).

Key results

• 2001: first direct evidence of neutrino flavor transformation from solar νe to νμ/ντ through comparison of CC and ES rates.
• 2002: neutral-current measurement confirmed that the total ⁸B neutrino flux matches the Standard Solar Model — resolving the solar neutrino problem unambiguously.
• Subsequent salt phase (2004) and NCD phase (2006) refined the extraction of individual channels.
• Provided the definitive oscillation parameter region (LMA-MSW) confirmed by KamLAND in the same year.

Significance

SNO's heavy-water technique delivered the second of the two independent oscillation discoveries that earned the 2015 Nobel Prize in Physics (Arthur McDonald). Unlike Super-K's zenith-angle argument for atmospheric neutrinos, SNO's multi-channel solar-neutrino comparison made flavor transformation directly visible. The former SNO cavern is now the central site of SNOLAB, a world-leading underground physics facility.

Background: why heavy water

The solar neutrino problem was established by the Homestake chlorine experiment in the 1970s and confirmed by Kamiokande, SAGE, GALLEX, and Super-K through 1998. None of those experiments could distinguish between a deficit caused by flawed solar models and a deficit caused by flavor transformation: they all measured either ${\nu }_e$ only (Cl, Ga) or a ${\nu }_e$-weighted combination (elastic scattering).

Herb Chen at UC Irvine proposed in 1984 that heavy water could provide the decisive test. On deuterium, neutrinos can undergo three different interactions with different flavor sensitivities:

• Charged current: ${\nu }_e+d\to p+p+e^{-}$ — only ${\nu }_e$
• Neutral current: ${\nu }_x+d\to p+n+{\nu }_x$ — all active flavors, equal weight
• Elastic scattering: ${\nu }_x+e^{-}\to {\nu }_x+e^{-}$ — ${\nu }_e$ weighted $\sim 6\times$ heavier than ${\nu }_{\mu }$, ${\nu }_{\tau }$

By measuring all three rates, one could extract the total flavor-summed flux and the ${\nu }_e$ fraction independently. If oscillation was the cause of the deficit, NC would match the Standard Solar Model while CC would not.

Construction

The SNO collaboration, led by Arthur McDonald at Queen’s University, began construction in 1990. The detector was sited at a 2 km depth in the Creighton Mine near Sudbury, Ontario — at the time one of the deepest active detectors in the world. A 12-m-diameter acrylic vessel held 1 kt of heavy water, on loan from the Canadian nuclear industry. Outside the vessel, a geodesic array of 9,456 PMTs viewed inward; beyond them, 7 kt of ultrapure light water provided shielding.

Construction completed in 1999 and data-taking began immediately.

The three phases

SNO ran three distinct phases, each optimized for a different aspect of the neutral-current measurement.

Phase I (pure D₂O, 1999–2001). The neutron produced by NC interactions was detected through its capture on deuterium, producing a 6.25 MeV gamma. The signal was small but clean.

Phase II (salt, 2001–2003). 2 tonnes of NaCl were dissolved in the heavy water. Neutron capture on ${}^{35}$Cl released higher-energy gamma cascades (8.6 MeV total) with a higher capture efficiency, improving NC statistics and energy reconstruction.

Phase III (Neutral-Current Detectors, 2004–2006). An array of 40 ³He proportional counters was deployed inside the acrylic vessel. Neutrons were counted directly by each tube, providing an NC measurement independent of the PMT system.

The result

Comparison of the three phases gave consistent results that cleanly separated the fluxes: ${\Phi }_{CC}^{{\nu }_e}=1.76_{-0.10}^{+0.10}\times 10^6{\text{ cm}}^{-2}{\text{s}}^{-1}$ ${\Phi }_{NC}^{{\nu }_x}=5.09_{-0.43}^{+0.44}\times 10^6{\text{ cm}}^{-2}{\text{s}}^{-1}$ The NC flux matched the Standard Solar Model prediction of ${\Phi }_{SSM}=(5.05\pm 0.91)\times 10^6$ cm⁻² s⁻¹. The CC flux was about one third of the total. The flavor-transformation interpretation was immediate and incontrovertible.

Legacy: SNOLAB

The heavy water was drained and returned to its owners. The SNO cavern was incorporated into SNOLAB, a greatly expanded underground research facility that now hosts experiments including SNO+ (tellurium-loaded scintillator, double beta decay), DEAP-3600 (argon dark matter), PICO, SuperCDMS, and HALO. The 2 km depth and radon-controlled cleanrooms make it among the best underground laboratories in the world.

The successor experiment SNO+ reuses the acrylic vessel with liquid scintillator instead of heavy water, targeting neutrinoless double beta decay in tellurium-130.