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Neutrinos from an Atomic Bomb

February 16, 2015

[The text below is taken from today’s post on my blog, “A Quantum Diaries Survivor” – T.D.]

Less than three weeks separate us from the XVI Neutrino Telescopes, a very interesting conference held in Venice every two years. The physics of neutrinos is a very special niche in the realm of particle physics, one not devoid of cunning experimental techniques, brilliant theoretical ideas, and offering possible avenues to discover new physics. Hence I am quite happy to be attending the event, from where I will also be blogging (hopefully with the help of a few students in Padova).

Confined home to recover from a flu, today I was lazily browsing a nice book by George Trigg, “Landmark experiments in twentieth century physics” when I came across the description of how the neutrino was discovered, by Reines and Cowan in 1956. I knew the story, but had forgotten about an amusing detail. I believe that detail is interesting for you too, so here it is.
 In the fifties neutrinos were a well-established entity in the rapidly developing field of particle physics: the successes of Fermi’s theory of beta decay of nuclei was extraordinary evidence that neutrinos did exist; yet a direct experimental proof of their existence was still lacking. While previously the technology was not yet mature enough to allow the design of a detection experiment, by the mid-fifties the tools were there.
To detect neutrinos, which are incredibly elusive particles, capable of traversing unimaginable amounts of matter without interacting at all, one needs a very large flux, or a very large and background-free detector -or better, both. What was most important, though, was a large flux which could be turned on and off at will: in that case, the signature would be the surplus in the counting rate of neutrino-like interactions in the detector (whatever that was) when the extra source of neutrinos was active.
Nuclear plants could produce a large flux of electron antineutrinos, and were indeed what was used by Reines and Cowan to detect them. The collision of an electron antineutrino with a proton produces a positron and a neutron: they used both final state bodies in their successful experiment. There are nice accounts of that experiment around, so I will not discuss it here further. Instead let me mention the amusing alternative that the scientists caressed for a while.
If one concentrates on the simpler detection of the positron produced in the antineutrino-proton interaction, as was proposed at first, nuclear plant fluxes would be insufficient to yield rates large enough above backgrounds. So the idea was initially to detect neutrinos with a detector located a few hundred feet away from a nuclear explosion. Cowan described the setup as follows:

“We would dig a shaft near ‘ground zero’ about 10 feet in diameter and about 150 feet deep. We would put a tank, 10 feet in diameter and 75 feet long on end at the bottom of the shaft. We would the n suspend our detector from the top of the tank, along with its recording apparatus, and back-fill the shaft above the tank. As the time for the explosion approached, we would start vacuum pumps and evacuate the tank as highly as possible. Then, when the countdown reached ‘zero’, we would break the suspension with a small explosive, allowing the detector to fall freely in the vacuum. For about 2 seconds, the falling detector would be seeing antineutrinos and recording the pulses from them while the earth shock [from the blast] passed harmlessly by, rattling the tank mightily but not disturbing our falling detector. When all was relatively quiet, the detector would reach the bottom of the tank, landing on a thick pile of foam rubber and feathers.”

Later, they would be able to recover the detector and get the readings, finding out whether indeed a surplus of positrons had been seen.
I think this is a brilliant setup, and as much as I abhor nuclear explosions and the use that has been made of them since 1945, I think it would have been quite cool if we had used one of them to discover the neutrino. The actual experiment by Reines and Cowan was no less cunning and beautiful, but lacks of drama in comparison.
One question remains in my mind: why was it necessary to use a “small explosion” to release the detector ? This really looks like a bit too much explosive for an experiment IMHO…
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