Nobel prize to J. D. Cockcroft and E. T. S. Walton awarded in 1951 "for their pioneer work on the transmutation of atomic nuclei by artificially accelerated atomic particles''
Cockcroft, J.D.; Walton, E.T.S.; Disintegration of Lithium by Swift Protons
Nature 129 (1932) 649;
In a previous letter to this journal we have described a method of producing a steady stream of swift protons of energies up to 600 kilovolts by the application of high potentials, and have described experiments to measure the range of travel of these protons outside the tube. We have employed the same method to examine the effect of the bombardment of a layer of lithium by a stream of these ions, the lithium being placed inside the tube at 45° to the beam. A mica window of stopping
power of 2 cm of air was sealed on to the side of tube, and the existence of radiation from the lithium was investigated by the scintillation method outside the tube. The thickness of the mica window was much more than sufficient to prevent any scattered protons from escaping into the air even at the highest voltage used. 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 the highest voltage used, namely 400 kilovolts. At this point many hundreds of scintillations per minute were observed using a proton current of a few microampers. No scintillations were observed when the proton was cut off or when the lithium was shielded from it by a metal screen. The range of the particles was measured by introducing mica screens in the path of the rays, and found to be about eight centimetres in air and not to vary appreciably with voltage. To throw light on the nature
of these particles, experiments were made with a Shimizu expansion chamber, when a number of tracks resembling those of -particles were observed and of range agreeing closely with that determined by the scintillations. It is estimated that at 250 kilovolts, one particle is produced for approximately 109 protons. The brightness of the scintillations and the density of the tracks observed in the expansion chamber suggest that the particles are normal
-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 -particles, each of mass four and each with an energy 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. Experiments are in progress to determine the effect on other elements when bombarded by a stream of swift protons and other particles.
Related references More (earlier) information appears in J. D. Cockcroft and E. T. S. Walton, Nature 129 (1932) 242;
p 7Li 2He
0.125,0.250 MeV (Tlab)
First evidence of nuclear reactions with accelerated protons. Cockcroft-Walton accelerator.