Collider: The Search for the World's Smallest Particles - Paul Halpern (2009)
Chapter 4. Smashing Successes
The First Accelerators
What we require is an apparatus to give us a potential
of the order of 10 million volts which can be safely
accommodated in a reasonably sized room and
operated by a few kilowatts of power. We require too an
exhausted tube capable of withstanding this voltage . . .
I see no reason why such a requirement cannot be
—ERNEST RUTHERFORD (SPEECH AT THE OPENING OF
THE METROPOLITAN-VICKERS HIGH TENSION LABORATORY,
MANCHESTER, ENGLAND, 1930)
The Soviet People’s Commissariat of Education issued its coveted stamp of approval, permitting one of its most brilliant physicists, George Gamow, the opportunity to spend a year at Cavendish. He had almost missed the chance due to an odd medical mix-up. During his clearance check-up, his doctor inadvertently reversed the digits of his blood pressure, flagging him for heart disease. Once that error was cleared up, he got the green light. The Rockefeller Foundation generously offered to support his travel and expenses. Fellowships funded through oil wealth weren’t exactly the revolutionary means to success Lenin had anticipated, but the Soviet Union at that time viewed the admission of one of its native sons to the world’s premier nuclear physics laboratory as a triumph for its educational system.
It is lucky for the history of accelerators that Gamow managed to make it to England. His theoretical insights would offer the critical recipe for breaking up atomic nuclei and place Cavendish in the forefront of the race to build powerful atom smashers. Thanks in part to his contributions, and to the magnificent experimental work of his colleagues, Cavendish would become for a time the leading center for nuclear research in the world.
The Leningrad-trained physicist arrived in Cambridge in September 1928 and quickly found lodging at a boardinghouse. When a friend visited him soon thereafter he was astonished: “Gamow! How did you manage to get this house?”
At Gamow’s perplexed look, his friend pointed to the name of the building. By sheer coincidence it was called the “Kremlin.”
Several weeks after joining Cavendish, Gamow experienced one of its director’s legendary bursts of temper. One day, without prior explanation, Rutherford called Gamow into his office. Red-faced, he started screaming about a letter he had just received from the Soviet Union. “What the hell do they mean?” he bellowed as he shoved it at Gamow.
Gamow read over the letter. Written in scrawled English, it said:
Dear Professor Rutherford,
We students of our university physics club elect you our honorary president because you proved that atoms have balls.
After Gamow patiently explained that the Russian word for atomic nucleus is similar to its word for cannonball, and that the letter was probably mistranslated, Rutherford calmed down and had a hearty laugh.1
Among the first items Gamow procured at Cambridge were instruments that were ideal for striking spherical projectiles and hurling them toward distant targets. It was a set of golf clubs—par for the course at a collision laboratory. Gamow’s instructor in the sport was John Douglas Cockcroft, a young Cavendish researcher and avid golfer.
Born in Todmorden, England, in 1897, Cockcroft took a circuitous path to physics. His father ran a cotton-manufacturing business, but, like Rutherford and Marsden, Cockcroft opted for science over textiles. He began studying mathematics at Manchester University, but then World War I broke out and he joined the British army. Returning to Manchester upon the armistice, he switched to electrical engineering and found a job in the field. Finding that career path personally unfulfilling, he enrolled at St. John’s College, Cambridge, and made his way to Rutherford’s lab.
In golf, it can be frustrating when there’s a hill right in front of the green, occluding the direct path to the hole. In that case, you need a bit of strategy to figure out what club to use and how hard to swing it in order to clear the barrier. Tap the ball with not enough force, and it’s liable to fall short.
Cockcroft worked on a problem in nuclear physics that offered a similar kind of challenge. He wanted to hurl particles toward nuclear targets with the goal of exciting them to higher energy levels and possibly breaking them into subcomponents. If smashing them together broke them up, seeing what came out would allow him and his colleagues to learn things about the makeup of an atom they couldn’t learn in any other way. Blocking the path to the nucleus, however, was the barrier caused by the mutual electrostatic repulsion of positively charged particles and the positive nuclear charge. They naturally push each other apart—a formidable obstacle to overcome—like the north poles of bar magnets resisting being brought together, only far stronger.
Gamow knew just how to handle the issue theoretically. Plugging the parameters corresponding to protons and alpha particles (the particles radioactive atoms such as uranium give off) into his “quantum tunneling” formula, Gamow discovered that the former would need sixteen times less initial energy than the latter for the same probability of penetration. The choice was clear: protons offer much more economical projectiles. If protons could be induced to move fast enough, a few might pass through the force barrier around an atom and smash into its nucleus. What exactly would happen once they reached their targets was unknown, but, convinced by Gamow, Rutherford decided it would be worthwhile to try. It would be the one major step Rutherford took that was driven by theoretical predictions.
Already involved in planning the details of an atom smasher was an adept young experimentalist, Ernest Thomas Sinton Walton. Born in Dungaravan, Ireland, in 1903, he was the son of a traveling Methodist preacher. In 1915, Walton enrolled in a Methodist boarding school where he excelled in the sciences. Following his graduation in 1922, he became a student at Trinity College in Dublin, where he received a master’s degree in 1927. Upon being awarded an Exhibition of 1851 scholarship to Cambridge, he joined the group at Cavendish and soon became one of Rutherford’s trusted assistants.
In late 1928, Walton came across an extraordinarily innovative research paper by Norwegian engineer Rolf Wideröe that described his attempts to accelerate particles by means of a device called the ray transformer. Wideröe’s mechanism combined several basic concepts in electromagnetic theory. It starts with the idea of an electromagnetic coil: a current-carrying wire wrapped in a loop that produces a magnetic field in its vicinity. If the wire has a changing current, then the magnetic field changes over time. Then, according to Faraday’s law of induction, the changing magnetic field produces a second current in any wire that happens to be nearby. If that second wire is in a loop too, the setup is known as a transformer—a familiar system for transferring power from one wire to another. In a way, it’s analogous to the spinning of a bicycle’s pedals giving rise to the turning of its wheels—with the chain representing the varying magnetic field connecting the two.
Wideröe’s principal innovation was to replace the second wire with electrons accelerated through a vacuum-filled ring. These electrons would be removed from atoms and propelled through space by what is called the electromotive force produced by the changing magnetic field. To keep the electrons moving in a loop, like race cars on a circular track, he envisioned a central magnet that would steer them round and round. Unfortunately, in trials of his machine at Aachen University in Germany, he found that “islands of electrons” built up in the tube, sapping the revolving electrons’ energy. For some reason, the magnet couldn’t keep the electrons moving smoothly, though he couldn’t figure out why. The best Wideröe could manage given the turbulence was to get the electrons to circle around the loop one and a half times.
Frustrated by the problems with the circular track, Wideröe finally decided to abandon the project and turn to a different scheme. Borrowing a concept he found in a 1924 article by Swedish physicist Gustav Ising, he pursued the idea of a linear accelerator and built a small prototype, about one yard long. Rather than a ring, it used a pair of “drift tubes” (straight, isolated, vacuum-filled pipes) in which particles would be sped up by successive “kicks” of an electric field. These boosts were arranged rather cleverly—allowing the particles to be lifted up to higher speeds twice by use of the same voltage difference—something like the continuously ascending stairways in some of Escher’s paintings. Just when the particles seemed to reach the top, there was more to climb.
Voltage, electric potential energy per charge, is a measure of how easy it is for particles of specific mass and charge to accelerate from one place to another; the higher the voltage difference, the greater the acceleration, all other factors being equal. In other words, voltage is a measure of how steep that staircase is—and how much of a boost it gives.
Particles began their journeys through a drift tube with high voltage (twenty-five thousand volts) at the entrance point and low voltage at the exit. This voltage difference caused them to speed up. When the particles were halfway through and already moving quickly, Wideröe tricked them by reversing the voltage difference, setting the formerly low voltage to high. Because they were already moving at high speeds it was too late for them to turn back. They rushed through the tail end of the first tube, across a gap, and then on to the start of a second tube, where the same voltage difference (once again, due to an initially high voltage and a final low voltage) accelerated them even farther. Because it used the same voltage difference twice, Wideröe’s method doubled the impact of the boost, enabling a lower voltage source than otherwise required.
At the end of the second tube, Wideröe placed a photographic plate to record the streaks produced by the high-velocity particles as they impacted. Experimenting with potassium and sodium ions as the projectiles, he was able to run them through his device. The ions were made by stripping atoms of their outer electrons. The positively charged ions were then compelled by the voltage differences to accelerate through the tubes before hitting the plate. After collecting enough data, Wideröe incorporated his findings into a doctoral thesis for Aachen University. The thesis was published in a journal his Ph.D. adviser edited.
Fascinated by Wideröe’s work, in December 1928, Walton proposed to Rutherford the idea of building a linear accelerator at Cavendish. Rutherford was keen to devise such a device that could look inside one of the lighter elements, such as lithium. (Lithium is the third element in the periodic table after hydrogen and helium and its atom is now known to have three protons and four neutrons in its core.) The next month, Gamow gave a talk that presented his barrier penetration formula to the group. Cockcroft was eager to apply this formula to the issue of penetrating the lithium nucleus with protons. Estimates showed that it would take several hundred thousand electron volts to do the job. By human standards, even 1 MeV (one million electron volts) is an extraordinarily tiny amount of energy—approximately one billionth of a billionth of a single dietary calorie (technically, a kilocalorie). Elementary particles obviously don’t have to worry about slimming down—however for them it’s quite an energizing burst!
Upon hearing these results, Rutherford called Cockcroft and Walton into his office. “Build me a one-million-electron volt accelerator,” he instructed. “We will crack the lithium atom open without any trouble.”2
Soon Cockcroft and Walton were hard at work building a linear accelerator that they would locate, when it was complete, in a converted lecture hall. They rigged up a straight tube with a specially designed high-voltage power supply, now known as the Cockcroft-Walton generator. It included a mechanism known as a voltage multiplying circuit that included four high-voltage generators stacked in a ladderlike formation twelve feet high. Capacitors (charge-storing devices) in the circuit helped oost a relatively modest input voltage to an overall voltage of up to seven hundred thousand. Propelled by this high voltage, protons would be accelerated by the electric forces through an evacuated tube and collide with nuclear targets on the other end—with any disintegrations recorded as sparks on a fluorescent screen placed inside the vacuum.
A Cockcroft-Walton generator, one of the earliest types of accelerator. This example is retired and located in the garden of Microcosm, CERN’s science museum.
In 1931, Walton received his Ph.D. from Cambridge. With the Cavendish accelerator on the brink of completion, Rutherford certainly couldn’t afford to lose one of its principal architects. Walton was appointed Clerk Maxwell Research Scholar, a position he would hold for an additional three years while continuing to work with Cockcroft and Rutherford.
Cavendish was far from the only player in the race to split the atom. Physicists around the world were well aware of what Rutherford was trying to do and hoped to unlock the nucleus themselves with their own powerful atom smashers. Aside from scientific interest, another motivation that would become increasingly important was anticipation of colossal energy locked inside the atomic core. Einstein’s famous equation for the equi-valence of energy and mass, E = mc2, indicated that if any mass were lost during a nuclear disintegration it would be converted into energy—and this could be formidable. In 1904, even before Einstein’s finding, Rutherford had written, “If it were ever possible to control at will the rate of disintegration of the radio elements, an enormous amount of energy could be obtained from a small amount of matter.”3 (In 1933, he would qualify this statement when, in a prediction uncharacteristically off the mark, he expressed the opinion that atomic power could never be controlled in a way that would be commercially viable.)
A highly innovative thinker who would become a key participant in the development of nuclear energy was the Hungarian physicist Leo Szilard. In December 1928, while living in Berlin, Szilard took out a patent for his own concept of a linear accelerator. Like Ising and Wideröe, Szilard envisioned an oscillating (direction switching) electric field that would prod charges along. In his patent application, titled “Acceleration of Corpuscles,” he described a way of lining up charged ions so they ride the crest of a traveling wave forcing them to move ever faster:
With our arrangement, the electric field can be conceived of as a combination of an electric field in accelerated motion from left to right and an electric field in decelerated motion from right to left. The device is operated in such a way that the velocity of the accelerated ion equals, at each point, the local velocity of the field moving left to right.4
Curiously, Szilard never pursued his design. He developed patent applications for two other accelerator schemes that he similarly never followed up on. History does not record whether or not his patent applications were even accepted—conceivably the patent officers were aware of the earlier papers by Ising and Wideröe.
Around the same time as Cockcroft and Walton began to build their apparatus, American physicist Robert Jemison Van de Graaff developed a simple but powerful accelerator model that, because of its compactness and mobility, would become the nuclear physics workhorse for many years. Born in Tuscaloosa, Alabama, in 1901, Van de Graaff began his career on a practical track. After receiving B.S. and M.S. degrees in mechanical engineering from the University of Alabama, he worked for a year at the Alabama Power Company. He could well have remained in the electrical industry, but Europe beckoned, and in 1924, he moved to Paris to study at the Sorbonne. The great Marie Curie herself taught him about radiation—acquainting him in her lectures with the mysteries of nuclear decay. His savvy won him a Rhodes scholarship, enabling him to continue his studies at Oxford. There he learned about Rutherford’s nuclear experiments and the quest to accelerate particles to high velocities. Oxford awarded him a Ph.D. in physics in 1928.
In 1929, Princeton University appointed Van de Graaff as a national research fellow at the Palmer Physics Laboratory, the center of its experimental program. He soon designed and built a prototype for a novel kind of electrostatic generator that could build up enormous amounts of charge and deliver colossal jolts. Its basic idea is to deliver a continuous stream of charge from a power source to a metal sphere using a swift, insulated conveyor belt. Van de Graaff constructed his original device using a silk ribbon and a tin can; later he upgraded to other materials. Near the bottom of the belt, a sharp, energized comb connected to the power source ionizes its immediate surroundings, delivering charge to the belt. Whisked upward, the charge clings to the belt until another comb at the top scrapes it off and it passes to the sphere. A pressurized gas blankets the entire generator, creating an insulated cocoon that allows more and more charge to build up on the sphere.
Using the Van de Graaff generator as an accelerator involves placing a particle source (a radioactive material or an ion source, for example) near the opening of a hollow tube, each situated within the sphere. The voltage difference between the sphere and the ground serves to propel the particles through the tube at high speeds. These projectiles can be directed toward a target at the other end.
Van de Graaff worked continuously at Princeton and later at MIT to increase the maximum voltage of his generators. While his prototype could muster up to eighty thousand volts, an updated model he presented at the American Institute of Physics’s inaugural banquet in 1931 stunned the dining guests (fortunately not literally) by producing more than one million volts. A much larger machine he assembled on flatbed railroad cars in a converted aircraft hangar in South Dartmouth, Massachusetts, consisted of twin insulated columns, each twenty-five feet high, capped with fifteen-foot-diameter conducting spheres made of shiny aluminum. Its colossal power inspired the New York Times headline on November 29, 1933, “Man Hurls Bolt of 7,000,000 volts.”5
According to Greek mythology, Prometheus stole the secret of fire from the gods, offering humanity the sacred knowledge of how to create sparks, kindle wood, light torches, and the like. Yet even after that security breach, mighty Zeus reserved the right to hurl thunderbolts at his foes, illuminating the heavens with his terrifying power. Through Van de Graaff ’s generator, even something like the awe-inspiring vision of lightning became scientifically reproducible, albeit on a smaller scale, ushering in a new Promethean age in which colossal energies became available for humankind’s use. Given such newly realized powers, perhaps it is not surprising that many horror films of the day, such as Frankenstein (1931) and Bride of Frankenstein (1935) most notably, offered sinister images of gargantuan laboratories spawning monsters electrified by means of colossal generators.
Why rely on expensive generators for artificial thunderbolts, if the real thing is available in the skies for free? Indeed, though lightning is, of course, highly unpredictable and extremely dangerous, several physicists of that era explored the possibility of using lightning itself to accelerate particles. During the summers of 1927 and 1928, University of Berlin researchers Arno Brasch, Fritz Lange, and Kurt Urban rigged up an antenna more than a third of a mile across between two adjacent peaks in the southern Swiss Alps near the Italian border. They hung a metallic sphere from the antenna and wired another sphere to the ground to measure the voltage difference between the two conductors during thunderstorms. During one lightning strike, more than fifteen million volts passed through the device, according to the researchers’ estimates. Sadly, during their investigations Urban was killed. The two survivors returned to Berlin to test discharge tubes with the potential to withstand high voltages. Brasch and Lange published their results in 1931.6
Lightning strikes, even of the artificial kind, are usually one-time-only affairs. Whenever great quantities of charge build up, it creates a huge voltage drop that maintains itself as long as the collected charge has nowhere else to go (if the apparatus is isolated or insulated, for example). Like cliff divers, particles plunging down the steep potential difference experience a force that accelerates them. But once they reach the ground, that’s it—end of story.
However, as Wideröe pointed out in his “ray transformer” proposal, particles forced to travel in a ring, instead of a straight line, could be accelerated repeatedly each time they rounded the loop, building up to higher and higher energies. Although, after his initial experiments failed, Wideröe ceased working on the idea of a circular accelerator, his article inspired an extraordinary American physicist, Ernest Orlando Lawrence, to pursue this vital approach.
Lawrence was born in the prairie town of Canton, South Dakota, in 1901. His parents, Carl and Gunda, were both school-teachers of Norwegian descent. Carl was the local superintendent of schools and also taught history and other subjects in the high school; Gerda instructed in mathematics. Ernest was a cheerful baby, whom the neighbors across the street, the Tuve family, contrasted with their own colicky, crying boy born six weeks earlier, Merle.
Practically from birth Ernest and Merle were best friends. They would pull various pranks together, such as once dumping rubbish on another neighbor’s porch. The neighbor happened to be home and snatched Merle, before he escaped, through a hole in her fence. Meanwhile, Ernest managed to get away. They shared a code of honesty and tried not to fib, even when they were mischievous.
When the boys were only eight years old, they became interested in electrical devices. Practically all of their waking hours, aside from school and chores, were spent hooking up crude batteries into circuits, connecting these power sources to bells, buzzers, and motors, and testing which combinations worked best.
Tall and lanky, Lawrence earned the childhood nickname “Skinny,” which he didn’t seem to mind. His interests were as narrow as his build. Aside from tennis, he was little interested in sports and participated grudgingly in athletic activities, mainly when prodded by his father. Nor, as a teenager, was he much inclined to go on dates and other social events. Rather, he buried himself in his studies so that he could graduate from high school a year early, while continuing to spend his free time, along with Tuve, assembling various mechanical and electrical apparatuses. To earn money for switches, tubes, and other radio equipment, he spent one summer working on an area farm—a job he detested. The farmer he worked for had a low opinion of his skills, complaining, “He can’t farm worth a nickel.”7
Despite his limitations in many areas beyond science, Lawrence’s single-mindedness would prove a great strength. Like a magnifying glass on dry wood, whatever topics Lawrence’s bright blue eyes did focus on would be set ablaze with his extraordinary energy and intuitive understanding. One of the first to recognize his talents was Lewis Akeley, dean of electrical engineering at the University of South Dakota, where he completed his undergraduate studies. Lawrence had transferred there in 1919 from St. Olaf College in Minnesota with the goal of preparing for a career in medicine, but Akeley steered him toward physics. Akeley was so impressed by Lawrence’s phenomenal knowledge of wireless communication that he decided to experiment with a new teaching arrangement. For senior seminar he asked Lawrence, the only upper-level physics major, to prepare and deliver the lectures himself. While Lawrence was speaking, Akeley sat smiling as an audience of one, marveling that he was lucky enough to get to know perhaps the next Michael Faraday.
Meanwhile, Tuve was at the University of Minnesota and persuaded Lawrence to join its graduate program in physics. There, Lawrence found a new mentor, English-born physicist W. F. G. Swann, from whom he learned about the latest questions in quantum physics. Swann was a bit of a restless soul, an accomplished cellist as well as a researcher, who hated stodginess and valued creative thinking. Unhappy in Minnesota, he moved to Chicago and then to Yale, inspiring Lawrence to follow. Lawrence received his Ph.D. from Yale in 1925 and continued for three more years as national research fellow, working on new methods for determining Planck’s constant and the charge-to-mass ratio of the electron. Along with another research fellow, Jesse Beams, he developed a highly acclaimed way of measuring extremely short time intervals in atomic processes. They showed that the photoelectric effect, in which light releases electrons from metals, takes place in less than three billionths of a second—lending support to the idea that quantum events are instantaneous.8
With his Ph.D. in hand, Lawrence finally found time to socialize, albeit in a manner unusual for a postdoctoral researcher. The daughter of the dean of the medical school, Mary Kimberly “Molly” Blumer, who was then only sixteen years old, needed a date for her high school prom. Word got out, and Lawrence agreed to be her escort. He was impressed with her quiet thoughtfulness, and after the prom he asked her if he could see her again. Politely, she said he could stop by, even though at that point in her life she understandably felt awkward being wooed by a man nine years her senior. Each time he visited her house, she would take any measure not to be alone with him—making sure her sisters accompanied them at all times. Sometimes she would even hide out in a family fishing boat in the Long Island Sound and refuse to return to shore. It is a tribute to his perseverance that they would eventually get married.
Lawrence was courted in a different way by a rising star in the academic world, the University of California in Berkeley. Berkeley offered him an assistant professorship. When he turned it down in favor of remaining at Yale, Berkeley raised its offer to an associate professorship—a rank usually reserved for more seasoned faculty. Lawrence then decided to accept, thinking Berkeley would offer quicker advancement and a greater chance of working directly with graduate students. Some of his stuffier colleagues at Yale were taken aback that he would even consider switching to a non-Ivy League institution. “The Yale ego is really amusing,” Lawrence wrote to a friend. “The idea is too prevalent that Yale brings honor to a man and that a man cannot bring honor to Yale.”9
In August 1928, Lawrence drove out west in an REO Flying Cloud Coupe to assume his new position. After crossing the American heartland and reaching the rolling Berkeley hills, he took time to admire the beauty of the Bay Area and the exciting cultural jumble of San Francisco. The campus, dominated by a Venetian-style bell tower, offered a different kind of splendor. Although rooted in European architectural themes, it seemed refreshingly bright and modern—a far cry from pompous East Coast tradition.
Blessed with ample space and support for his work, he resumed his studies of the precise timing of atomic processes. Then, barely seven months after he started, his research took an unforeseen turn. Around April Fools’ Day of a year devastating for investors but auspicious for high-energy physics, Lawrence was sitting in the Berkeley library browsing through journals. Wideröe’s article leapt to his attention as if it were spring-loaded. It was the diagrams, not the words, that he noticed at first—sketches of electrodes and tubes arranged to propel particles.
Of Wideröe’s two accelerator designs, the linear dual-tube arrangement and the ringed “ray transformer” scheme, Lawrence found the latter more appealing. Lawrence realized immediately that a straight-tube setup would be limiting, providing only a few kicks before particles reached their targets. By curving the tubes into semicircles with electrifying gaps in between, and bending particle paths with a central magnet, he saw that he could jolt the particles again and again. He noted the fortuitous coincidence in magnetism that for a given particle steered in a circular loop by a magnetic field, if the field is constant the ratio of the particle’s velocity to its orbital radius—a quantity known as angular velocity—similarly remains the same, even if the particle speeds up. Because angular velocity represents the rate by which an object travels around a circle, if it is constant then the object passes the same point in equal intervals of time—like a racehorse passing a grandstand precisely once a minute. This regularity, Lawrence, determined, would ensure that a voltage boost peaked at regular intervals (oscillating in the same rhythm as the orbits) could accelerate particles around a ring to higher and higher energies until they reached the level needed to penetrate a target nucleus. Lawrence realized the problem with Wideröe’s design, and his pileup of electrons, was all in timing the voltage boosts.
Lawrence shared his design with Berkeley mathematician Donald Shane, who verified that the equations checked out. When Shane inquired, “What are you going to do with it?” Lawrence excitedly replied, “I’m going to bombard and break up atoms!”
The following day, his exuberance grew even greater when additional calculations confirmed that particles in his planned accelerator could continue to circle faster and faster no matter how far they spiraled outward from the center of the ring. As he strutted through campus like a proud peacock, a colleague’s wife distinctly heard him exclaim, “I’m going to be famous!”10
As he was accustomed to during childhood, Lawrence couldn’t wait to run his idea past Tuve, who was then working at the Carnegie Institution in Washington. Tuve was dubious about the scheme’s practicality. Ironically, the best friends were becoming rivals—following separate tracks in the race to split the nucleus. Along with Gregory Breit and Lawrence Hafstad, Tuve was involved in efforts to crank Tesla coils—paired wound coils for which lower voltage in one induces high voltage in the second—up to ultrahigh energies. However, these devices were extremely hard to insulate and wasted a lot of energy. After Van de Graaff developed his high-voltage generators, Tuve recognized their promise and began constructing his own.
Because of doubts expressed by Tuve and other colleagues, Lawrence was at first hesitant to try out his concept, which he initially called the magnetic resonance accelerator and later became known as the cyclotron. It took some prodding by a noted scientist to get him going. Around Christmas 1929, he sat down for some bootleg wine (it was Prohibition) with German physicist Otto Stern—who was visiting the United States at the time—and outlined his scheme. Stern became excited and urged Lawrence to turn his design into reality. “Ernest, don’t just talk any more,” he urged. “You must . . . get to work on that.”11
Placing himself in direct competition with Cockcroft, Walton, Van der Graaff, Tuve, and other nuclear researchers, Lawrence hadn’t a moment to spare to get his accelerator up and running. He took his first Ph.D. student, Niels Edlefsen, aside and inquired, “Now about this crazy idea of mine we’ve discussed. So simple I can’t understand why someone hasn’t tried it. Can you see anything wrong with it?”
Edlefsen responded that the idea was sound. “Good!” said Lawrence. “Let’s go to work. You line up what we need right away.”12
Under Lawrence’s supervision, Edlefsen assembled a hodgepodge of materials found around the lab into a working prototype. A round copper box, cut in half, served as the two electrodes—wired to a radio-frequency oscillator that offered a cyclic voltage boost. Edlefsen encased the device in glass and centered it between the four-inch poles of a guiding electromagnet. Finally, he sealed all of the connections with sticky wax. Completed in early 1930, it wasn’t very elegant. Nevertheless it was sufficient, after some tinkering, to get protons circulating—much to Lawrence’s delight.
A thirty-seven-inch early cyclotron at the Radiation Laboratory, now the Lawrence Berkeley National Laboratory.
For energies high enough to break through nuclear barriers, Lawrence realized that he needed a bigger machine with a more powerful magnet. Fortunately an industrial executive, who also taught at the university, offered him an eighty-ton magnet that had been gathering dust in a warehouse about fifty miles south in Palo Alto. Built for radio transmission, it had been rendered obsolete by technological advances.
The generous donation prodded Lawrence to find a more spacious setting suitable for a much larger accelerator. He got lucky again; in 1931, an old building on campus was about to be demolished and he was given permission to use it. Christened the Radiation Laboratory (and nicknamed the “Rad Lab”), it and its successor buildings would serve for decades as his dedicated center for research. It would eventually be renamed in honor of its founder and is now known as the Lawrence Berkeley National Laboratory.
Another pressing issue was how to move the colossal magnet to the lab. When yet another donor offered him funds to that effect, Lawrence scored a triple play in the tight budgetary age of the Great Depression. He finally had ample space and equipment to construct a powerful machine.
In 1932, a banner year for nuclear physics, remarkable experiments around the world cast powerful spotlights on the murky inner workings of atoms. At Columbia University, chemist Harold Urey discovered deuterium, an isotope of hydrogen with approximately twice the mass of the standard version. James Chadwick’s identification of the neutron, found through meticulous observations at Cavendish, explained why deuterium is twice as massive as its similarly charged brother: the heavier isotope is bloated with extra neutrons. Speculations arose as to whether neutrons are particles in their own right, or alternatively protons and electrons somehow clumped together to make an electrically neutral particle.
A couple of different theories had been bandied about, and only experimentation could tell which of the theorists has guessed right. For example, beta decay is when a radioactive substance gives off electrons. Those electrons, some suggested, must be coming from neutrons breaking into protons and electrons. (We now know that it is the weak interaction that is mediating a transformation involving the quarks that form protons and neutrons, along with the electron and a neutrino.)
Carl Anderson’s discovery of the positron offered another possible explanation for the relationship between neutrons and protons. He found the positron in cloud chamber photographs taken at Caltech of positively charged cosmic rays (radiation from space passing through Earth’s atmosphere) with the same mass as the electron. We now know a positron is the antimatter version of an electron, but at the time, Anderson wondered if the neutron is fundamental, and the proton an amalgamation of a neutron and a positron. Testing these alternatives would require precise measurements of the masses of protons and neutrons, to see if one was sufficiently heavier than the other to accommodate an electron or positron. (Indeed, as we now know, the neutron is heavier, but is composed of quarks, not protons and electrons.)
While Lawrence, along with Wisconsin-born graduate student M. (Milton) Stanley Livingston, toiled on the larger cyclotron, word came of victory in the race to split the lithium nucleus. The first to reach the finish line were Cockcroft and Walton, using the Cavendish linear accelerator. Walton recalled the moment of discovery when they finally bombarded the lithium target with protons and observed the stunning results:
On the morning of April 14, 1932, I carried out the usual conditioning of the apparatus. When the voltage had risen to about 400,000 volts, I decided to have a look through the microscope which was focused on the fluorescent screen. By crawling on my hands and knees to avoid the high voltage, I was able to reach the bottom of the accelerating tube. To my delight, I saw tiny flashes of light looking just like the scintillations produced by alpha particles which I had read about in books but which I had never previously seen.13
After observing what surely looked like the decay of lithium, Walton called Cockcroft into the lab, who agreed with that explanation. Then they invited Rutherford to crawl into the chamber and check out the scintillations himself. They turned off the voltage, and he ducked inside. When Rutherford came out, he said:
Those scintillations look mighty like alpha particle ones and I ought to be able to recognize an alpha particle scintillation when I see one. I was in at the birth of the alpha particle and I have been observing them ever since. 14
Uncharacteristically, Rutherford asked Cockcroft and Walton to keep the news a “dead secret” until they could conduct more measurements. As Walton explained in a letter to his girlfriend, Freda Wilson (whom he would marry in 1934):
He [Rutherford] suggested this course because he was afraid that the news would spread like wild fire through the physics labs of the world and it was important that no lurid accounts should appear in the daily papers etc. before we had published our own account of it.15
Cockcroft and Walton ran the experiment further times using a cloud chamber to record the alpha particle tracks. (Recall that a cloud chamber is a box full of vapor for which charged particles passing through create a visible misty trail.) Calculating the masses before and after the collision, they confirmed that each lithium nucleus, with three protons and four neutrons, had been cajoled by an extra proton to break up into two alpha particles, each of two protons and two neutrons. They’d literally cut the lithium ions in half!
Moreover, the energy released during each hit corresponded precisely to the mass difference between the initial and final states, times the speed of light squared. Their experiment confirmed Einstein’s famous formula. Satisfied with the accuracy and importance of their results, they published their findings in the prestigious journal Nature. For their exemplary work, Cockcroft and Walton would share the 1951 Nobel Prize for physics.
The news from Cambridge didn’t dampen Lawrence’s spirits. He had much to celebrate. For one thing, he had just married Molly and was on his honeymoon. His dogged persistence in romance as well as in science had finally paid off. The coy young woman had grown to love her awkward but accomplished suitor. They would have a large family together—four girls and two boys.
Another cause for Lawrence’s optimism was his strong conviction that he was at the forefront of a new scientific era. Ultimately, he realized, cyclotrons could yield much higher energies than linear accelerators could muster, and would thereby be essential for future probes of the nucleus. He wasted no time in confirming Cockcroft and Walton’s lithium results with an eleven-inch cyclotron. The larger device in the Rad Lab with the eighty-ton magnet was still under completion. When it was ready in March 1933, Lawrence bombarded lithium with protons and generated a bounty of highly energetic alpha particles—ricocheting back with impressive range. He also struck a variety of elements with deuterons, producing protons of Olympian stamina—some sprinting up to fifteen inches. By that point, he was more than ready to share his results with the physics community at large.
The Seventh Solvay Conference, held in Brussels during the last week of October 1933, was a milestone for discussion of the remarkable advances in nuclear physics. Among the scientific luminaries present were quantum pioneers Bohr, de Broglie, Pauli, Dirac, Heisenberg, and Schrödinger. The Parisian contingent included Marie Curie, along with her daughter and son-in-law, Irene Joliot-Curie and Frédéric Joliot, each an esteemed nuclear chemist and future Nobel laureate.
From Russia came Gamow—the start of his permanent exile, as it turned out. Two years earlier, he had returned to his home-land by way of Copenhagen. Unhappy living under Stalin’s iron thumb, he and his wife had attempted to escape across the Black Sea to Turkey but had been foiled by foul weather. Remarkably, an invitation by Bohr allowed both of them to slip into Belgium, where Gamow announced to his surprised host that they would never go back.
The Cavendish contribution to the meeting was impressive. Headed by Rutherford, it included Cockcroft, Walton, Chadwick, and Blackett. Finally, though Lawrence was the lone American attendee, his presence was vital, in as much that cyclotrons represented the future of nuclear exploration and that the United States would for decades be the principal testing ground for such devices.
Cockcroft delivered the conference’s first talk, “The Disintegration of Elements by Accelerated Protons.” Listening eagerly to his every word was Lawrence, keen to demonstrate the superiority of the cyclotron in handling the job. Perusing Cockcroft’s handout, Lawrence noted a statement that “only small currents are possible” from the cyclotron and emphatically crossed it out. In the margin he wrote, “Not true,” expressing his clear impatience with Cockcroft’s assertions.16
When it came time for discussion, Lawrence was quick to respond. He presented an account of his own device and argued that it offered the best way forward to explore the nucleus. He also offered his own estimation of the mass of the neutron—controversially, much lower than Chadwick’s value. Further experiments conducted by Tuve later that year would demonstrate that Lawrence was wrong; a mistake he would frankly acknowledge. The neutron turned out to be slightly bulkier than the proton.
After Solvay, Lawrence traveled to England and spent a pleasant couple of days at Cavendish. Rutherford warmly welcomed him and led him on a personal tour. After some heated discussions about lithium bombardment results, Rutherford said of Lawrence, “He’s a brash young man but he’ll learn.”17
Lawrence tried to convince Rutherford to build a cyclotron at Cavendish. Chadwick and Cockcroft joined in the chorus, arguing that it was the only way for the lab to remain competitive. Rutherford would not budge. He had a preference for homemade equipment and was reluctant to import another group’s idea. Moreover, he disliked trolling for funds and knew a cyclotron would be expensive.
Rutherford’s reluctance cost him dearly. In 1935, Chadwick, frustrated with the lack of progress, departed for a position at the University of Liverpool where he began to solicit funds for a cyclotron. On a visit to Cambridge in the summer of 1936, he and his former mentor were barely on speaking terms. Around the same time, Australian-born physicist Mark Oliphant, another of Rutherford’s protégés, was offered a position at the University of Birmingham, which he would assume the following year. Pressed by the loss of some of his top researchers, an embittered Rutherford finally agreed to let Cockcroft construct a cyclotron at Cambridge.
While Rutherford hedged, Lawrence was busy collecting funds to build an even larger cyclotron at Berkeley. Tremendously successful at fund-raising, he had no trouble continuing to expand the Radiation Laboratory’s work. Oliphant, who would visit there and get to know Lawrence as well as Rutherford, explained the difference in their styles: “The Cavendish laboratory, under Rutherford and his predecessors, was always short of money. Rutherford had no flair and no inclination for raising funds. . . . Lawrence, on the other hand, had shrewd business sense and was adept at raising funds for the work of his laboratory.”
Oliphant pointed out that Lawrence, who originally was on a premed track at university, had the savvy to foresee the medical applications of cyclotrons and how these could be used to draw funding. In a 1935 letter to Bohr, Lawrence wrote, “As you know, it is so much easier to get funds for medical research.”
Unlike Rutherford, who suggested and personally supervised almost every experiment his lab undertook, Lawrence liked to delegate authority. He had exemplary management skills that impressed his benefactors in government and industry and enabled his lab to expand. As Oliphant noted:
His direct approach, his self-confidence, the quality and high achievement of his colleagues, and the great momentum of the researchers under his direction bred confidence in those from whom the money came. His judgment was good, both of men and of the projects they wished to undertake, and he showed a rare ability to utilize to the full the diverse skills and experiences of the various members of his staff. He became the prototype of the director of the large modern laboratory, the costs of which rose to undreamt of magnitude, his managerial skill resulting in dividends of important scientific knowledge fully justifying the expenditure.18
On October 19, 1937, Rutherford died of a strangulated hernia. Having been raised to peerage six years earlier, he was buried with the honors accorded his position as “Right Honourable Lord of Nelson.” His coat of arms reflected both his national and his scientific heritage: images of a New Zealand kiwi bird and a Maori warrior, along with a motto borrowed from Lucretius, “Primordia Quaerare Rerum (To seek the first principles of things).” Fittingly, his ashes were interred in a grave at Westminster Abbey next to the final resting places of Newton and Lord Kelvin.