Homestake Experiment

The Homestake experiment in a mine one and a half kilometres deep in South Dakota
The Homestake experiment in a mine one and a half kilometres deep in South DakotaPhoto: US Department of Energy, public domain, Wikimedia Commons

The Homestake experiment was the first attempt to detect neutrinos from the Sun. Raymond Davis ran it from 1968 to 1994 in a gold mine in South Dakota. It produced a result that nobody could explain for thirty years — and was thus the beginning of one of the most productive troubleshooting exercises in the history of physics.

The task

Up into the 1960s, nuclear fusion in the Sun's interior was a well-founded theory, but nobody had ever observed it directly. The light that reaches us comes from the surface; the energy took tens of thousands of years to get there. It reveals nothing about what is happening in the core right now.

Neutrinos do. They leave the core unimpeded and are here a good eight minutes later. Whoever counts them is watching the Sun burn.

John Bahcall had calculated how many of them ought to arrive. Davis's task was to find them.

The set-up

The method rests on a nuclear reaction that Bruno Pontecorvo had proposed: an electron neutrino turns a chlorine-37 nucleus into an argon-37 nucleus and an electron. The threshold lies at 814 kiloelectronvolts — high enough that only the rare high-energy neutrinos from boron-8 decay are captured, that is, less than one per cent of all solar neutrinos.

The chlorine source was perchloroethylene, ordinary dry-cleaning fluid. 615 tonnes of it stood in a steel tank, 1,478 metres underground on the 4850-foot level of the Homestake mine near Lead in South Dakota. The rock above keeps out cosmic muons, which would otherwise have swamped any signal.

The real difficulty

Bahcall's calculation gave about one argon atom every two days in this tank. Over a measuring run of two months, then, a few dozen atoms accumulated — in 615 tonnes of liquid, that is, among some 10³⁰ other atoms.

Most specialists thought this hopeless. Davis was a chemist, not a physicist, and for a chemist it was not an impossible task, merely a difficult one.

His method: every few weeks, helium was blown through the tank. The noble gas carries the equally noble argon with it, while the perchloroethylene stays behind. The argon was frozen out in a cold trap, transferred into a tiny counting tube and detected there individually via its radioactive decay. Argon-37 has a half-life of 35 days — short enough to count almost everything within a few months, long enough to survive the extraction.

Nor was that all: Davis demonstrated that his method really does find every atom. He put a known, tiny quantity of argon into the tank and retrieved it again. The yield was over 90 per cent — a positive control about as strict as one could wish for.

The result nobody wanted

From 1968 onwards there were numbers. Davis found about one third of what Bahcall had calculated.

That was not a close result but a gaping shortfall. And the obvious explanations suggested themselves: either Bahcall was calculating wrongly, or Davis was measuring wrongly.

Both checked for decades. Bahcall refined his solar model further and further and had it confirmed independently by helioseismology — from the oscillations of the solar surface the interior structure can be derived, entirely without neutrinos. Davis repeated his calibrations and checked whether his tank might be responding to something else; among other things he placed a tank next to a nuclear reactor and found, as expected, nothing, because reactors emit antineutrinos and not neutrinos.

Neither found an error. Because there was none.

The resolution

The solar neutrino problem remained open until 2002. Then the Sudbury Neutrino Observatory showed that the missing neutrinos had not vanished but had merely been transformed — into kinds that Davis's chlorine tank could not respond to in principle.

Add all three kinds together and Bahcall's figure is right.

Davis's measurement had therefore been correct from the outset. It had simply measured something other than everyone assumed — and precisely in that lay the discovery.

Aftermath

Davis received the Nobel Prize in 2002, together with Masatoshi Koshiba. He was 87 years old at the time.

The site itself has found a remarkable continuation. In the same tunnels the far detector of DUNE is being built today — four modules with 68,000 tonnes of liquid argon between them. Where a single tank of dry-cleaning fluid once stood, the question of CP violation will in future be settled.

Sources

  • R. Davis Jr., D. S. Harmer, K. C. Hoffman: Search for Neutrinos from the Sun, Physical Review Letters 20, 1205 (1968).
  • B. T. Cleveland et al.: Measurement of the Solar Electron Neutrino Flux with the Homestake Chlorine Detector, The Astrophysical Journal 496, 505 (1998).