Nov 20, 2007
Cockcroft's subatomic legacy: splitting the atom
This year marks the 75th anniversary of the world's first, and probably most famous, accelerator-based subatomic physics experiment. In splitting the atom at the Cavendish Laboratory, it brought a remarkable scientist, John Cockcroft, to the world's attention.
Fission de l'atome: l'héritage de Cockcroft
En avril 1932, John Cockcroft et Ernest Walton réalisaient la première fission de l'atome, à Cambridge (Royaume-Uni). La séquence qui a conduit à cette expérience révolutionnaire a depuis été répétée maintes fois, pour d'autres expériences en physique des hautes énergies: définition des paramètres des faisceaux de particules, en s'appuyant sur la théorie et la phénoménologie du temps, innovation et développement des technologies nécessaires à la création des faisceaux, et enfin utilisation des faisceaux pour réaliser des expériences à l'échelle subatomique, afin de mieux connaître la matière. Cockcroft lui-même a ensuite joué un rôle important dans la création de grands laboratoires d'accélérateurs, en particulier le CERN, en 1954.
In April 1932 John Cockcroft and Ernest Walton split the atom for the first time, at the Cavendish Laboratory in Cambridge in the UK. Only weeks earlier, James Chadwick, also in Cambridge, discovered the neutron. That same year, far away in California, Carl Anderson discovered the positron while working on cosmic rays. So 1932 was a veritable annus mirabilis in which experiments discovered, and worked with, nucleons; exploited Albert Einstein's relativity and energy-mass equivalence principle; took advantage of the newly emerging quantum mechanics and its prediction of "tunnelling" through potential barriers; and even verified the existence of "antimatter" predicted by Paul Dirac's relativistic quantum theory of the electron. It is hard to think of a more significant year in the annals of science.
The experiment by Cockcroft and Walton split the nucleus at the heart of the atom with protons that were lower in energy than seemed possible, by virtue of quantum mechanical tunnelling – a phenomenon new to physics. In 1928 George Gamow had applied the new quantum mechanics to show how particles could tunnel through potential barriers, and how this could explain the decay of nuclei through alpha emission. He also realized that tunnelling could lower the energy required for an incident positively charged particle to overcome the Coulomb barrier of a target nucleus. It was this insight that underpinned the commitment of Cockcroft and Walton.
The entire sequence of events that led to the pioneering experiment (the specification of particle beam parameters based on contemporary theoretical application and phenomenology; the innovation and development of the necessary technologies to create such beams; and the use of the beams to do experiments on a subatomic scale to achieve a deeper understanding of the structure and function of matter) have been repeated many times as high-energy physics has advanced with the construction of accelerators to the current Standard Model of particles and forces. That Cockcroft realized the immense potential of accelerators in research, and in particular for progress in fundamental physics, is manifest in his instrumental role in later years to establish large accelerator laboratories, in particular CERN in 1954.
Cockcroft was born on 27 May 1897 to a family of cotton manufacturers in Todmorden, straddling the Lancashire–Yorkshire border in northern England. In his early years he experienced a varied educational background. He studied mathematics at Manchester University in 1914–1915, but the First World War interrupted his studies with service in the Royal Field Artillery. After the war, he returned instead to the College of Technology in Manchester to study electrical engineering. Later he joined the Metropolitan Vickers ("Metrovick") Electrical Company as an apprentice for two years, but subsequently went to St John's College, Cambridge, and took the Mathematical Tripos in 1924. This wide-ranging education ser