Strange and Contrary Particles
On January 21, 1954 the operating crew began the first pumpdown of the Bevatron tank. It took a day to bring the huge system to 10e-1 mm Hg, four days to reach 5x10e-5, another four to hit 3x10e-5. Many tiny holes were found, mostly in the welds and some too small to deflate a soap bubble. The second pumpdown, lasting 16 days, reached 1.5x10e-5 mm Hg. Four days into the evacuation, on February 2, the crew injected a pulse from the linac and led it around the tank. Two weeks later the first acceleration was achieved, for a few milliseconds; on April 1 the beam reached 6.1 BeV; a week later a generator shorted, and the great machine went down for two months.
|A Cockroft-Walton accelerator (right) fed protons to an Alvarez linac (center) for injection into the Bevatron at 10 MeV.|
When the beam came back, Lofgren's crew worked to increase its strength and reliability. By October 1954 intensity had reached 10e10 protons/pulse, a hundred thousand times what it had been in June. The machine worked for a little over a third of its scheduled 55 hours/week, and of that third about a third went to physics research. Beginning in November, running time rose to two-thirds of scheduled time except for protracted shutdowns, which averaged about a week per quarter. The beam could deliver 11 pulses a minute at the redesign energy of 6.2 BeV. The Bevatron, though still temperamental, had become available for research.
Most of the experiments performed between November 1954 and September 1955 concerned proton scattering, pion production and scattering, and the life history of the K meson. A mixed pion and kaon beam became available by the end of the year. With fast electronic detectors, experiments could be done on the relatively few kaons present, about one for every 10l protons incident on a polyethelene target in the Bevatron tank. That was enough to aggravate, and to help resolve, one of the oddest puzzles of high-energy physics. The puzzle had been set by cosmic-ray physicists. Since 1947 they had collected evidence for the existence of heavy mesons, each having about 1000 times the electron mass, and all quite different in manner of decay. When the Bevatron began to operate, six different sorts of K particles, as the mesons were called collectively, had been distinguished according to their descendants. The question arose whether physicists had to accept a half-dozen kaons as elementary, or whether the several sets of descendants represented not independent ancestors, but the several ways in which a single indecisive one might divide its estate of energy. The physicists who discussed the matter at the fifth Rochester Conference on High Energy Physics in 1955 decided that "the situation was neither clear nor were all the answers easily forthcoming through cosmic radiation investigations." Hence the great utility of the Bevatron's kaon beam.
To help resolve the problem, several groups at the Laboratory sought to determine the masses and half lives of the several K particles or modes. Gerson Goldhaber of Segre's group and his associates exposed an emulsion stack to a stream of particles from a target in the Bevatron beam and examined the tracks of kaons that came to rest in the emulsion. They and a group led by R. W. Birge confirmed that all kaons had about the same mass, including the so-called tau meson, which decayed into three pions. Furthermore, they found that the lifetimes of the kaons, from creation in the target to decay in the emulsion, were about the same for all sorts of K+ mesons, except for tau, which they did not determine. Alvarez and his students confirmed the last finding another way, by detecting kaons and their products electronically; and, together with Sulamith Goldhaber, he made it plausible that the T+ lived about as long as the other K+ mesons.
The confluence of these vital statistics strengthened the view that there is one sort of kaon with several ways of disappearing. But a weighty objection confronted this conclusion. One of the K+ forms, called theta, disintegrates into two pions. As Richard Dalitz demonstrated, if theta and T are the same, its two different decay modes, to two or to three pions, cannot both conserve parity. That nature makes no distinction in the parity, or mirror symmetry, in such decays was then firmly believed. Another puzzle confirmed at Berkeley: the behavior of K+ mesons differs strikingly from that of K mesons. The positive form never, and the negative form often, provoke nuclear reactions giving off particles heavier than protons (hyperons). Yet both sorts of mesons have the same mass and lifetime.
Both these puzzles -- the amazing similarity between T and theta, and the equally odd dissimilarity between K+ and K- -- helped prompt profound contributions to theory. Murray Gell-Mann and others introduced a new quantum number, hypercharge or strangeness, the selection rules of which prevented K+ from participating in the sort of nuclear debauch the K- enjoyed. Berkeley emulsion groups played a part in confirming Gell-Mann's systematics. As for the likeness of T and theta, T. D. Lee and C. N. Yang declared it an identity and sacrificed parity in certain "weak interactions" involving mesons and hyperons.
The pursuit of kaons, though exciting and rewarding, had an air of deja vu: once again a great accelerator made possible the detailed study of particles first found in cosmic rays. Another quest beckoned, the detection of a particle of fundamental importance then not yet found among nature's products. Several groups began to look for the antiproton early in 1955. Two hoped for a quick victory using detectors that had worked well for mesons. One, under Chaim Richman, stuck emulsions in a beam of negative particles from a metal target. Another, under Wilson Powell, used a cloud chamber. They both hoped to find evidence of the end of the career of antiprotons in annihilation explosions. They found nothing.
Another alternative was to detect the negative proton directly by evaluating its momentum and velocity and thence deducing its mass. Lofgren's group had placed pop-up targets in the magnet aperture that could be thrust abruptly into the beam. Negatively charged shrapnel from the collision, primarily pions, came through a window in one of the straight sections of the Bevatron and thence through a bending magnet, which operated on the derived beam as a prism does on light. Segre's group built a system of three magnetic quadrupole lenses to focus whatever antiprotons they might catch onto a suitable set of detectors. The other group aiming at direct detection, Lofgren's, also used the Segre group's magnetic analyzer, which selected for a momentum of 1.19 BeV/c to within two percent. Concurrently, a team in Segre's group led by Goldhaber sought evidence of the annihilation of antiprotons in an emulsion stack inserted in the analyzed beam.
Two methods of finding the velocity of particles in the analyzed beam presented themselves. One was to time their flight between encounters with two scintillation devices. The other used a series of Cerenkov counters that could determine v/c (v the particle velocity) to within a few percent. The Laboratory had pioneered the development of these agile counters. In unusually fruitful thesis work, completed in 1951, R. L. Mather had designed the world's first Cerenkov detector for protons. The accuracy with which he could determine velocity recommended his system to Segre, who was one of his advisors; together they used it to improve the range-energy estimates for the evaluation of the speeds of charged particles in emulsions and cloud chambers. The Cerenkov counter came into common use at the Laboratory to discriminate between particles with like charges and equal momenta but different velocities.
|Antiproton detecting setup at the Bevatron, 1955.|
Lofgren's group used scintillation counters in coincidence merely to define the trajectory of a particle through their counter telescope. Three Cerenkov devices did the measuring work. Since the Segre group's magnetic analyzers would select antiprotons with velocities just above 3 c/4, the telescope contained one counter, of polystyrene, with a threshold at 0.76c. In order to discount the faster ¼- and K- mesons that pour through the analyzer, the telescope had Cerenkov devices of water and lucite with thresholds above 0.78c. With the scintillation and polystyrene counters in coincidence and the water and lucite counters in anticoincidence, the telescope would register negative protons of appropriate velocity and reject the accompanying noise. After passing the third scintillation counter the now identified antiproton would enter a block of lead glass, where, if all went badly for it, it would annihilate itself and an ordinary proton, producing a shower of secondary fast particles that would light up the lead glass counter. After testing their telescope on ordinary protons, Lofgren's group set it looking for negative ones at the end of July 1955.
|Day-to-day results of the antiproton experiment were displayed on a chalk-board in the Bevatron.|
By then Segre's group had constructed a more elaborate detecting system. Two scintillation counters fed a circuit that timed the passage of a charged particle between them, about 50 billionths of a second for an antiproton with v-3 c/4. Any one of the 50,000 negative pions that accompanied each antiproton traversed the same distance in even less time and could easily be distinguished from the heavier particle by the advanced coincidence circuitry designed by Clyde Wiegand. The arrangement also registered accidental coincidences when one meson tripped the second counter 50 billionths of a second after another had tripped the first. To rule out these spurious events a Cerenkov counter, with threshold above the velocity of the antiproton and below that of the mesons, was placed in anticoincidence with the scintillation counters. (It played the part of the water and lucite counters in the Lofgren group's telescope. A second Cerenkov counter, designed by Wiegand and Owen Chamberlain, had a mirror system that confined its counts to particles moving at between 0.75c and 0.78c. By the first of August the detecting system of the Segre group was ready for testing at the Bevatron.
Planning for that moment had started toward the end of 1954. Segre's group acknowledged "very useful suggestions" concerning focusing that were then contributed by Oreste Piccioni of Brookhaven, an expert on quadrupole magnets and beam extraction, who visited the Laboratory in December and the following January. In April 1955, on Alvarez's initiative, Piccioni was invited to join Lofgren's group, which he did in September, hoping to try "a pet experiment based on time-of-flight with counters.... discussed at length with Segre, Chamberlain, and the others [of Segre's group]." Meanwhile Segre and his colleagues had proceeded. When Piccioni arrived at Berkeley it was only a question whether the antiproton would be sighted first through the telescope of Lofgren's team, or through the more sensitive combination of counters of Segre's group, or in its death throes in their emulsion stack.
|The first annihilation star ("Faustina") of an antiproton found in film exposed by the Segre group, 1955.|
Time on the Bevatron did not come for the asking. The Laboratory physics division set priorities for the big machine to which its users conformed in negotiating schedules under Lofgren's diplomatic management. Various contingencies affected the implementation of the proposed schedule; the machine might not work, the appointed group might not be prepared, the preceding experiment might be prolonged. Log sheets from the earliest days of physics research on the Bevatron show both the ideal and the real worlds, the scheduled experiments and those performed. During the first week of August Segre's group was scheduled for three of the six days of Bevatron operation, and ran for five; during the second and third weeks it had no time, while Lofgren's and Powell's groups sought antiprotons in their own ways; on August 29 Segre's group returned and ran as scheduled until the Bevatron broke down on September 5. On the 21st, a week after operating crews had revived the machine, Lofgren's group was to begin a four-day hunt for the antiproton. Instead it ceded its time to Segre's group, which immediately got its first antiproton counts. For the next month the entire research effort at the Bevatron went to confirming and extending the counts. The physics division decreed that Segre's equipment would remain in place indefinitely; and money was found to increase nominal operating hours from 65.5 to 81 a week.
The experiments following up the Segre group's discovery of the antiproton centered on a search for the cataclysmic explosion that should mark the end of the career of an antiproton captured by an unfortunate nucleus. Lofgren's and Moyer's groups joined forces to find the sort of evidence of annihilation in a Cerenkov counter that Lofgren's group had sought earlier. Segre's group pressed forward with the scanning of emulsion stacks in collaboration with a group under Edoardo Amaldi in Rome. The Rome team found the first annihilation star, whose visible energy (the combined energy of all ionizing fragments) amounted to above 826 MeV, an amount deemed appropriate-for an explosion initiated by an antiproton. Other persuasive terminal events did not come to light before the Segre group's equipment was removed just before Christmas to make way for scheduled experiments on K mesons. A subsequent exposure to a beam of slower antiprotons, some of which ended their lives in the emulsion, created stars more generously, including one with a visible energy greater than the rest energy of the proton. The large group by then engaged in the experiment recommended their observations as a "conclusive proof that we are dealing with the antiparticle of the proton." The discovery brought Segre and Chamberlain the Nobel prize in physics for 1959.
|Surrounding Edward Lofgren (center) are discoverers of the antiproton (left to right) Emilio Segre, Clyde Wiegand, Owen Chamberlain, and Thomas Ypsilantis.|
The Segre group had announced their discovery of the antiproton by electronic counters in a letter sent to the Physical Review on October 19, 1955. The previous day Lawrence and Willard Libby, acting chairman of the AEC, had informed a wider audience. They intimated that the discovery, by fulfilling "one of the important purposes" for which the Bevatron was built, justified the expenditure. Nine months later the machine's steward, Lofgren, announced that the Bevatron was "obsolete in design and in a few years will not even be in the class of high energy physics." It was time for the next turn in the spiral of high energy.