Reines and Cowan: Project Poltergeist
Twenty-six years passed between Pauli proposing the neutrino and anyone catching one. The reason was not lack of interest but arithmetic: the interaction is so weak that any sane estimate of the required apparatus came out absurd. Reines and Cowan's contribution was to notice that if you cannot improve the odds per neutrino, you can supply an enormous number of neutrinos and watch for a signature that nothing else can imitate.
The Bomb Proposal
The problem in 1951 was flux. Detecting a neutrino requires an enormous number of them arriving at a target, and the only sources available were nuclear. Reines and Cowan's first concrete plan was to suspend a detector in a shaft near an atmospheric nuclear weapons test, drop it into free fall at the moment of detonation so that it experienced no vibration, and recover it afterwards from the debris.
The plan was serious, worked out in detail, and approved. It is worth registering how strange that is - and also that it was a reasonable response to the numbers. A bomb produces a brief, colossal burst of antineutrinos, and a brief burst has the advantage that anything not synchronised with it can be ignored.
They abandoned it after realising that a nuclear reactor produces a continuous flux which, integrated over months of running, exceeds what a single detonation delivers - and does so without requiring the detector to survive a nuclear explosion.
The episode is a useful reminder that the history of physics contains a great many discarded plans that were not stupid, only superseded.
The Signature
The reaction they looked for is inverse beta decay: an antineutrino strikes a proton and produces a neutron and a positron. What makes it identifiable is not either product on its own but the sequence.
The positron annihilates almost immediately with an electron, producing two gamma rays of a precise energy flying in opposite directions. The neutron wanders for a few microseconds, slowing down, until it is captured by a cadmium nucleus, which then emits its own characteristic gamma rays.
So the detector sees a flash, then a second flash a few microseconds later, with specific energies. Random background produces flashes constantly, but producing that exact pair in that exact interval by chance is vanishingly unlikely. The delayed coincidence is the fingerprint.
The apparatus was accordingly simple in concept: tanks of water with dissolved cadmium chloride, sandwiched between tanks of liquid scintillator watched by photomultiplier tubes. The design principle survives in modern reactor neutrino experiments almost unchanged.
1953, 1956 and the Telegram
A first run at the Hanford site in 1953 produced a signal that was not convincing. The background from cosmic rays was too high and the result could not be separated cleanly from it. They rebuilt with better shielding and moved to the Savannah River Plant, where the detector could be placed twelve metres underground and eleven metres from the reactor core.
The 1956 run worked. The signal appeared when the reactor was running and disappeared when it was shut down, which is the control that mattered most.
On 14 June 1956 they sent Pauli a telegram informing him that his particle had been definitely detected. Pauli, then in Zurich, replied that everything comes to those who know how to wait. He died two years later.
The Nobel Prize for the work was awarded in 1995, thirty-nine years afterwards. Reines received it. Cowan had died in 1974, and the prize is not awarded posthumously - one of several cases where the rule has produced an outcome nobody defends on the merits.
What It Established
The immediate result was that the neutrino existed as a particle rather than as a bookkeeping device. That distinction had been genuinely open for a quarter of a century.
The method mattered as much as the finding. Delayed coincidence remains the standard technique for reactor antineutrino detection, and the basic layout of target plus scintillator plus photomultipliers is recognisable in Borexino, JUNO and every experiment in that family.
It also set the scale of the problem for everyone who followed. Reines and Cowan recorded roughly three events per hour from a reactor eleven metres away. Every later experiment, from Homestake to IceCube, has been an argument about how to get a useful number of events out of a flux that yields so few.
Reines continued working on neutrinos for the rest of his career and was involved in the detection of neutrinos from supernova 1987A, thirty-one years after the first detection.
Frequently asked questions
How did they detect a particle Pauli called undetectable?
By using an enormous flux from a nuclear reactor and looking for a signature nothing else produces: a positron annihilation followed a few microseconds later by a neutron capture, both at specific energies.
Did they really plan to use a nuclear bomb?
Yes. Their first approved plan was to drop a detector down a shaft near an atmospheric weapons test so that it fell freely during the detonation. They abandoned it once they realised a reactor delivers more integrated flux over months of running.
What was the telegram?
On 14 June 1956 they wired Wolfgang Pauli that his particle had been definitively detected. Pauli replied that everything comes to those who know how to wait.
Why did only Reines receive the Nobel Prize?
Because it was awarded in 1995, and Clyde Cowan had died in 1974. The Nobel Prize is not awarded posthumously.
Is the method still used?
Yes. Delayed coincidence remains the standard technique for detecting reactor antineutrinos, and the layout of target, scintillator and photomultipliers is recognisable in modern detectors.
