The Telescope in the Ice Page 7
One was a self-effacing, painstaking, and as events would show, exceedingly patient man by the name of Ray Davis, a physical chemist at Brookhaven National Laboratory on Long Island, where the stated mission was to explore “peaceful uses of the atom.” In those good old days scientists were actually encouraged to go exploring. When Davis arrived and asked the chairman of the chemistry department what he ought to do, he was told to go figure it out himself.
Off he went to the library, and a lifelong fascination with neutrinos was born. For the first few years, he worked on an indirect method for detecting the particle, and in 1951 he began working on a practical realization of Pontecorvo’s direct radiochemical method.
Davis’s stated goals for his first set of experiments were to try to detect neutrinos coming from the Sun and to see whether or not “reactor” antineutrinos would interact in the same way as the solar variety. Now, the radiochemical method consists of sweeping a small number of argon atoms from a large tank of target liquid sometime after any neutrino interactions have occurred, so it does not reveal the direction of the incoming neutrinos. Thus Davis’s instruments could not be called telescopes. Nevertheless, by focusing on the Sun he was embarking on the very first experiment in neutrino astronomy.
He chose simple dry cleaning fluid as his target liquid, perchloroethylene, a compound that contains four chlorine atoms, and he built two detectors that were relatively large for the time, one employing 200 liters of the stuff (science-speak for a 55-gallon oil drum) and the other 3,900 liters or a thousand gallons.
Employing the High Flux Research Reactor at Brookhaven as what he hoped would be an antineutrino source, he measured a noticeable signal with the larger instrument, which he attributed not to neutrinos but to stray protons from the reactor. He then buried the tank about nineteen feet deep, a good distance from the reactor, hoping to shield it from cosmic rays, and the signal disappeared. This second step, incidentally, initiated the peculiar tradition that survives to this day of locating neutrino detectors and telescopes in mineshafts, tunnels, and the like, in unexpected and remote locations, often in the mountains.
Davis then carried on in the august tradition of cosmic ray physics by taking his smaller instrument to the top of 14,265-foot Mt. Evans in Colorado, where he measured another spurious signal that he attributed to cosmic rays. He was, as I say, a painstaking and cautious man. When he finally published his results in 1955, he made no claim to have detected neutrinos, anti- or otherwise, although he did use his null result along with the admittedly low sensitivity of his instrument to provide an upper limit for the neutrino brightness of the Sun. (Limits are an important matter in physics, as we shall see: If you don’t see something, but you know how sensitive your instrument is, you can say that whatever source you’re examining isn’t bright enough to give you a signal, and this can tell you something about its physics.) He would later tell the story of how one of the paper’s reviewers was unimpressed by his upper limit, because his experiment was too insensitive to have any bearing at all on the question of the existence of neutrinos. The reviewer had illustrated his point by observing that “one would not write a scientific paper describing an experiment in which an experimenter stood on a mountain and reached for the moon, and concluded that the moon was more than eight feet from the top of the mountain.” Indeed, Davis’s own results would later demonstrate that his first limit was high by a factor of about 15,000—or, in plain English, that his instrument was 15,000 times too insensitive to detect neutrinos from the Sun.
He went back to the drawing board.
* * *
The other man to take up the challenge was a physicist about four years younger than Davis, named Fred Reines. Reines had started out as a theorist. He had earned his doctorate in 1944 with a thesis on “The Liquid Drop Model of Nuclear Fission.” This being a timely topic (publication was withheld until after the war), he was recruited directly from school to the Manhattan Project in Los Alamos, too late to contribute significantly to the bombs that were used in the war. He remained at Los Alamos afterward, participating in many of the bomb tests that took place in the South Pacific, and eventually rose to the level of director for the Operation Greenhouse tests on the Eniwetok Atoll, which prepared the ground for the hydrogen or fusion bomb.
In 1951, tired of such mission-oriented work and yearning for something more fundamental, he asked his boss, who was also open-minded, if he could take a sabbatical in order to “ponder.” Reines later “moved to a stark empty office, staring at a blank pad for several months searching for a meaningful question worthy of a life’s work. It was a very difficult time. The months passed and all I could dredge up out of the subconscious was the possible utility of a bomb for the direct detection of neutrinos.” Enrico Fermi happened to be visiting Los Alamos that summer, so Reines “went down the hall, knocked timidly on the door and said, ‘I’d like to talk to you a few minutes about the possibility of neutrino detection.’ [Fermi] was very pleasant, and said, ‘Well, tell me what’s on your mind?’ I said, ‘First off as to the source, I think that the bomb is best.’ After a moment’s thought he said, ‘Yes, the bomb is the best source.’ So far, so good! Then I said, ‘But one needs a detector which is so big. I don’t know how to make such a detector.’ He thought about it some and said he didn’t either. Coming from the Master that was very crushing.” Reines returned to his blank pad.
Several months later, he and a Los Alamos colleague named Clyde Cowan were flying east on business when their plane developed engine trouble and they were forced to land in Kansas City. Finding themselves at loose ends for several hours while the engine was being repaired, they batted around ideas about challenging problems and finally agreed to “work on the neutrino.” Cowan “knew as little about the neutrino as I did,” wrote Reines, “but he was a good experimentalist with a sense of derring-do. So we shook hands and got off to working on neutrinos.”
They actually did design a detector for use with a bomb. The plan was to suspend it in an underground vertical shaft, pumped free of air, about two hundred feet from the bomb tower, and release it as the bomb was detonated, so that it would be falling in a vacuum and therefore unaffected as the shock wave passed through. After landing on a bed of feathers and foam rubber, it would begin detecting the copious antineutrinos emitted by the numerous fission byproducts produced by the explosion. A few days later, when the surface radioactivity had diminished enough, the detector would be dug up and the results would be read from a recorder.
This audacious concept never “flew,” however. When they presented it at a seminar at Los Alamos, a colleague suggested they replace the bomb with a reactor. They swiftly developed a new concept, and queried Fermi again, by letter. This time the master was more optimistic: “Certainly your new method should be much simpler to carry out and have the great advantage that the measurement can be repeated any number of times.… I do not know of any reason why it should not work.”
Their method was entirely different from Pontecorvo’s and had the added advantage of aiming specifically at the antineutrinos that were expected to emerge from reactors.
Their detector was arranged in the manner of a club sandwich, with two “meat” or “target” layers and three “bread” or “detection” layers, one in the middle of the meat layers and the other two on the top and bottom. The target layers were tanks of cadmium chloride dissolved in water, and the detection layers were tanks of liquid scintillation material, monitored by light detectors. (Scintillation materials give off light when charged particles or gamma rays pass through them.)
Their method was based on the inverse of the Joliot-Curies’ beta decay process involving the positron: an antineutrino from the reactor collides with a free proton in the water of the target layer, which changes to a neutron and ejects a positron. This results in two flashes of light. The first comes almost immediately, as the positron finds a nearby electron and they mutually annihilate, sending off two gamma ray photons in exactly oppos
ite directions. This is why the target layers are each sandwiched by two detection layers: the annihilation event will light up the two adjacent scintillation tanks simultaneously. The newborn neutron then stumbles around in the water of the target tank for about five millionths of a second before being captured by one of the dissolved cadmium nuclei, and this gives off a second flash of gamma rays, which lights up the same two scintillation tanks. The five-microsecond delay time acts as a signature for the neutrino interaction, which helps distinguish it from the background created by the charged cosmic ray particles that will inevitably rain through the detector and stray neutrons and protons from the reactor.
Cowan and Reines ran their first set of experiments in the early spring of 1953 at a reactor in Hanford, Washington, that had been used to produce plutonium for the bomb that was dropped on Nagasaki. Although they were plagued by a high cosmic ray background, they observed an increase in signal when the reactor was on as opposed to off, and after a few months convinced themselves that the increase was real. In November, they announced the “probable” detection of the free neutrino. This was a bit of a stretch, and one guesses that Reines was the one who pushed for it. He was far more aggressive than Cowan, who was modest and devoutly Catholic, and overshadowed him all through their collaboration—as he did most of the people he worked with. Cowan later wrote that the “evidence would not yet [have stood] up in court” and reflected that only in retrospect did it appear “genuine.”
It was big news, nevertheless, and it soon filtered across the Atlantic to the man who had invented the particle more than two decades earlier.
A young post-doc who was working with Pauli at the time, one William Barker, writes that when word reached Zürich, a group of friends and faithfuls joined the great man in a walk up the Üetliberg, a hill near the city, for a celebratory dinner. “On the way down, [Konrad] Bleuler and I noticed that Pauli was a little wobbly from the red wine we had at dinner. (He had graciously responded to many individual toasts.) Bleuler said to me: ‘Take his left arm—I’ll take his right arm, we can’t afford to lose him now.’”
This celebration was a bit premature, as there were real problems with the experiment, but Pauli didn’t need much of an excuse to throw a party, generally speaking.
* * *
Cowan and Reines went back to Los Alamos to improve their detector, and soon received a tip from the theorist John Archibald Wheeler that the most powerful reactor in the world was nearing completion at the Savannah River nuclear reservation in Aiken, South Carolina. In the fall of 1955, they caravanned with their families across the country to run their second set of experiments. There was a summer camp feeling in the air, since they had shared the tip with Ray Davis and he was running his second series of experiments side-by-side with theirs. Davis came up empty again, but Cowan and Reines struck gold.
The new reactor produced many more neutrinos, and their improved methods also helped tamp down the various backgrounds. By early June 1956, they had what they felt was a definitive result. To give a sense of the neutrino’s bashfulness, they estimated that the reactor was sending about twelve trillion electron antineutrinos through each square centimeter of their detector every second, but they detected only about three inverse beta decays per hour.
“We were done,” Cowan wrote. “The experience of knowing a fact new to mankind and knowing it for a while all alone is an unforgettable one. The neutrino existed as an objective, demonstrable fact of nature. The great laws of conservation stood firm. And our small group had had the privilege of sharing in the work that made them so.”
This time they felt confident enough to notify Pauli directly. On the fourteenth of June, they sent a telegram to Zürich:
We are happy to inform you that we have definitely detected neutrinos from fission fragments by observing inverse beta decay of protons. Observed cross section agrees well with expected six times ten to minus forty-four square centimeters.
It had to be forwarded to Pauli, who was attending a meeting at CERN, the European accelerator laboratory on the outskirts of Geneva. When he received this definitive announcement of his brainchild’s birth, twenty-six years after he had conceived of it as “a desperate remedy” for the crisis in beta decay, he interrupted the proceedings to read the telegram aloud and make a few impromptu remarks. He then responded to Cowan and Reines by night letter, a less expensive form of overnight telegram, quoting a Chinese proverb: “Thanks for message. Everything comes to him who knows how to wait.” But the Pauli effect may have been at work again, for the letter never arrived! A copy was finally sent to Reines thirty years later by Charles Enz, Pauli’s last assistant.
* * *
There remained the small matter of the bet with Walter Baade, involving the case of champagne. At a meeting on neutrinos that took place at the Royal Society, London, in 1967, astronomer Fred Hoyle remarked that Pauli “paid up—as I happen to know since I drank some of the payment.” Pauli, true to form, drank his share as well.
* * *
One would think that a discovery of this magnitude would be an occasion for unreserved celebration, but the little particle had other ideas in mind. As Charles Enz put it, “Several experiments [soon] discovered a birth defect.”
Cowan and Reines probably had detected the neutrino, but they had overreached again in publishing a hard number for the so-called cross-section. A cross-section gives a measure of the likelihood that an interaction involving a collision or some similar sort of interaction will occur. It has the dimensions of an area. One way to get a grip on the idea is to think of a window made of glass hard enough that if a kid throws a baseball at it, it will break only one in ten times. The cross-section for hitting the window will be its area, pure and simple, while the cross-section for breaking it will be a tenth of that.
Aside from publishing the number—6 × 10−44 square centimeters, as in the telegram to Pauli—they had gone a step further and claimed that this value was “within 5 percent” of the theoretical prediction for the cross-section, with an error of plus or minus about 10 percent. This put the theoretical value comfortably inside their experimental range.
Even as they were savoring the knowledge of their new discovery “all alone,” however, two Chinese American theorists on the east coast, Tsung-Dao Lee from Columbia University and Chen-Ning Yang from the Institute for Advanced Study, were beginning to suspect that the neutrino (or, more precisely, the weak force) might have a surprising quality that would increase the theoretical value by a factor of two. When Reines became aware of their idea, he stuck to his guns and stoutly defended his and Cowan’s analysis. Within six months, Lee and Yang’s suspicion was confirmed by other experiments and the discrepancy with the Savannah River number became difficult to ignore. Reviewing their methods, Cowan and Reines then realized that they had “grossly overestimated the detection efficiency.” In 1958, they carried out a third series of tests with methods improved yet again and came up with a number almost twice the original, snapping into line with the new theory.
But the damage had been done. Reines’s initial defensiveness, along with what might have seemed a habit of coming up with numbers to match the theory of the day, had induced a certain mistrust, and in some cases dislike, that persisted for decades. There were even those who suspected that he and his partner hadn’t actually detected the neutrino. And Reines’s behavior in subsequent years did nothing to defuse the controversy. It was four decades before half the Nobel Prize in Physics was awarded for the discovery, by which time Cowan had died, so that Reines received their share of the prize alone. It might be fair to say that his aggressiveness in staking claims and his combativeness in defending them robbed his self-effacing partner of a Nobel Prize.
Reines, who died in 1998, three years after receiving the prize, was “a man of imposing physical stature” and outsized character as well. John Wheeler once described him as “talented in both theory and experiment, a bear of a man given to thinking big about nearly impossible pr
oblems as he paced up and down in his oversized shoes.” He was undoubtedly one of the great experimentalists of the twentieth century, and his restless footprints wind through all the choicest glens in neutrino physics and astronomy—as we’ll see. He had a great love of poetry, wrote poems himself, and was talented enough as a baritone to have faced a career choice between opera and physics in his youth. Later in life, he sang in the Shaw Chorale with the Cleveland Symphony Orchestra under the direction of the composer and the legendary conductor George Szell. John Learned, one of the cofounders of DUMAND, who is a singer himself, says that “Fred’s voice was deep and rich and just much better than most mortals.” But Reines had a dark side as well. He was extremely competitive, even with his own students, whom he rarely supported, and he earned numerous enemies over the years.
Learned remembers Reines telling him that he and Cowan weren’t “trying to measure any physical parameters” in their first experiment at Savannah River, “only to show that they had found the elusive neutrino.” “With a little humility and openness,” Learned adds, “this could have all been avoided.”
* * *
The writer-physicist Jeremy Bernstein refers to the year and a half following Lee and Yang’s conjecture as the Glorious Revolution. (At the time, some referred to it with less political correctness as the Chinese Revolution.) It shook physics as it hadn’t been shaken since the discovery of fission. The two theorists had realized that the weak force—and the neutrino in turn—might violate one of the most hallowed laws of physics: the law of mirror symmetry, which says that a mirror image of any physical system should behave exactly as the real thing. The technical term for this is parity.
What prompted Lee and Yang to call parity into question was the behavior of a new breed of particle that had recently turned up in cosmic ray showers, with a hitherto unobserved quality that was eventually named strangeness. Two of these strange beasts, the theta (θ) and the tau (τ), appeared to be identical—they had every “personal” quality in common, such as mass, spin, and lifetime—except that the theta decayed into two pions, while the tau decayed into three. “Physicists would have been happy to put the theta and the tau down as identical,”* writes Bernstein, except that this would have violated parity conservation, a law that “was not to be tampered with lightly.”