Walton · 1932
Cockcroft and Walton split the atom
In the early afternoon of Thursday the fourteenth of April 1932, in the small high-voltage laboratory at the back of the Cavendish Laboratory in the basement of the New Museums building on Free School Lane in Cambridge, the Irish-born twenty-eight-year-old physicist Ernest Walton and his English-born senior collaborator John Cockcroft, then thirty-four, fired the world's first artificial particle-accelerator beam at a lithium-metal target and produced the first artificial nuclear disintegration in the history of physics. The accelerator (the Cockcroft-Walton voltage-multiplier circuit and the long evacuated glass column that delivered a beam of about three hundred kiloelectronvolt-energy protons against the lithium target) had been built across the previous three years on the floor of the Cavendish high-voltage room. Walton, at the controls on the afternoon of the fourteenth, watched through the small lead-glass observation window at the back of the column as the lithium target on the receiving end produced the characteristic faint blue scintillations of alpha-particle emission, the signature in the zinc-sulphide screen behind the target of the splitting of the lithium-7 nucleus into two helium-4 nuclei by the incoming proton beam. He left the observation window, walked across the basement laboratory to fetch Cockcroft, and the two men confirmed the observation across the next hour. They had artificially split the atom. The result, published in Nature on the thirtieth of April 1932 under the title Disintegration of Lithium by Swift Protons, was the foundational experimental result of modern nuclear physics, the first demonstration that one element could be artificially transmuted into another, and the central single technical breakthrough on which the Manhattan Project of 1942 to 1945 and the nuclear age of the twentieth century would rest. Cockcroft and Walton shared the Nobel Prize in Physics in 1951 for the discovery.
An age is rarely opened by two men in a basement laboratory in front of a single zinc-sulphide screen. The faint blue scintillations Walton saw through the lead-glass observation window on the afternoon of the fourteenth of April 1932 were a few flashes of light on a small chemical screen at the back of an evacuated glass tube. The man at the controls had to walk across the laboratory floor to get his collaborator. Two men confirmed the observation across the next hour. They went home that evening having opened the nuclear age and having only the small set of scintillation counts on a Cavendish log-book to show for it.
THE CAVENDISH HIGH-VOLTAGE ROOM
Ernest Thomas Sinton Walton was born at Abbeyside in the small Waterford coastal town of Dungarvan on the sixth of October 1903, second son of the Reverend John Walton, a Methodist minister of the South Munster circuit, and Anna Sinton. The Walton family moved with the father's Methodist postings through his boyhood (Belfast, Cookstown, Tullamore, Letterkenny, Banbridge), and Ernest was schooled at the Methodist College Belfast from 1915. He took the Royal Junior Trinity Scholarship to Trinity College, Dublin, in 1922, took a first-class BA in mathematics and physics in 1926, and on the strength of the Trinity research-prize was awarded the 1851 Exhibition Scholarship that sent him to Cambridge in October 1927 in his twenty-fourth year as a postgraduate research student under Ernest Rutherford at the Cavendish Laboratory.
He was paired in late 1928 with the Yorkshire-born senior research student John Douglas Cockcroft, then thirty-one, on Rutherford's instruction to attempt the construction of a high-voltage proton-accelerator capable of producing nuclear disintegration. Rutherford had announced at the 1927 Royal Society Anniversary Discourse that the central single unsolved problem of experimental physics was the artificial transmutation of elements by accelerated charged particles, and the Cavendish high-voltage programme was Rutherford's institutional commitment to the problem. Cockcroft and Walton worked together for the next three and a half years in the small high-voltage room at the back of the Cavendish basement on the construction of the apparatus that would carry their joint names.
THE VOLTAGE MULTIPLIER
The technical problem was the generation of sufficient electrical voltage to accelerate protons to energies of several hundred kiloelectronvolts, the threshold (on George Gamow's 1928 quantum-mechanical calculation of the tunnelling probability for proton penetration of the nuclear Coulomb barrier) at which artificial nuclear disintegration would become observable. The existing 1928 electrostatic-generator technology (the Van de Graaff generators that the American physicist Robert Van de Graaff had begun developing at Princeton from 1929) could not deliver the required voltage in a continuous beam.
Cockcroft and Walton solved the voltage problem by the cascade-rectifier circuit (the Cockcroft-Walton voltage-multiplier circuit) that became one of the foundational circuits of twentieth-century electrical engineering. The circuit used a series of capacitors and diode-rectifiers arranged in a cascade ladder, with each stage doubling the input alternating voltage; eight cascade stages in series produced a steady direct-current output voltage of approximately seven hundred kilovolts from an input alternating voltage of approximately two hundred kilovolts. The cascade circuit, built on the high-voltage room floor across 1929 to 1931 with copper-wire-and-glass-bottle capacitors and German-manufactured selenium-rectifier diodes, was operational by late 1931.
The associated proton-beam apparatus was a long vertical evacuated glass column running from a hydrogen-ion source at the top to a target chamber at the bottom, with the cascade voltage applied across the column to accelerate the protons down the column at the cathode-end target. The target chamber contained a thin foil of the element to be bombarded (the candidate elements were lithium, beryllium, boron and carbon), backed by a zinc-sulphide scintillation screen and a small lead-glass observation window through which the experimental physicist could watch the screen for the scintillations characteristic of alpha-particle emission from the bombarded nucleus.
THE FOURTEENTH OF APRIL
Walton was at the controls of the apparatus on the afternoon of the fourteenth of April 1932 (it was Cockcroft's day to teach the undergraduate physics class in the lecture-room two floors above; the apparatus had been fully assembled by lunch-time, and Walton was running the first beam-on test with a lithium target). He brought the cascade voltage up to four hundred kilovolts at one-thirty in the afternoon, opened the proton-beam gate at one-forty, and ran the beam against the lithium target for approximately fifteen minutes while watching the zinc-sulphide screen through the lead-glass observation window.
The scintillations began at approximately one-fifty-five. The faint blue flashes on the zinc-sulphide screen (each flash one alpha-particle striking the screen) appeared in the characteristic pattern of the lithium-disintegration reaction Gamow had predicted four years earlier: the lithium-7 nucleus, absorbing the incoming proton, splitting into two alpha-particles travelling outward at approximately eight million electron volts of kinetic energy each. The pattern was unambiguous. Walton observed it across the next ten minutes through the observation window, was certain by two-ten in the afternoon that the disintegration was real, closed the beam-gate, switched off the cascade voltage, and walked up the basement stairs to the Cavendish lecture-room to fetch Cockcroft. He returned with Cockcroft at two-thirty. The two of them ran the beam again across the next forty-five minutes with Cockcroft at the observation window and Walton on the cascade controls; Cockcroft confirmed the scintillations independently. They had artificially split the atom.
THE NATURE PAPER
They reported the result to Rutherford at the Cavendish staff meeting on the sixteenth of April. Rutherford verified the observation in person at the apparatus on the eighteenth of April (the famous Rutherford visit to the high-voltage room, in which the seventy-year-old founder of nuclear physics climbed under the apparatus to look into the observation window himself and emerged with the comment they look mighty like alpha-particles to me, the Walton-Cockcroft Cavendish-folklore line). The paper was written across the next ten days and submitted to Nature on the twenty-eighth of April under the title Disintegration of Lithium by Swift Protons. It appeared in Nature on the thirtieth of April 1932.
The paper is on every modern syllabus of the foundational experimental results of twentieth-century physics. It was the first artificial transmutation of one element into another (lithium plus proton produces two helium-4 nuclei plus 17.3 megaelectronvolts of energy), the first experimental confirmation of Einstein's mass-energy equivalence equation E equals m c squared (the measured kinetic energy of the emerging alpha-particles matched, within the experimental precision of the time, the mass-loss between the lithium-plus-proton input and the two-helium output), and the foundational technical platform on which the next generation of higher-energy particle accelerators (the cyclotron at Berkeley from 1932, the synchrocyclotrons of the 1940s, the synchrotrons of the 1950s, the Large Hadron Collider of the modern era) was built.
Cockcroft and Walton shared the Nobel Prize in Physics in 1951 for the discovery. Walton returned to Trinity College, Dublin, in 1934 in his thirty-first year as Fellow and from 1946 to 1974 as Erasmus Smith's Professor of Natural and Experimental Philosophy, the senior physics chair in the Republic of Ireland. He served as a member of the Pontifical Academy of Sciences and held honorary degrees from a dozen universities. He died at Belfast on the twenty-fifth of June 1995 in his ninety-second year and is buried at the Deansgrange Cemetery in Dublin. The Walton name in modern nuclear physics carries the weight of the afternoon at the Cavendish high-voltage room on the fourteenth of April 1932.