The Telescope in the Ice Read online

  When Pontecorvo arrived in America, his fellow via Panispernan Emilio Segrè found him a job in the oil industry in Tulsa, Oklahoma, where he used his knowledge of neutron scattering to invent several methods for prospecting for radioactive materials, including uranium. This had strategic implications, since the most difficult part of actually making a fission bomb is to produce a so-called critical mass of weapons-grade uranium or plutonium. The lion’s share of the material resources in the Manhattan Project was devoted to this task.

  In 1943, Pontecorvo took a position at a research laboratory at McGill University in Montreal that was an arm of Tube Alloys, the secret Anglo-Canadian nuclear program, and put his inventions to use. The Brits and Canadians being U.S. allies, of course, Tube Alloys was effectively an arm of the Manhattan Project. At McGill, Pontecorvo also played a central role in designing what was probably the most advanced nuclear reactor in the world at the time, the NRX (Nuclear Reactor X) heavy water reactor in Chalk River, Ontario.

  During a breezy, anecdotal talk that he gave in Paris in 1982 on “The Infancy and Youth of Neutrino Physics,” Pontecorvo recalled that it had occurred to him in the mid-1940s “that the appearance of powerful nuclear reactors made free neutrino detecting a perfectly decent occupation.” In May 1945, a few months before the first manmade nuclear explosion, Trinity, took place in the New Mexican desert, he proposed the first experimental method for detecting the particle in a technical report for the Chalk River Laboratory, which remained classified for four years, probably owing to several appearances of the word pile.

  The basic idea was to bombard a solution of some well-chosen substance with neutrinos, which would interact via inverse beta decay with a vanishingly small fraction of the dissolved nuclei and transform them into a new, radioactive substance that could be separated out of the solution and quantified. Each transmuted nucleus would represent the product of one inverse beta decay event and thus be the clear signature of the death of a free neutrino. Scanning through the known radioisotopes, Pontecorvo found chlorine to be “by far the best” target nucleus, because it would transform into a radioisotope of argon, an inert noble gas that could easily be separated out. Another advantage was that this isotope has the relatively long half-life of thirty-five days (reverting back to chlorine via positron beta decay), so there is no great hurry in carrying out the separation. It can be done periodically, and the amount produced since the previous separation measured with a Geiger counter.

  In his now-legendary report, Pontecorvo alluded to three potential neutrino sources, “a powerful reactor [which he saw as the most promising], a concentrate of radioelement(s) extracted from a reactor and … the Sun.”

  In 1939, Hans Bethe, one of the pair that had previously suggested that the neutrino was effectively undetectable, had produced a general theory of energy production in stars that showed that the Sun should be an incredibly bright neutrino source. In a nutshell, all stars are powered by nuclear fusion, the binding together of small nuclei—single protons and alpha particles mostly—into larger nuclei, and virtually every step in the cycle produces neutrinos. (“The nuclear reactions that produce the neutrinos also cause the sun to shine,” observed the subsequent solar neutrino theorist John Bahcall.) The vast majority of the trillion or so neutrinos that are passing through your body as you read these words were born in our nearest star, and they do so day and night, of course, since they pass through the Earth as easily as a bullet passes through fog.

  * * *

  It should be obvious by now that good physicists are often decades ahead of their time, and this is doubly true in neutrino physics, where progress is so hard-won and long in coming. Even in the 1940s, long before it was detected, quite a lot was known about the particle, although this knowledge was seasoned with sensible doubt, since it had given up so few experimental facts. It was believed, for example, that nuclear reactors should emit antineutrinos, while the Sun should emit neutrinos.

  These were the early days of particle physics. Strange new creatures were cropping up almost yearly, mostly in cosmic ray instruments sitting on mountaintops. They were being classified into groups, and new rules about their behavior were beginning to emerge.

  In 1945, the same year in which Pontecorvo invented his detection method, the theorists Abraham Pais and Christian Møller coined the term lepton, from the Greek lep, meaning slight or slender, to characterize the lightest two known particles, the electron and the neutrino. Aside from their relatively light weight (at the time it was suspected that the neutrino weighed nothing at all) leptons are also distinguished from nucleons by being unaffected by the strong nuclear force; they feel only the weak force.

  One of the first new rules to emerge was the notion of lepton conservation. Consider the beta decay process that had led Pauli to invent the neutrino in the first place. When the unstable carbon-14 nucleus decays to nitrogen-14, a neutron changes to a proton, and a lepton, an electron, is produced. Since there were no leptons in the picture beforehand, lepton conservation dictates that the neutrino that is produced along with the electron must be an antilepton: an antineutrino. So the neutrino that Pauli invented was actually the antiparticle. And since this is the form of beta decay that takes place in nuclear reactors, they give off the antiparticle, too—in quantity.

  A neutron changing to a proton in the original form of beta decay. There are no leptons and there is no electric charge before the decay. After the decay, the negative charge of the electron offsets the positive charge of the proton, and the electron antineutrino offsets both the lepton number and the flavor of the electron.

  Now, on the face of things, Pontecorvo’s detection method should not be sensitive to the antiparticle. In his scheme, a stable chlorine-37 nucleus, with seventeen protons and twenty neutrons, is transformed into a radioactive argon-37 nucleus, with eighteen protons and nineteen neutrons: a neutron is changed to a proton. By electric charge conservation, the creation of this proton must be accompanied by the creation of an electron, and since the electron is matter as opposed to antimatter, the particle that initiated the reaction must have been matter as well: a neutrino. Since the Sun emits neutrinos, Pontecorvo’s method should be sensitive to those.

  But this is where Ettore Majorana’s rarified conjecture comes in. If he was right and the neutrino and antineutrino are identical, Pontecorvo’s method should be sensitive to both.

  * * *

  In 1945, a few months after the war ended, the Pontecorvo family moved to Chalk River to be near Dad’s new workplace in the reactor complex. At about the same time, the three Italians who had been working in hiding revealed their surprising news about the muon, and Pontecorvo’s interest was piqued. “That was indeed an intriguing particle,” he recalled in Paris many years later. “I found myself caught in an antidogmatic wind and I started to put lots of questions.” He set up a cosmic ray laboratory in Chalk River in collaboration with the Canadian physicist E. P. “Ted” Hincks, and over the next several years they made a series of discoveries that answered all of his questions and more.

  The upshot was that the muon turned out to be the third lepton. It has the same charge and spin as the electron, it is similarly affected by the weak but not the strong force; in fact, it has so much in common with its lighter cousin that it is often described as a heavy electron. It is unstable, with a lifetime of 2.2 millionths of a second, whence it decays into an electron and two other particles. At that early stage of the game, Pontecorvo guessed correctly that these would be a neutrino and an antineutrino, and this led him to make another wise guess: that the neutrino might carry a sort of identity card that associated it with either the muon or the electron. “For people working on muons in the old times,” he recalled in Paris, “the question about different types of neutrinos has always been present.”

  If a single muon, which is a lepton, decayed into three leptons, and one of those was an electron, then lepton conservation said that the other two must cancel each other out: they must be a
lepton and an antilepton, in other words, a neutrino and antineutrino. But when a particle and its antiparticle find themselves in close proximity they usually annihilate and give off other particles. Since he and Hincks had found that the two uncharged products of muon decay don’t annihilate, Pontecorvo deduced that some hitherto unobserved quality must make them different and that it might have something to do with the difference between the muon and the electron.

  A muon decays into three particles. Before the decay, there is one lepton of muon flavor, with a negative electric charge. After the decay, the electron carries the electric charge, the muon neutrino carries the muon flavor, and the electron antineutrino offsets the flavor of the electron and the lepton number of either the electron or the muon neutrino. Thus lepton number, lepton flavor, and electric charge are conserved.

  Following this train of thought: to conserve “muon-ness,” which is now known as muon “flavor,” the new neutrino must be a muon neutrino, and to conserve electron flavor, which was zero before the decay, the antineutrino produced in concert with the new electron must be an electron antineutrino. And we can now say precisely what it was that Wolfgang Pauli intuited back in 1930: since an electron is produced in the original form of beta decay, the accompanying neutrino must be an electron antineutrino.

  To bring this all back to earth, or perhaps Antarctic ice, it so happens that flavor has important implications for neutrino astronomy. For a muon neutrino can initiate inverse beta decay in the same way that its electron counterpart can, with the important difference that it will give birth to a muon, rather than an electron. And a muon happens to be much more easy to detect as it flies through the ice than an electron is. Muon detection was the basic principle of operation for the Antarctic Muon And Neutrino Detector Array, aka AMANDA, and it is still the bread and butter for IceCube. The muon is the workhorse of neutrino astronomy.

  * * *

  In 1947, while Pontecorvo and Hincks were still pursuing these investigations, the tracks of a new particle, the pion, were revealed in photographic emulsions exposed on mountaintops in the Pyrenees and the Bolivian Andes. It was soon realized not only that this was the field particle Yukawa had postulated but also that the pion decayed into the muon—which explained why only the latter had been found at lower altitudes.

  The pion also plays a key role both in neutrino astronomy and in experimental neutrino physics, since it provides the most obvious mechanism in particle accelerators, either manmade or cosmic, for producing high-energy neutrinos. When a proton is accelerated in an electromagnetic field, either on Earth or in the cosmos, and then collides with some other particle, such as a photon or an atomic nucleus, it will give birth to a pion. If that pion is uncharged, it will decay into two gamma ray photons and back to a proton. If it’s charged, it can decay by two pathways: to a muon, a muon neutrino, and a neutron, or to an electron, three neutrinos, and a neutron. So the way to make a “neutrino factory” here on Earth is to direct a manmade proton beam at a target or “beam dump” that will generate pions, and manipulate the charged pion beam and its decay products in such a way as to produce a clean beam of neutrinos. Cosmic accelerators, such as active galactic nuclei, supernova remnants, and their siblings, are presumed to accelerate protons and other nuclei in their own ways, and these particles will collide with cosmic beam dumps to produce pions and, in turn, the cosmic high-energy neutrinos that IceCube is looking for.

  * * *

  Bruno Pontecorvo was juggling quite a few balls during these productive years. Not only were he and his Swedish wife, Marianne, raising a family, they also moved at least four times. In 1948, after turning down several offers from top universities in the United States and Italy, he accepted a senior position at the main national laboratory for applied nuclear research in England, the Atomic Energy Research Establishment in Harwell, and the family moved back across the Atlantic.

  By then, the most unwieldy of the balls he was keeping in the air was the suspicion surrounding the fact that he and Marianne were ardent communists. Like many Italian intellectuals, he had joined the communist party in 1936 with the advent of the Spanish Civil War. The couple had met in Paris when he was working with the Joliot-Curies, who were also active in the communist cause, and the Pontecorvos had worked against Nazism and Fascism during their years there. Bruno’s brother, Gillo, who would achieve international renown as a filmmaker (best known for The Battle of Algiers), was a member of the Italian communist party, and a cousin also held a high post in the party.

  The year after the Pontecorvos moved to England, the Soviet Union detonated its first atomic bomb, an exact copy of the American version, and in March 1950 the German physicist Klaus Fuchs, a socialist true believer who also worked at Harwell, was convicted of passing nuclear secrets to the Soviets. A wave of anti-communist hysteria swept the west. Over the summer, U.S. Senator Joseph McCarthy commenced his infamous campaign against the communist “fifth column.” All of this brought more pressure to bear on Bruno Pontecorvo.

  In September 1950, he, his wife, and their three young children took a holiday to Italy and quietly disappeared. About a month later, the British government was forced to admit that one of the top nuclear physicists in the country had most likely defected to the Soviets. It made front-page news in Britain.

  People who lived during that incendiary time still remember Pontecorvo as being a spy. He was lumped in with Fuchs, Julius and Ethel Rosenberg (who were sentenced to death in the United States about six months after his defection), Kim Philby, and others as one of the top turncoats of the Cold War era. When the Rosenbergs were convicted, Time ran an article under the title “SPIES: Worse than Murder” that placed him in the inner circle of evil and explained how his alleged crimes (for which they offered no evidence) fit into a global communist conspiracy.

  No evidence has ever been found that Pontecorvo was a spy. The contemporary science historian Simone Turchetti, who is probably the foremost expert on the “Pontecorvo Affair,” believes he was innocent. Recently released documents demonstrate that British government officials were aware of his innocence but didn’t disclose it because they were involved in delicate negotiations with the United States about the transfer of nuclear technology to Britain and the myth of his guilt served the agendas of powerful U.S. political factions and agencies, such as the FBI, which they had no good reason to resist. The witch hunts served many political ends, of course, and had no particular use for the truth.

  It was five years before Pontecorvo made his whereabouts known. In 1955, in a press conference at the leading nuclear and high-energy physics laboratory in the Soviet Union, the Joint Institute for Nuclear Research in Dubna, near Moscow, he claimed that he had left England because he was terrified of the witch hunts and “the pressure put on him by security forces during vetting.” He said “he had defected to correct the balance between East and West and that he had only ever worked on the peaceful uses of atomic energy.” After 1978, when the Soviets finally allowed him to travel to the west, he campaigned for nuclear disarmament.

  One can never know, of course, but Turchetti has found many documents that support Pontecorvo’s innocence, including records of meetings he called with his superiors in order to discuss his apprehensions before he made his fateful decision. He would certainly have been an asset to the Soviet nuclear energy and weapons programs—and perhaps he was, after he defected. It seems unlikely that he was beforehand.

  It was also a bad career move. For, in defecting, this world-class scientist left the vanguard of his field for a backwater. He would continue to make important theoretical contributions from behind the Iron Curtain, but the Soviet Union fell far behind the west in the development of accelerator technologies, which is where particle physics went for the rest of the century. Eleven people, all experimentalists, have received Nobel Prizes based on Pontecorvo’s theoretical work in neutrino physics. (Thus it is ironic that he was basically an experimentalist.) Had it been possible for him to remain in the west
—and had he lived long enough—it seems likely that he would have shared in at least one.

  He died in Dubna in 1993. In accordance with his will, half of his ashes were buried there and the other half in Rome.

  * * *

  If Wolfgang Pauli conceived of the neutrino and Enrico Fermi gave it life, then Bruno Pontecorvo gave it personality. His guess that it came in different flavors turned out be correct: the muon neutrino was discovered in 1962. And in 1958, behind the Iron Curtain, he also intuited what may be the particle’s weirdest and most ghostlike property: that it will change flavor, or “oscillate,” as it speeds along.

  There are now three charged leptons: the electron, the muon, and the tau particle, which was discovered in 1975. The tau is the heaviest of the three, weighing in at about 3,500 times the mass of the electron, and its neutrino was finally detected in 2001. So there are six leptons altogether, in three pairs, and each has an antiparticle.

  Oscillation, which will only occur if the neutrino has mass, means that an electron neutrino will change into, say, a muon neutrino, then perhaps a tau neutrino, then back into an electron neutrino, and so on, as it races along—something like watching your dog change into a cat and then an ocelot and then back into a dog in the course of his morning walk. Pontecorvo’s 1958 conjecture was proven true forty years after he made it and five years after he died, by an instrument very much in the tradition of AMANDA and IceCube. But that’s getting ahead of the story.

  3. From Poltergeist to Particle

  Neutrinos induce courage in theoreticians and perseverance in experimenters.

  —MAURICE GOLDHABER

  By the early 1950s, the stage was set for detecting the neutrino. The challenge was taken up by two men with very different personalities.