Doug Reilly's neutrinos back then and today

Submitted by Ellie Swain Ballard on Friday, 2/21/2014, at 10:37 AM

At the end of 1968, after completing my PhD at Johns Hopkins U in Baltimore, Peggy Hauschka and I drove from Baltimore to La Jolla, where I was about to start on my post-doc years. We passed by Cleveland, where my former Stearns roommate Doug Reilly happened to be doing his PhD work in physics at Western Reserve U in Cleveland. He was looking for neutrinos which according to theoreticians should emanate from the core of the sun, and hit the earth in amounts of several neutrinos per day, per square meter. Doug had built a detector at the bottom of one of Cleveland’s salt mines, some 700 ft down, or was it 700 meters? Anyway, Doug took me down into the salt mine, where it was cool and salty, with Caterpillar shovels roaring back and forth through cavernous tunnels, to provide Cleveland its salt supply for wintry roads. 

Doug's device consisted of a basin with solvent, something like 500 gallons as I recall, surrounded by large photomultipliers. The idea was that the neutrinos could pass through the 700 m or ft of soil above the salt mine, whereas various cosmic rays would be filtered out so that a small number of neutrinos could be seen and counted without a huge background of other particles. Doug’s neutrinos would be captured in the solvent basin, where the interaction between neutrinos and solvent (or some capturing molecule; Doug might tell us more) produced  flashes that were then detected by his photomultipliers which translated the light flashes to little current pulses, bla, bla, bla. Doug’s cabin was filled with 1960’s electronic gear to interpret and store the information generated by the photomultipliers.

Doug had been at it for several years, but he had not yet seen any pulses that looked like they were produced by neutrinos. The theoreticians went back to their estimates and changed a few of their assumptions resulting in lower neutrino fluxes. They presumably were able to write a few elegant papers for Physics Reviews, Nature or PNAS, but Doug now had to figure out how to produce a thesis. This he did by looking for another particle, more easily detected, namely muons. He received his PhD, but the excitement and thrill came only years later, accompanied still later by a 1995 Nobel prize to Fred Reines. I never forgot that visit and that story, because what Doug must have gone through did not seem easy to me: descending into that salty cave day after day, the noise not only of the large Cats, but also huge fans to circulate air through the tunnels. And then to wait for the telltale pulse that should have said: “yes, I am one of the neutrinos, and there will be more…”

Yesterday I and several others sat and listened to 3 physicists who came to explain why they needed funding for the Dutch contribution to a neutrino telescope being built by a European consortium. This device too has to sit behind a massive cosmic ray filter, and the neutrinos have to produce flashes that can be seen by photomultipliers. The difference with Doug’s experiment was one of scale. The Europeans are building a system that has a volume of one cubic km, or about a billion times more than Doug’s basin. It is hard to do that in a mine. The solution is to build this device at the bottom of the Mediterranean Sea, which has clefts more than 3000 m deep, so that a layer of 2500 m water shields the device. Why the size? This increases the overall sensitivity, which Doug could have used, and it also allows the determination of the angle at which the neutrinos penetrate the Med, necessary for the telescope function. Information about neutrino sources throughout outer space will apparently help to say something about the sources and nature of dark matter in our universe. We put their request high on our list. Of course, this being Europe, there still remains the question as to whether the telescope will be built along the coast of France, Italy or Greece. So, as you summer along the Riviera or sail among the Greek isles in the years to come, think of that submerged equipment tracing our evolution while you sip a glass of wine. 

By the way, if you want to know more about related matters, read Just Six Numbers: The Deep Forces that Shape the Universe, by Martin Rees, one of the most exciting books about our larger environment. Really fantastic. 

And if you want to know a little more about neutrinos, maybe this powerpoint helps -