Arthur McDonald: Counting the Neutrinos Nobody Could Detect
Every solar neutrino experiment before Sudbury could see only electron neutrinos, which meant none of them could distinguish between neutrinos going missing and neutrinos changing into something invisible to the detector. The design McDonald led resolved that by measuring both quantities in the same tank at the same time, which is why the result was decisive rather than suggestive.
The Design Problem
Raymond Davis had found a third of the predicted solar neutrinos in 1968 and the deficit had stood unexplained for three decades. The difficulty was structural: chlorine, gallium and water Cherenkov detectors all responded predominantly to electron neutrinos, so a deficit was equally consistent with the neutrinos never being produced and with their having turned into something the detector could not register.
Distinguishing those requires counting the invisible ones, which sounds impossible and is not. Ordinary water contains hydrogen, whose nucleus is a single proton. Heavy water contains deuterium, a proton bound to a neutron, and that extra neutron makes a second reaction available - one that breaks the deuteron apart and is driven equally by all three neutrino types.
So a heavy water detector produces two independent counts from the same flux at the same time: how many electron neutrinos arrived, and how many neutrinos arrived in total. The comparison between them is the measurement, and no assumption about the Sun is needed to make it.
A third channel, elastic scattering off electrons, sits between the two and provides a cross-check. Together they turn a single ambiguous number into an internally consistent set.
Building It
The heavy water was not purchased. A thousand tonnes of it, worth several hundred million Canadian dollars, was lent from the national reserve maintained for nuclear reactors, used for seven years and returned.
It was held in a transparent acrylic vessel twelve metres across, suspended inside a barrel-shaped cavity filled with 7,000 tonnes of ultrapure ordinary water, watched by around 9,600 photomultiplier tubes, at the 6,800-foot level of an active nickel mine near Sudbury - about 2,070 metres of rock overhead, deeper than any comparable detector of its era.
Contamination discipline was extreme, because a trace of uranium or thorium in the acrylic or the water produces neutrons indistinguishable from the flavour-blind signal the whole experiment depended on.
The collaboration ran in three phases, each using a different method to detect those neutrons: the heavy water alone, then with dissolved salt to improve neutron capture, then with dedicated neutron detectors installed. Three independent methods, three consistent answers - a deliberate design choice rather than an afterthought.
The Result
Published in 2001 and confirmed in 2002, the arithmetic was unusually clean. The flavour-blind channel measured a total solar neutrino flux closely matching the standard solar model. The electron-neutrino-only channel measured roughly a third of it.
The neutrinos were all arriving. Two thirds had simply stopped being electron neutrinos somewhere between the Sun's core and Ontario. The solar model had been right, Davis's chemistry had been right, and the missing ingredient was oscillation - which requires neutrinos to have mass.
Takaaki Kajita had reached the same conclusion from a different direction three years earlier, using atmospheric neutrinos and the distance they had travelled through the Earth. Two experiments, different sources, different technologies, different systematic errors, one answer.
That convergence is why the conclusion was accepted quickly and has not been seriously challenged since. It is also the standard worth carrying elsewhere: a measurement becomes a discovery when an independent experiment that could have contradicted it does not.
Since
McDonald is professor emeritus at Queen's University in Kingston, Ontario. The site became SNOLAB, an expanded underground facility hosting dark matter searches and other experiments, and SNO itself was converted into SNO+ by replacing the heavy water with liquid scintillator to search for neutrinoless double beta decay.
The result left the absolute neutrino mass open, since oscillation measures differences between squared masses rather than the masses themselves. That question passed to direct-measurement experiments such as KATRIN.
What Sudbury settled was the thirty-year-old solar neutrino problem, and it settled it in a way that vindicated everyone: the astrophysicists who modelled the Sun, the chemist who counted the argon atoms, and the theorists who had proposed oscillation decades earlier and waited.
Frequently asked questions
What made the Sudbury experiment different?
Heavy water. Deuterium's extra neutron allows a reaction that responds equally to all three neutrino types, alongside a separate reaction sensitive only to electron neutrinos. One detector could therefore count both independently.
Where did a thousand tonnes of heavy water come from?
It was lent from Canada's national reserve held for nuclear reactors, worth several hundred million Canadian dollars, used for seven years and returned. Purchasing that quantity was not realistic.
What did the result show?
That the total flux of all three neutrino types from the Sun matched the standard solar model, while the electron-neutrino component was about a third of it. The missing neutrinos had changed type rather than disappeared.
How does it relate to Kajita's result?
Both concluded that neutrinos oscillate. Kajita used atmospheric neutrinos and distance travelled; Sudbury used solar neutrinos and flavour composition. The independence of the two is why the conclusion held, and they shared the 2015 Nobel Prize.
What happened to the detector?
The site became SNOLAB, an expanded underground laboratory. SNO itself was converted into SNO+ by replacing the heavy water with liquid scintillator, to search for neutrinoless double beta decay.
