The Neutron and the Bomb, page 30
Towards the end of the academic year, the Chadwicks had rented a small cottage in North Wales from a local farmer, which they hoped to use at weekends and for holidays with the girls. It seems that James, at least, got little enjoyment from it that first summer. In August 1938, he13 still felt ‘just completely worn out when even talking tires me’. By September he thought he was improving steadily, but in November told Feather that he was ‘determined to be well for the beginning of next term. I diet, take medicines before and after meals, have injections once a week, go to bed at 10 o’clock and do as little work as possible. It sounds as if I had one foot in the grave but it is not quite as bad as that’.14 He consulted Professor Cohen in Liverpool, but the exact diagnosis seems to have been elusive. He was never in robust health after his experiences in the camp at Ruhleben, and had some type of chronic digestive problem. There was also a tendency, from this time on, for Chadwick to become ill at times of personal stress and use it as a reason to withdraw from everyday life, sometimes for prolonged periods. He also suffered from chronic back pain, and ‘My God, my back!’ became ‘almost a catch phrase among the research students’ in Liverpool.9 Sometimes, he would forget himself and on one occasion was seen positively to skip down the corridor from his office, after using his terrible back pain as an excuse to terminate an interview with a research student, who had come to him to register a complaint. Such interviews, at the best of times, would be marked by long silences as Chadwick carefully considered his opinion and decided how to express it. This deliberate manner came to be valued by his junior colleagues as they realized that ‘his judgement was invariably to be trusted’. Holt9 summed up his style of leadership as follows:
The role of leader in the conventional sense did not come naturally to him. He organised the Department as he wanted it to be, assigned people their roles, and then left them to play their part without interference. If there were serious problems or difficulties, of course he was there to advise and help, but one was made to feel responsible for one’s own particular job, and such was the natural respect that one felt for Chadwick that one strove to do well and to earn his approval at the end.
The main forum for discussion in the department was at tea time, when all the research staff would sit round a long table in the small library outside Chadwick’s office. This was the opportunity to dissect a scientific problem or any issue of general interest, but was not a time for ill-considered opinions when the Professor was present. Chadwick’s views were always carefully thought out (indeed he would prefer to keep silent when he was unsure of his ground), and he expected others to adhere to the same exacting standards in their conversation. Any trespass against these would be reproved by a sardonic glance, rendering any verbal rebuke superfluous. As at the Cavendish, he would scrutinize all scientific papers before they were submitted for publication, and often helped his juniors rewrite large sections until the material was in a well-turned and unambiguous form. Chadwick’s forbidding presence did not extinguish all the natural exuberance of his young staff. One afternoon he visited the basement to find a doctoral student, Gerry Pickavance, spreadeagled on the floor with his boilersuit firmly nailed to the boards so that all he could do was to lift his head. Assuming his most lugubrious manner, Chadwick approached the helpless figure and eyed him with curiosity. He then began asking Pickavance a series of questions about his research and progress with the cyclotron, before returning upstairs. On re-entering his office, he remarked to his secretary, Miss Lloyd-Jones, that ‘the boys have been up to their tricks again’.9
The issue of Nature on 16 January 1939 carried a letter from Lise Meitner and her nephew Otto Frisch15 headed, ‘Disintegration of uranium by neutrons: A new type of nuclear reaction’. Lise Meitner had ended her long and productive partnership with the chemist Otto Hahn in Berlin the previous summer. After the German annexation of Austria in March 1938, she was no longer protected by her Austrian nationality and became subject to the anti-semitic laws of Nazi Germany. Although she had grown up as a protestant, as Frisch16 said ‘her honesty did not allow her to conceal her Jewish descent’ and with the help of several colleagues she was smuggled into Holland. From there she spent a short time as a guest of the Bohrs in Copenhagen before moving on to Sweden, where she had been given a position by Manne Siegbahn at the Nobel Institute in Stockholm. At Christmas time, she had received a letter from Hahn, telling her the results of some experiments that he had completed since she left. With another radiochemist, Strassmann, Hahn had found that when uranium is bombarded with neutrons one of the products was barium, a much lighter element far away from uranium in the periodic table. All previous scattering experiments involving α-particles or neutrons had given rise to, or been interpreted as giving rise to, products which were of similar atomic number to the bombarded element. Otto Frisch17 had gone to visit her in Sweden to cheer her up and found her ‘at breakfast brooding over a letter from Hahn.’ He described the unfolding of their ideas.
I was skeptical about the contents — that barium was formed from uranium by neutrons — but she kept on with it. We walked up and down in the snow, I on skis and she on foot (she said and proved that she could get along just as fast that way), and gradually the idea took shape that this was no chipping or cracking of the nucleus but rather a process to be explained by Bohr’s idea that the nucleus was like a liquid drop; such a drop might elongate and divide itself... Lise Meitner worked out the energies that would be available from the mass defect in such a breakup. She had the mass defect curve pretty well in her head. It turned out that the electric repulsion of the fragments would give them about 200 MeV of energy and that the mass defect would indeed deliver that energy...
After a few days, Frisch had returned to Copenhagen where he was working and ‘just managed to tell Bohr about the idea as he was catching his boat to the US.’ Bohr’s reaction was instantaneous — he actually struck his forehead and exclaimed: ‘Oh, what fools we have been. We ought to have seen that before.’ Frisch asked a visiting American biologist what term was used to describe the division of a cell into two daughter cells, and the word ‘fission’ in this physical context (not yet ‘nuclear fission’) first appeared in the Meitner-Frisch letter to Nature:15
... an entirely different and essentially classical picture of these new disintegration processes suggests itself. On account of their close packing and strong energy exchange, the particles in a heavy nucleus would be expected to move in a collective way which has some resemblance to the movement of a liquid drop. If the movement is made sufficiently violent by adding energy, such a drop may divide itself into two smaller drops... It seems therefore possible that the uranium nucleus has only small stability of form, and may, after neutron capture, divide itself into two nuclei of roughly equal size... The whole ‘fission’ process can thus be described in an essentially classical way...
The heavy uranium nucleus plus the captured neutron have a combined mass slightly larger than the combined masses of the two fission fragments or nuclei. By Einstein’s mass-energy equivalence, the fission process liberates a large amount of energy and the two fragments (both positively charged nuclei) fly apart with that energy. When Chadwick3 read the letter, he immediately accepted the findings and the proposed new mechanism of fission because he knew Hahn and Meitner so well and trusted their work. He wondered whether he and Goldhaber might have made the observation themselves back in 1935, when they had bombarded uranium with slow neutrons. In fact, it seems extremely unlikely that they would have succeeded in making the crucial chemical identification of barium, even if their experimental method had been modified to allow them to isolate it. Frisch had also been aided by Bohr’s 1937 liquid drop model of the nucleus in making his own brilliant conceptual leap; so it was a regret which should not have troubled Chadwick for long. Nor did it, for Chadwick was busy with his own affairs, and at the time ‘did not see any interesting consequences from it [fission]... It seemed to me that if something could be done with it, it would be a technical development rather than a search for new physical facts.’3
Although the cyclotron had played no part in the discovery of fission, there had been a remarkable convergence among the main nuclear physics laboratories across Europe during the late 1930s towards acquiring cyclotrons. As a result of Chadwick’s endeavours, Liverpool was in the vanguard of this movement. In France, Joliot18 had become convinced of the need for new particle accelerators in 1935; by 1937 he was installed as Director of the Laboratoire de Synthèse Atomique at Ivry and had commissioned the construction of a large magnet. With the help of one of Lawrence’s acolytes, the French cyclotron started to produce a beam of deuterons within two years. In Copenhagen, Bohr had become very interested in the biological applications of radioactivity and had also secured a large grant to construct a cyclotron. He too borrowed a young physicist from Berkeley for a year, who spent ‘most of his time with his feet upon a table, his chair tilted back, and on his lap a drawing board on which he drew unperturbably one component after another of [the] cyclotron’.19 Similar activities were underway at the Cavendish, Birmingham, the Nobel Institute in Stockholm and in Leningrad. Strangely, there was none under construction in Germany; Chadwick’s old friend Walther Bothe was given permission to commission a cyclotron at the Kaiser Wilhelm Institute in 1938, but the necessary resources were never provided. Other physicists in smaller laboratories wanted to join the cyclotron club, and one of these, Joseph Rotblat20 from Warsaw, decided to come to Liverpool to see how to build one.

The 37-inch Liverpool cyclotron, completed in 1939. (courtesy of the Physics Department, University of Liverpool)
In many ways, the natural place for Rotblat to go would have been to Joliot-Curie’s laboratory at Ivry. His professor in Warsaw was Ludwik Wertenstein, a pupil of Madame Curie: she had remained the honorary director of the Warsaw laboratory until her death in 1934 and Wertenstein knew her family well. Rotblat had learned French at school, but spoke no English. Paris seemed a more glamorous city than Liverpool, but Rotblat had the feeling that the Liverpool machine was at just the right stage of construction; if he went there, he could observe and participate in the crucial stages and then be able to take his expertise back to Poland. Physics in Poland was in a very poor state, but Rotblat was ambitious, and determined to establish a proper school of nuclear physics in Warsaw. Although they had access to only 30 mg of radium in solution, Rotblat21 and Wertenstein pumped off the radon gas into a tube filled with beryllium powder, creating a minute neutron source. Using this simple device, they managed to compete with Fermi’s prestigious group in the discovery of new radionuclides. They also made the first direct observation of the inelastic scattering of neutrons (where the neutron not only changes direction but loses energy in a collision with a target nucleus) and this was the subject of Rotblat’s doctoral thesis. He had used gold as the original scatterer, but had also run experiments using a uranium target. As soon as he heard about fission in February 1939, Rotblat was in a position to replicate the Hahn-Meitner experiment, which he did within a week. He was particularly interested to see if more neutrons would be emitted than absorbed in the fission process and soon found that this was so. The same question had occurred to several other physicists22 and three teams, headed by Joliot, Fermi, and Szilard, independently published affirmative answers in March 1939. Rotblat followed them in April with his own paper in which he suggested that about six neutrons are liberated per fission29. ‘From this discovery’, Rotblat21 found, ‘it was a fairly simple intellectual exercise to envisage a divergent chain reaction with a vast release of energy. The logical sequel was that if this energy were released in a very short time it would result in an explosion of unprecedented power.’ Rotblat21 found the thought so frightening that his ‘first reflex was to put the whole thing out of my mind, like a person trying to ignore the first symptom of a fatal disease in the hope that it will go away. But the fear gnaws all the same, and my fear was that someone would put the idea into practice.’
Leaving Warsaw with a small scholarship which was meant to support him for a year, Rotblat was thirty years old and had never before travelled outside Poland. When he arrived by train at Liverpool’s Lime Street station, he was shocked by the atrocious slums which provided the visitor with his first impressions of the city. His first impressions of the physics department were scarcely more favourable.20
I expected that Chadwick would be the head of a modern department, and I would be impressed by the equipment and so on. But when I came, it did not take very long to realise that the Physics Department was two departments, cohabiting in the same building, but with very little contact between the two. One was the teaching side, the other was the research side. The teaching side was mostly lecturers who were left over from the days of Wilberforce... not concerned in research and... quite happy to go on slowly, teaching the old fashioned type of teaching. Then quite separate, almost physically too because most of the other research was in the basement with the cyclotron — the most up to date equipment there. And the two parts hardly mixed. Chadwick had put in a few people, Kinsey, Kempton and Walke, and they were mostly involved with the cyclotron. But the only link, apart from Chadwick as a figurehead, was in the students who came back and began to work. Pickavance, for example, and afterwards Holt who came to work when the cyclotron started... I was shocked when I went to the teaching lab and discovered they had no a.c. [alternating current]. A teaching lab with no a.c, only d.c. Can you imagine in 1939... how could you teach electricity? It was almost as though you ran a transportation firm and used a cart and horse.
Despite his lack of English, Rotblat was instantly accepted by the members of the department and on his second weekend there was invited to Sunday tea by Chadwick. He thought that this was a normal courtesy and spent the afternoon in Aigburth with Professor and Mrs Chadwick and their two daughters. There were awkward silences and comical misunderstandings, as when Chadwick asked Rotblat23 if his rooms were ‘nice’; Rotblat thought he had said ‘noisy’, and replied: ‘No, no, no — very quiet’. The tea passed pleasantly enough and Rotblat was astonished by the reaction when he went to work on Monday. He was besieged for details about the house and about Aileen because no other member of staff in Chadwick’s four-year tenure had received such an invitation. Chadwick, unlike Rutherford, was never a gregarious man and kept his professional and personal lives completely separate. Aileen was a devoted wife and mother, but, unlike Lady Rutherford, did not see it as her role to entertain her husband’s junior colleagues. She could not forget that she was from Liverpool’s aristocracy and, according to Rotblat,20 ‘did not want to mix with the masses and the workers’. The essentially classless world of science was foreign to her. The family became very fond of Rotblat and would take him to the cottage in North Wales for weekends. There he became Chadwick’s fishing companion and also found Aileen to be a warm person ‘once you got over the class distinction and snobbery’.20
As a research student in his early twenties in Warsaw, Rotblat23 had investigated some short-lived radioisotopes of silver. The lack of facilities in Wertenstein’s department were such that he would perform the neutron bombardment in one place, then have to run down two flights of stairs and into another laboratory to test the sample in a shielded Geiger counter. He was nimble footed and used to take each flight of stairs in one leap so that the whole trip used to take seven seconds. After hundreds of jumps downstairs, he developed stress fractures in his tibia, and money was found from somewhere to build a chute to drop the silver foil from the neutron source to the Geiger counter. By the time he came to Liverpool he was a little older and wiser, and when Chadwick set him the task of investigating another very short-lived isotope, he devised a more sedate and elegant coincidence technique, which established that the isotope had a half-life of only a few microseconds. He completed the project within the first two months and Chadwick was duly impressed. He offered Rotblat the Oliver Lodge Fellowship which was the most prestigious in the department, having first been held by Charles Barkla. To Rotblat, at that stage, the promise of income meant more than the honour. His scholarship from Poland was worth £120 for the year, paid quarterly, and the Fellowship would exactly double his emolument. On hearing Chadwick’s offer, Rotblat20 said, ‘Oh good, this means I shall be able to bring my wife.’ Chadwick, who had not previously known of her existence, retorted: ‘Good God, you won’t be able to live on this.’ But Rotblat was convinced they would manage and made plans to return to Warsaw later in the summer to fetch her.
