Abstract and Keywords
John Cockcroft and Ernest Walton designed and built their eponymous linear accelerator at the Cavendish with crucial help from scientists and engineers at the Metropolitan-Vickers company in Manchester. In April 1932, they produced 400-kilo-electron-volt protons with which they split the lithium nucleus into two alpha particles. Ernest Lawrence, stimulated by an article in German on the linear acceleration of positive ions, realized they would execute circular trajectories in a superposed perpendicular magnetic field, thereby conceiving the cyclotron principle. By January 1932, he and M. Stanley Livingston had built a 10-inch-diameter cyclotron with which they produced 1.2 million-electron-volt protons. These new accelerators transformed experimental nuclear physics. These two inventions and discovery of the deuteron, neutron, and positron garnered five Nobel Prizes. That Americans received three was a harbinger of the momentous shift occurring in the geographical center of experimental and theoretical nuclear physics.
The three new particles, the deuteron, neutron, and positron, entered into the nuclear reactions produced by the two new machines that were invented in 1932, the Cockcroft–Walton accelerator and the cyclotron. The new particles and new machines transformed the face of nuclear physics.
John Douglas Cockcrofta was born in Todmorden, north of Manchester, on May 27, 1897, the first of five sons of the owner of a small cotton mill and his wife, the daughter of a cotton manufacturer and former teacher and talented singer.1 Both parents instilled in their children a deep work ethic, and all were successful in life.
Cockcroft worked in his father’s mill and walked in the countryside as a youth, developing his manual skills and closely observing his natural surroundings. He received his elementary education at the Church of England school in nearby Walsden and Roomfield Grammar School in Todmorden, and his secondary education at the Todmorden Secondary School. He had good teachers in physics and mathematics and developed a particular interest in atomic physics while reading a book that described the work of J.J. Thomson and Ernest Rutherford. He was an outstanding student, and although his father probably hoped his eldest son would run the mill someday, he recognized his son’s exceptional academic abilities and agreed with his teachers that he should go on to university. He won a three-year scholarship to the University of Manchester at age seventeen in the summer of 1914,2 a time when cars “were still few and far between,” when the “first men had flown, but for only a few minutes,” and when few people took seriously the “suggestion that messages might be sent from one part of the world to another by radiowaves, without a physical connection.”3
Cockcroft intended to go into experimental physics but was advised to first study mathematics, so in his first year at Manchester he was especially influenced by “that most charming of mathematicians,” Horace Lamb. However:
As some light relief I attended the first-year lectures in physics. These lectures increased steadily in noisiness until one day the storm broke, and Rutherford was brought in to restore order. I still remember the immediate impression that here was a great man who was not going to stand any nonsense; thereafter the lectures were delivered by Rutherford in perfect quiet except for the applause which greeted the beautiful demonstrations of [William] Kay, the laboratory steward.4
At the end of his first year, since there was no end in sight to the Great War, he volunteered to serve with the Young Men’s Christian Association (YMCA) at an army camp in North Wales, and in November 1915 enlisted for military service. He was assigned to the Royal Field Artillery in March 1916, trained as a signaler, and repeatedly saw combat. In the summer of 1917, in the Third Battle of Ypres (known familiarly as Passchendaele), his battery lost twenty-four men killed or wounded in one night, and he was the sole survivor at a forward observation post. In the spring of 1918, he was sent to an Officers’ Training Unit and then commissioned Second Lieutenant. Incredibly, he survived the war, with its horrific loss of life and limb, without a scratch. His psychological wounds, however, ran deep. His memories of machine guns “with their devilish mechanical chatter” and of the “sickly smell” of poison gas never left him.5 He formed a deep and enduring hatred of war.6
Cockcroft was demobilized in January 1919, two months after the Armistice and returned to Manchester to resume his studies. He recalled:
When the war came to an end, I expected to have forgotten all the mathematics I had learned before my three years in the Army. However, I found that as soon as I started working again at the Manchester College of Technology my mathematics came back with a surprising completeness. I also felt that my three years in the Army had deepened and increased my capacity for scientific work rather than damaged it, as I approached it with a greater maturity of outlook.7
Cockcroft switched from mathematics and physics to electrical engineering, because he judged it offered more immediate prospects of getting a job.8 He received his B.Sc. Tech. in June 1920. His advisor and head of department, the gifted and kindly Miles Walker, then persuaded him to become a College Apprentice in the Metropolitan-Vickers Electrical Company (Metro-Vick) in Trafford Park, southwest of the Manchester city center. He worked in its Research Department, which was under the directorship of Arthur P.M. Fleming, who had introduced the College Apprentice program after the Great War.9 Cockcroft recalled:
My first piece of electrical research as a College Apprentice was to design a permanent magnet. Well, this hadn’t been in the College lectures [at Manchester], but I found that Maxwell’s equations were a better guide than any lecture or handbook, and it was quite a thrill to find that the magnet really did work.10
Cockcroft’s work blossomed, and Walker persuaded Fleming to allow him to work on a topic that would turn into a thesis for his M.Sc., which he received in June 1922. Walker (p.185) then urged him to apply for a scholarship to his old college, St. John’s College, Cambridge. Cockcroft received a Sizarship at St. John’s, a Miles Walker Studentship, a grant of £50 per annum from Fleming, and some supplementary funds from one of his aunts. Fleming was a “kind and generous man”: His grant to Cockcroft was the first of many that he made to young scientists and engineers who had worked for him at Metro-Vick.11
Walker also wrote a strong letter of recommendation for Cockcroft to Rutherford, to whom he presented himself when he arrived in Cambridge in October 1922.
I remember going to see him in the old Maxwell Wing of the laboratory and finding him sitting, as he so often did, on a stool. He received me very kindly, and gave me authority to devote such time as I could spare from mathematics to work in the advanced practical [physics] class.12
Then, “looking at me with those penetrating eyes, he promised to take me into the Cavendish if I got a ‘first’.”13 Cockcroft already had two Manchester degrees, so he was permitted to bypass the general first-year course for Part I of the Mathematical Tripos and go directly to Part II, a two-year course. He took mathematics and physics courses in his first year and specialized topics in applied mathematics and theoretical physics in his second year. He also attended, on his own initiative, the advanced course on experimental physics.14 His tutor for the Tripos was the demanding mathematical physicist Ebenezer Cunningham; that, he said, was the most difficult work he ever did in his life. He passed the Tripos with a B* (highest honors) in June 1924. Along the way, he joined literary, musical, and other clubs, including the Heretics, a select society he described as “non-religious but highly respectable; of the Ten Commandments, it held that only six need be attempted.”15
Rutherford accepted Cockcroft as a research student at a time, he recalled, “when the zinc sulphide scintillation screen, … the gold-leaf electroscope, and other pre-war primitive instruments were the standard tools of the Cavendish.” He took James Chadwick’s Attic Course, in which the difficulty of seeing faint scintillations “drove home to us the difficulty of Rutherford’s experiments.”16 He also learned how to blow glass and produce high vacua. By the end of his second year, he was secure enough financially from scholarship and other support to marry (Eunice) Elizabeth Crabtree, whom he had known since childhood, and whose family also were cotton manufacturers. They married in Bridge Street United Methodist Church in Todmorden on August 26, 1925, exactly two weeks after James Chadwick and Aileen Stewart-Brown married in Liverpool, a rapid succession of weddings that prompted much merriment by their mutual Russian friend (and Chadwick’s Best Man), Peter Kapitza.17
In the fall of 1924, Rutherford asked Cockcroft to work with Kapitza in his attempt to produce high magnetic fields using a large alternating-current (AC) generator to send electrical pulses through a copper coil with an air core. Cockcroft told Kapitza that Metro-Vick made such generators, and he and Miles Walker drew up the specifications for one. Cockcroft also designed coils that could generate high magnetic fields with maximum efficiency and minimum stress.18 The generator and its ancillary equipment (p.186) was constructed with a £8000 grant from the British Department of Scientific and Industrial Research (DSIR). It was installed in a new Magnetic Laboratory in a shed in the Cavendish courtyard in July 1925, which was formally opened on March 9, 1926. It delivered a transient magnetic field of 300,000 gauss at the center of the coil, the highest man-made magnetic field created up to that time.19
Towards the end of 1924, Cockcroft began working on his Ph.D. research, which challenged his glass-blowing skills, never very good, to the maximum, so his work progressed slowly. He finally received his Ph.D. on September 6, 1928. By then he was thoroughly integrated into the life of the Cavendish.20 He had been elected to the Kapitza Club in 1924, which he had told his fiancée Elizabeth “consists of 12 members—all the bright young sparks of the Cavendish.”21 Between 1924 and 1933 Cockcroft gave a talk or read a paper at the Kapitza Club on at least twenty occasions. He also was invited to join the select ∇2V (del squared V) Club, which had been set up by Paul Dirac and met twice during term. It consisted of twenty-five established physicists, including prominent theorists like Arthur Eddington and Ralph Fowler.22
Cockcroft was “of medium height with a slight, though athletic, figure,”23 and loved to take walks and play and watch cricket. He enjoyed listening to fine music and was well informed about modern sculpture and art. He read widely and had a deep interest in both ancient and modern architecture. He was highly disciplined: With “his economy of words and by never wasting time,” he could “deal efficiently, and almost simultaneously, with a large number of totally different problems,”24 seemingly without effort, helping everyone who asked. He kept track of his ideas and duties by making detailed entries in a small, black, ever-present, loose-leaf notebook, writing in a “mixture of script and shorthand” that was “almost illegible to those not familiar with it.”25 George Gamow once teased him that when his letters arrived he assembled a special commission of English-speaking people and specialists in Egyptian and Babylonian scripture to decipher them, and only then passed them on to fellow physicists.26 Cockcroft answered questions with a minimum of words, often just a “Yes” or “No.” He said “nothing, or ‘yes’ when he meant ‘perhaps’ or ‘perhaps’ when he meant ‘no’,”27 but usually made an entry in his small notebook and got back to the questioner later. He was a good listener, but disliked gossip and off-color stories. He was a devoted husband and family man. He and his wife were not spared heartbreak. Their first child, Timothy, died in October 1929 at the age of two from a severe attack of asthma.28 Their loss was mitigated only when their daughter Dorothea was born three years later. They eventually raised a family of four daughters and one son.
Cockcroft’s research took an entirely new turn in 1928, a year after Ernest Thomas Sinton Walton entered the Cavendish. Walton, six years younger than Cockcroft, was born in Dungarvan (County Waterford), on the southeast coast of Ireland, on October 6, 1903. (p.187) His father was a Methodist minister whose ministry took him, his wife Anne Sinton, and their son Ernest to Rathkeale (County Limerick), where his mother died, and to County Monaghan. Walton attended day schools in Banbridge (County Down) and Cookstown (County Tyrone) in Northern Ireland. In 1915, at the age of twelve, he entered Methodist College in Belfast as a boarder, where he excelled in science and mathematics. And, he recalled:
Tools have always had a fascination for me. As a boy and as a student, any money which came to me at Christmas and at birthdays was invariably spent on tools. This fascination arose undoubtedly from the power to do and make things, which the possession of tools gave me. They could be used to put new ideas into concrete form and they could produce machines and instruments not available on the market.29
Walton met his future wife, Freda Wilson, the daughter of a Methodist minister, in Belfast; they married in 1934. They were lifelong pacifists. Like the Cockcrofts, they too experienced heartbreak: they lost a son in December 1936.30 They later raised a family of two sons and two daughters.
In 1922, at age 19, Walton received a renewable scholarship and other support to enter Trinity College, Dublin.31 His total annual income was about £80, almost enough to cover his expenses at a time when a good three-piece suit cost £4 or £5.32 He lived in College rooms, studying by the light of an oil lamp he himself had to purchase. He took a full load of courses, and in the last term of his fourth year was given the task of making a new cloud chamber work, and taking photographs of alpha-particle tracks in it.33 He graduated with first-class honors in both mathematics and physics in 1926, carried out further research on hydrodynamics, and received his M.Sc. in 1927.
Walton was “an extremely likeable man, full of humour and of original ideas, exceptionally clever with his hands and quite capable of making spare parts for watches.”34 With his exceptional ability in mathematics, and with the full support of his mentor, versatile Irish geophysicist John Joly, he was awarded one of the highly competitive 1851 Exhibition Overseas Research Scholarships to go to the Cavendish Laboratory in October 1927. “I had some difficulty in finding the famous laboratory,” he mused, “for it was an unpretentious building tucked away inconspicuously up a side street and no passers-by seemed to have heard of it.”35 During his first year, he attended lectures by J.J. Thomson, C.T.R. Wilson, Aston, and Rutherford—whose lectures he found to be infectious but not well prepared. Rutherford assigned a bench to him in a ground floor room of the Cavendish alongside Cockcroft and another Metro-Vick physicist-engineer, Thomas Allibone, and around the corner from where Chadwick was working.
During the first (Michaelmas) term, Walton gained experience in making radioactive measurements and producing high vacua in Chadwick’s Attic Course, and at the end of the term Rutherford sent for him to discuss potential research topics. “He asked me,” Walton recalled, “if I had any suggestions to make, and I said that I would like to try producing fast particles.”36 Since it seemed impossible to accelerate heavy charged particles to energies high enough to disintegrate nuclei, Walton suggested a method for (p.188) accelerating electrons by induction in a circular electric field, which Rutherford judged to be too difficult to achieve then, so he suggested another possibility, of producing such a field with a high-frequency current in a circular coil.
Unbeknown to Walton, Rutherford was particularly receptive to his ideas, because the possibility of bombarding nuclei with high-energy particles was much on his mind. Allibone had had experience in high-voltage work at Metro-Vick, and by the fall of 1927 had built a high-voltage Tesla transformer at the Cavendish,37 having learned that when working with high voltages “a strict discipline of thought and action is the only way of avoiding electrocution.”38 Allibone’s success prompted Rutherford to argue for a new line of research in his presidential address before the Royal Society on November 30, 1927.
It would be of great scientific interest if it were possible in laboratory experiments to have a supply of electrons and atoms of matter in general, of which the individual energy of motion is greater even than that of the α-particle. This would open up an extraordinarily interesting field of investigation which could not fail to give us information of great value, not only on the constitution and stability of atomic nuclei but in many other directions.
It has long been my ambition to have available for study a copious supply of atoms and electrons which have an individual energy far transcending that of the α and β-particles from radioactive bodies. I am hopeful that I may yet have my wish fulfilled, but it is obvious that many experimental difficulties will have to be surmounted before this can be realised, even on a laboratory scale.39
Walton tried for almost a year to get the electrical induction method to work, first alone then, beginning in November 1928, with the help of Cockcroft. Success eluded him.
On realising that I would probably not be able to make the induction method work, I turned my mind to devising other indirect methods and suggested to Rutherford the method of the linear accelerator in early December 1928. … The idea was new to Rutherford, who after making a few quick simple calculations, agreed that the method was feasible and worth trying.40
Although new to both Rutherford and Walton, Norwegian engineer Rolf Wideröe had already designed a linear accelerator in 1927, for his doctoral thesis at the Technical University (Technische Hochschule) in Aachen, building on an idea published three years earlier by Swedish physicist Gustaf Ising.41 Wideröe published his design in December 1928, and when Walton saw it he “decided to try to be quicker off the mark next time.”42
George Gamow provided a crucial impetus. In pursuing his theory of alpha decay, he solved the inverse problem in Bohr’s institute at the end of October 1928, calculating the probability for an alpha particle of two different energies, that of a polonium and an RaC alpha particle, to penetrate a nucleus of atomic number Z.43 He sent a mimeographed copy of his paper to Cockcroft,44 hoping it would prompt an invitation to the Cavendish. Bohr assisted by engaging the help of Ralph Fowler, Douglas Hartree, and Nevill Mott on Gamow’s behalf. Their efforts succeeded: Rutherford wrote to Bohr on December 19, enclosing an invitation to Gamow. Gamow left Copenhagen for Cambridge on January 4, 1929, and stayed there until February 12.45
(p.189) On Tuesday evening, January 29, Gamow presented his theory of alpha-particle penetration at a meeting of the Kapitza Club, and on Thursday afternoon, January 31, he gave a talk on it at a meeting of the Cavendish Physical Society.46 Cockcroft heard both talks and Walton and Allibone heard the second one. “I well recall,” Allibone wrote,
returning from the Gamow’s colloquium at the Cavendish Physical Society, to the room in which Cockcroft, Walton, and I worked, and Walton and I stood round Cockcroft as he put figures into Gamow’s new formula—1 μA [microamp] of protons seemed a sensible figure—accelerated to, let us say, 300 000 V [volts], and let them bombard a target of lithium [actually boron]; making generous allowances for loss of protons as the beam emerged … the number penetrating the energy barrier seemed sufficient to give an observable number of disintegrations. … It is not often that theory had guided experiment as clearly as this.47
Neither Walton nor Allibone had heard of Gamow’s theory before, while Cockcroft had already read the mimeographed copy of Gamow’s paper,48 and had sent a memorandum to Rutherford summarizing Gamow’s calculations.49
Cockcroft (Figure 8.1) converted Gamow’s formula for the probability of penetration of a doubly charged alpha particle into one for a singly charged proton, and then calculated the probability that a proton with an energy of 300 keV (kilo electron volts) would penetrate a boron nucleus. He found that around one in a million would make a close collision with it and many would be disintegrated.50 Thus, contrary to what Rutherford thought, it should not be necessary to have protons of a few MeV (million electron volts) in energy, but only of a few hundred keV, to disintegrate the boron nucleus or other light nuclei.
(p.190) Cockcroft was so excited and inspired by this prediction that, even before Gamow’s talks, he had secured the largest induction coil in the Cavendish, and had begun to build an accelerator tube. Rutherford was skeptical of success—he was still calling for an accelerating potential of ten million volts in February 1930.51 He nevertheless backed Cockcroft and also decided that Walton should stop working on his electrical induction method, and instead should help Cockcroft build an accelerator tube and proton source.52
Cockcroft and Walton began collaborating in early 1929, exploiting their complementary knowledge and skills. Cockcroft had a strong background in mathematics and electrical engineering but was not skilled in constructing apparatus. He also had other balls in the air, in keeping with his lifelong ability to work on several projects simultaneously: He was Steward (or Bursar) of St. John’s College and in that capacity was supervising the reconstruction of its beautiful portal, devising a numbering system for the thousands of bricks and stones that had to be first removed and then replaced.53 He also was continuing to assist Kapitza in his magnetic laboratory. Allibone recalled that he and Walton
kept an eye on John Cockcroft’s apparatus and saved it times without number from total collapse. John would come in early in the morning—well, not too early—switch on [vacuum] pumps … and then dash out to Kapitza’s laboratory, or to St. John’s or elsewhere, completely forgetful of the need to turn on the water or something else, and one or other of us would find his apparatus in a critical state just before it disintegrated.54
Walton, by contrast, loved to work with his hands and assumed the lion’s share in constructing their accelerator and associated apparatus. He and Cockcroft “had many happy discussions about our work” without “any sign of discord between us.”55 Cockcroft also was “very good at locating unusual apparatus needed for our work,” and his “background of mathematics and engineering was very valuable” in designing their apparatus. Cockcroft also persuaded Rutherford to request a £1000 grant from the Royal Society to buy a 300 kilovolt transformer and auxiliary equipment to rectify alternating current (AC) and thereby deliver direct current (DC) to their hydrogen discharge tube, their source of protons.56
By then both Cockcroft and Walton had sufficient income to support themselves and their families. Cockcroft had been elected a Fellow of St. John’s College in the fall of 1928, which was renewed in January 1931. He also was appointed University Demonstrator in 1930 and lectured on electrodynamics to third-year physics students.57 Walton’s 1851 Exhibition Overseas Research Scholarship expired in June 1930, after which he received a Senior Research Award, tenable for four years, from the British DSIR. He also was awarded a Clerk Maxwell scholarship.58 Without this university and government financial support, neither Cockcroft nor Walton would have been able to pursue their research in those Depression years.
(p.191) Between early 1929 and early 1930, Cockcroft and Walton built a transformer–rectifier system to produce a steady potential of 300 kilovolts, and a hydrogen discharge tube capable of withstanding that voltage. Their work was crucially advanced by Cockcroft’s connections to Metro-Vick, but by then Thomas Allibone and two other scientists, Brian Goodlet and Cecil Burch, had far more extensive ties to Metro-Vick, of vital importance.
Allibone received his Ph.D. in physics in 1926 at Sheffield University while also working in the Metallurgy Department of Metro-Vick.59 Then, having gained experience with high voltages, he wrote to Rutherford, proposing to accelerate electrons through high voltages to produce nuclear transmutations using a 600 kilovolt Tesla transformer that Fleming, Director of the Research Department of Metro-Vick, had offered to provide.60 Rutherford accepted Allibone’s proposal, and Allibone went to the Cavendish in October 1926 on a Wollaston Studentship at Gonville and Caius College, an award that was facilitated by George McKerrow, a graduate of Gonville and Caius and Fleming’s right-hand man as scientific liaison to Cambridge and other British universities.61 Fleming, as for Cockcroft, also provided additional financial support for Allibone.
Allibone’s “fierce Tesla coil” soon “produced 500,000-volt sparks, to the annoyance of the Corpus [Christi] dons across the way.”62 He then constructed vacuum tubes that could withstand about 600 kilovolts in oil and 450 kilovolts in air for accelerating electrons. Cockcroft recalled:
I remember Rutherford putting a crystal in the emerging electron beam with his own hands and watching the bright fluorescence with joy. I wonder what dose of X-rays he received—we had no health physicists to take care of us in those days.63
In any event, Cockcroft and Walton now knew they could use such vacuum tubes in their three bulb-shaped rectifiers, each of which was thirty centimeters in diameter at its center. Allibone designed them,64 and the Jena Glaswerke produced them.
Brian Goodlet, nicknamed “Proteus,” after Charles Proteus Steinmetz, for his great mathematical ability, was born in Russia and “had shot his way down the Nevesky Prospect in St. Petersburg during the revolution.”65 He became a British subject, was taught mathematics at Sheffield University by Allibone’s father, and rose to become head of the High-Voltage Laboratory at Metro-Vick. He designed a 350 kilovolt transformer for Cockcroft and Walton that could be disassembled, moved through the entryways, and reassembled in the laboratory room.66
Cecil Burch, always known as Bill, was born in Oxford on May 12, 1901, the fifth child and third son of the Professor of Physics at University College, Reading, and his wife, the headmistress of a finishing school in Oxford, who had a deep love of literature, music, and languages.67 Burch attended preparatory school in Oxford from 1907 to 1914 and then Oundle School near Peterborough from 1914 to 1918. He recalled that at both he probably spent more time studying Greek and Latin than all other subjects combined.
The classical and literary emphasis had caused me to think … largely in terms of quotations (as I cynically put it to myself, if there is a Latin quotation for it, it is possibly true; if as a (p.192) hexameter or an elegiac couplet, truer still, and if as a Greek iambic, it is really true); the Greeks and Romans gave a lot of thought to moral philosophy and that part of it concerned with emotion, and this has not changed very much.68
Burch’s emotions were profoundly tested in the summer of 1918 when, just as he was going to one of his final examinations, he received a letter notifying him that his oldest brother Raymond had been killed in action.
Burch was an outstanding student, and Gonville and Caius College awarded him a Senior Scholarship worth £80 per annum. He and his next older brother Francis achieved a second in Part II of the Natural Sciences Tripos in 1922, after which both received a two-year College Apprenticeship in Fleming’s Research Department at Metro-Vick, which launched their careers. In 1927 Burch
began the work with which his name will always be associated—his work and his products became household names among physicists—the development of oils and greases with extremely low vapour pressures, products to which, with his knowledge of Greek, he gave the name apiezon (α (privative) and πίεξον (pressure)).69
Burch produced apiezon oil by an innovative evaporation–distillation process, which immediately found industrial applications: Metro-Vick sold the patent for Burch’s pot still for distilling pharmaceutical products to the British Drug Houses for £300,000.
Burch also immediately realized that his new apiezon oil could replace mercury vapor in vacuum diffusion pumps, and he and a colleague designed a series of new ones, which soon were found in physics laboratories everywhere. Metro-Vick supplied Cockcroft and Walton with several of them in 1930, before they had been put on the market for sale commercially.70 Patrick Blackett captured the spirit of these developments.
A rapid change is taking place in the technique of experimental physics. New methods are constantly being invented, and each new advance of technique increases our knowledge of the physical world by making possible experiments which before were technically impossible. In part these changes come from within the laboratories themselves. … But to an important extent the technique of the experimental physicist is influenced by the technical achievements of industry. The relation is reciprocal. A discovery in a laboratory in one decade leads to an industry in the next. And the purely commercial products of the industry may provide ready-made the instruments to extend the field of the technically possible.71
Cockcroft and Walton had everything ready to go by the middle of March 1930. They produced a several-microamp mixed beam of protons and molecular hydrogen ions in their discharge tube, accelerated them to an energy of 280 keV with their transformer–rectifier system, and bombarded beryllium and lead targets in their small experimental chamber. They wanted to see if the protons would produce secondary high-energy radiation, presumably gamma rays, like those Bothe and Becker had found when bombarding boron with polonium alpha particles. They reported their experiments in a paper Rutherford communicated to the Proceedings of the Royal Society on August 19, 1930. They (p.193) had observed “very definite indications of a radiation of a non-homogeneous type … using a gold leaf electroscope as a detector.”72
After finishing these experiments, Cockcroft and Walton’s transformer failed internally.73 That failure, and their inability to produce more than 280 keV protons, probably was what stimulated Cockcroft to think about other ways of generating a high DC voltage starting with a modest AC transformer voltage. Walton recalled:
Cockcroft had the idea of modifying the Schenkel circuit.74 … My contribution to the circuit theory … was in showing that the circuit could be thought of as a method of maintaining equal potentials across each of a number of capacitors connected in series. The arrangement could therefore be regarded basically as a transformer for stepping up or stepping down a D.C. voltage. Indeed we took out a patent on this aspect of the circuit as we thought that it might be used for the long distance transmission of electric power by D.C.75
Their new voltage-multiplier circuit consisted of four rectifiers that charged four capacitors in parallel and discharged them in series, converting an input AC voltage V into an output DC voltage 4V through which protons could be accelerated. Cockcroft calculated all of the important characteristics of their soon-to-be-famous circuit.
Cockcroft and Walton knew that before they could construct their new linear accelerator, they would have to relinquish their laboratory room in May 1931, when it would be required by the Physical Chemistry Department.76 They therefore moved their equipment into the Balfour Library, a large basement lecture room in the Cavendish from which the seating had been removed. That greatly increased the space available to them, and allowed them to aim for voltages much higher than 280 kV (kilovolts). They erected their new system there, which consisted of a column of four rectifiers, four columns of tall metal capacitors, and the accelerating tube with the observation hut below (Figure 8.2). Decades later, Walton declared:
I have never regarded the move to the larger room … as anything but fortunate. The higher voltage enabled us to investigate immediately elements up to about fluorine. For most work we could produce far more disintegrations than we could cope with. This meant that we were operating the apparatus mostly well below its limits, which meant that breakdowns were not very frequent. If we had remained in the old room with its low ceiling, observations would have been difficult. Indeed we would have had to lie down on the floor to see the scintillations—not a relaxed position for reliable counts.77
Cockcroft and Walton found that their glass bulb-shaped rectifiers tended to puncture at voltages higher than 300 kV, so they now used a column of four glass cylindrical rectifiers, each three feet long, fourteen inches in diameter, and separated by tinned sheet-iron plates. They made the joints airtight by using their fingers to press a new Apiezon plasticene compound into them, which Burch at Metro-Vick had supplied before it was placed on the market,78 and they evacuated the entire column with a Burch Apiezon oil-diffusion pump. They connected their rectifier–capacitor system to the accelerating tube, which consisted of two sealed glass cylindrical tubes similar to those in their column of rectifiers. At the top of this was their proton source, a hydrogen discharge (p.194) tube supplied by a separate 60 volt transformer. Blackett described what was involved in constructing their column of rectifiers.
The exact shape and position of the electrodes is of great importance and can only be found by trial and error. To have the complete tubes made especially by commercial firms would be very expensive and many weeks’ delay might result from the necessity of making some trivial alteration to the electrodes. Even to cement these tubes together with sealing-wax would involve building an oven some fifteen feet high to heat the tubes to the softening point of the wax, and (p.195) many hours would be required for the heating and subsequent cooling. But the introduction of plasticine as a sealing material has so facilitated the whole technique that when, for instance, a rectifier filament burns out, the great tubes can be dismantled, the filament replaced, the tubes re-erected and resealed, and an X-ray vacuum again obtained in an hour or so.79
The entire structure was built with additional grants totaling £600.80 It was huge and demanded prodigious effort to construct—mostly by Walton and their laboratory assistant, Willie Birtwhistle, because Cockcroft was mostly occupied with other matters in the summer and fall of 1931. An international conference on the history of science was held in London in June 1931 that was attended by a large Soviet delegation,81 and two months later Cockcroft went on his first extended scientific tour of the Soviet Union. Then, on his return to Cambridge, he was secretary of the organizing committee of a large celebration commemorating the centenary of the birth of James Clerk Maxwell,82 which opened in London on September 30 with the unveiling of memorial tablets to Maxwell and Michael Faraday in Westminster Abbey, and then moved to Cambridge where many dignitaries, including J.J. Thomson, Max Planck, and Niels Bohr, gave lectures in the old Maxwell wing of the Cavendish.83
At last, the construction of the huge accelerator was finished, the joints in the rectifier column and accelerator tube were sealed, it was “outgassed” and evacuated, and Cockcroft and Walton brought their accelerator into operation. It was a time-consuming process.
When the apparatus is first connected to the transformer … considerable quantities of gas are evolved from the walls. The voltage has to be increased slowly with intervals of a few seconds between the different evolutions of gas to allow the pumps to clear the tubes. Thus it may take a whole day’s operation before the full voltage can be applied to the apparatus. After this outgassing process is complete, however, full voltage can usually be obtained within 30 minutes of starting the pumps and within a few minutes of first applying the potential. The tube will then run continuously without trouble.84
They determined the accelerating voltage by positioning two large aluminum spheres (seventy-five centimeters in diameter) one above the other (Figure 8.2, left), connected one to the top and the other one to the bottom of the accelerator tube, and raised or lowered the upper sphere until a spark was produced in the air gap between them. Knowing the voltage at which air breaks down and becomes electrically conducting, they then calculated the accelerating voltage.
Cockcroft and Walton published a preliminary report of their experiment in Nature on February 12, 1932,85 and a full report that Rutherford communicated to the Proceedings of the Royal Society on February 23.86 They achieved an accelerating voltage of about 690 kV, which when added to the 20 kV in their hydrogen discharge tube gave a total proton energy of about 710 keV. The protons passed through a thin mica window at the end of their accelerator tube and into the experimental chamber, to bombard various light elements. They found that if any X rays or gamma rays were produced, their intensity was within “the limits of error of the experiment.”87 In other words, they found no evidence that protons or hydrogen molecular ions could excite the high-energy gamma rays that Irène (p.196) Curie and Frédéric Joliot had just reported finding when polonium alpha particles bombard beryllium or boron.88 That, Cockcroft said later, had incorrectly given them “the fixed idea that the gamma rays would be the most likely disintegration product.”89
Cockcroft and Walton continued their experiments in February and March of 1932, with beryllium as a target. They then interrupted them to clear up some trouble with the rectifiers, to try using helium instead of hydrogen in their discharge tube, and to carry out some magnetic-deflection experiments to determine the proportion of protons to hydrogen molecular ions in their beam. This work lasted until April 12—and drew the wrath of the impatient Rutherford. As Walton recalled,
Rutherford came in one day and found us doing magnetic deflections experiments and told us that we ought to put in a fluorescent screen and get on with the job, that no-one was interested in exact range measurements of our ions.90
Rutherford, in other words, demanded that Cockcroft and Walton stop messing around and get some physically meaningful results.
Cockcroft and Walton put in a lithium target on April 14. Walton recalled the events of that day.
I carried out the usual [preliminary] procedures while Cockcroft went to the Mond Laboratory to help Kapitza. When the voltage reached a fairly high value, I left the control table and crawled across the room to the little hut under the apparatus. On looking through the microscope, I immediately saw scintillations which seemed very like what I had read about the appearance of α-particle scintillations but which I had never previously seen. After applying a few simple checks, I telephoned Cockcroft, who returned immediately and confirmed my observations. We then got Rutherford to come along and observe them.91
Walton added further details on another occasion.
With some difficulty we manoeuvered him [Rutherford] into the rather small hut and he had a look at the scintillations. He shouted out instructions such as, “Switch off the proton current”; “Increase the accelerating voltage” etc., but he said little or nothing about what he saw. He ultimately came out of the hut, sat down on a stool and said something like this: “Those scintillations look mighty like α-particle ones. I should know an α-particle scintillation when I see one for I was in at the birth of the α-particle and I have been observing them ever since.”92
Cockcroft and Walton’s experiments, in fact, probably were the last success of the human scintillation counting technique. They began counting at a proton energy of 252 keV and went down stepwise to 126 keV.93 Walton continued:
During his visit …, Rutherford swore us both to strict secrecy and this surprised me at the time. It was a wise precaution as it enabled us to get a lot of work done quickly without any interruption from visitors. By working late in the evenings we soon accumulated essential information about the disintegrations and by Saturday evening, 16th April, we had even seen the tracks of the particles in an expansion chamber. At about 9 or 10 o’clock that evening, we went round to Rutherford’s house to report on the results and a letter to “Nature” was drawn up there.94
(p.197) Twelve days later, on April 28, Rutherford took his two protégés (Figure 8.3) to a meeting of the Royal Society in London, where he announced their pioneering experiments, and then proudly took them as his guests to dinner at the Royal Society Dining Club.95 Arthur P.M. Fleming, when interviewed by the press, “spoke without exaggeration” about the crucial help his department at Metro-Vick had provided the young scientists.96 Cockcroft and Walton published a report on their experiments in Nature on April 30, noting that they had placed a lithium target at 45° to the direction of the proton beam.
On applying an accelerating potential of the order of 125 kilovolts, a number of bright scintillations were at once observed, the numbers increasing rapidly with voltage up to … 400 kilovolts. At this point many hundreds of scintillations per minute were observed. … The range of the particles was … found to be about eight centimetres in air and not to vary appreciably with voltage.97
Cloud-chamber experiments confirmed their range. Moreover:
The brightness of the scintillations and the density of the tracks observed in the expansion chamber suggest that the particles are normal α-particles. If this point of view turns out to be correct it seems not unlikely that the lithium isotope of mass 7 occasionally captures a proton and the resulting nucleus of mass 8 breaks into two α-particles, each of mass four (p.198) and each with an energy of about eight million electron volts. The evolution of energy on this view is about sixteen million electron volts per disintegration, agreeing approximately with that to be expected from the decrease of atomic mass involved in such a disintegration.98
It was a thrilling discovery.99 C.P. Snow saw
John Cockcroft, normally about as given to emotional display as the Duke of Wellington, skimming down King’s Parade and saying to anyone whose face he recognized: “We’ve split the atom! We’ve split the atom!”100
Scientists everywhere congratulated Cockcroft and Walton. Their popular acclaim was enhanced by the coincidental opening of a new play, Wings over Europe, in London’s West End, which portrayed a young scientist who split the atom with dire consequences for mankind.101
In their full report, published in the Proceedings of the Royal Society in June 1932,102 Cockcroft and Walton carried out the above calculation in detail. They took the mass of the proton (1H1) to be 1.0072 amu and the mass of the alpha particle (2He4) to be 4.0011 amu, assumed that the nuclear reaction was 3Li7 + 1H1 → 22He4 + Q, and concluded:
The mass of the Li7 nucleus from [J.-L.] Costa’s determination is 7.0104 [amu] with a probable error of 0.003. The decrease of mass in the disintegration process is therefore 7.0104 + 1.0072 − 8.0022 = 0.0154 ± 0.003. This is equivalent to an energy liberation [Q] of (14.3 ± 2.7) × 106 volts.103
From the observed range in air of the two alpha particles, which by conservation of momentum were emitted with equal energy in opposite directions (as they verified in separate experiments), they found that the alpha particles acquired a total energy of 17.2 MeV, which was “consistent with our hypothesis” on the nature of the disintegration process.
Cockcroft and Walton’s experiment therefore was not a test of Einstein’s mass–energy relationship, E = mc2. They just used Einstein’s relationship in their analysis, assuming it to be valid. A true test of Einstein’s relationship was carried out a year later by experimental physicist Kenneth Bainbridge, at the Bartol Research Foundation of the Franklin Institute in Philadelphia, where he developed a precision mass spectrometer to determine isotopic masses. His goal was to gain a better understanding of nuclear structure, but as a byproduct he saw that he could test Einstein’s relationship. He reported his test on June 16, 1933, in a Letter to the Editor of The Physical Review, bearing the explicit title, “The Equivalence of Mass and Energy.”104 He assumed the energy of the incident proton to be 0.270 MeV and that of the two alpha particles to be 17.24 MeV, so the gain in energy was 16.97 MeV or 0.0182 amu. Then, instead of using Costa’s value for the mass of the Li7 isotope, he used his own precise value of 7.0146 ± 0.0006 amu, and Aston’s values for the masses of the helium and hydrogen nuclei. He calculated the change in mass to be 0.0181 ± 0.0006 amu and concluded: “Within the probable error of the measurements the equivalence of mass and energy is satisfied.”105
(p.199) A few days after Bainbridge reported his results, he gave a talk on them in Chicago at a joint meeting of the American Physical Society and the American Association for the Advancement of Science, in conjunction with Chicago’s “Century of Progress” exhibition. A reporter for Science News commented:
From the latest atom-smashing comes proof that Einstein was right. … Using the world’s largest mass spectroscope, … Dr. K.T. Bainbridge … weighed with extreme accuracy the newly discovered heavy-weight hydrogen and the two varieties of lithium atoms. … Dr. J.D. Cockcroft, present at the meeting, was delighted to learn that the atom rearranging he did with Dr. E.T.S. Walton in [the] Cavendish Laboratory last year upholds the Einstein Law. … Dr. Bainbridge’s figures show that the mass lost was transformed into energy as the Einstein law requires.106
Aston called Bainbridge’s achievement “a noteworthy triumph in the experimental proof of the fundamental theory of Einstein of the equivalence of mass and energy.”107
Lawrence and Tuve
Ernest Orlando Lawrence was born in Canton, South Dakota, in the southeastern corner of the state, on August 1, 1901, the older of two sons of Carl Gustav and Gunda Lawrence (née Jacobson); their younger son, John Hundale, was born on January 7, 1904. Their father was a graduate of the University of Wisconsin who returned to Madison for graduate study in history and physics and became Superintendent of Canton City Schools while also teaching in high school. Their mother, five years younger than their father, was a graduate of a state teachers’ college, where she specialized in mathematics and was hired by her husband as a second high school teacher in Canton. Both were devout Lutherans of Norwegian ancestry.108 Ernest and John attended public schools in Canton until 1911, when their father was elected State Superintendent of Public Instruction. He sold their house in Canton and moved his family to Pierre, the state capitol.
Ernest Lawrence’s closest friend in Canton was Merle Anthony Tuve, who was born on June 27, 1901,109 just over a month before Ernest. His father, Anthony G. Tuve, had been President of Augustana College since 1890, six years after its predecessor, a Norwegian Lutheran seminary and academy, moved from Beloit, Iowa, to Canton. His mother, Ida Marie Tuve (née Larsen), taught music at the college. Merle was the second of their four children (three sons and one daughter). He and Ernest Lawrence were inseparable as children and as grade school students in Canton. They were engrossed in electricity before the age of nine, spending hours building motors and batteries, reading popular magazines on electricity, and toying with ham radio. “Spark coils, bells, buzzers, and motors littered the Lawrence house and the Tuve basement.” Lawrence “seemed unable to get home from school fast enough to get back to his electrical apparatus.”110
Lawrence’s and Tuve’s friendship endured, but their life trajectories diverged in January 1911, when Lawrence’s father moved his family from Canton to Pierre. Tuve’s (p.200) father died seven years later in the horrific influenza pandemic of 1918, in which an estimated twenty to fifty million people perished. His mother and the children moved to Minneapolis, where Merle received his B.S. at the University of Minnesota in 1922 and his A.M. in 1923, both in physics.111
Lawrence entered St. Olaf College in Northfield, Minnesota, in the fall of 1918 but one year later transferred to the University of South Dakota in Vermillion, where he received his A.B. in chemistry with high honors in 1922. Lewis Akeley, Dean of the College of Engineering, tutored Lawrence privately in physics and convinced him to pursue graduate work in physics at the University of Minnesota. Lawrence remained deeply grateful to Akeley throughout his life: “On the wall of Lawrence’s office [at Berkeley], Dean Akeley’s picture always had the place of honor in a gallery that included photographs of Lawrence’s scientific heroes: Arthur Compton, Niels Bohr, and Ernest Rutherford.”112
Merle Tuve also encouraged his friend Ernest to do graduate work in physics at Minnesota, where Henry Erikson, Chairman of its Department of Physics, was developing a strong faculty.113 In 1916, he had hired two young instructors in physics, Arthur Holly Compton, who had just received his Ph.D. at Princeton, and John (Jack) Torrence Tate, who had received his Ph.D. in Berlin in 1913. In 1918, Erikson appointed William Francis Gray Swann, an accomplished cellist who had received his D.Sc. at University College London in 1910, as Professor of Physics. In 1922, Erikson appointed John Hasbrouck Van Vleck, who had just received his Ph.D. at Harvard, as Assistant Professor of Physics. Tuve received his A.M. under Tate in 1923, and Lawrence received his A.M. under Swann in 1923. Lawrence never forgot his debt to Swann “for his part in setting him on his course” by giving him “the thorough grounding in electrodynamics and magnetism that was basic to his future achievements.”114
In the middle of 1923, Swann left Minnesota for the University of Chicago and invited Lawrence to accompany him. Lawrence made some progress on his doctoral research at Chicago but realized he would not be able to complete it in at least a year. Still, he was encouraged by Arthur Holly Compton, who himself had just left Washington University in St. Louis for Chicago.
I derived tangible evidence and assurance that not all the important discoveries had been made but that science was alive and throbbing with growth, that I might indeed be able somehow to contribute in the never-ending search for knowledge.115
Compton, on his part, was greatly impressed by Lawrence.
He had an extraordinary gift of thinking up new ideas that seemed impossible of achievement and making them work. In our conversations in the laboratory our relations had been more those of research colleagues than those of student and teacher.116
Swann surprised Lawrence again at the end of the academic year, telling him that he was going to leave Chicago in the fall to accept a professorship at Yale. He again invited Lawrence to accompany him, and suggested that he apply for a Sloane Fellowship at Yale. Lawrence happily agreed.
(p.201) Lawrence arrived in New Haven, Connecticut, in September 1924, where Swann welcomed him to Yale. He also took him to meet John Zeleny,117 who had risen through the academic ranks at Minnesota and in 1915 had been appointed Professor and Head of Physics in the Sheffield Scientific School at Yale. Lawrence passed the required French examination at the end of September, and the required German examination soon thereafter, both “by the slimmest of margins.”118 He passed his oral doctoral examination at the end of October and began work on his thesis, investigating the photoelectric effect in potassium vapor. He finished it at the end of January 1925, and Swann submitted it to The Philosophical Magazine for publication.119 In February, Swann nominated Lawrence for a National Research Council (NRC) Fellowship. The rules required that it be held at another university, but an exception was made for Lawrence, to enable him to continue his experiments at Yale for at least another year.120 He received his Ph.D. at commencement on June 17, 1925.
Lawrence continued his experiments on the photoelectric effect but also began a new series with Jesse Beams (Figure 8.4), who was born in Belle Plaine, Kansas, in 1898, and had received his M.A. in mathematics at the University of Wisconsin in 1922 and his Ph.D. in physics at the University of Virginia in 1925.121 Lawrence and Beams quickly (p.202) became close friends, and they merged their areas of experimental expertise, Lawrence’s on the photoelectric effect and Beams’s on the measurement of very short time intervals by chopping light into “very short segments” and passing it through a rapidly operating [Kerr cell] shutter.b They concluded that light quanta are less than three centimeters in length, and that an electron absorbs a light quantum photoelectrically in less than 10−10 second.122 Lawrence gave a talk on their experiments at a meeting of the American Physical Society in Philadelphia at the end of December 1926,123 where it became “by far the most discussed issue of the meeting.”124
Swann left Yale in May 1927 to become the first director of the Bartol Research Foundation in Philadelphia, which he soon moved to nearby Swarthmore. One year later, Lawrence also decided to leave Yale to accept—after protracted negotiations—an associate professorship at the University of California at Berkeley. He was promoted two years later, becoming the youngest full professor on the Berkeley faculty. His future colleague Luis Alvarez placed his decision to leave Yale within its academic context.
It is difficult for one starting on a scientific career today to appreciate the courage it took for him to leave the security of a rich and distinguished university and move into what was, by contrast, a small and only recently awakened physics department. In later life, when he needed to reassure himself that his judgment was good even though he disagreed with the opinions of most of his friends, he would recall the universally dire predictions of his eastern friends; they agreed that his future was bright if he stayed at Yale, but that he would quickly go to seed in the “unscientific climate of the West.”125
Merle Tuve, Lawrence’s close friend, also had progressed in his career. After receiving his A.M. at Minnesota in 1923, he held a one-year instructorship in physics at Princeton University and then transferred to The Johns Hopkins University in Baltimore, where he received his Ph.D. in 1926. He had spent the summer of 1925 working in the Department of Terrestrial Magnetism of the Carnegie Institution of Washington, where its acting director, John Fleming, offered him an appointment to begin on July 1, 1926, to work with Gregory Breit to construct a Van de Graaff electrostatic generator.126
Tuve was working on this project when Lawrence visited him on his way to Berkeley one afternoon in the late spring or early summer of 1928. Tuve (Figure 8.5) had a “vivid memory” of their discussion.
I asked Ernie what research he was going to do at Berkeley. He responded, rather vaguely, with some small notions about high-speed rotating mirrors, chopping the tails off quanta and other single-shot ideas. I then talked to him like a Dutch uncle.c I said it was high time for him to quit selecting research problems like choosing cookies at a party; it was time for him to pick a field of research that was full of fresh questions to be answered, and sure of (p.203) rich results after techniques were worked out. I said that any undergraduate could see that nuclear physics using artificial beams of high-energy protons and helium ions was such a field, and that he should stake out a territory there to work and to grow in. … I told him to consider carefully the possible indirect methods of accelerating particles (linear accelerators, etc.). … [My] primary heavy-footed emphasis to Ernie was on the nuclear disintegration of light elements, as first done by Rutherford in 1919, and on billiard-ball scattering of α-particles, as per Rutherford in 1908–1911. … Ernie was very sober and did not seem to resent the rather harsh way I went after his quantum tails. I asked him to join in the questing, and he seemed pleased. I think he was vaguely searching for an identifiable field full of specific problems, and this discussion clearly impressed him.127
Lawrence was indeed impressed, and was warmly welcomed to Berkeley by local physicists and the prominent physical chemist Gilbert N. Lewis. He soon felt at home in the scientific and social life of the university.128
Lawrence extended his experimental research on the photoelectric effect with his graduate student Niels Edlefsen, and on other investigations with two other graduate students. He also spent many hours reading journals—and had an epiphany.
One evening early in 1929 as I was glancing over current periodicals in the University library, I came across an article in a German electrical engineering journal by Wideröe on the multiple acceleration of positive ions.129 Not being able to read German easily, I merely looked at the diagrams and photographs of Wideröe’s apparatus and from the various figures in the article was able to determine his general approach to the problem—i.e. the multiple acceleration of the positive ions by appropriate application of radiofrequency oscillating voltages to a series of cylindrical electrodes in line. This new idea immediately impressed me as the real answer which I had been looking for to the technical problem of accelerating positive ions, and without looking at the article further I then and there made estimates of the general features of a linear accelerator for protons in the energy range above one million volt electrons. Simple calculations showed that the accelerator tube would be some meters in length which at that time seemed rather awkwardly long for laboratory purposes. And accordingly, I asked myself the question, instead of using a large number of cylindrical electrodes in line, might it not be possible to use two electrodes over and over again by bending the positive ions back and forth through the electrodes by some sort of appropriate magnetic field arrangement.130
Thomas (Tom) Johnson, whom Lawrence had come to know well at Yale and who had joined Swann as assistant director of the Bartol Research Foundation in 1927, was on leave in Berkeley in early 1929. He recalled:
I was with Ernest that night, probably in March or April 1929, when he first saw the Wideröe article and we discussed the ideas it invoked. I believe it was the first time the idea of a multiple acceleration had occurred to Ernest, and the next day we discussed the matter again. The idea of bending the orbits in a magnetic field occurred to Ernest immediately on reading Wideröe, but that evening he was worried about synchronization and it was not until the next day that he realized the periods would be the same for successive orbits.131
Lawrence’s former graduate student James Brady picked up the story.
I was one of the first to hear of Lawrence’s idea of the cyclotron [Figure 8.6]. He came into my research room (room 216, Le Conte, at that time) one morning and asked me to come to the blackboard. He said he had been reading a German article by Wideröe the evening before, which gave him a new idea. He proceeded to write a few simple equations on the board, and he pointed out that the radius of the particle orbit disappeared from the equations relating the magnetic field and the frequency of the oscillations. He pointed out that resonance would be maintained regardless of the radius. From his remarks to me, I think he was greatly influenced by Rutherford’s 1919 paper on the disintegration of nitrogen by alpha particles, in which Rutherford predicted the possibility of a variety of disintegrations if one had available laboratory-accelerated particles.132
The two Ernests did share scientific and personal characteristics but also had sharp differences, according to their mutual physicist friend Mark Oliphant.
Both men were extroverts and good “mixers” in company. … Neither was a good speaker or lecturer, yet each influenced and inspired more colleagues and students than any other of his generation. Both built great schools of physics that became peopled with other great (p.205) men. … Each was most generous in giving credit to his junior colleagues, creating thereby extraordinary loyalties.
Rutherford and Lawrence were self-confident, assertive, and at times overbearing, but their stature was such that they could behave in this way with justice, and each was quick to express contrition if he was shown to be wrong. Neither Rutherford nor Lawrence could tolerate laziness or indifference in those who worked with them ….
But there was one great difference. Rutherford enjoyed what has been called smoking-room humor. Although his own memory for such stories was not good, his great roar of booming laughter was to be heard after dinner as he savored the subtlety of some lewd tale. I never heard Lawrence swear, under any circumstances, and his reaction to off-color humor was not encouraging.
Both Lawrence and Rutherford could be devastatingly blunt and uncompromising when faced with evidence of lack of integrity, or of gullibility, in scientific work.133
As James Brady testified, he was present when Lawrence wrote down the equation for a particle of mass m and charge e moving with a velocity v in a circular orbit of radius r perpendicular to a superposed magnetic field of intensity H, namely, (mv2/r) = (Hev/c), where c is the velocity of light. It follows that the particle’s time of travel t in half of its (p.206) orbit, t = (πr/v) = (πmc/He), does not depend on its radius r and velocity v. Moreover, its kinetic energy T at the periphery of its orbit is T = (mv2/2) = (r2H2e2)/(2c2); in other words, it is proportional to the square of its radius r and to the square of the magnetic field intensity H, so larger sizes and bigger magnets are better. Lawrence’s future colleague Edwin McMillan judged this to be “the single most important invention in the history of accelerators; it brought forth a basic idea of great power, and one capable of later elaborations and variations.”134
Lawrence was now off and running. He gave his graduate student Niels Edlefsen the task of making a glass vial, sealing it with wax, and coating it internally with silver, to make two hollow, facing D-shaped electrodes—dees—to be connected to a high-frequency oscillator circuit. Protons inside the dees circle in ever-increasing orbits, and are incrementally accelerated each time the polarity of the dees is changed when they cross the gap between them. Lawrence (Figure 8.7) explained this at a meeting of the National Academy of Sciences in Berkeley on September 19, 1930, and he and Edlefsen published (p.207) a brief account in Science on October 10, 1930.135 “It was a failure,” Lawrence said, “but it did show promise.”136
That promise was fulfilled by M. Stanley Livingston (Figure 8.8). Born in Broadhead, Wisconsin, on May 25, 1905, Livingston was the only son of four children of a minister and his wife, a descendant of an influential Dutch family. They moved to southern California in 1910, where Livingston grew up in Burbank, Pomona, and San Dimas. His father became a high school teacher and later principal. His mother died when he was twelve years old, and a few years later his father remarried, giving him five half-brothers.
Livingston graduated from high school and entered Pomona College in 1921, where he initially majored in chemistry but also took physics courses toward the end of his studies, receiving his A.B. with a double major in chemistry and physics in 1926. He then went east to Dartmouth College in Hanover, New Hampshire, where he received his M.A. in physics in 1928, stayed on for another year, and then chose Berkeley over Harvard for his doctoral studies.137
Livingston first met Lawrence in 1929 as a student in Lawrence’s electricity and magnetism course.
I was greatly impressed with his enthusiasm and his vivid personality. He seemed always to emphasize the important concepts and conclusions, but took a rather cavalier attitude toward factors of 4π or other details in theoretical developments.138
In the early summer of 1930, Livingston asked Lawrence to propose a topic for his doctoral thesis. He “suggested a study of the resonance of hydrogen ions with a radiofrequency electric field in the presence of a magnetic field,” which Edlefsen had investigated for his Ph.D. thesis. Livingston reworked Edlefsen’s apparatus and then, “with Lawrence’s continued enthusiastic interest and supervision,”139 he built a vacuum chamber with a brass ring around it and flat brass covers, mounted electrodes inside it, and made everything vacuum tight with sealing wax. The radiofrequency electrode, a hollow dee to which he could apply 1000 volts at a variable frequency, faced a “dummy dee” in the chamber. At its center was a source of hydrogen molecular ions (H2+) that were accelerated incrementally to its periphery and were observed in a shielded “collector cup.”
In the summer of 1930, in the midst of this demanding work, Livingston courted Lois Robinson, a Berkeley graduate student in English, who recalled: “We mostly did our courting over tea and toast, after he left the lab at 11 o’clock in the evening.”140 They married on August 8, 1930, in the Pomona College Chapel;d their daughter Diane (Dee) was born in 1935 and their son Stephen in 1943.
In November 1930, Livingston “first observed sharp peaks in the collector current as the magnetic field was varied over a narrow range at the calculated resonance frequency.141 Then, by “working straight through the Christmas and New Years holiday,”142 he obtained 13 keV H2+ ions on January 2, 1931. Three months later, Lawrence told Livingston to start writing his thesis and get his degree to make him eligible for an instructorship in the fall. Livingston wrote his thesis in two weeks and defended it on April 14, 1931.143
Livingston used a stronger, ten-inch-diameter magnet shortly thereafter and produced 80 keV H2+ ions. As an important byproduct, he observed and understood how the electric field between the dees and the superposed magnetic field focus the ions as they spiral outward. Lawrence reported these results at a meeting of the American Physical Society in Washington, D.C., from April 30 to May 2. 1931. He commented:
These preliminary experiments indicate clearly that there are no difficulties in the way of producing one million volt ions in this manner. A larger magnet is under construction for this purpose.144
To finance its construction, Lawrence received a $1000 grant from the National Research Council,145 and additional grants from the Chemical Foundation and the Research Corporation for Science Advancement, which the Berkeley physical chemist and philanthropist Frederick Gardner Cottrell had founded in 1912.
(p.209) During the summer and fall of 1931, Livingston installed the ten-inch-diameter magnet in Room 339 of Le Conte Hall, the departmental home of the physics department. He connected the dees to a radiofrequency oscillator supplied by the Federal Telegraph Company in San Francisco, which Leonard F. Fuller, its Vice President and Chairman of Berkeley’s Department of Electrical Engineering,146 had arranged. He produced 0.5 MeV H2+ ions in December 1931 and 1.22 MeV protons in January 1932. Livingston never forgot
the day when I had adjusted the oscillator to a new high frequency and, with Lawrence looking over my shoulder, tuned the magnet through resonance. As the galvanometer spot swung across the scale, indicating that protons of 1-MeV energy were reaching the collector, Lawrence literally danced around the room with glee. The news quickly spread through the Berkeley laboratory and we were busy all that day demonstrating million-volt protons to eager viewers.147
They submitted a full report on their pioneering achievement to The Physical Review on February 20, 1932, where it was published on April 1.148
By that time, Lawrence had another reason to be excited. On August 16, 1931, he had met with Robert G. Sproul, President of the University of California, who had agreed to assign an old civil engineering testing laboratory near Le Conte Hall to Lawrence for his research.149 The Princeton physicist Joseph Boyce, who was visiting Berkeley, described the move to John Cockcroft, his friend and former colleague at the Cavendish Laboratory, in a letter on January 8, 1932.
Lawrence is just moving into an old wooden building back of the physics building where he hopes to have six different high-speed particle outfits. One is to move over the present device by which he whirls protons in a magnetic field and in a very high frequency tuned electric field and so is able to give them velocities a little in excess of a million volts. … The fourth is a whirling device for protons in a magnet with pole pieces 45 inches in diameter. … Lawrence is a very able director, has many graduate students, adequate financial backing, and … has achieved sufficient success to justify great confidence in his future.150
Luiz Alvarez was intimately familiar with Lawrence’s demanding work ethic.
The great enthusiasm for physics with which Ernest Lawrence charged the atmosphere of the Laboratory will always live in the memory of those who experienced it. The Laboratory operated around the clock, seven days a week, and those who worked a mere seventy hours a week were considered by their friends to be “not very interested in physics.” The only time the Laboratory was really deserted was for two hours every Monday night, when Lawrence’s beloved “Journal Club” was meeting.151
Lawrence had installed the new magnet with its forty-five-inch-diameter core in his new laboratory in December 1931, which the Federal Telegraph Company had built and transferred to the university.152
These accomplishments formed the foundation of Lawrence’s scientific life. He established the foundation of his personal life when he and Mary Kimberly Blumer married in (p.210) historic Trinity Church on the New Haven Green on May 14, 1932.153 Molly, as she was always called, was the oldest of three daughters of George Blumer, Dean of the Yale Medical School, and Mabel Louise Blumer née Bradley. Lawrence had met her in June 1926, fifteen months after he had arrived at Yale, on a blind date for her commencement dance at nearby Gateway School.154 She was sixteen, he twenty-five. She was a brilliant student and went on to obtain her bachelor’s degree with honors in bacteriology at Vassar in 1930, after which she began graduate study at Radcliffe but took most of her courses at Harvard Medical School. They were engaged in August 1931, and after marrying made their home in Berkeley and raised a family of two boys and four girls. “With Ernest and the children, Molly Lawrence created a home that was famous throughout the world of physics for its warmth and hospitality.”155
Meanwhile, Lawrence and Livingston “had barely confirmed” the results they had published in The Physical Review on April 1, 1932, when they read the July 1 issue of the Proceedings of the Royal Society, in which Cockcroft and Walton announced their disintegration of lithium with 500 keV protons.156 (They seem not to have read Cockcroft and Walton’s preliminary report in the April 30, 1932, issue of Nature.157) Lawrence and Livingston could not repeat—or even check—Cockcroft and Walton’s pioneering experiment, because they “did not have adequate instruments to observe disintegrations.”158 Lawrence therefore “sent an emergency call” to his physicist friends at Yale, Donald Cooksey and Franz Kurie, asking them to come to Berkeley. They joined forces there with Lawrence’s graduate student Milton White to continue the work that his former graduate student James Brady had begun. They assembled the necessary counters and instruments and confirmed Cockcroft and Walton’s disintegration of lithium with 500 keV protons.159 They also noted that Lawrence’s colleague Robert Oppenheimer had calculated, “along the lines of Gamow’s theory, the probability that an alpha-particle will be liberated by a proton striking … lithium,”160 finding good agreement with their experiments.
Livingston had the forty-five-inch-diameter core of the Federal Telegraph magnet “machined to form flat pole faces initially tapered to a 27½-inch diameter,”161 thereby moving to the next stage in Lawrence’s program of building machines of ever larger diameters to produce ever higher energies. By 1935, as he and two colleagues noted in a footnote, that machine had come to be called a “cyclotron … as a sort of laboratory slang.”162
Livingston reflected that, throughout this work,163 he was the mechanic who built things with his hands that gave Lawrence full satisfaction, but Lawrence, who was the most dramatic person he had ever known,164 always
was the leader and the central figure, enthusiastic over each new result, intent on each new technical problem, in and out of the laboratory at all hours up to midnight, convinced that we were making history and full of confidence for the years ahead.165
Physicists knew it was only a matter of time before Lawrence would receive a Nobel Prize, which he did in 1939, but he could not go to Stockholm to receive it on December (p.211) 10, because Germany had invaded Poland on September 1. He received it in an award ceremony at Berkeley on February 29, 1940.166
In November 1939, congratulations had poured in when physicists learned that Lawrence would receive the Nobel Prize. One came from physicist Lee DuBridge, Dean of the Faculty of Arts and Sciences at the University of Rochester, who sent a limerick that was immediately posted on Lawrence’s blackboard.
- A handsome young man with blue eyes
- Built an atom-machine of great size,
- When asked why he did it
- He blushed and admitted,
- “I was wise to the size of the prize.”167
Five Nobel Prizes in Physics
Each of the discoverers of the three new particles and the inventors of the two new accelerators in the year spanning 1931–2 was awarded a Nobel Prize: Harold C. Urey (Chemistry, 1934) “for his discovery of heavy hydrogen”;168 James Chadwick (Physics, 1935) “for his discovery of the neutron”;169 Carl D. Anderson (Physics, 1936) “for his discovery of the positron” (shared with Victor F. Hess “for his discovery of cosmic radiation”);170 John D. Cockcroft and Ernest T.S. Walton (Physics, 1951) “for their pioneer work on the transmutation of atomic nuclei by artificially accelerated atomic particles”;171 and Ernest O. Lawrence (Physics, 1939) “for the invention and development of the cyclotron and for results obtained with it, especially with regard to artificially radioactive elements.”172 That was a harvest of Nobel Prizes never equaled for scientific achievements in such a short period of time. That two of the three discoveries and two of the three inventions were made by Americans was a harbinger of the momentous shift that was occurring in the scientific center of gravity of nuclear physics.
(8.) Cockcroft interview by Thomas S. Kuhn, May 2, 1963, p. 4 of 20.
(14.) Cockcroft interview by Thomas S. Kuhn, May 2, 1963, p. 5 of 20.
(20.) Hartcup, Guy and Thomas E. Allibone (1984), pp. 31–3.
(24.) Oliphant, Mark L.E. and Lord Penney (1968), p. 143.
(32.) Walton, Ernest T.S. (1987), p. 44.
(44.) Cockcroft interview by Thomas S. Kuhn, May 2, 1963, pp. 14–15 of 20.
(48.) Cockcroft interview by Thomas S. Kuhn, May 2, 1963, p. 15 of 20.
(56.) Hartcup, Guy and Thomas E. Allibone (1984), p. 43.
(65.) Allibone, Thomas E. (1984b), p. 156.
(67.) Allibone, Thomas E. (1984a), pp. 4–5.
(81.) Science at the Cross Roads (1931).
(82.) James Clerk Maxwell: A Commemoration Volume 1831−1931 (1931).
(83.) “The Clerk Maxwell Centenary Celebrations” (1931).
(89.) Quoted in Hartcup, Guy and Thomas E. Allibone (1984), p. 50.
(97.) Cockcroft, John D. and Ernest T.S. Walton (1932b).
(104.) Bainbridge, Kenneth T. (1933c); see also Bainbridge, Kenneth T. (1933b).
(106.) “The Chicago Meeting” (1933).
(114.) Childs, Herbert (1968), p, 65.
(b) A Kerr cell is a vessel filled with an optically active liquid in which the plane of polarization of light is rotated by a superposed oscillating electric field, so that the light either does or does not pass through an appropriately oriented analyzer as the field changes polarity, thus turning the cell into a light shutter.
(c) One of at least a dozen pejorative insults the English hurled at the Dutch during the Anglo-Dutch wars in the seventeenth century, this one meaning, according to Webster’s, “a person who bluntly and sternly lectures or scolds someone, often with benevolent intent.”
(138.) Livingston, M. Stanley (1969b), p. 22.
(140.) Quoted in Website “The Sante Fe Report Reporter from Santa Fe, New Mexico.”
(d) I thank our family friend Carla Hunter Gates for locating an announcement of their impending marriage in The New York Times.
(141.) Livingston, M. Stanley (1969b), p. 25.
(154.) Website “Lab Mourns Death of Molly Lawrence, Widow of Ernest O. Lawrence.”
(159.) Lawrence, Ernest O., M. Stanley Livingston, and Milton G. White (1932).
(163.) Amaldi, Edoardo (1984), pp. 57–62; Heilbron, John L. and Robert W. Seidel (1989), pp. 18–44, 71–116; Mladjenović, Milorad (1998), pp. 90–5; Dahl, Per F. (2002), pp. 49–52, 61–9, 75–9; Fernandez, Bernard (2013), pp. 296–301, for other accounts.
(164.) Livingston interview by Charles Weiner and Neil Goldman, August 21, 1967, p. 23 of 73.
(169.) Nobel Foundation (1965), p. 331.