The Neutron and the Bomb, page 29
34 Rutherford, M. (19/10/37). Letter to J. Chadwick. CHAD II, 1/17, CAC.
35 Eve, A.S. (1939). Rutherford, p. 428. Macmillan, New York.
36 Chadwick, J. (1937). Lord Rutherford, obituary. Nature, 140, 749-50.
10 ~ The post-heroic age
Niels Bohr1 said of Rutherford that, like Galileo, ‘he left science in quite a different state from that in which he found it’. The statement is incontrovertibly true if one remembers that in 1895, when Rutherford first arrived at the Cavendish as a graduate student on an 1851 Exhibition Scholarship, the world knew nothing of X-rays, the electron, the proton, the neutron or radioactivity. He extended knowledge of all these phenomena and more by his own work, and in his lifetime also saw fundamental advances made by many of his students, for which some share of the credit is rightly given to him. To Rutherford himself, it seemed that he was living in ‘the heroic age of physics’.2 He had achieved a degree of public recognition and fame which went beyond his status as the Cavendish Professor and Nobel prizewinner. He had become the embodiment of science throughout the British Empire, and his elevation to the peerage served to confirm his rank as the equivalent of the Law Lords or Lords Spiritual25.
To Chadwick, it seemed almost an impertinence that anyone should replace Rutherford as Director of the Cavendish. Rutherford had confided in Chadwick3 when he left for Liverpool in 1935 that he would be retiring in two or three years time, and would like Chadwick to succeed him. Chadwick was surprised and responded that he ‘was not good enough to succeed him and at the same time I didn’t think there was anyone good enough in this country’ and he also said to Rutherford that ‘it would not happen’. The university authorities obviously needed to take a more practical line and set out to find a new director. Chadwick4 wrote to Feather on 23 January 1938: ‘I saw in the Times that applications were asked for by February 1st. But I cannot imagine why it is thought necessary to ask for applications or how any man can think himself worthy.’ The choice of Cavendish Professor is made by a group of electors, and there is no doubt that Chadwick’s name was considered by them, although he never applied for the job. He3 was asked informally to submit his ideas about the way the Cavendish should be developed, and replied that it should be expanded considerably and should diversify into molecular physics as well as continuing the tradition of nuclear physics. In the event, the man who had succeeded Rutherford to the Langworthy Chair in Manchester, Sir Lawrence Bragg, also followed him as the Cavendish Professor in Cambridge. Chadwick5 again shared his feelings with Feather: ‘I was by no means surprised, I almost expected it. I should like to have been asked, I still feel rather disappointed. But I was very relieved that I had not to leave here.’ According to Peierls, most physicists had expected Chadwick to be appointed to the Cavendish Chair, and their rationalization was that the electors did not choose him because they were worried that he would not be able to cope with the social aspects of the position: playing host to visiting dignitaries and taking a full part, as Rutherford had done, in the affairs of the Royal Society and other learned bodies. Reflecting 30 years later, Chadwick3 had this to say:
I was not dismayed when I was not chosen. In fact, I was rather relieved, because I did realize my limitations... I was in a sense relieved and pleased, too, that Lawrence Bragg had been chosen, which I knew would happen. At the same time I must confess to a slight feeling of disappointment in spite of saying that I was relieved. The two things can exist together.
There was little time for Chadwick to dwell on his slight disappointment since there were growing demands for his services in Liverpool. In February 1938 the Liverpool Daily Post announced the formation of a commission of leading medical men and university academics under the leadership of Lord Derby to report on cancer research and treatment in the city. The remit of the commission6 was broad and it was charged with paying particular attention to:
1. The best use, consistent with the welfare of patients, of the hospital accommodations and research facilities available in the voluntary and municipal hospitals in the Liverpool area and, in particular, in the Liverpool Radium Institute and Hospital for Cancer.
2. The most promising lines of investigation in regard to cancer (its causes and treatment) capable of being pursued in the Liverpool area.
3. The feasibility of co-ordinating, if that should seem desirable, the various activities under some unifying board or authority or by some other method.
The six-man commission included three of the most formidable intellects to be found in Liverpool: the new Vice-Chancellor, Arnold McNair, a distinguished jurist who knew Chadwick well from Caius College, where he was also a Fellow; the Professor of Medicine, Henry Cohen, and the Professor of Physics, James Chadwick. He, of course, was excited about the possible medical applications of the cyclotron, especially in view of the encouraging snippets of information he was learning from Lawrence in Berkeley. There seemed to Chadwick several possible avenues to be explored using cyclotron products: there was the direct effect of neutrons on malignant tumours, but even more intriguing to him was the production of artificial radio-isotopes that could be used as tracer substances to study biochemical and physiological processes and which might themselves have some therapeutic use as anti-tumour agents.
One of his former students, Douglas Lea, with whom he had made an unsuccessful search for the neutrino a few years before, had left the Cavendish and was making a name as a pioneer in the new field of radiation biology. Rutherford ‘showed no interest whatever in medical applications’ and had regarded Lea as a loss to physics. Chadwick3 now thought of attempting to recruit him to Liverpool because he intended ‘to push the biological side strongly’. In the event, Lea did not come to Liverpool and Chadwick never appointed a scientist with a major interest in the biological effects of radiation. He was finding it difficult to hire and retain experienced staff. E.J. Williams held the Leverhulme Fellowship for about 18 months, which was longer than his predecessor Norman Feather, but left Liverpool early in 1938 to take up the chair at Aberystwyth. Bernard Kinsey was promoted in his place, although Chadwick would have preferred to expand the department. He7 would have liked to have appointed Egon Bretscher, a Swiss experimental physicist then working at the Cavendish, but did not for fear that ‘there would have been serious opposition — not I think from Faculty [of Science], but from higher up — on the grounds of nationality’. In Chadwick’s opinion, the burghers of Liverpool were not as liberally minded as their pre-war counterparts in Manchester, where of course there had been a small but influential group of German descent. He also needed to replace James Rice, the reader in theoretical physics, who had died in harness in 1936. Rice had begun his career as a schoolmaster and always took a large teaching role in the department. Chadwick wanted to replace him with a more academically active theoretician. He first thought of Walter Heitler, then a lecturer in Bristol, but decided ‘it was no use thinking about Heitler, because the appointment of a German would not have been agreeable to some of the people in the University’.3 He then turned to Cambridge and approached Homi Bhabha, a young man from a wealthy Indian background, who was a member of Gonville and Caius College.
Bhabha came up to see me. I would very much have liked Bhabha to come, but I thought he was too good. Well, you see, some teaching had to be done, and the quality of our students was not the quality of the students in Cambridge, and I thought that much of it would be drudgery to a man like Bhabha, who was a most exceptional man. He was a painter and a poet and had extremely wide interests — not merely interests but far more than that — and I didn’t feel that however much I liked him it was fair.3
Ultimately, Chadwick settled on another talented Cambridge man, Maurice Pryce, who had studied under Fowler and Dirac. His name was put forward to Chadwick by Kinsey — they had been undergraduates together at Trinity College.8 Although Chadwick had no illusions about the relative strengths of Liverpool students, the burgeoning research programme was beginning to attract some of the more able ones to stay on and do postgraduate research. John Holt9 was one such and began his research career in 1938. In the best tradition of a Chadwick nursery course, he was set to work learning to make Geiger counters which had to be filled with argon gas and ‘an essential whiff of alcohol vapour, without which they refused to function properly’. Geiger had sent Chadwick two original tubes a few years earlier, and Holt found that their performance could be improved by minor alterations, such as using end caps of brass rather than of ebonite. It was not long before Holt made his first significant scientific contribution, which resulted from the close cooperation that had built up between Liverpool and Lawrence’s Berkeley laboratory. One of Lawrence’s team, Luis Alvarez, had just described a novel form of radioactive decay called K-electron capture. Harold Walke brought back, by rail and sea, some new isotopes with long half-lives manufactured at Berkeley so that their properties could be investigated in Liverpool. Holt9 found one isotope, vanadium 47, ‘which emitted only soft X-radiation, the first example of a K-capture decay with no accompanying nuclear radiation. It was very convincing evidence for the new process.’ He had the satisfaction of using his home-made equipment for the crucial observations. ‘The matter was clinched by some absorption measurements which I was able to make with a counter having a window of Cellophane, which showed that the X-rays were characteristic of the daughter element titanium.’ Cellophane, rather than the usual mica, was very transparent to the soft X-rays in question.
Chadwick continued to rely on Lawrence for information and guidance concerning the cyclotron; he peppered Lawrence with detailed questions10 in preparation for an evening discourse he was scheduled to give at the Royal Institution at the end of May 1938. He wanted to know the maximum voltage and current that could be obtained, how rich it was as a source of neutrons compared with radium and beryllium, the weight of the magnet, the diameter of the pole pieces and the maximum strength of the magnetic field. After apologizing for asking so many questions (the answers to which he thought would be of interest to those more concerned with applications of the machine rather than the physics), Chadwick continued:
I hope your new apparatus is really big. I feel that one ought to make a serious attempt to get up to 60 or 70 million volts (i.e. ~ 137 mc2) — perhaps not just yet but one ought to prepare for it. I feel sure that with such particles we should begin to learn the true mechanics of the nucleus. Of course nature provides us with such particles in the cosmic rays but in most niggardly fashion. I think the phenomena in the cosmic rays point the way to us.
I am afraid there is very little chance of being able to build a big cyclotron in this country for some time to come. We have had endless trouble in getting our small one. Even now we have not yet got the tank [vacuum chamber] from Metrovick. It is now a year since the drawings were Finally completed and handed to them. For various reasons I cannot quarrel with them but I am sorely tempted to do so. Kinsey’s patience was long ago exhausted.
He closed with some words of praise for the two members of his staff who had been trained by Lawrence.
You will be pleased to hear that Kinsey is doing well. He has been extraordinarily useful and I have been able to leave most of the cyclotron work to him. He was very unsettled for some time — rather naturally, for the laboratory was in a very bad state and big changes have to be made cautiously here — but he seems much happier now. I am glad, for I like him very much and I think he has good prospects.
Walke has joined in well. He is a nice lad and I am glad to have him and to learn my previous opinion of him was wrong. He is finding his past (i.e. his publications before he came to you) rather hard to live down. But he will do it.
The reason for the exasperating delays in the construction work was that Metropolitan-Vickers had become very busy with new defence contracts from the government. Chadwick did not know this until later, although he paid several visits to the factory to try to hurry them along. On these occasions, Chadwick3 ‘could see my bits of apparatus lying about the place waiting for their turn, but their turn never coming because they were big. I think there was only about one lathe in the place which would take them.’ At the same time, he was upgrading the laboratory workshop in Liverpool, buying new lathes, but was short of skilled technicians. The workshop did manage to build the two 10 kW radiofrequency power oscillator valves that operated the machine, but like the vacuum chamber from Metrovick when it was finally installed, were a constant source of trouble because of vacuum leaks. The most reliable piece of equipment was the 12 kV DC power supply for the valves: this was a mercury pool rectifier set supplied by the German firm of Siemens. Chadwick3 was able to obtain it ‘at a very reasonable price’ because ‘at that time the Germans were very anxious to get hold of money from abroad’.26
Lawrence’s programme was forging ahead and in his reply to Chadwick’s enquiries, he mentioned11 that his latest cyclotron was intended for ‘the primary purposes of medical research, requiring openness and accessibility’ in its design. He could not contain his excitement at the dramatic breakthroughs that might be achievable in this sphere.
There can be no question at all as to the importance of the artificial radioactive substances and neutrons for medical research and therapy and I should think that your biophysical friends in London would undertake the construction of a cyclotron for this line of endeavor. As an illustration which should not be mentioned in public in as much as the experiments are still in progress and it will perhaps be another year before definite publications will be made, I would mention that at the present time my brother John Lawrence, is treating with radio-phosphorus a patient suffering from myelogenous leukemia, with remarkable results. Recently he had been studying leukemia in mice and found that radio-phosphorus is selectively taken up by not only the bones and lymphatic tissue but also, to an extraordinary degree by the diseased white blood cells. For example, he found that in the spleen of the diseased animal the uptake of radio-phosphorus is something like 5 times as great per gram of tissue as in the spleen of a normal animal. This suggested the clinical possibility of treating the human disease, and beginning early in January he gave, over a period of about two months, a patient suffering from myelogenous leukemia a total of 70 millicuries of radio-phosphorus. At the beginning the patient’s white blood count was 600,000 while the red cells were 2.5 million. Shortly after the beginning of the administration of the radio-phosphorus the white blood count steadily dropped, the myeloblasts falling much more rapidly than the other cells, while the red count steadily went up to normal. Several weeks ago the patient’s blood picture had got to the point where it was not far from normal, the total white blood count being about 8,000, while the red count was 5 million, with less than half of one per cent of the white cells being diagnosable as diseased cells. The radio-phosphorus treatment has been stopped and now the patient is being watched to see what will happen next. Dr. John and all of the medical people feel that this patient’s response to radio-phosphorus has been remarkable but they feel on the other hand that there is no evidence that the phosphorus has cured the disease, and I am afraid that if my brother knew that I had mentioned it to you he would scold me. However, I do mention it to you privately as an illustration of one of the first cases where one of the artificial radioactive substances, serving as a tracer element, has revealed fundamental information about a disease, that is to say — in this instance, a profound disturbance of the phosphorus metabolism of the diseased cells; and secondly, that radio-phosphorus has actually been used now in the treatment of disease with results at least as good, if not better than anything that has been achieved with x-rays or radium.
The letter reveals how closely Ernest Lawrence followed his brother’s medical research and his remarkably detailed knowledge of the results; it would take little effort to turn the passage quoted above into a report that would be suitable for publication in a medical journal. Equally, one can sense the measured approach of John Lawrence, first making accurate observations in animals about the abnormal metabolism of phosphorus in malignant cells. Although the transition to a clinical application was extremely rapid, Dr Lawrence did not delude himself about the effectiveness of the new treatment and would make no major scientific report for two years.
In the period leading up to his Royal Institution discourse, Chadwick7 showed himself to be on edge, complaining to Feather about ‘too many university meetings and too many lectures, and all my weekends booked up so that we can’t get away’. The address to the Royal Institution12 was polished and well received. After reviewing the principles by which cyclotrons work, Chadwick told the audience that the machines in Liverpool and Cambridge were nearly ready for use. He showed pictures of the Liverpool machine under construction and rattled off some statistics:
our Liverpool magnet contains 46 tons of iron and 8 tons of copper. (The copper was generously presented to me by the directors of British Insulated Cables Ltd.). The diameter of the pole faces is about 36 inches and the air gap between them is 8 inches. The power consumption in normal running conditions will be from 40 to 50 kW, under full load about 70 kW. The maximum field under the conditions of experiment, that is, with a working gap of 5 inches, is about 19,000 gauss.
He briefly recapitulated the advances in the field of artificial radioactivity since the Curie-Joliots’ discovery of the phenomenon, and mentioned that about two hundred new isotopes had since been isolated, some of which had special interest. The applications of the cyclotron that Chadwick wished particularly to bring to their attention were those concerned with biology and medicine. He explained that neutrons from a cyclotron caused a more dense pattern of ionization in living tissue than did X-rays, and therefore might be expected to be biologically more active when given in equal doses (i.e. the same total energy absorbed). He also stated that some elements, boron for example, absorb slow neutrons very effectively. Although animal tissues did not ordinarily contain such elements in any significant amount, it might be possible to inject boron into tumours and then irradiate them with slow neutrons causing selective devastation of a small volume of disease.27 Chadwick then moved on to the subject of radioactive indicators, which, despite Lawrence’s enthusiastic reports about the possibilities of radioactive isotopes for cancer treatment, he correctly foresaw would have the greatest impact on physiology and medicine. The cyclotron could be used to produce isotopes of many of the elements commonly found in living tissue, which were chemically identical to their natural analogues and therefore handled by the body in an identical fashion. He cited experiments by his friend from Manchester days, Hevesy, who was now working in Bohr’s Danish institute, and had shown by feeding radioactive phosphorus to rats that ‘the mineral matter of bone is in a dynamic state, in which the bones are continually losing phosphorus atoms and taking up others which are later, in their turn, replaced.’ To Chadwick ‘it is clear that this method of radioactive indicators has many interesting possibilities, for its power and delicacy make it possible to attack problems which have so far been inaccessible to experiment’.28
