The Magicians

by Marcus Chown

  A neutrino had very little chance of being stopped by a proton in an atom; the way to boost the chance was to put together lots of atoms. Reines estimated that, with a detector mass of about a tonne, it would be possible to detect a handful of neutrinos, but neither he nor Fermi had any idea of how to go about it.

  The fact that Fermi had not ridiculed his idea gave Reines confidence that detecting the neutrino was possible, but the problem was that he was just one man with an obsession. That changed when he flew to a meeting in Princeton, New Jersey. The plane had engine trouble and was forced to land in Kansas City. Travelling with him from New Mexico was a physicist called Clyde Cowan, who had worked with the British on radar in the Second World War and had arrived at Los Alamos in 1949. Although Reines and Cowan had been part of the same American bomb teams, they had never had a chance to talk properly. Now, as they strolled through the streets of Kansas City while waiting for their plane to be fixed, they hit it off.

  Their conversation quickly turned to fundamental physics and the question: What was the hardest experiment in the world? Both men agreed it was the detection of the neutrino. The fact that everyone thought it was impossible made it appealing to them, and they imagined the buzz of achieving something that everyone said could never be done. There and then, the two men decided to work together on detecting neutrinos. Reines had found his partner in crime.

  Back at Los Alamos, there was much enthusiasm for the venture, which resulted in the creation of a neutrino group in late 1951. Since the neutrino was a fleeting ghost that barely haunted the world of physical reality, the quest to detect it was christened ‘Project Poltergeist’.

  The way to snare a neutrino, as Bethe and Peierls had already realised, was via inverse beta decay. On rare occasions, an antineutrino interacted with a proton, creating a neutron and a positron in the process. The positron would quickly run into an electron, since electrons are ubiquitous in matter, and ‘annihilate’ with it. There would emerge two high-energy photons, or gamma rays, flying away in opposite directions. It was these gammas – proxies for the antineutrino – that Reines and Cowan intended to detect; proving the existence of the antiparticle would automatically prove the existence of the particle, in this case the neutrino.

  A year earlier, in 1950, several teams had discovered transparent liquids that emit flashes of light when a charged subatomic particle or gamma ray flies through them. The light flashes from such ‘liquid scintillators’ were weak, but could be boosted by placing ‘photomultipliers’ all around the scintillator, which converted the light into a measurable electrical signal.

  The neutrino detector envisioned by Reines and Cowan would incorporate tanks of liquid scintillator and a bath of water. The protons in the water would provide a large number of targets for the antineutrinos. The pair of gamma rays created in the interaction of an antineutrino and a proton would fly out through tanks of liquid scintillator on either side of the water bath, and photomultipliers arranged around each tank would detect them.

  The experiment was little more than a dull piece of plumbing, but the place where Reines and Cowan intended to locate it was anything but dull. And it was here that the fearless thinking and sheer chutzpah of the two physicists came to the fore. An atomic bomb creates a blisteringly hot fireball capable of erasing a city, and Reines and Cowan planned to place their detector a mere fifty metres from the centre of such an inferno.

  Nothing in the open could survive such a blast, but Reines and Cowan envisaged placing the neutrino detector in a vertical shaft ten feet in diameter and 150 feet deep. They would pump the air out of the shaft and, at the very instant the bomb went off, would let the detector drop. During its two-second fall, not only would it be shielded from the ferocity of the fireball by the surrounding ground, but because it was in free fall, it would be protected from the potentially catastrophic shockwave thundering through the earth. At the bottom of the shaft, the fall of the apparatus would be cushioned by a thick bed of foam rubber and feathers. Reines and Cowan intended to retrieve the detector several days later, when radiation levels would be low enough to risk a quick in-and-out foray.

  The extraordinary plan was granted approval by Norris Bradbury, director of Los Alamos, and work even started on digging the 150-foot-deep shaft to house the detector at the bomb test site in Nevada. But then, in the autumn of 1952, Jerome Kellogg, leader of the Los Alamos Physics Division, asked Reines and Cowan whether it might be possible to carry out the experiment with a nuclear reactor rather than a nuclear bomb. At first sight, it did not look promising – a nuclear reactor was a weaker source of neutrinos than a nuclear explosion by a factor of a thousand. However, when Reines and Cowan investigated it in detail, they were surprised to find that such a neutrino experiment was indeed possible.

  When an antineutrino hit a proton, it created not only a positron, which could be detected by the two gamma rays created by its annihilation, but a neutron. The key thing, Reines and Cowan realised, was to detect the neutron as well as the positron. The neutron could be detected by taking a substance like cadmium that acted like a neutron-sponge and adding it to the liquid scintillator. Each neutron would ricochet from nucleus to nucleus, before burying itself in a cadmium nucleus after about five millionths of a second and shedding its surplus energy as a gamma ray.

  The electronics connected to the photomultipliers could be arranged to register a response only to a signal consisting of two gamma rays (from the annihilation of the positron) followed by a single gamma ray (from the capture of the neutron). This ‘delayed coincidence’ signal was so distinctive that it was unlikely to be mimicked by any other particle process going on in the detector. The ability to reject other confusing signals would make the detection of neutrinos at a nuclear reactor possible, even though it was a far weaker source of the particles than a nuclear explosion.

  A nuclear reactor had other advantages over a nuclear explosion. Rather than providing an ultra-brief window of a second or two in which to detect neutrinos, it could be monitored continuously for weeks, months or even years. Furthermore, there was no risk of the experiment being incinerated or of harm to anyone who retrieved it from a radiation-scarred landscape.23

  In the early spring of 1953, Reines and Cowan’s team loaded their vehicles with a 300-litre neutrino detector, barrels of liquid scintillator and racks of electronics, and headed for the plutonium-producing reactor at the Hanford Engineer Works in Washington state. America’s newest and largest reactor was expected to generate the largest flux of antineutrinos. If it had been possible to see neutrinos with the naked eye, the reactor would have glowed like a second sun.

  But at Hanford, Project Poltergeist hit a show-stopper. The neutrons the team was looking for turned out to be not the only neutrons in town; it became clear that there were others coming from fissioning nuclei in the reactor core. To absorb them and stop them reaching the detector, the team built a thick wall of paraffin, borax and lead around their experiment. It worked, but then they encountered another problem: the neutrons from the reactor core were not the only source of a signal that mimicked the signal they were looking for. There was another source, and it came from space.

  Cosmic rays are high-energy nuclei created by exploding stars and other violent cosmic events. At the top of the Earth’s atmosphere, they slam into nuclei of atoms and create ‘secondary’ particles, which shower down through the atmosphere like a fine rain. The most penetrating of all the particles are ‘muons’, a form of heavy electron. Cosmic ray muons slammed into nuclei in the shield that Reines’ team had built around their experiment, creating sprays of neutrons. Unfortunately, these neutrons were ten times more abundant than those expected from the neutrons produced by neutrinos. ‘The lesson of our work was clear: it is easy to shield out the noise men make, but impossible to shut out the cosmos,’ said Cowan. ‘We felt we had the neutrino by the coattails, but our evidence would not stand up in court.’

  Reines and Cowan were disappointed, but not defea
ted. They at least knew that the technology they were using worked; all they needed was a nuclear reactor that was better shielded from the confusing signals from cosmic rays. Finally, they found one in P Reactor at the Savannah River Plant. By virtue of the fact that it was buried twelve metres deep in the ground, it was perfectly shielded from the menace from space. In November 1955, Project Poltergeist moved to South Carolina.

  Savannah River, South Carolina, 14 June 1956

  Reines drove past the sign a joker on the team had put up – ‘DANGER, DO NOT STAND CLOSE TO FENCE – HIGH NEUTRINO FLUX’ – and parked next to the empty truck that had brought a load of wet sawdust to the site.24 The control room, with its humming generator, was in a trailer dwarfed by the concrete hulk of the reactor. Cables snaked across the ground, carrying the electrical signals up from the scintillator tanks twelve metres below and creating a trip-hazard that Reines had to be careful to avoid. Inside, Cowan was sitting in front of a wall of oscilloscopes, switches and racks of glowing vacuum tubes, monitoring the output from the detector.

  With its team of almost a dozen and a mountain of accompanying equipment, the ten-tonne detector was the biggest physics experiment on the planet. Nobody before had dared carry out anything as complex, but nobody else had learnt their craft while testing the weapons of Armageddon or had at their disposal the financial resources, machine shops and technology of Los Alamos. This was big science. It was a vision of the future: one day much of physics would be done like this, in laboratories that spanned national borders, employed thousands of researchers and cost tens of billions of dollars.

  The final design for the Project Poltergeist apparatus was a kind of double-decker sandwich. Two layers of water with cadmium chloride added to it acted as the neutrino target, and these were interleaved with three layers of liquid scintillator. Positrons produced by neutrinos interacting with protons in the water would be detected almost immediately via back-to-back gamma rays in the adjacent scintillator tanks, and neutrons produced by the same neutrinos would reveal themselves five microseconds later via another burst of gamma rays in the same tanks.

  Project Poltergeist had been running for 1,371 hours. Not only was the gamma ray signal it had measured four times bigger than the background level, it was five times bigger when the reactor was switched on than when it was switched off. Every hour they detected three neutrinos.

  The possibility remained, however, that neutrons from nuclear fissions in the reactor were penetrating the eleven metres of concrete shielding around the reactor and creating spurious gamma rays in their experiment. So overnight, while Reines had slept, the rest of the team had piled bags of wet sawdust against the wall of the reactor. Their material of choice – a tribute to the cuisine of South Carolina – had actually been black-eyed peas, but wet sawdust was easier and cheaper to obtain in the required quantities.25

  If some of the gamma rays they were detecting were coming from neutrons produced by the reactor, the extra shielding of the wet sawdust should have stopped them, reducing the signal by a factor of ten. ‘Any change in the signal?’ asked Reines. Cowan, looking up from an oscilloscope, grinned. ‘No change.’ It was exactly what Reines had wanted to hear.

  Outside the trailer, the whole team had assembled: Richard Jones and Forrest Rice, who had installed the detectors and lead shielding; F. B. Harrison, the expert in large liquid scintillators; Austin McGuire, who had designed the tank farm containing the scintillator; Herald Kruse, who had been responsible for interpreting the oscilloscope traces; and Martin Warren, the gopher. All of them looked exhausted, but they were euphoric and pumped each other’s hands, slapping each other on the back. They had overcome the final hurdle and achieved the impossible. After five years of sweat and struggle, they had detected the elusive neutrino.26

  There remained only two things to do: send a telegram and then pack up their equipment and drive back to Los Alamos.

  *

  Pauli received the telegram on 14 June 1956: ‘We are happy to inform you that we have definitely detected neutrinos from fission fragments by observing the inverse beta decay of protons … Frederick Reines, Clyde Cowan.’

  The next day, Pauli replied from the Swiss Federal Institute of Technology in Zurich: ‘Frederick REINES, and Clyde COWAN, Box 1663, LOS ALAMOS, New Mexico. Thanks for message. Everything comes to him who knows how to wait. Pauli.’

  With the proof of the neutrino that he had predicted a quarter of a century earlier, Pauli well and truly joined the ranks of the magicians. Who in their most extravagantly wild dreams would have imagined something as insubstantial, ghostlike and downright weird as the neutrino? Pauli had predicted it for the sole reason that it was what the mathematical logic was telling him. The neutrino simply had to exist because, without it, radioactive beta decay made no sense at all.

  Pauli announced the discovery of the neutrino at a symposium at CERN, the European laboratory for particle physics near Geneva, the week after he received Reines’ telegram. Reines, in his Nobel Prize acceptance speech in 1995, would recount that Pauli celebrated with a case of champagne.27 That certainly made a good story since Pauli had, of course, bet a case of champagne that the neutrino would never be detected, though sadly it was not true.28

  Reines and Cowan, for their part, were rather more sober. Standing outside P Reactor in the South Carolina sunshine, they and their team celebrated their success not with flutes of champagne but with paper cups of Coca-Cola.29

  *

  For the neutrino, it was only the beginning of the story. Hold up your hand. About one hundred billion neutrinos pass through your thumbnail every second. Eight and a half minutes ago they were in the heart of the Sun. Solar neutrinos are produced in prodigious quantities by sunlight-generating nuclear reactions.

  Remarkably, Reines and Cowan’s team was not the only one with the temerity to attempt the impossible feat of detecting the neutrino; it was not even the only one to do it at the Savannah River Plant. In 1954, a team led by Raymond Davis, the American chemist and physicist, had installed a detector filled with 3,800 litres of cleaning fluid – carbon tetrachloride – in the basement of one of the nuclear reactors. The idea behind the detector had been suggested by Bruno Pontecorvo, a onetime colleague of Enrico Fermi’s who had defected to the Soviet Union. Occasionally, a neutrino would interact with a chlorine nucleus in the cleaning fluid, turning it into a nucleus of argon, a gas which could be easily separated. The amount collected would correspond to the number of neutrinos detected.

  Unfortunately, Davis had suffered similar problems to Reines and Cowan at Hanford. His detector was not sufficiently shielded from the confusing effect of cosmic rays and so he lost out in the race to detect the neutrino. But he was nothing if not persistent. In the mid-1960s, he located a detector of 400,000 litres of cleaning fluid 1.5 kilometres underground in the Homestake gold mine in Lead, South Dakota. His aim was to detect neutrinos from the core of the Sun, and incredibly, he succeeded, becoming the first person to see into the heart of a star.

  But there was a problem. Unexpectedly, Davis registered only between one third and a half of the neutrinos which were predicted by the theory of energy generation in the Sun. Was there something wrong with our understanding of the Sun, or with our understanding of neutrinos?

  Davis’s conundrum triggered a wave of other experiments to check his anomalous result, and he is credited with giving birth to the field of ‘neutrino astronomy’. The belief of pretty much everyone was that Davis was wrong. Contrary to expectations, however, the new experiments confirmed that there was indeed a shortfall in the number of neutrinos coming from the Sun.30

  The ‘solar neutrino puzzle’ had an extraordinary solution, which was eventually confirmed by the Sudbury Neutrino Observatory in Ontario, Canada: there are three types of neutrino. These are the electron neutrino; the muon neutrino, discovered in 1962 at Brookhaven, New York; and the tau neutrino, discovered in 2000 at Fermilab near Chicago. Nobody knows why nature has chosen to triplicate i
ts neutrinos – along with all its other basic building blocks, the quarks – but crucially, the observatory could detect all three types. What it showed, in 2006, was that there was no shortfall in neutrinos, as long as the numbers of the three types were added together.

  As early as 1957, Pontecorvo had suggested neutrinos might come in different types, or ‘flavours’, and that, flying through space on their way from the Sun to the Earth, they might morph from one type to another. Imagine a dog walking along a street and changing into a cat after one hundred metres, a rabbit after another one hundred metres and back into a dog after a further one hundred metres. Say, for some weird reason, your eyes can only spot dogs: you would see them only a third of the time. So it was with neutrinos. Davis’s experiment was sensitive only to electron neutrinos, but the neutrinos arriving at his detector in the Homestake mine were in the guise of electron neutrinos only a third of the time.31

  Such neutrino ‘oscillations’ have implications for the mass of the neutrino, which many had assumed was zero. According to Einstein’s special theory of relativity, only a massless particle like the photon can travel at the ultimate cosmic speed limit – the speed of light – and, for such a particle, relativity predicts that time slows to a standstill. The photon cannot therefore change since change is something that can only happen in time. However, the neutrino emphatically does change, oscillating between its three flavours. The implication is that it must travel slower than the speed of light and therefore have a mass.32

  The mass of the neutrino is, not surprisingly, hard to measure. It appears to be at least 100,000 times smaller than that of the electron, which was formerly the lightest known subatomic particle. This suggests that neutrinos acquire their mass in a different way to all the other fundamental particles, which get theirs by interacting with the ‘Higgs field’ (see chapter ‘The god of small things’). The Higgs is a key component of the Standard Model of particle physics, a quantum description of nature’s three non-gravitational forces. Although very successful, the Standard Model fails to predict the masses of the fundamental particles or the relative strengths of the fundamental forces and is widely believed to be an approximation of a deeper, more satisfactory theory. The hope among physicists is that if they can understand how the neutrino gets its mass, they might gain important clues about this elusive ‘theory of everything’.