CERN Courier July/August 2016 History Ghosts in the machine Los Alamos National Laboratory Christine Sutton describes the pioneering 1956 experiment that proved the existence of the neutrino, and how subsequent particle-beam experiments at CERN and elsewhere contributed to unearthing a further two neutrino types. In July 1956, in a brief paper published in Science, a small team based at the Los Alamos National Laboratory in the US presented results from an experiment at a new, powerful fi ssion reactor at the Savannah River Plant, in South Carolina. The work, they wrote, “verifies the neutrino hypothesis suggested by Pauli”. Clyde Cowan, Fred Reines, Kiko Harrison, Herald Kruse and Austin McGuire had demonstrated for the fi rst time that it was possible to detect neutrinos, setting in motion the new fi eld of neutrino phys- ics. The key ingredients were an intense source and a big detector, with more than a touch of ingenuity and patience. More than two decades previously, in 1930, Wolfgang Pauli had proposed that the “energy crisis” in nuclear beta decay – pre- sented by the continuous energy spectrum of the emitted electron – would be solved if the decaying nucleus also emitted a second, Fred Reines, left, and Clyde Cowan, at the controls of the undetected particle. This would allow the energy released to be Savannah River experiment, which discovered the electron shared between three objects, including the recoiling nucleus, and antineutrino in 1956. so yield electrons with a range of energies, just as observed. The new particle had to be neutral and have a relatively small mass. neutrinos produced during tests of atomic bombs to make a direct Pauli called his proposal “a desperate remedy”, in part because he detection of the elusive particle. He was soon joined in this strange thought that if such a particle did indeed exist, then it “would prob- pursuit by Clyde Cowan, a fellow researcher at Los Alamos, after ably have long ago been seen”. they were stranded together at Kansas Airport, where the conver- Nevertheless, Enrico Fermi took the possibility seriously and sation turned to the “supreme challenge” of detecting neutrinos. based his seminal work on beta decay, published in 1934, on a Reines had an idea to place a detector close to a bomb-test tower point-contact interaction in which a neutron decays to a proton, and use the timing of the detonation as a “gate” to minimise back- electron and (anti)neutrino: n → p e– ν–. Soon afterwards, Hans ground. But what kind of detector? He and Cowan decided on the Bethe and Rudolf Peierls calculated the cross-section for the recently developed medium of liquid scintillator, which could both inverse reaction in which a neutrino is absorbed, but when they act as a target for the inverse beta-decay reaction ν– p → e+ n, and found a value of about 10–44 cm2, the pair concluded that no one detect the emitted positrons via their annihilation to gamma rays. It would be able to detect neutrinos (Bethe and Peierls 1934). What was an audacious plan, not only in taking advantage of a bomb test they did not count on was the discovery of nuclear fi ssion – which but also in scaling up the use of liquid scintillator, which until then on a macroscopic scale produces copious numbers of neutrinos – or had been used only in quantities of about a litre. Reines and Cowan the ingenuity of experimentalists and, later, accelerator physicists. named it “Project Poltergeist”, to refl ect the neutrino’s ghostly nature. Notoriously, nuclear fi ssion was fi rst applied in the atomic bombs Remarkably, the Los Alamos director gave approval for the used towards the end of the Second World War. A few years later, experiment. However, in late 1952, Cowan and Reines were urged in 1951, Fred Reines, a physicist who had worked on the Manhat- to reconsider the more practical idea of using antineutrinos from a ▲ tan Project at Los Alamos, began to think about how to harness the nuclear reactor. The challenge was to work out how to reduce the 17 PWAug16Ad_IBIC_full.indd 1 23/06/2016 16:04 CERNCOURIER www. V OLUME 5 6 N UMBER 6 J ULY /A UGUST 2 0 1 6 CERN Courier July/August 2016 CERN Courier July/August 2016 History History Roy Kaltschmidt/LBNLRoy 1963, thanks to these innovations, CERN had what was at the time l the world’s most intense neutrino beam. ~νe In the 1970s, the combination of the neutrino beam from the PS and Gargamelle – the large bubble chamber built at the Saclay A Laboratory by a team led by André Lagarrigue – led to the discov- II γ γ 3–10 μs ery of weak neutral currents (CERN Courier September 2009 p25), Los Alamos National Laboratory γ thereby providing crucial experimental support for the unifi cation e+ n Cd* of the weak and electromagnetic forces. The neutrino experiments B with Gargamelle also produced key evidence about the existence III p of quarks and, in particular, their fractional charges (CERN Cou- rier April 2014 p24). Then, in 1977, the Super Proton Synchrotron γ γ γ γ (SPS) became the source of neutrino beams at higher energies, and for the next 21 years a series of experiments in CERN’s West Area Fig.2. One of eight detectors at the Daya Bay Reactor Neutrino used neutrinos in experiments covering a broad range of physics, Experiment in China, which are situated within 1.9 km of six Fig.1. (Left) The detector used at Savannah River consisted of three 1400-litre tanks of liquid scintillator (I, II and III), each viewed from neutral currents and the quark structure of matter through nuclear reactors. A larger follow-up experiment called the by 100 phototubes. The smaller tanks (A and B) contained the targets of 200 litres of water doped with cadmium. (Right) The quantum chromodynamics to neutrino oscillations (CERN Cou- Jiangmen Underground Neutrino Observatory (JUNO) is principle of the delayed-coincidence method for detecting the electron antineutrino in the experiment at Savannah River. rier December 1998 p28). currently under development. Around that time, physicists at Fermilab were closing in on a third – backgrounds, because the antineutrino fl ux from a reactor would 1956). After completing many checks, on 14 June 1956 Reines and neutrino type. The DONUT experiment (Direct Observation of antineutrinos (νe) per second and experiments based on the same be thousands of times smaller than that from a nuclear explosion. Cowan sent a jubilant telegram to Pauli in Zurich, informing him the NU Tau) detected neutrinos produced at the Tevatron, and in liquid-scintillator concept continue to provide essential contribu- – Reines and Cowan realised that in addition to looking for positron that they had “defi nitely detected neutrinos from fi ssion fragments 2000, the collaboration announced the discovery of the tau neutrino. tions to neutrino physics by looking for the “disappearance” of the νe. annihilation, they could also detect the neutrons through neutron by observing inverse beta decay of protons”. At the time, Pauli was Although experiments at CERN’s Large Electron–Positron collider Sixty years after the fi rst detection of the neutrino, and more than capture – a process that is delayed for several microseconds, thanks in fact at a meeting at CERN, to where the telegram was forwarded, had already established from precise measurements of the Z boson 80 years after the particle was tentatively predicted, experiments to the neutron’s random walk through a medium prior to inter- and he reportedly interrupted the meeting to read out the good that there are three light neutrino types, the observation of the tau with neutrinos continue to have a leading role in particle physics. acting with a nucleus. In particular, the addition of cadmium to news, later celebrating with a case of champagne (Reines 1979). neutrino completed the leptonic sector of the Standard Model. Today, experimentalists around the world are vying to determine the detector would increase the likelihood of capture and lead to Ten years later, CERN was again setting records for neutrino precisely the mixing parameters of the neutrino, including the the emission of gamma rays. The signature for inverse beta decay The move to accelerators beams, with the CERN Neutrinos to Gran Sasso (CNGS) project, masses. The measurements may prove to hold the answers to some would then be a delayed coincidence between two sets of gamma At the time of the neutrino’s discovery, laboratories such as CERN which directed an intense beam of muon-neutrinos (νμ) to two exper- key questions in the fi eld – ensuring that the “supreme challenge” rays: one from the positron’s annihilation and the other from the and Brookhaven were on their way to building proton synchrotrons iments, ICARUS and OPERA, in the Gran Sasso National Labora- of creating and detecting neutrinos will remain a worthwhile and neutron’s capture. that would have suffi cient energy and intensity to form beams of tory in Italy about 730 km away. CNGS followed the same principle exciting pursuit for the foreseeable future. The detector for Project Poltergeist contained 300 litres of liq- neutrinos via decays of pions and kaons produced when protons as CERN’s early record-breaking beam, this time with protons from uid scintillator with added cadmium chloride, viewed by 90 pho- strike a suitable target. The muons produced in the decays could the SPS. Following fi rst commissioning in 2006 (CERN Courier ● Further reading tomultiplier tubes, and was set up in 1953 at a new reactor at the be stopped by large amounts of shielding, allowing only neutrinos November 2006 p20), the facility ran for physics from 2008 to the H Bethe and R Peierls 1934 Nature 133 532.