E.T.S. Walton – Nobel Laureate
A brief summary of the career of Ernest Thomas Sinton Walton is provided in the section on the Erasmus Smith’s professors of natural and experimental philosophy. While studying for his PhD in Cambridge with Ernest Rutherford, Walton worked on the development of particle accelerators. This led to the Cockcroft-Walton linear accelerator described in their first Nature paper in February 1932. They reported accelerating protons from a discharge in hydrogen to a velocity of 1.16 x 109 cm/s.
Soon after on April 14, 1932 Walton observed the signature of alpha particles after bombarding a lithium target: the lithium broke into two helium nuclei. Walton described this moment “We had rigged up a scintillator screen of willemite. If there were any fast particles coming out they would produce scintillations in this material. I left the control bench – the apparatus was giving voltages of something of the order of six or seven hundred thousand volts – and I crawled across the room on my hands and knees in order to avoid the high voltages and went into a little hut that we had built under the apparatus. We were quite safe from these high voltages and also shielded from the X-rays that were being produced in the apparatus. When I looked in through the microscope I could see a whole lot of little stars suddenly appearing and just as suddenly disappearing.”
As Walton explained “… a proton was going into the nucleus of an atom. Now a proton has a mass of one unit on the atomic scale. Some of the lithium atoms, in fact the majority of them, have a mass of seven units. So when you add one to seven you get eight units. A lot of surplus energy is available and the atom that has been formed in this way immediately blows asunder and two-alpha particles come out, each of them of mass four; and they come out in opposite directions with the same velocity.”
Lithium + Hydrogen = Helium + Helium + Excess Energy (17 meV)
This was the first time that Einstein’s mass/energy E=mc2 was verified directly in nuclear reaction. c is the speed of light so when even a small mass, m, is destroyed a very large amount of energy, E, is released. Their experiment also verified Gamov’s predictions of quantum tunnelling using the new theory of wave mechanics. The Nobel Prize citation reads their work “profoundly influenced the whole subsequent course of nuclear physics [and] stands out as a landmark in the history of science.”
Van de Graaff particle accelerator at TCD in the 1950s.
After Walton returned to the Physics department in Trinity in 1934. In the 1950s he constructed, with his colleague in physics, Bobble Elliott, a van de Graaff particle accelerator for inducing nuclear reactions. In this device the large metal conductor is charged up to a very high voltage by using an insulating conveyor belt to carry positive charge on to the conductor. Positively charged protons or heavier atoms are produced by ionisation inside the large conductor and are accelerated to ground potential by the electric field in the accelerator tube, which is displayed in the Fitzgerald Library. The specially shaped metal conductors inside the tube act as Einzel lenses to refocus the particle beam at each stage.
The accelerator tube with the specially shaped metal conductor Einzel lenses.
Sources
- Vincent McBrierty (2003), Ernest Thomas Sinton Walton 1903-1995, Trinity College Dublin Press.
- The image of the Cockcroft-Walton accelerator is courtesy of the Cavendish Laboratory.
- The image of the Walton’s Van de Graaff accelerator in the Physical laboratory is courtesy of Ian Elliott.