Search for Neutrinos from the Sun

The chlorine technique

The reaction proposed by Pontecorvo in 1946, ${\nu }_e+{}^{37}\text{Cl}\to {}^{37}\text{Ar}+e^{-}$, has a threshold of 0.814 MeV, making it sensitive to the high-energy tail of the solar neutrino spectrum — primarily ${}^8$B and CNO-cycle neutrinos. The daughter ${}^{37}$Ar decays back to ${}^{37}$Cl with a 35-day half-life by electron capture, releasing 2.82 keV Auger electrons that can be counted in a low-background proportional chamber.

Davis’s approach was to run the target — tetrachloroethylene (C₂Cl₄, used commercially as cleaning fluid) — for ~2 months, then sweep the dissolved argon out of the tank with helium, cryogenically trap it, and count the ${}^{37}$Ar decays over the following months in a shielded low-activity proportional counter.

The 1968 result

Three runs completed in the 1967–1968 period gave a measured capture rate of roughly $0.52\pm 0.08$ ${}^{37}$Ar atoms per day, corresponding to $2.5\pm 0.4$ solar-neutrino units (1 SNU = $10^{-36}$ captures per target atom per second). John Bahcall’s contemporaneous Standard Solar Model predicted approximately 8 SNU.

The factor-of-three discrepancy was immediately recognized as either:

1. A solar-physics problem: the central solar temperature might be lower than assumed, suppressing ${}^8$B production
2. A particle-physics problem: solar ${\nu }_e$ might transform into something the detector could not see
3. An experimental problem: subtle systematics in the extraction or counting

Option 3 was methodically excluded by repeated cross-checks and refinements. Options 1 and 2 remained live for three decades.

Three decades of puzzle

Davis continued the measurement at Homestake for nearly thirty years, refining systematics and accumulating runs. The deficit persisted at about one-third of the Standard Solar Model prediction, robust to all experimental improvements.

Independent experiments using different targets — SAGE and GALLEX with gallium (sensitive to lower-energy pp neutrinos), Kamiokande and Super-Kamiokande with water Cherenkov detectors — each also found deficits, though with different magnitudes reflecting their different energy ranges.

The pattern — deficit present at all energies but with energy-dependent magnitude — was eventually recognized as the signature of MSW oscillation in the Sun.

Resolution

The 2001–2002 SNO measurements, enabled by a heavy-water target that could separate charged-current from neutral-current channels, showed that the total flavor-summed ${}^8$B neutrino flux matched the Standard Solar Model. The electron-neutrino fraction was about one-third — perfectly consistent with the deficit Davis had measured for thirty years. Two-thirds of the solar ${\nu }_e$ had transformed into ${\nu }_{\mu }$ and ${\nu }_{\tau }$ in flight.

Davis received the 2002 Nobel Prize in Physics, sharing it with Masatoshi Koshiba (Kamiokande) and Riccardo Giacconi (X-ray astronomy). He was 88 at the time of the award.

The Homestake measurements remain the canonical example of how a persistent, carefully controlled experimental anomaly can eventually force a rewriting of fundamental physics — in this case the recognition that neutrinos have mass.