by Close, Frank
The NRX became critical on July 21, 1947; by 1949 it was the most powerful neutron flux in the Western world, a status it held for several years. By that time, however, Pontecorvo had indeed moved to Harwell, to work on the British reactor program. Why he chose this course is one of the questions that came up following his defection: Was it a personal choice or part of a clever strategy orchestrated by others? As we shall see, there were seemingly legitimate personal reasons for Pontecorvo’s decision. On the other hand, its timing also fits uncannily with events involving Nunn May and Fuchs, and some regard this as being more than a coincidence.
EIGHT
PHYSICS IN THE OPEN
1945–1948
IN 1945, WITH PEACE RESTORED, BRUNO PONTECORVO HAD SOME TIME to devote to pure science.
By the 1940s the fundamental building blocks of atoms had been identified as electrons, protons, and neutrons. In addition there was the neutrino, theoretically predicted by Wolfgang Pauli, which was likened to an electron but without electric charge or mass.
Although the idea of neutrinos had been around for two decades, and Enrico Fermi had incorporated them into his 1934 theory of beta radioactivity, no one had been able to establish their existence experimentally. It was now that Bruno began what turned into a lifelong quest: to understand this ghostly and enigmatic particle.
THE GHOSTLY NEUTRINO
Today, neutrinos are the focus of investigations both in particle physics and in a new branch of science: neutrino astronomy. Pontecorvo’s interest in the subject had begun while he was still a student, when he had learned about the theory of the neutrino and beta decay from his teacher and mentor, Enrico Fermi. According to Fermi’s theory, a nucleus undergoing beta decay emits a neutrino; however, when a neutrino hits an atomic nucleus, this transmutation can happen in reverse: the neutrino becomes a negatively charged electron, while the nucleus increases its positive charge. In effect, the atom moves one place forward in the periodic table, becoming an entirely new element, all due to the action of the neutrino.
Thus, like H. G. Wells’s Invisible Man jostling the crowd, a neutrino may reveal its presence by bumping into something. The problem was that, according to Fermi’s theory, the chance of this happening was so small that it would be extremely difficult to detect. Most people believed it to be impossible.
In 1945 Bruno began to think about neutrinos a great deal. He was ideally placed to make a breakthrough. Having set Fermi on the road to nuclear power in 1934, Bruno was now designing a nuclear reactor. These experiences in nuclear technology inspired a thought: if Fermi’s theory were correct, a uranium reactor should make over a million billion neutrinos each second.
In Pontecorvo’s opinion, the goal of finding the neutrino was “not out of the question.” Although there is almost no chance of capturing a neutrino, almost is not the same as none. An individual neutrino produced by beta decay may travel for several light-years without interruption, but when you have an intense source producing billions of neutrinos each second, one or two might occasionally get caught in the atomic net of a sophisticated detector. He believed that, with “modern experimental facilities” such as a nuclear reactor, it could be done.1
Bruno considered the requirements. He realized that, to capture elusive neutrinos, the target must be large, so its material would have to be cheap. A liquid was ideal, as its volume would be limited only by the size of the container. If the atoms resulting from neutrino capture were radioactive, their decays could be recorded by a suitable detector, giving proof of their transitory existence. The fact that they had existed at all would then be evidence that a neutrino had struck.
Cleaning fluid is cheap, and contains chlorine atoms in the form of carbon tetrachloride; sporadic hits by neutrinos will convert chlorine into argon, one atom at a time. Argon is chemically inert, and can be extracted simply by boiling the liquid. The argon atoms created by the neutrino collisions are mildly radioactive, and survive long enough for these metaphorical needles in a haystack to be extracted and identified. If you wait long enough, the amount of argon created will be large enough to be measured, and the neutrino’s existence will be confirmed. This, in a nutshell, is the basic strategy for pinning down the neutrino’s existence, as articulated by Bruno Pontecorvo in his publicly available report, written in 1946.
However, this report derives from an earlier document, written on May 21, 1945, which shows that Pontecorvo did not develop these ideas alone.2 First, the original 1945 paper credits the idea of using carbon tetrachloride to Jules Guéron. Guéron, a squat Frenchman with a chubby face and high forehead, was a chemist at Chalk River. He was also an expert in radioactivity. It was Guéron, too, who suggested that the production of radioactive argon atoms could be key to the endeavor.
Second, the pursuit of the neutrino appears to have inspired wide interest at the Chalk River laboratory. Guéron’s observation about radioactive argon inspired a further discussion between “the author and Dr. Frisch.” This led Bruno to conclude that argon’s advantage of being chemically inert made the carbon tetrachloride strategy “the most promising method according to Dr. Frisch and the writer.”3
Bruno estimated that several cubic meters of cleaning fluid would give him a reasonable chance of winning the lottery if the fluid was located near a nuclear reactor. A small quantity of radioactive argon atoms would be produced, their number revealed by the amount of radiation they emitted. From this, one could deduce how many neutrinos had struck. Thus the idea that the vicinity of a nuclear reactor would be a good place to search for neutrinos was original to Bruno; many details, however, were not. And the actual discovery of the neutrino—in 1956, by two American physicists, Frederick Reines and Clyde Cowan—did not use this technique, as the chlorine-argon method was not actually appropriate for the emissions from a reactor. No one knew then what we do today: reactors produce antineutrinos—the antimatter counterparts of neutrinos—rather than neutrinos themselves.
Finally, we come to the idea of solar neutrinos, for which Bruno subsequently became famous. In the body of his original paper, Pontecorvo had focused exclusively on the opportunities offered by a nuclear reactor; there is no mention of solar neutrinos at all. He had concluded that a reactor with just a little more power than the one being designed in Canada might produce enough neutrinos to give success. At the end of the paper, however, after Bruno’s signature, there is a footnote. This appendage is an afterthought, inspired by Maurice Pryce, a British theoretician at Chalk River.
Pryce pointed out to Bruno that if the sun is indeed powered by nuclear fusion, as astrophysicists theorized, it could be irradiating the earth with a neutrino flux of 10 billion neutrinos per square centimeter every second. Bruno Pontecorvo credits Maurice Pryce unambiguously with this suggestion: “Dr. Pryce pointed out to the author that the flux of neutrinos from the sun is quite considerable.”
Thus, the father of the solar neutrino idea is Maurice Pryce. However, at the time, he and Bruno dismissed it because the intensity of solar neutrinos at the earth’s surface would be “too low for an experiment of the type suggested.” They estimated that a flux a million times stronger than this would be required for success.
Such was the sensitivity to anything “nuclear” that this paper was immediately classified as secret, and remained so for two decades.4 On September 4, 1946, however, these attempts at secrecy became moot when Bruno gave a talk at a nuclear physics conference at Chalk River, which was subsequently published as a paper. It is this latter version that has entered the public record and become famous. As this document does not mention the source of the solar neutrino idea, or the roles of Guéron and Pryce, Pontecorvo has been credited with a string of ideas that actually originated elsewhere.5
IN 1948, AMERICAN CHEMIST RAY DAVIS JOINED BROOKHAVEN National Laboratory in Long Island, New York, a facility that specializes in the research of fundamental science. During the war Davis had become an expert on chemical explosives, and at its close he had joined the US
Atomic Energy Commission to work on radiochemistry—the chemistry of radioactive materials. Davis’s new boss at Brookhaven advised him to visit the library, read the research literature, and “choose a project that appeals to you.”
His good fortune, which determined his life’s work, was to find in the library an article about neutrinos.6 Davis had heard mention of this hypothetical particle, but that was the sum of his knowledge. Here was a chance to learn something. As he began to read the article, he quickly discovered that no one else was much wiser. He had stumbled upon a field that was wide open, and rich in problems. Then his excitement grew. The paper briefly mentioned that in 1946 Bruno Pontecorvo had suggested a method of finding the neutrino using chlorine and radiochemistry. Davis realized that this was right up his alley.
For Davis, Bruno’s proposed experiment seemed all too easy, and so it was. His first attempt at the Brookhaven reactor failed because the impacts of cosmic rays were so numerous that it was impossible to discern any faint signals of neutrinos coming from the reactor. In 1955 he built a large detector, using 4,000 liters of carbon tetrachloride, at the powerful reactor near the Savannah River in South Carolina; Davis dutifully shielded the detector from cosmic rays, but saw no sign of the neutrino. This eventually helped confirm that reactors produce antineutrinos, to which the chlorine-argon method is not sensitive, rather than neutrinos.7
Davis missed out on discovering the neutrino, but by 1956 the existence of the ephemeral particle had been confirmed, and the subtle difference between the processes at work in a nuclear reactor, which produce antineutrinos, and those in the sun, which produce neutrinos, were also understood. Thus, in 1959, Davis set out to use Bruno Pontecorvo’s method for a slightly different application: looking for neutrinos that have come from the sun.
Forty years would elapse before the quest for “solar neutrinos” was completed to everyone’s satisfaction, and Davis, at the age of eighty-seven, received his Nobel Prize having spent his career “doing just what I wanted and getting paid for it.”
WHO ORDERED THAT?
While at Chalk River, Pontecorvo had an insight that now forms a cornerstone of the Standard Model of particle physics. Its genesis involved cosmic radiation.
During the 1930s and 1940s, cosmic rays—high-energy particles from outer space—revealed varieties of matter previously unknown to scientists. For nuclear physicists, interested in the basic particles from which matter is built, cosmic rays became the new frontier. Earth’s upper atmosphere is being continuously bombarded by extraterrestrial radiation, including the nuclei of elements, many of which were produced during the explosion of distant stars. When these rays hit the upper atmosphere their energy dissipates as they disrupt atoms in the air, creating showers of less powerful subatomic particles. These finally reach the ground as a gentle “rain,” which is interesting to scientists but poses no real hazard to humans.
Among the debris produced in collisions between the primary rays and the atmosphere were new forms of matter, previously unknown. The first example of antimatter—the positron, or antielectron—was discovered in cosmic rays as early as 1932. Other discoveries included the muon in 1937, as well as the so-called “strange” particles and the pion, both in 1947.
Electromagnetic forces give rise to radiation, such as light, which in quantum theory occurs in staccato bundles—the particles known as photons. In 1935, Japanese theorist Hideki Yukawa predicted that a similar phenomenon should arise from the strong nuclear force that holds atomic nuclei together; agitate an atomic nucleus violently and, under certain circumstances, it might radiate energy in the form of the particles later known as pions.
Within two years, cosmic rays had revealed novel particles that seemed to confirm Yukawa’s theory. This discovery formed the frontier of fundamental physics as World War II began, and was still the frontier as physicists, including Bruno Pontecorvo, took up open science again in 1945. The cosmic rays seemed to have completed the fundamental understanding of atomic nuclei and particles.
However, scientists soon realized that they had caught the wrong suspect. Three young Italians, hiding from the Germans in Rome toward the end of the war, had set up a makeshift laboratory in a basement.8 They used an array of Geiger counters to reveal the passage of cosmic rays; their hope was to measure the lifetime of Yukawa’s novel particles. Yukawa’s theory predicted that the negatively charged versions should be attracted by the positively charged nuclei of atoms, and thus would be captured and absorbed by the strong force before having a chance to decay. Positively charged particles, by contrast, should be repelled and then decay. The Italians’ experiment succeeded, but with a surprise: they saw both negative and positive versions decay. This implied that the novel particle had no affinity for the atomic nucleus, and so could not be the pion that Yukawa had predicted. The eventual discovery of the pion in 1947 completed the understanding of the powerful forces of the nucleus, but left scientists with an enigma: What was this other particle that had been discovered in 1937?
To distinguish it from the pion, physicists gave it a name: muon. A name is a form of classification, and provides a sense of control perhaps, but is not an explanation. The discovery of the pion now thrust the question of the muon’s identity to center stage. This seems to have been the moment when Bruno shifted his focus from nuclear reactors to cosmic rays and the nature of the muon.
As we have seen, Bruno was familiar with the phenomenon of beta radioactivity, which causes the transmutation of elements in nuclear reactors, and, we now know, in stars. Nature also plays this sequence in reverse: it is possible for a proton in a nucleus to capture an electron. This is known as the “inverse beta process.” Bruno noticed that an atomic nucleus captures a muon in a similar fashion, and took the brave step of assuming this similarity to be significant. His conclusion: the muon is a heavier version of the electron. “Who ordered that?” physicist Isadore Rabi famously exclaimed, and more than thirty years would pass before the beginnings of an answer emerged.
A first step toward answering Rabi’s question was to find how muons are born. Experiments quickly established that the muon is produced when pions decay. Bruno proposed that this process is fundamentally analogous to ordinary beta radioactivity. He then turned his attention to how muons decay. Unlike electrons, which are stable, muons only live for about a millionth of a second before decaying into other particles. Bruno addressed the question of what the resulting particles might be. An electron is the only electrically charged particle that is lighter than the muon, and because the muon is electrically charged, the only way that the electric charge can survive when the muon decays is if an electron is created. To balance the total energy and momentum in the transmutation, one or more other particles must be created also. The most likely candidates were a single photon, some previously unknown massive particle, or a pair of neutrinos.9 If a muon was simply a heavy form of an electron, the muon should be able to shed energy by radiating a photon, thereby converting to an electron and a photon. If the muon’s relationship to the electron was more subtle, however, this decay would be very rare or absent. To decide between these alternatives, he devised an experimental test.
If a muon decayed into an electron and a photon, the latter pair would emerge back-to-back, with each carrying a specific amount of energy. If the decay produced an electron and two neutrinos, however, the energy could be shared among the particles in a variety of ways. So Bruno proposed a test: measure the energy of the electron in hundreds of examples of muon decay, and determine whether its energy is always the same (suggesting the presence of a photon), or whether it has a range of values (suggesting the alternative). He completed a paper that laid out this proposal in June 1947, and it was published later that year.10
Having proposed the idea, Bruno then began to set up the experiment, along with his colleague Ted Hincks. The basic idea was to have the electrons hit a sheet of material that would absorb them. The more energy an electron has, the more material it can pass t
hrough. So they measured the quantity of electrons that managed to penetrate the material, to see how this varied with its thickness. If the decay of muons always produced electrons with the same energy, as in the electron-photon scenario, the quantity of electrons getting through the absorber would suddenly drop when its thickness reached a critical level. However, the quantity would change gradually if the electrons’ energies covered a range of values, as would be the case if each electron were accompanied by two neutrinos.
Their experimental results appeared in 1948.11 Pontecorvo and Hincks were able to show that the muon does not decay into an electron and a single massive particle, but the question of whether the electron was accompanied by a photon or two neutrinos remained unresolved. In any event, Jack Steinberger, an American physicist working in Chicago, beat them to the finish line. Steinberger conducted a similar experiment, and published his findings in the same journal.12 He was the first to show that the scenario in which a muon decayed into an electron and two lightweight particles (neutrinos) fitted best with the experimental results.13 This would not be the last time that Steinberger beat Bruno to a crucial discovery.
THE UNIVERSAL WEAK FORCE
In the seventeenth century, Isaac Newton realized that the rise and fall of ocean tides, the orbits of the moon and planets, and the descent of apples to the ground were all manifestations of the universal force of gravity. In 1947, halfway through the twentieth century, Bruno Pontecorvo proposed that a variety of apparently diverse phenomena in atomic physics could be due to a universal “weak” force. These included the transmutation of the elements in beta decay, the production and decay of the muon, the instability of the pion, and the behavior of “strange particles.”
It would be disingenuous to push the comparison with Newton too far. In both cases, a variety of disparate phenomena were recognized to have a common fundamental origin. In the case of gravity, Newton both recognized its universal character and worked out the quantitative laws governing its behavior. Pontecorvo made a qualitative insight regarding the existence of the weak force, and performed experiments that helped establish its reality. However, another quarter of a century would pass before the quantitative theory of the weak force was established by others. Nonetheless, this time span itself bears witness to Pontecorvo’s foresight in recognizing the presence of the universal weak force, which choreographed the dance of these particles.
