“Now I know what the atom looks like.” – Ernest Rutherford
During the 1920s, the only particles that were available to physicists for bombarding atoms were alpha particles emitted by naturally radioactive elements, such as radium. Rutherford had opened the door to study of the atom with the alpha particles but, as he said, he hadn’t really smashed the atom yet, just nicked it. To explore the inner workings of the atom, scientists needed more energetic particles to bombard atoms to force them to reveal their secrets.
Rutherford encouraged one of his researchers, John Cockcroft, to initiate a high-voltage accelerator program with the assistance of a student, Ernest Walton. Cockcroft was somewhat of an odd duck at the Cavendish Laboratory; he had both a background in mathematics and training as an electrical engineer, and his theoretical grasp of problems and ability to design and build complex equipment were a real asset to Rutherford. Walton was a few years younger than Cockcroft and had come from Dublin to the Cavendish to complete a Ph.D. Cockcroft, with assistance from Walton, devised an instrument in 1929 that could multiply voltages to hundreds of thousands of volts so that they could use the electric field from this high voltage to accelerate protons to extremely high velocities. Cockcroft had been working with a former Cavendish alumnus, the theoretical physicist George Gamow, who had predicted that the nucleus of the atom could be split by protons of much lower energy levels than seemed possible, by virtue of the quantum mechanical “tunneling” phenomenon.
Figure - The apparatus used by Ernest Rutherford in his atom-splitting experiments, set up on a small table in the Cavendish Laboratory.
The day that transformed atomic physics was April 14, 1932, when Cockcroft and Walton bombarded lithium with high energy protons and produced alpha particles. Rutherford had been pushing the two for results as they had been working on the particle accelerator for years. Cockcroft was the lead investigator and Walton took care of the day-to-day work in the laboratory. Walton recalled later the events of that fateful day, “When the voltage and the current of protons reached a reasonably high value, I decided to have a look for scintillations. So I left the control table where the apparatus was running and I crawled over to the hut under the accelerating tube. Immediately I saw scintillations on the screen. I then went back to the control table and switched off the power to the proton source. On returning to the hut no scintillations could be seen. After a few more repetitions of this kind of thing…I then phoned Cockcroft, who came immediately.” The results of the experiment were that the lithium nuclei of mass seven hit by a proton of mass one and disintegrated into two alpha particles (helium nuclei) of mass four. The atom had truly been split this day, as this was the first nuclear reaction initiated through artificially accelerated particles rather than with the aid of natural radioactivity.
Figure – Informal meeting of top European scientists in May 1932 in Münster, Germany: standing from left to right: Georg de Hevesy, Mrs. Geiger, Lise Meitner, Otto Hahn; sitting from left to right: James Chadwick, Hans Geiger, Ernest Rutherford, Stefan Meyer, and Karl Przibram.
The experiment revealed another very important result in that the total mass of the end product was different from the original mass of the hydrogen. The small mass change had been converted into energy in the violence of the atomic collisions. The amount of energy produced corresponded to the amount of energy that should be expected from Albert Einstein’s famous equation E=mc2, which is that the energy is equal to the mass multiplied by the square of the speed of light. This was the first experimental proof of Einstein’s 1905 theory of relativity.
The year 1932 had indeed been a banner year for Rutherford and his “boys” at the Cavendish Laboratory as they had discovered the neutron, split the atom, and validated Einstein’s theory. For their efforts, Cockcroft and Walton would win the 1951 Nobel Prize in physics "for their pioneer work on the transmutation of atomic nuclei by artificially accelerated atomic particles."
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Tragedy struck the Rutherford family two days before Christmas in 1930 when their only child, Eileen, died at age 29. She suffered a fatal embolism nine days after giving birth to her fourth child. Eileen was married to Ralph Fowler, a mathematical physicist at the Cavendish Laboratory. After Eileen’s death, Rutherford spent more time with his four grandchildren and grew closer to his son-in-law, who was part of the old man’s Sunday golf foursome. All four of Eileen and Ralph’s children went on to lead distinguished careers in the fields of science, medicine, and engineering. The tragedy cast a long shadow over Rutherford’s elevation to the peerage in the New Years honors list for 1931, when he became a baron titled “Ernest, Lord Rutherford of Nelson.” He cabled his mother back in New Zealand with the news, saying, “Now Lord Rutherford. More your honor than mine.”
In mid-October 1937, Rutherford was trimming trees in his wife’s garden when he fell from a low branch. The next day he was in great pain and doctors diagnosed the problem as an old umbilical hernia that had been aggravated by the fall from the tree. An emergency surgery had little effect, and Lord Rutherford died with his wife at his side on October 19, 1937. He was buried in Westminster Abby near Sir Isaac Newton and Lord Kelvin. Lady Rutherford lived out the remainder of her years in Christchurch, New Zealand. The New York Times published an obituary for Lord Rutherford, writing, “It is given to but few men to achieve immortality, still less to achieve Olympian rank, during their own lifetime. Lord Rutherford achieved both. In a generation that witnessed one of the greatest revolutions in the entire history of science, he was universally acknowledged as the leading explorer of the vast infinitely complex universe within the atom, a universe that he was first to penetrate.”
The End
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- Doug