by Brian Cathcart, Viking. Hardback ISBN 0670883212, £14.99.
The “fly” in question is the nucleus and the “cathedral” is the atom, and this is the account of “how a small group of Cambridge scientists won the race to split the atom”. The story begins in 1909, after Ernest Rutherford’s student, Ernest Marsden, found that when alpha particles are scattered by a gold foil the Rutherford formula is not exactly satisfied. There is therefore evidence that the nucleus is not just a point, but a “fly”. After moving from Manchester to Cambridge, Rutherford and his collaborators wanted to know more about what is inside the “fly”.
In the Cavendish Laboratory at Cambridge in 1927 there were two lines of approach to the study of the nucleus: the traditional one using naturally produced particles such as alphas, gamma rays, etc, and another that was trying to accelerate particles artificially in the laboratory.
James Chadwick was following the first approach, and this led him to the discovery of the neutron in 1932 and to the Nobel prize in 1934. In 1930 two German physicists, Walther Bothe and Herbert Becker, had reported that by bombarding beryllium nuclei with alpha particles from a polonium source they had registered the emission of a powerful neutral radiation. Later, the Joliot-Curies reproduced the phenomenon and proved that the mysterious radiation could knock protons out of a block of paraffin – something that gamma rays of available energies could not have done. When their “Note aux comptes rendus de l’Academie des Sciences” arrived in Rome in January 1932, according to what Gian-Carlo Wick, who was present, told me, Ettore Majorana exclaimed: “Stronzi (idiots), they have not understood that it is the neutron.” Soon after, Chadwick proved by careful experimentation that it really was the neutron, with a mass close to the proton. Another important experiment belonging to the same category, but not mentioned in the book, is the photodisintegration of the deuteron into a proton and a neutron by Chadwick and the then young Maurice Goldhaber (who is now 93 years old). They showed that the neutron was slightly heavier than the proton.
The other approach, trying to accelerate particles, marked the beginning of a new era that CERN continues, through its past achievements and its future projects. This is why the detailed account given in this book, including the successes and failures and the personal lives of the protagonists, seems so interesting.
The main protagonists were a young Irishman, Ernest Walton, a Briton named John Cockcroft, their friend Thomas Allibone from the Metropolitan-Vickers laboratory, and naturally Rutherford himself, who said in his address as president of the Royal Society in November 1927: “It would be of great scientific value if it were possible in laboratory experiments to have a supply of electrons and atoms of matter in general, of which the individual energy of motion is greater even than that of the alpha particle. This would open up an extraordinary new field of investigation that could not fail to give us information of great value, not only in the constitution and stability of atomic nuclei but also in many other directions.”
Cockroft and Walton worked very hard within the limits allowed by the rules of the Cavendish Laboratory, which closed at 6 p.m. and during holidays. But the friendly competition between Europe and the US was already fierce, a little like we see today! In the US Lawrence and Livingstone were working on their cyclotron, Van de Graaf had the machine that bears his name, and Lauritsen had his X-ray machine. There were also competitors in Europe, such as Greinacher, who independently of Cockcroft and Walton had the idea of voltage multiplication.
At this point it is necessary to mention a theoretician of Russian origin, George Gamow, who, as the author explains very well, played an important role in predicting that the attempts of Cockcroft and Walton would be successful. Gamow was probably the first to realize that quantum mechanics applied not only to the electrons running around the nucleus but also to the constituents of the nucleus. Before the discovery of the neutron he was extremely unhappy at having electrons inside the nucleus because their wavelength was much larger than the size of the nucleus. He produced a beautiful explanation of the alpha decay of nuclei by the tunnelling of alpha particles through the Coulomb barrier of the nucleus, a typical quantum mechanical effect. The alpha particles don’t need to have an energy as high as the classical barrier but can “borrow” energy for a short time to cross it. This was very important for the Cambridge machine builders because it meant that protons can penetrate inside the nucleus without having the full energy to cross the barrier. Even Rutherford, who had some repulsion for theory, liked this.
The Cockcroft-Walton accelerator finally started working at the beginning of 1932, but the protons they directed at beryllium and lithium targets did not seem to produce any clear effect. They were looking for gamma rays and saw practically none. This was a serious disappointment and they feared that Lawrence, with a higher energy, would win the race. Rutherford was beginning to get irritated. So they tried using a scintillation detector of zinc sulphide that had been used in the past to detect alpha particles and then they saw a beautiful signal, which was immediately interpreted by Rutherford to be a pair of alpha particles. This was at 800,000 volts, clearly below the classical Coulomb barrier. Then in complete agreement with Gamow’s theory, they lowered the voltage to 150,000 volts and still saw the effect. In this case Rutherford broke the rule of closing the laboratory at 6 p.m. The time of the “night shift” was approaching.
The reaction observed was p + 7Li → α + α with a kinetic energy release of 8 MeV. It was a tremendous success and as the subtitle of the book says, they “won the race to split the atom”. The press jumped on that, but Cockcroft and Walton disliked the statement that this could be “a new source of energy”. In fact the press was right. The discovery of the fission of uranium was not too far away and this kind of proton-induced fission, except for the fact that it uses light elements, is not fundamentally different from what is proposed now by Carlo Rubbia as a new source of energy. Later, long after the unfortunate death of Rutherford due to delays with a hernia operation in 1937, Cockcroft and Walton received the Nobel prize in 1951.
There are also many other people who are rightly quoted in the book, such as Kapitza, who after spending several years in Cambridge was forced by Stalin to stay in the USSR; Blackett, who started cosmic-ray research with Occhialini; and the theoreticians – Dirac of course, but also Mott, Massey, Hartree and so on.
To end this review I would like to complete the postscript of the book, in particular regarding the links of Cockcroft with CERN. A biography of Cockcroft by Ronald Clark says (on page 101) that he “loaned someone from Harwell to build one of the accelerators of CERN”. This accelerator was the Proton Synchrotron (PS), and the “someone” was John Adams. Cockcroft directed radar research in Malvern during the Second World War and one of the people he hired was an engineer called John Adams (once, John complained to me that journalists thought he was originally a “mechanic”). Another was Mervyn Hine (who died very recently, see his obituary in “Faces and Places”). When the war was over Cockcroft retained these two people at the Atomic Energy Research Establishment at Harwell, where they worked on accelerator research; they then moved to CERN with the fantastic success that we all know. Until 1992 the pre-injector of the PS was a Cockcroft-Walton accelerator, and Cockcroft was a member of the CERN Scientific Policy Committee from 1956 until 1961. So there is a link between Rutherford, Cockcroft and Adams for which we must have a great deal of gratitude.
