CHS-17 March 1985

STUDIES IN CERN HISTORY

High-energy physics from 1945 to 1952/ 53

Ulrike Mersits

GENEVA 1985 The Study of CERN History is a project financed by Institutions in several CERN Member Countries.

This report presents preliminary findings, and is intended for incorporation into a more comprehensive study of CERN's history. It is distributed primarily to historians and scientists to provoke discussion, and no part of it should be cited or reproduced without written permission from the Team Leader. Comments are welcome and should be sent to:

Study Team for CERN History c/oCERN CH-1211 GENEVE23 Switzerland

© Copyright Study Team for CERN History, Geneva 1985

CERN-Service d'information scientifique - 300- mars 1985 HIGH-ENERGY PHYSICS from 1945 to 1952/53

I. The scientific situation in 'elementary particle physics' around 1945/46 I.1. Cosmic-ray physics I.2 Nuclear physics

II. Institutional changes in nuclear physics due to the war

III. The post-war accelerator programmes III.1. The principle of phase stability III.2. The United States III.3. Great Britain - the leading country in Europe III.4. Continental western Europe III.5. AG focusing - another step into higher energy regions

IV. Experimental particle physics: developments from 1946 to 1953 IV.1. The leptonic nature of the mesotron and the detection of the pi­ meson (1946/47) IV.2. The artificial production of charged and uncharged pi-mesons (1948/49) IV.3. The complexity of the mass spectrum (1947-1953) IV.3.1.The V-particles IV.3.2.The heavy mesons IV.3.3.The Bagneres-de-Bigorre Conference (1953)

V. The theoreticians: from the first Shelter Island Conference to the Rochester Conferences (1947-1952)

VI. Concluding remarks HIGH-ENERGY PHYSICS FROH 1945 TO 19521531

High-energy physics (or elementary particle physics) in general, and CERN in particular, have now reached an enormous size. To be able to understand better why in the early fifties such an enterprise was under­ taken, why, having chosen high-energy physics as a topic for investigation, it took the form that it did, we want to pursue the development of this part of physics from the immediate post-war years up to the period when CERN was founded. Thus we will study in particular detail those aspects of the evolution of the field which were of relevance to the setting-up of CERN, although it is D..Q.t. our intention to give a complete description of the development of high-energy physics.

Regarding the situation immediately after the war, two rather different aspects are of major interest for our purposes, the scientific situation and the institutional changes which had taken place due to the war. These two aspects will be studied in Chapters I and II.

For the period after 1946 we will present the three major avenues of development which were collectively responsible for the shaping of elementary particle physics in the fifties. These were: the development of accelerators, the experimental data collected on these elementary particles, and the theoretical assessment of the events. Each of these parts has its own internal evolution, and we will study them in Chapters III, IV and V respectively.

Finally, having done this we will then be in a position to present a more comprehensive picture of the whole situation, showing the points of interaction between the different courses of events. In this way it will be possible for us to see roughly when, how and why elementary particle physics 2

had become an independent discipline using accelerators as its main tools - a discipline of high prestige and considerable attractiveness, which was regarded with fascination by many of the physicists.

Before we enter this study, it seemed necessary to us to clarify two notions rather frequently used namely nuclear physics and elementary particle physics (high-energy physics). If we use them in the way we under­ stand them nowadays, nuclear physics means the study of the atomic nuclei, of the reactions occuring when they are bombarded with particles like protons, neutrons or photons, of radioactive decays, etc. The decisive energies would therefore be those given by the binding energy of the protons and neutrons in the nucleus, i.e. several MeV (up to roughly 200-300 MeV). Elementary particle physics means the study of the particles building up the nucleus and those found elsewhere in nature and their interactions with each other. The spatial dimensions of the objects under study are smaller than in the case of nuclear physics so that the energies needed for studying them are much higher than those necessary there. They start at some 300 MeV (threshold for meson production) and have in fact no upper limit.2

Now that we are aware of how these notions are used today we must pay special attention to the way they were used in the different stages of their development, as this is to a certain extent indicative of the stage of evolution of the discipline.

I. The scientific situation in 'elementary particle physics' around 1945/46

In the immediate post-war period, notions like elementary particle physics or high-energy physics did not in fact exist. However, if we wish to study the origins of this discipline by the end of the Second World War, we have to take account of all parts of physics which deal with the constitution of matter in general, i.e. with the particles out of which matter is built and the way in which they interact with each other. This leads us to study two different approaches to such investigations, namely cosmic-ray physics on the one hand and nuclear physics on the other. The division is, as we will see, more important for experimental work. For the 3

theoreticians such a division did not exist they were doing nuclear physics in a rather broad sense, taking the basic experimental data from both sources.

Almost a year after the end of the Second World War the first big European conference in the field of particle physics took place. It was held from 22-27 July 1946 at the Cavendish Laboratory in Cambridge and was entitled Fundamental Particles and Low Temperature. 3 Its task was twofold to provide a meeting place for the international community of physicists and to help in orientating them in the steadily growing field.

It was important as a meeting place because international contacts had become impossible or at least rather difficult during the war. Thus the conference offered a welcome opportunity to establish new contacts or to re-establish old ones. In all some three hundred physicists attended the conference, one third of them from foreign countries. Many European states, excluding Germany for well-known reasons, as well as other countries like the USA, the USSR, China, and India were represented.

The conference's designation combined two completely different subjects of which only the first is of interest for our purpose. Even for this section the title Fundamental Particles was rather general. In the preface to the proceedings this was explained by the fact that 'the subject of Fundamental Particles is nowadays so wide that the arrangement of the programme was not easy. The time available for the whole Conference could well have been filled by the discussion of any one of a number of topics which in fact were compressed into a single session or less. But, at the first post-war gathering, one was reluctant to narrow down the field too much.' This conference was meant to give as broad an overview as possible to demonstrate all the possibilities for further investigation.

This section of the conference comprised five sessions in all on 'General introduction and survey', 'Mesons and cosmic rays', 'Experimental techniques', 'Nuclear forces and relativistic particles' and 'Theory of Heisenberg's s-matrix' respectively. We now wish to study the different 4

approaches to these topics by both the cosmic-ray physicists and the nuclear physicists. In this way it will be possible for us to see the 'state of the art' in these fields.

I ..1 Cosmic-ray. p h ys1cs . 4

Before further use is made of the term 'cosmic-ray physics', we should clarify what this meant in the mid-forties. Basically, one could divide the work of the cosmic-ray researchers into two big categories according to the type of questions they were trying to answer:

1. What are the constituents of cosmic radiation? 2. What is the origin of cosmic radiation and what are its effects on Earth?

The investigations dealing with the first question are of primary interest to us. We will not deal with the second question, but it should be mentioned that this question came to represent the main task for the cosmic-ray physicists after the particle aspect of their work had been taken over by physicists working with accelerators.

Let us begin by looking at the field from a scientific angle with a summary of the particles known at the time. Altogether eight particles were known by 1945, namely the electron, the positron, the proton, the neutron, the photon, the neutrino, the positive and negative mesotron (named mu-meson by Powell in 1947). It should be mentioned here that, due to the similarity in mass, the aesotron was (wrongly) thought at the time to be the field quantum mediating the strong interaction, as predicted by Hideki Yukawa in 1935. 5

One of the five sessions at the Cambridge Conference was exclusively devoted to mesons and cosmic radiation. Twelve talks were given and physicists like P. M. S. Blackett, G. Bernardini, B. Feretti, L. Janossy, L. Leprince-Ringuet and J. G. Wilson took the opportunity to survey part of the work they had done during the war years and to present their current investigations. Two of the main topics were the 5 production of mesons, (i.e. to look for the primary particles in cosmic radiation) and their decay. The latter topic gave rise to the question of why the positively charged particles were decaying into a positron and one neutrino (as it was thought at the time) while the negative ones were easily absorbed by atomic nuclei. Cosmic-ray showers were also discussed and, last but not least, a quarter of all talks was devoted to the problem of meson mass measurements.

In the third and fourth sessions the results from cosmic-ray studies were also presented, for example the work by E. Amaldi et al. on 'The elastic scattering of fast neutrons by medium and heavy nuclei', and the work by C. F. Powell and G. P. S. Occhialini on 'The scattering of fast neutrons by protons'.

Both the purely scientific content of the talks and the comments, which give an impression of the way the work was done, are rather interesting. The difficulties physicists faced in carrying out experiments were frequently referred to and considerably underline the advantages accelerators were to offer physicists.

These difficulties can be divided into three groups:

1- Inaccuracy in measurement

This problem existed from the very beginnings of cosmic-ray work and meant that the results obtained were perhaps more of a qualitative than of a quantitative nature. As J. G. Wilson said, 'identification of all these [detected particles] depends upon the limitations of our technique, which have up to now allowed only individual measurements correct to about 10\' .6 This problem was impeding particle identification and the drawing of conclusions, thus making it difficult to provide guidance in theoretical consi­ derations. This also explains why these physicists spent a great deal of their time on developing better detection devices. These detection devices and also the experience the physicists had gained in using them were later of decisive importance to the physicists working with accelerators. 6

2- Low reproducibility

owing to the nature of the particle source, namely cosmic radiation with its low particle flux density, some events occurred too arbitrarily and infrequently to be able to provide good sta­ tistics and satisfactory results. Thus we find Leprince-Ringuet closing his talk on 'Mass measurements of mesons by the method of elastic collision' with the statement that 'this photograph is in favour of [ a cosmic-ray particle of 990m mass ], but is not e sufficient to confirm, the existence of a heavy meson',7 largely because it was only a single photograph. Even two years later in September 1948 Reitler again referred to this problem in a talk at a conference on 'Cosmic Radiation' in Bristol 8 He described the difficulty in his talk on 'Cosmic-ray mesons and meson theory' in the following way: 'Each of these items of evidence [for heavy mesons] is rather convincing, [ ... ], but as long as there is only one example for each event we cannot be quite sure about the existence of these particles'. A further direct consequence was that any research proved somewhat time consuming.

3- Theoretical background

The third difficulty which occurred and which was clearly recognized was the lack of theoretical background for the experi­ mental work. Since Yukawa's prediction of a field quantum mediating the strong interaction in 1935, no other predictive theory had been put forward. As already mentioned, this theory had been erroneously 'satisfied' in 1937 by the discovery of the mesotron in cosmic radiation. 9 As Yukawa put it in his Nobel speech in 1949: 'At that time (1937], we came naturally [our emphasis] to the conclusion that the meson which constituted the main part of the hard component of cosmic rays at sea level was to be identified with the aesons which were responsible for the nuclear force.• 10 One could say that the general attitude of many of the physicists was not to •expect' a variety of further elementary particles, largely because there was no 'need' for them, in so far as they were not necessary to explain 7

anything. This attitude was also one reason why the erroneous identification of the mesotron and Yukawa's meson could persist for so many years (1937-1947), although even at the time the experimental results achieved seemed to some of the physicists not to fit very well into the picture of strong interactions. 11

This problem of believing or disbelieving in the existence of many more particles of different masses was also reflected at the Cambridge Conference in 1946 by J. G. Wilson. He started his talk by saying that 'a crucial topic concerning fundamental particles today is that of existence [our emphasis], particularly with reference to the meson group. Are mesons of a unique mass? Are there several types of mesons differing in mass? Or, is a continuous distribution of masses possible? These are questions for which the theoretician is not able to offer much assistance, and so they must primarily be the concern of the experimental physicist' .12

What consequences did these three problems have for the work of the cosmic-ray physicists? Both inaccuracy in measurement and low repro­ ducibility meant that their results were rather a weak basis for theo­ retical work. However, theoretical guidance was necessary for evaluating results and partly also for conceiving experiments. This weak interplay between experiment and theory was also a source of anxiety for the theore­ tical physics community. Thus, for the time being success for the cosmic-ray physicists was linked to the improvement of their apparatus rather than to any sort of deeper theoretical understanding.

To conclude, we will summarize the main contributions of cosmic-ray physics to particle physics. Without doubt the central question occupying the physicists was to establish the law describing the nuclear force. For this reason, two main areas of investigation were fundamental. First, one had to understand the production of the cosmic-ray mesons, to measure their main features like mass, charge, spin and lifetime and to study their interaction with matter. In carrying out these investigations, it was assumed that the mesons were responsible for mediating the nuclear force. Consequently an understanding of such particles could lead to an 8

understanding of the nature of strong interactions. Secondly, nucleon­ nucleon interactions were extensively investigated. Considerable information was collected on each of these aspects but physicists were not in a position to establish definite lines of interconnection between them. At the time their work could thus be compared to the attempts to complete a puzzle without knowing what the final picture would look like, although they had already found some of its main pieces.

Despite all the problems and difficulties mentioned above, we should not overlook the considerable amount of data and results presented at the Cambridge Conference in 1946. It is true that physicists complained that much of their data was rather inaccurate and perhaps not sufficiently situated within a theoretical framework. However, we should not forget that this was the only way to get information on the constitution of matter and that the field had a very stimulating influence on many physicists. It was therefore of crucial importance.

I.2. Nuclear physics13

In order to grasp the notion of 'nuclear physics' as used in 1946, we wish to contrast it with its present-day meaning. The major difference is possibly the fact that 'nuclear physics' in 1946 had a much broader, much more comprehensive meaning. In contrast to today, its meaning was not strictly associated with the idea of a specific energy region. Rather it was the type of phenomena investigated and the kind of questions asked which determined whether the work was 'nuclear physics'. In fact, the whole complex of research relating to questions on the basic structure of matter and the laws behind it, i.e. nuclear forces, mesons, field theory, etc., was regarded as nuclear physics.

The most significant feature that the talks on experimental nuclear physics, presented at the Cambridge Conference, had in common was that they all covered the topic neutron physics. Four talks on this were given at the session under the heading 'Experimental techniques', and at no other. Herbert Anderson, Enrico Fermi, Leona Marshall and Walter Zinn were 9

the speakers and they presented the work which they had performed under contract with the Manhattan project in the Metallurgical Laboratory at the Un1vers1ty . . o f ch' icago. 14

Generally speaking, owing to the war-time atomic energy project in the United States, physics related to fission, and thus neutron physics, and work on uranium isotopes, etc. had been the major topics of research for many of the physicists. About a hundred new unstable isotopes had been created using accelerators (which at that time achieved a maximum of roughly 20 MeV), and a number of new elements had also been found, of which the most well known was plutonium-239.

In fact, the Cambridge Conference clearly reflects the situation in experimental nuclear physics by the end of the war. On the one hand we have a somewhat selected presentation of the war work, owing to the fact that many documents were still classified at the time. On the other hand, results from basic research are completely lacking. No mention was made of the research topics begun before the war using existing accelerators such as nucleon-nucleon scattering or measurements of the main features of the deuteron, for example.

To conclude, we could say that as far as pure fundamental research was concerned, the investigations in this field ceased to a great extent during the war, although a large number of open questions still remainded unanswered. On the other hand, fields like isotope physics and neutron physics were flourishing, as we have seen, owing to the fact that they were of immediate significance for the war effort.

Turning to the contribution of theoretical nuclear physics to the 1946 Cambridge Conference, we find that on the basis of the number of talks given this was the dominant part. Nearly three complete sessions out of five were devoted to this field, and Niels Bohr, Max Born, and many other leading European theoreticians were among the speakers.

The Opening Address was given by Bohr with a talk on 'Problems of elementary-particle physics' . 15 He dealt rather generally with the self-energy problem of point-like charges, electron-pair production, and the 10

treatment of field quanta such as mesons, but he offered no solution to any of these questions. The other speakers in this first session were Wolfgang Pauli on 'Difficulties of field theories and the field quantization', P.A. M. Dirac on 'The difficulties of quantua electro­ dynamics' and, as the last speaker, Max Born gave a talk on 'Relativistic quantum mechanics and the principle of reciprocity'. As the preface to the proceedings of the conference, emphasized the operative word was 'diffi­ culty', which occurs in the title of nearly every paper in the session. As the preface put it: 'The passage of the years has not altered the basic fact that the subject under discussion transcends the limits of well established theory and that no entirely satisfactory new point of departure is known.' Thus theory was presented as having reached something of an impasse.

The fourth session on 'Nuclear forces and relativistic particles', which was almost entirely theoretical, was opened by Leon Rosenfeld. As an introduction to his talk on 'The two-nucleon problem', he gave the following illustrative general picture of the situation in theoretical physics with regard to the problem of nuclear forces:

'A survey of the problem of nuclear forces must at the moment remain inconclusive. On the theoretical side, progress is hampered by the present imperfection of the quantum theory of fields of forces; besides the fundamental problems of self-energies and proper magnetic moments which affect any treatment of the elementary particles themselves, peculiar difficulties are encountered, as we shall see, even in the derivation of the law of interaction between such particles. On the empirical side, the situation is perhaps more hopeful, inasmuch as we can expect information of decisive value from experiments extending only slightly further than those at present available; but the existing evidence is not yet sufficient to allow us to make a choice between a number of theoretical possibilities.•16 11

The subsequent talks covered topics like strong coupling meson theories, the collisions of neutrons and protons with deuterons, possible new equations for fundamental particles, and the possible existence of aass spectra of fundamental particles. They were given by G. Wentzel, H. S. W. Massey, R. A. Buckingham, A. Proca, and c. M¢ller.

The last session was entirely devoted to the S-matrix theory, which had been proposed by Werner Heisenberg in 1943. 17 W. Reitler, C. M¢ller and E. C. G. Stueckelberg presented talks on this subject.

Summarizing, we can say that the general research aim was to find a consistent formalism to describe the 'behaviour' of these elementary particles, and so to develop quantum field theory to the stage at which it would have overcome its current difficulties and could be used to understand cosmic-ray phenomena, to treat field quanta like mesons theoretically and to describe the ways particles transform into each other. There was no real prospect of a rapid solution to these. On the other hand, in reading these papers something else emerges. We frequently find the more or less ex­ plicitly stated hope that help for the theoretical problems would come from the experimental side. Max Born, for example, said this very clearly in the introduction to his talk: 'I wish, however, to stress the point that I expect the real solution of our difficulties from the discovery of new facts in the domain of high energy which will be experimentally explored in the near future. ' 18 Leon Rosenfeld also expressed the same hope in saying that theoreticians 'would reuqire evidence from a domain of energies still much higher than those we have been considering, but not outside the range of the modern accelerators. 119 In other words what was needed were well-defined experiments with accurate results to serve as a basis for the theoreticians to decide in which directions to proceed.

* * * 12

On reading interviews with physicists working in this field at that time and on studying the actual work they did, we see that after the war one could choose between two distinct directions of research in nuclear physics. As P. A. Morrison put it at a conference in 1967 one could: 20

- either work on the structural phenomenological complexities of the nucleus;

- or try a more refined ultimate analysis of matter.

This of course did not mean that there was a strict division within the community of physicists doing nuclear physics, and some physicists in fact worked for quite a while in both fields. However, joining the first group (and continuing along those lines in the following years) would mean remaining a nuclear physicist (in today's sense). Joining the second would ultimately lead to becoming an elementary particle physicist.

II. Institutional chanqes in nuclear physics due to the war21

On looking at the manner in which high-energy physics has been carried out since the fifties, two main characteristics stand out. First, investigations in this field of physics are generally undertaken in big laboratories in which large, very expensive and technically highly sophisticated equipment is concentrated. Secondly, the investigations are undertaken in increasingly large collaborations of physicists and technicians - a fact which is connected to the last point. In this section we will briefly sketch the origines of this approach to high-energy physics, thus the start of 'big science', which go back to the Second World War.

As is well known, the detection of nuclear fission by 0. Hahn and F. Strassmann in 1938/39 together with the outbreak of the Second World War brought about an abrupt change in the whole character of nuclear physics. Investigations hitherto regarded as basic research were henceforth considered solely from the point of view of practical applicability. The 13

feasibility of the atomic bomb had been rooted and much of the war effort rather was concentrated in this direction for the purpose of military superiority.. . 2 2

In fact there were two large war projects in the allied countries, namely the development of radar and research in connection with the development of an atomic bomb. A large number of American physicists and emigrants from Europe were involved in one or the other of the projects. Big collaborations were set up with the best available staff, and previously unthinkable amounts of money were placed at the physicists disposal.

As already mentioned, this new style of working doubtless had an enormous influence in the post-war period. We shall therefore investigate both the positive features, underlined by the 'success' achieved during the war, and the negative ones, which caused considerable anxieties within the physics community. This will enable us to counterbalance the arguments used and to understand certain developments which took place later.

The new method of doing nuclear physics research had two rather striking advantages

- First, a large number of very good physicists provided with an accumulation of various tools, all concentrated in one place, was a very good basis for highly efficient work;

- Secondly, close collaboration was established between various fields like theoretical physics, experimental physics and electrical engineering. Thus, there was cross-fertilization between these different fields, which led to the relatively rapid development and improvement of the research tools required.

The best known example of a big war-time laboratory is of course Los Alamos (New Mexico). 23 Having started its work in March 1943, by spring 14

1945 its staff numbered 2000. Los Alamos was a special weapons laboratory established by General Leslie R. Groves and under the leadership of J.Robert Oppenheimer. Many of the best American and emigrant physicists, both theoreticians and experimentalists, gathered there to work on the development of the bomb. The average age of the staff was very low and thus a whole new generation of physicists and technicians gained their experience there and returned to 'normal' life with very high expectations for their future work - expectations which were understandably entirely different from those they had had before the war. Their attitude had mainly changed in three respects:

First, their financial position was different. Some kinds of research work had become very expensive, but during the war scientists had seen that adequate funding opened up many promising possibilities. The pre-war threshold for the cost of an experiment had now been exceeded by several orders of magnitude. Many physicists wanted to continue to work on this scale.

Secondly, their position vis-a-vis the government had changed. Scientists had demonstrated to themselves and to the government that such large-scale projects could work, and work efficient­ ly. Even more important was that as a result of the war nuclear physics had demonstated its practical applicability for both peaceful and military purposes. Support could thus be expected from official quarters.

Thirdly, the relationship between industry and physics had been reinforced. Through experience physicists became increasingly conscious that a good exchange of ideas and attitudes between industry and the scientists could be rather useful. Many technical improvements had been achieved such as radar, fast electronics, etc. which could now be used for nuclear physics. The industrialists, on the other hand, were also interested in co-operation, because of the possibility of practical appli­ cations from this type of pure research. 15

Having briefly sketched the main positive effects of the war on the style of research and on the attitudes of the scientists, we now intend to constrast them with the doubts and anxieties expressed at the same time. The main question for many physicists was of course that of their moral responsibility once they had seen the consequences of their work. However, here is not the place to speak about this problem. We prefer to concentrate on the way scientists thought about their future work in nuclear physics.

Throughout the war, physicists had been doing strongly oriented research and thus they felt that a great deal of fundamental physics remained to be done. As the Director of the American Institute of Physics expressed it, 'it is serious enough that the best efforts of physicists have been for several years diverted from fundamental research. The advance of basic physics has probably been retarded by the war, whatever may be said about the stimulation of its application to industrial purposes. •24

The question of how to re-start doing 'real' physics after the war was therefore a very pertinent one. This explains why in May 1944 a Conference on the problems of physics in the post-war period was held in Philadelphia, 25 attended by some sixty American physicists to discuss topics such as physics education, relation between industry and physics and problems of the future financing of science. It is interesting to see that many considered these changes in the style of research to be rather dangerous for the future of nuclear science. Their comments can be grouped around three topics.

1. - Difficulties of running large-scale institutions under normal conditions

Some physicists foresaw difficulties in drawing up a programme for a very big institution, in organizing it and in finding the same unanimity among physicists to work along the lines agreed. During the war there was pressure from without, i.e. from the government, coupled with the inner desire to help win the war. The physicists 16

were struggling together to reach their aim. Now, in peace-time these pressures would no longer be present, and this could to some extent lead to their disorientation in a big organization.

2. - Financial dependence of science

Scientists planning big research projects for the post-war period needed financial support from industry or the state. This fact raised questions like 'Who is going to own the results of research?' and 'Will there be enough freedom to do investigations in whatever f ie. ld one c hooses?'. 26 L. C. Dunn of Columbia. University . . expressed this even more explicitly in his talk entitled 'To what extent is government financial aid for training and research desirable?' He concluded his argument as follows: 'Now, in conclusion, I should like to raise the question whether the chief problem really is in the method of financing training and research; or whether it is not the fundamental question of the divorce of support and control, that is, the maintenance of freedom for research workers and the prevention of local interference and favoritism which everyone fears as the government enters new fields of science. 127

3. - Centralization of research

This topic covers two types of slightly different arguments. First, there was the complaint that there would be no place for indi­ vidualism in this new way of doing research. For some people work in smaller groups seemed more fruitful. Secondly, the necessity for decentralization was stressed by F. J. Kelly (Division of Higher Education, U.S. Office of Education) because only in this way will people 'keep up their sense of responsibility•.28

We do not want to go into further details on these aspects. Our purpose was simply to give an impression of the situation in nuclear physics at the end of the war. In parts of the following chapter we will see how this new way of doing nuclear physics was converted into reality, especially in the United States. 17

29 III. The post-war accelerator programme

The aim of this chapter, which will be divided into five parts, is to convey an idea of the situation in the accelerator field around 1950 to 1952.

We will begin with the invention of the principle of phase stability in 1945, which was the basis for the developments in the following years. Then, in the second part, we will study the situation in the United States. We will concentrate mainly on the accelerator programme of the two research establishments Brookhaven and Berkeley, giving only a short overview of the machines built at universities. These machines played an important role as models for European physicists.

We then intend to study the role of Great Britain in the construction of accelerators. Although in terms of machine energy her accelerators did not rank particularly high among accelerators in the world at the time of their completion, when originally planned some of them were the largest, or embodied technically novel features. Great Britain was thus the only European country whose evolution in this respect can be compared to that of the USA.

As for the accelerator programme of continental Europe, two features will prove to be characteristic: first, only a few countries pursued an extensive accelerator programme and secondly, the machines built were all in an energy region intended for pure nuclear physics and not for particle physics.

We will conclude this description by presenting the development of another new concept in accelerator design the alternating-gradient fo­ cusing principle. It provided the basis for accelerators of even higher energies which could be built within reasonable financial limits. This idea, formulated by a group at Brookhaven, is of particular interest as it was the principle upon which the CERN Proton synchrotron was built. 18

III.1. The principle of phase stability

In 1945 four types of accelerators were known: Van de Graaff, Cockcroft Walton, cyclotron and betatron. However, the cyclotrons, which were the type of accelerator able to produce the highest energies, soon reached their energy-limit.

This limitation, as Bethe and Rose pointed out in 1937, was due to the relativistic mass increase when a particle reaches very high velocities. Since the immediate consequence of this increase is a change in the frequency of the revolving particle, this frequency falls out of step with the frequency of the accelerating field, and the particles are decelerated. In practice this limit lies at roughly 25 MeV for protons. 30

A solution to this problem was proposed in 1945 by E. W. McMillan and V. Veksler independently, which came to be known as the principle of phase stability.

The main idea behind it was to compensate for the relativistic mass increase either by changing the accelerating high frequency voltage or the magnetic field strength during the acceleration of the particles. This idea made it possible to accelerate particles into the GeV region without encountering major difficulties. In this way it was not only possible to operate cyclotrons at higher energies, thus converting them into synchro­ cyclotrons, but also to build a completely new type of accelerator, the synchrotron.

The principle of this new machine was to keep the particles on a path of constant radius by varying both the magnetic field strength and the frequency of the accelerating H. F. voltage with increasing particle energy. One therefore needed only a ring of magnets, and not disc-like magnets as before. This had both technological and financial advantages, in the first case, because it would be easier to achieve a homogeneous magnetic field in the small region of the ring, and in the second because less aaterial would 19

be needed for construction. In addition, the dimensions of the vacuum chamber could also be reduced, making it easier to achieve a good vacuum. Furthermore, it should be stressed that this kind of accelerator could be used for accelerating both protons and electrons.

This idea, as we will see below, opened up completely new possibi­ lities and made it feasible to build enormous machines like the Bevatron at Berkeley and the Cosmotron at Brookhaven.

* * *

Before discussing the accelerator programmes of the various countries, we should review the situation of particle physics in general. As we have seen, the understanding of meson physics was just beginning. The real Yukawa meson had still not been found and there were only a few vague hints of other mesons of even higher mass than the mesotron. At the same time, the exact production mechanisms of known particles had not been completely clarified and estimates for production thresholds consequently proved rather difficult. 32 Nevertheless, there were high hopes of success in reproducing these cosmic-ray events and studying them in laboratory conditions.

III.2. The United States

As mentioned above, it is intended in this study to consider main­ ly the machines built at the two big high-energy research laboratories at Brookhaven and at Berkeley.

The idea of Brookhaven arose late in 1945 in discussions between Isidor I. Rabi and Norman Ramsey. 33 As the plan was to have a big research centre, nineteen of the major East-Coast research institutes and two industrial laboratories were invited to attend a meeting in January 1946. From that meeting it took only about two months for the decision to be taken: nine of the universities joined together to establish a research laboratory for nuclear science. 34 The intention was not only to do physics, 20

but also chemistry, biology, medicine and technical research, using both atomic piles and accelerators.

Let us now describe in some detail the machines envisaged. In the case of the pile, the aim was to construct a rather advanced type, which would provide a high neutron flux. But as this would take time for develop­ ment, a conventional pile was planned as a first step. This equipment would have various applications, e.g. in medicine for neutron therapy, for neutron physics 'to get a better insight into the nature of the atomic nuclei', and for the production of isotopes, which would be used in chemistry, physics, biology, and medicine. Remarkably, it was also decided that the piles would not be isolated from residential areas, as elsewhere. The idea was to find a solution to this delicate problem which could serve as a model for the power industry in the construction of atomic power stations.35

The accelerator programme proposed in the Initial Programme Report of December 1946 was the following: a Van de Graaff of 3.5 MeV, a 60-inch cyclotron, a synchro-cyclotron with 0.6-1 GeV energy, a 1-2 GeV electron synchrotron and a 'Super High Energy Accelerator'.

The plans for Brookhaven were thus somewhat ambitious, the hope being to cover the whole energy range with accelerators. But in fact only three of them were finally built: the Van de Graaff, the cyclotron and the big accelerator (now known as Cosmotron).

Let us now look at the reasons given in 1946 for the construction of the cyclotron and the big accelerator: 36

60-inch cyclotron: a conventional machine to complement the reactor, for the production of isotopes and to study the interaction of fundamental particles and the mechanism of nuclear reactions.

Super High Eneray Accelerators: The idea of having an accelerator in the region of 10 GeV was initially rather vague. No indication was given of what particles they intended to accelerate nor do they seem 21

to have been sure there would be no technical problems when accelerating to such high energies.

Clarification of these ideas took roughly a year. It was decided to have a proton synchrotron either in the region between 2 and 3 GeV or between 8 and 10 GeV. With a machine of this type a great deal of meson physics could be done, such as multiple meson production experiments, which would provide a greater understanding of the nuclear forces. More significantly, above a threshold energy of 5.6 GeV, nucleon-antinucleon pair production could probably be studied. This would be a step into a completely new field of part1c. 1e p h ys1cs. . 3 7

As it was intended to do particle physics during the years of design and construction of the accelerators, a cosmic-ray programme was launched. In fact, in the beginning (1947/48) the whole of the particle physics division consisted mainly of scientists and technicians, who had been working on cosmic rays at Aberdeen Proving Grounds. They had trans­ ferred a lot of their material like cloud-chambers, detectors, etc. to Brookhaven. Their speciality was the study of primary cosmic radiation, and a number of balloon flights up to about 100.000 feet had been undertaken. The importance of this group naturally diminished when the machines started 38 to work.

We now turn to developments at Berkeley at the same per10. d . 39

During the waryears, the Radiation Laboratory had been incor­ porated into the Manhattan Project, mainly for work on isotope separation, i.e. the separation of U-235 from U-238. This work was done with the 184-inch magnet, which had been installed with a view to building a giant new cyclotron. 40 With this machine, the so-called 'calutron', and with several others modelled on it, the amount of U-235 required for the bomb was produced. As a consequence of this war effort, the number of scientists grew enormously and the equipment to be used was improved. 22

Not surprisingly, when the war ended the plans to continue basic nuclear research were rather ambitious. E. 0. Lawrence's first aim was to convert the calutron into a svnchro-cyclotron, based on the newly developed principle of phase stability. This took only about a year and on 1st November 1946 the 184-inch synchro-cyclotron produced its first beam. From then on 195 MeV deuterons and 390 MeV alpha particles were available. (In the course of 1949 the machine was modified to produce 350 MeV protons.) With this machine it thus became possible to produce the mesons detected in cosmic. rad' iat1on. . 41

In addition to this machine, the Laboratory planned to enter completely new territory by constructing a synchrotron. McMillan's idea was to have an electron synchrotron, in the several hundred MeV range, which was intended to be the model for an even bigger machine of that type to be built later. This machine was completed by the beginning of 1949 and achieved an energy of 335 MeV.

The third part of this accelerator programme was Alvarez's proton linear accelerator. It would be the first proton linear accelerator and seemed to be rather a promising alternative to the synchrotrons. L. Alvarez, who had worked on the development of radar at MIT's Radiation Laboratory, 42 had realized that linear accelerators could be powered by searchlight­ directed radar sets which, he estimated, could result in output energies of roughly 1 GeV. He expected to get the necessary radar sets from army equipment. The final version of this machine was 40' long, producing 32 MeV protons. It was completed in 1948 and the beam produced was of both of high intensity (0.4 µA) and collimation. 43

However, the highlight of all the facilities was to be a 10 GeV proton synchrotron, planned in 1946 by W. M. Brobeck. 44 This would provide Berkeley with an outstanding tool, and without any doubt make it the leading institution in the world. But of course immediately the question of funding such a huge accelerator had to be faced. At that time (1946), the only 23 accelerator designed to reach energies in the GeV regions was the planned 1.3 GeV proton-synchrotron at Birmingham.

From 1st January 1947, the Atomic Energy Commission (AEC) took over the U.S. nuclear energy programme. When the proposal to build a 10 GeV accelerator was submitted to the AEC, it was soon clear that the costs would be too high. In the light of these considerations, the energy was first lowered to 5 GeV and then immediately raised to 6 GeV, so that it was at least above the threshold of nucleon pair production (5.6 GeV).

Meanwhile Rabi had brought Brobeck's proposal for a 10 GeV accelerator to Brookhaven, and was trying to convince scientists of the importance of such a project. It is said that some of them favoured the newly started 600 MeV synchro-cyclotron project (abandoned soon after), while others thought that 1 GeV or a little more would be in any case sufficient to do good meson physics. In the event, a design study on both a 2.5 GeV and a 10 GeV proton synchrotron was launched. 45 At the same time, Berkeley scaled down their design energy to 1.8 GeV, with plans to increase the energy step by step.

Early in 1948, Lawrence finally fixed the designed energy at 6 GeV for the reasons initially given. On 8 March 1948 the AEC took the final decision: a 6 GeV proton synchrotron (called BEVATRON) would be constructed at Berkeley and a 3 GeV machine of the same type (called COSMOTRON) at Brookhaven. 46

Apart from the purely technical data on the machines, three interesting details should be noted:

1. On the purely scientific level, it was at that time difficult to decide exactly what sort of physics one would be able to perform with what type of accelerator. We should keep in mind that it was only in March 1948, i.e. at the same time as the decision on the accelerators was taken, that the first mesons were artificially produced in the 184" synchro-cyclotron at Berkeley. Moreover, very little was known about heavy mesons. 24

For the big accelerators two energy regions seem to be of out­ standing interest: one at roughly 3 GeV, well above the threshold of meson production, thus making possible investigation of these processes at various energies, the other between 6 and 10 GeV, for the study of antiproton production.

2. On the technological level aany new techniques like radar were being incorporated into those programmes and were in fact absolutely crucial contributions.

3. The importance of obtaining funds for such large-scale projects is clearly demonstrated. Neither purely scientific considerations nor the ambitious plans were decisive - a long process of consideration and reconsideration of the project was necessary until a consensus on the financial and scientific levels could be reached.

The COSMOTRON was completed in July 1952 and was thus the first accelerator producing particles with an energy of more than 1 GeV and the first operating proton synchrotron. It opened up the field of heavy mesons to the particle physicists, whose only source of information until then had been cosmic. rad. iat1on. . 4 7

The BEVATRON was completed in 1954 and was the biggest proton accelerator for the next few years. The next generation of proton accelerators could already benefit from a completely new principle, the alternating-gradient focusing principle (see Chapter III.5.). 48

To complete this section on the United States, we want to give a list of the machines completed or under construction by 1952. We will re­ strict ourselves to synchro-cyclotrons and synchrotrons with energies of at least 100 MeV.

This table indicates the breadth of the accelerator programme and the intensive efforts made in this direction. 25

Laboratory Type of accelerator Energy Date of [MeV] completion

Univ. of Chicago synchro-cyclotron 450 1951 Carnegie Inst. of Techn. synchro-cyclotron 440 1952 Columbia University synchro-cyclotron 385 1950 Berkeley synchro-cyclotron 350 1946 University of Rochester synchro-cyclotron 250 1948 Harvard University synchro-cyclotron 125 1949

M. I. T. e - synchrotron 340 1950 Berkeley e - synchrotron 335 1949 Cornell University e - synchrotron 300 1951 University of Michigan e - synchrotron 300 1953 Purdue University e - synchrotron 300 1954 Brookhaven Cosmotron p - synchrotron 3.000 1952 Berkeley Bevatron p - synchrotron 6.400 1954

TABLE 1. SYNCHROTRDNS AND SYNCHRO-CYCLOTRDNS IN THE UNITED STATES4 9 26

III.3. Great Britain - the leading country in Europe

To improve our understand of the difference between the situation in Great Britain and the one on the European continent in the post-war period, we must go back to the war years. Many British physicists had been very active both in microwave-techniques and, together with the Americans in developing the bomb. Besides these activities they were carrying out nuclear physics research work under the Anglo-Canadian Project in Montreal. Thus when the war ended, Britain was in a situation similar to that of the United States, i.e. nuclear physics was a research subject of outstanding prestige and importance, especially with respect to military applications.

In the light of this, it is not surprising that as early as 29 October 1945 the Prime Minister announced that the government was 'to set up a research and experimental establishment covering all aspects of the use of atomic energy.' The responsibility for this project lay with the Ministry of Supply and Sir John Cockcroft was appointed as Director in January 1946. Harwell was chosen as the site for the Atomic Energy Research Establishment and April 1946 the building started. 50

What was the research programme for this centre? The basic idea of Harwell was to do applied nuclear physics, and so piles had priority. They would fulfil a double task, namely they would enable physicists to do basic research and, at the same time, produce fissile material. Although we do not want to go into details on this part of the programme we nevertheless wish to mention briefly the reactors built. The first was a graphite low energy experimental pile, soon to be called Gleep, which began to work in 1947. The second, called Bepo, was constructed with a view to reaching higher neutron flux densities and was thus more suitable for the production of isotopes. The isotopes produced with the piles found a ready market all over the world, and were mainly used for medical and biological purposes.51

We now wish to turn to our central point of interest, the accelerator programme. A variety of different types of accelerators was 27 planned at Harwell. Amongst them a Van de Graaff, a 110-inch synchro-cyclo­ tron, an electron synchrotron and a linear accelerator.52

The van de Graaf f could be regarded as standard equipment for a nuclear physics laboratory. It was largely built in the Engineering Unit at the Telecommunication Research Establishment in Malvern, and was then erected at Harwell. It reached an energy of 5 MeV.

The 110-inch synchro-cyclotron was the first machine in Great Britain to be built incorporating the recent discovery of the principle of phase stability. (The initial plan had been to build a 72" standard cyclotron.) The machine went into operation in December 1949 and was able to produce protons with a kinetic energy of 175 MeV. For roughly two years it was the largest operating accelerator in Europe. Its disadvantage was that the machine's energy was just at the threshold of meson production. As a consequence, the research programme was concentrated more around the question of the interactions between protons and neutrons and 'nuclear explosion' produced by high-energy protons and neutrons. 53

The second kind of cyclic accelerator was an electron synchrotron. The work on this machine at Harwell was influenced by the view that it should serve as a prototype for the planned 300 MeV electron syn­ chrotron at Glasgow. As a first step towards this goal, an already existing Betatron was modified. In August 1946, this convertion was completed and the world's first electron synchrotron produced 8 MeV electrons. The second step was then to construct a 30 MeV machine of the same type (diameter= 8 inch). In October 1947, it was put into operation.

The fourth machine in the Harwell programme was a linear accelerator for electrons, whose main feature was that it worked on the travelling wave principle, the first machine in the world to do so. In this accelerator the particles were made to 'ride' on the crests of very short electromagnetic waves. The realization of this idea was only 28

possible through developments in the radar equipment used for producing these electromagnetic waves. The first machine was 2 m long and produced a high intensity electron beam of 4 MeV energy.

To summarize, we wish to emphasize three main features of this programme. First, it was a comprehensive programme which tried to tackle many possibilities in accelerator construction. Secondly, Britain had both the technicians with adequate skills and the industrial resources to pursue such a programme (a fact which was not true for many other European countries). Last but not least, from the 'choice•54 of the machines it is rather clear that the direction of research was oriented towards nuclear physics, covering a wide range of aspects, and not towards meson physics.

Harwell's accelerators were only a part of the British nuclear physics programme. Early in 1946, the initiative was taken to provide the universities with adequate nuclear physics equipment. The Nuclear Physics Committee of the Ministry of Supply sent circulars to thirty universities and other institutions asking for an outline of their nuclear physics programme. The only five universities to reply with a request for large­ scale equipment were Birmingham, Cambridge, Glasgow, Liverpool and Oxford. All of them received grants from the Department of Scientific and Industrial Research for the construction and maintenance of the following acce­ lerators: 55

Birmingham: 1.3 GeV proton synchrotron

Cambridge: big linear accelerator56 (later abandoned)

Glasgow: 300 MeV electron synchrotron

Liverpool: 400 MeV proton synchro-cyclotron

Oxford: 140 MeV electron synchrotron 29

Let us now have a closer look at these five accelerators. A big proton-accelerator to be built at Birmingham had been suggested by M. L. Oliphant as early as 1943. His idea at that time was to use only a ring-shaped magnet for proton- acceleration, but we do not know how he intended to realize his idea. 57 When the war ended in 1945 he designed a powerful proton synchrotron (incorporating the principle of phase stability) which, at the time of its conception, was even in advance of Berkeley and Brookhaven. However, because of inadequate resources at Birmingham and owing to problems of housing the machine, the accelerator was finished after the American machine, reaching an energy of 1.3 GeV in July 1953, though with a very low intensity. 58 By this time, the Cosmotron in Brookhaven, with an energy of 3 GeV, had been completed.

The two electron synchrotrons in Glasgow and Oxford were both built with the advice of the Malvern-Harwell group, which had, as already mentioned, gained some considerable experience on this subject. The final version of the smaller machine at Oxford did not reach the design energy of 140 MeV, (it only got to 125 MeV) and was not generally regarded as a success. The Glasgow machine, on the other hand, was finished in 1954 and reached an energy of 350 MeV, 50 MeV above the design energy. This machine was referred to as 'most carefully designed and engineered' and as incorporating 'all the best features of the early machines' of its kind. 59

Last but not least a 156-inch synchro-cyclotron was planned for Liverpool, corresponding to a proton energy of 400 MeV. It had originally been conceived as a 60" cyclotron, but this was reconsidered when news of the principle of phase stability arrived from America. It went into operation in 1954 and was Europe's biggest synchro-cyclotron until 1957 when the CERN synchro-cyclotron (600 MeV) was finished.

To summarize, three of her four university acelerators under construction would provide Great Britain with the opportunity to do meson physics, although at the time when machines of much higher energies were already available in the USA. Nevertheless, these machines provided a good basis for starting a research programme in particle physics. 30

III.4. Continental western Europe60

We have seen the clear leadership of the United States in the building of accelerators and also the efforts undertaken by Great Britain to pursue a rather ambitious accelerator programme. As we will see, activities of this kind were more sporadic in continental western Europe. Below we wish to give an impression of the kind of accelerator programme pursued in various countries in Europe around 1951. 61 This date has been chosen because in 1951/52 major discussions took place on the future European laboratory (CERN).

Let us start with Sweden, since this was surely the country where there was most activity in building accelerators. By 1951 Sweden was in possession of two cyclotrons producing 7 and 25 MeV deuterons, both at the Nobel Institute for Physics. The later machine, finished in 1951, was one of the biggest conventional cyclotrons ever built. At the Royal Institute of Technology an electron synchrotron of 35 MeV and a betatron of 5.3 MeV were in operation. In addition, there were four electrostatic generators operating at other Institutes.

But the most outstanding of all these tools was the 200 MeV synchro-cyclotron at the Gustaf Werner Institute for Nuclear Chemistry in Uppsala. When completed at the end of 1951, it was Europe's biggest synchro-cyclotron (until the 400 MeV synchro-cyclotron at Liverpool was completed in 1954). Although its energy theoretically lay above the threshold for meson-production, the meson output near the threshold was much too low for meson physics.

The next two countries we will consider, namely the Netherlands and Switzerland, have one characteristic in colllllon. Their accelerator programme was 'backed' by large industrial firms like Philips in the case of the Netherlands and Brown Boveri and Oerlikon in the case of Switzerland.

In the Netherlands, the principal laboratory for atomic energy 31

development work was the Institute for Nuclear Research established in 1946 at the University of Amsterdam. Already before and during the war, C. J. Bakker and F. A. Heyn had begun to design a cyclotron while at Philips at Eindhoven. This was changed in 1946 to a synchro-cyclotron of 71" diameter, able to produce 28 MeV deuterons, which was erected at the University of Amsterdam and came into operation in 1949. Indeed Philips continued to build several accelerators of this type, as well as cascade generators, which were mainly sold abroad. In addition to this synchro-cyclotron, two further electrostatic generators (0.3 and 1 MeV) were installed in the Netherlands. 62

In the case of Switzerland, seven accelerators were in use by 1951, namely a cyclotron of 7.5 MeV protons and two Van de Graaffs of 0.85 and 1.5 MeV all at Zurich, two betatrons of 32 MeV, one at Zurich (since 1948) and one at Baden, a 1 MeV electromagnetic accelerator in Basel (finished in 1949) and a 1 MeV tensator at Zurich. In addition, there were plans for two synchrotrons of 100 MeV and 400 MeV respectively both at Brown Boveri at Baden. 63

The next two countries we would like to group together are Norway and France. Both had rather weak accelerator programmes since more emphasis was placed on their reactor programmes.

In 1951 Norway had four electrostatic accelerators reaching maximum energies of 1.5 MeV and one betatron producing 47 MeV electrons (at Bergen). Apart from these, further electrostatic generators with energies between 1 and 4 MeV were the only machines planned.64

By the end of the war, France had four electrostatic generators, the largest having an energy of 3 MeV, and one cyclotron of 32• producing 6.8 MeV protons. The French Commissariat a l'Energie Atomique (CEA) was founded late in 1945, its goal being the use of atomic energy for scientific, industrial and defence purposes. However, only two accelerators were planned in 1948 a standard cyclotron of 65• (producing 26 MeV deuterons) and a 3.5-4 MeV Yan de Graaff. The building of these machines was 32

primarily an engineering task and they were not in fact part of an experimenta. 1 nuc 1 ear p h ysics . programme using. acce 1erators. 65

In 1951 Belgium had only one operating accelerator, namely a Cockcroft Walton accelerator at Brussels (built by Philips). Apart from that three others were under construction: a 25-30 MeV proton linear accelerator, a 12 MeV cyclotron (deuterons), which had already been started in 1949 (finished in 1952) and a 1.2 MeV electromagnetic generator (built by Philips; finished in 1952). No further accelerators were planned. 66

The last two countries to be mentioned are Italy and Germany. In both these countries there was virtually no activity in accelerator construction immediately after the war. Germany's biggest accelerator by the end of the war was a cyclotron in Heidelberg, producing 9 MeV deuterons. Italy had by that time in fact no accelerators at all. Although we have not mentioned projects later than 1952 in the case of other countries, we wish to make an exception in the case of Germany and Italy as both countries had no activities at all until then. Italy launched a project in 1953 to construct a 1 GeV electron synchrotron at Frascati. Germany, which was not allowed to do applied nuclear physics could start only in the early fifties with plans to build a 50 MeV proton synchrotron and a 1/2 GeV electron synchrotron at Bonn.

Summarizing, it may be said that on the European continent the accelerators were all in an energy region appropriate only for nuclear physics experiments. None of them, not even the large synchro-cyclotron in Uppsala, had an energy well above the threshold for meson production, thus making them inadequate for particle physics activities. To conclude, we could therefore say, neither wanting to belittle the research done in the low energy region nor to deny that all these countries regretted not having access to big aachines, that in 1951 each of these countries on its own was far from having the technical, industrial or financial resources to build accelerators on the scale of the big American and British machines. 33

III.5. The AG focusing principle

By 1952 the major part of the post-war American accelerator programme had been realized. The synchro-cyclotrons and electron syn­ chrotrons built at various universities were finished, the big 3 GeV Cosmotron in Brookhaven went into operation and the 6 GeV Bevatron in Berkeley was well on the way. However, although the techniques used for all these machines could, in principle, be applied in even bigger machines, there were financial limits imposed by the fact that the costs rose exponentially with the energy.

A way out of the situation was proposed in a paper by E.D. Courant, M. S. Livingston and H. S. Snyder with the title 'The Strong­ focusing Synchrotron - A New High Energy Accelerator' which was sent to ~ Phvsical Review in August 1952. 67 Before going into details about this principle, it should be mentioned that this idea had already been proposed as early as March 1950. A Greek engineer, Nicolas Christofilos, sent his work on 'A Focusing System for Ions and Electrons' to the United States Patent Office. 68 Unfortunately, the value of this work was not initially realized. Thus it took another two years for the idea to be formulated at Brookhaven National Laboratories and its consequences realized.

We now wish to set out shortly the main features of the strong-focusing or alternating-gradient focusing principle' (called the AG principle for short). If one wished to make synchrotrons financially feasible at considerably higher energies, it was clear that a way had to be found of decreasing either the radius of the machine or the cross-section of the magnets, which depend on the size of the vacuum chamber needed. In a synchrotron, betatron and synchrotron oscillations usually occur, owing to deviations from the equilibrium orbit caused by angular and energy spread in the injected beam, scattering by the gas, magnetic inhomogeneities and freqency errors. These can only be stabilized by a radially decreasing magnetic field, and one with a field gradient n=-r/B*dB/dr which lies be­ tween O and 1. The idea of strong focusing was to build the magnet ring out of sections with a very high field index (field gradient n) of at least 34

n>100. The arrangement of these sectors is such that in the first the magnet field drops with increasing radius (positive n), so that the beam is vertically focussed while being at the same time radially defocussed. The following sector has a magnet field rising with increasing radius (negative n) which has vertical defocusing and radial focusing a consequence. Keeping up this sequence over the whole ring one achieves overall focusing of the beam. The decrease of the oscillation amplitude reached in this way makes it possible as a direct consequence, to decrease the size of the vacuum chamber used. As calculated in 1952 for the example of a 30 GeV accelerator, the aperture would be 1x2 inches. Financially this meant that, at a given cost, a machine with a ten times higher energy could be built.69

What we have described here is the basic idea underlying the new focusing principle. At this stage we do not want to explore the doubts, the problems, etc. which occurred as soon as attempts were made to put those ideas into practice. We will do this in detail in a report on the construction of the CERN proton synchrotron, the first machine to be built using the AG principle.

IV. Experimental particle physics: developments from 1946 to 195371

In our study of the period from 1946 to 1953, we were struck by one element immediately: the number of new particles, and the information on them, grew extraordinarily rapidly, and formed the basis on which particle physics was to become an increasingly important part of physics.

The development of 'elementary particle physics' in the seven years from 1946 to 1953 can be divided into three essential stages. The first was the detection of the pi-meson in cosmic radiation in 1947, the second was the first artificial production of these particles in 1948, and the last was the realization that the mass spectrum of elementary particles was more complex than had previously been thought. To illustrate the progress made in this field the International Congress on Cosmic Rays held in Bagneres-de-Bigorre in July 1953 is typical.72 It will help us to see the 35

effects of these different developments and the consequences of them for cosmic-ray physicists in particular.

We shall now study each stage and its consequences for particle physics as a whole.

IV.1. The leptonic nature of the mesotron and the detection of the pi-meson (1946/47)

In the first chapter we briefly referred to the fact that in 1937 when a particle of mass roughly 200 m was detected in cosmic radiation it e was identified with the field quantum mediating the strong interaction, as predicted by Yukawa. Although there was a disagreement in some points between the theory and the experiments, this identification was maintained. Before this 'meson-puzzle' could be solved, two steps were necessary. First, a clearer picture had to emerge of the features of the known cosmic-ray meson (the mesotron). Secondly, crucial improvements in detection devices were necessary.

The first step was taken by M. Conversi, E. Pancini and 0. Piccioni late in 1946. They carried out an experiment on the absorption of mesons of different charge (which had been separated magnetically) in both iron and carbon plates. It demonstrated that in iron all positive mesons disintegrated, whereas the negative ones were absorbed. However, using carbon as absorber a great deal of negative decay electrons coming from negative mesons were observed.73 Thus, it was concluded that the interaction of the mesotron with the nuclei was much too weak to play the role of the meson predicted by Yukawa, i.e. to mediate the strong interaction. Another important result of the experiment was that here the weak nature of the mesotron (later mu-meson) was clearly proved experimentally.

A big part of the credit goes to C. F. Powell for achieving success in the second stage. The British were leading in the development of nuclear emulsions. During the war the photographic aethod of studying 36

nuclear processes had already been used extensively by C. F. Powell and his collaborators. It was also Powell who played a prominent part in a small panel set up by the Ministry of Supply just after the end of World War II. The photographic firm Ilford was commissioned by the panel to produce emulsions for nuclear research. As early as mid - 1946, emulsions in four grain sizes and three sensitivities were available. With these it was even possible to detect particles which produced very little ionisation.74

It was these newly developed emulsions which made it possible for C. M. G. Lattes, H. Muirhead, G. P. S. Occhialini and C. F. Powell to detect the first two examples of the pi-meson in cosmic radiation in May 1947.75 They saw one meson stop and another meson leave the same spot with the energy of a few MeV. A few months later they already had 40 of these events. They had observed the decay of the Yukawa-meson, which they called the pi-meson, into the mesotron (as it had hitherto been called), which they now termed the mu-meson.

Despite the discovery of the pi-meson and the knowledge of its mass and decay scheme, many problems still remained. Of course, the particle transmitting the nuclear force had been found, but immediately the question arose as to what to do with the mu-meson. Firstly, this particle did not fit into any known theory, and, secondly, the decay of the pi-meson into the mu-meson did not fit into the simple beta-decay scheme proposed by Yukawa.

IV.2. The artificial production of charged and uncharged pi-mesons (1948/49)

This crucial step in the development of elementary particle physics took place roughly a year after the detection of pi-mesons in cosmic radiation. Using the newly-built 184-inch synchro-cyclotron at Berkeley, it had become possible to accelerate alpha particles up to energies of around 350 MeV. Bombarding a carbon target with these particles, c. M. G. Lattes and E. Gardener succeeded in producing and detecting charged pi-mesons, the first artificially produced mesons.76 It should be mentioned, parentheti­ cally, that their detection was only made possible by the recent develop­ ment of nuclear emulsions. 77 37

On 9 March 1948, the university and the AEC officially announced the success at Berkeley. The importance of this event was largely twofold:

First, it was a scientific success, because one was now able to start detailed investigations of these mesons. The resulting enthusiasm can be gauged from the reactions at various conferences where the new results were presented. To give just two examples: the second Shelter Island Conference held from 30 March to 2 April 194878 and the first post-war Solvay Congress held from 27 September to 2 Oc­ tober 1948. 79 At both conferences it was R. Serber, the leader of the theoretical group for the design and the planning of experiments at the 184-inch synchro-cyclotron at Berkeley, who presented the results. On both occasions the talk was received with great interest. Questions were asked on the lifetime of the meson, on the meson cross-section of the nucleus, on the statistics in n+ decay, etc. It should also be observed at this point that the problem of reproducibility, which had caused such difficulties in cosmic-ray work, had vanished. Only thirty seconds of bombardement with the cyclotron brought 100 times as many mesons as Lattes had been able to photograph in 47 days of cosmic-ray observation, figures which illustrate rather well the success of this new technique of producing mesons.

- Secondly, it to some extent justified the huge amounts of money already spent on the accelerator programme, and was a good argument for further funding. To illustrate this, we can quote a statement of E. 0. Lawrence who immediately took the opportunity to argue in favour of very big accelerators saying that, 'to exploit the knowledge which the meson may provide it will be necessary to construct super-giant cyclotrons.' 'The Times' reporting this event, hinted that the study of mesons might 'lead in the direction of a vastly better source of atomic energy than the fission of uranium.' This again shows the positive response which any sort of research even loosely connected to applied nuclear research looked in the post-war years. 80 38

What exactly did this event mean for the cosmic-ray physicists? Their feelings were in fact rather ambivalent. In one respect, others had trespassed on their domain of physics and had replaced their rather unreliable source of particles with machines offering advantages such as reproducibility, high particle flux densities and the possibility of well­ aimed experiments. Powell expressed this feeling very clearly at a conference on Cosmic Radiation held at Bristol in September 1948. He mentioned that 'before the observations [on pi-mesons] were completed, however, news came of the artificial production of mesons at Berkeley. It seemed clear that the possibility of making measurements of much higher precision would thus become available, and further experiments in this field were temporarily abandoned at Bristol.' The time had come to rethink the work to be done. 81

On the other hand, he tried at the same time to emphasize the role which cosmic-ray physics would still play: 'At the present time, the importance of the study of cosmic radiation appears to be associated with the fact that the individual particles of which it is composed are of ten found to be many thousands of times more energetic than any we can generate artificially in the laboratory. In spite of the relatively small number of particles in the incoming stream, we are thus enabled to investigate new types of processes which cannot be observed in any other way. 182

Thus cosmic-ray physicists had to leave part of their investigations, namely low energy meson physics, to the physicists working with accelerators.

A further important related event took place early in 1950. The planned electron synchrotron had been completed at Berkeley. With this accelerator it had become possible to produce the first neutral pi-mesons. The 335 MeV electron beam produced bremsstrahlung photons which were directed on to a carbon target leading to a reaction: 39

0 -y+p---+ p+n

I I 2 "Y 0 -y+n---+ n+n

The n° was detected by J. Steinberger, W. Panofsky and J. Steller83 via its two decay photons. This was the first particle produced by an accelerator which had not previously been detected in cosmic radiation. The hypothesis that such a particle existed had already been proposed in 1938 on the basis of considerations of the charge independence of nuclear forces.

After this first artificial production of mesons in 1948, the number of accelerators able to do meson physics grew quickly. As we can see from Table 1, six accelerators, i.e. four synchro-cyclotrons and two electron synchrotrons, all in the United States, were already in a position to produce mesons artificially by the end of 1950.

Although the neutral pi-meson was also detected in cosmic radiation soon afterwards, from the late forties onwards all precise information on the properties of pi-mesons and mu-mesons, like mass, lifetime and spin was obtained with accelerators. 84

To conclude, by 1951 the state of knowledge on the pi- and mu-mesons was as follows: 40

Particle Mass [m ] Spin e Lifetime [s] Decay Scheme

+ -8 + + 1r 276 0 2, 6 .10 1r - µ + u 0 0 1r 265 0 < 5.10- 14 1r - 21 - -8 -- 1r 276 0 2,6.10 1r - µ + u + -6 + + µ 210 1/2 2.22.10 µ - e + 2u - -6 -- µ 210 1/2 2.22.10 µ -e + 2u

TABLE 2. HAIN PROPERTIES OF THE PI- AND HU-HESONS C1951J 85

IV.3. The complexity of the mass-spectrum

The first hint that there would be other particles heavier than the mesotron came as early as 1944. L. Leprince-Ringuet and M. Lheritier reported the detection of a new particle in cosmic radiation with a mass about 990 times the mass of an electron, and consequently much heavier than the mesotron. However, as already pointed out in the first chapter, since there was only one photograph of this event and the measurement was rather inaccurate, the result was not particularly convincing.86

In fact, it took another three years before other new heavy and unstable particles in cosmic radiation were detected (in 1947). The hunt for other new particles was now on and would reach its climax in the fifties. 41

IV. 3 . 1. T h e V-partlc. 1 es B7

In 1947 G. D. Rochester and C. C. Butler working at Manchester University published a paper on a new kind of event they had found in their Wilson chamber in the course of investigation of cosmic-ray induced pene­ trating showers. A neutral particle entered the chamber and decayed into two charged particles, an event having a characteristic V-shaped track. More­ over, also a charged particle occurred leaving a V-shaped track when de­ caying. The mass of the neutral particle as measured was between 800 and 1000 m , while the charged one was estimated to be close to 1000 m . These e e new events were followed with great interest by the physics community, and promised to reveal something new and hitherto completely unknown. In fact these were the first examples of the (present) particles K0 and K+ , so the first members of the family of 'strange particles' (as they were later called). However, although the Wilson chamber continued in operation at Man­ chester, no further events of this form were detected in the following three years. BB

1950 Seriff et al. found a further 34 V-shaped tracks and this kind of particle could now be reliably regarded as established. Being asked to name the new particles, Blackett in consultation with C. D. Anderson baptized both the charged and the neutral particles V-particles.B 9

In the same year, the Manchester Group transported their cloud chamber to the top of the Pie du Midi de Bigorre and by March 1951 they had detected 36 v0 and 7 v• . Starting more systematic investigations on these V-particles, one came to the conclusion that there were in fact two different neutral V-particles each with its own mass and decay scheme. The

V1 °, a kind of superneutron, with a mass of roughly 2250 me' decayed as follows

v 0 p + 'If 1 - 42

0 The v2 , with a mass of 950 me' decayed thus:

v 0 + 2 • + n

However, these decay schemes were not absolutely clear and it was therefore still possible (although the mass values differed so much) to assume that there was only one v0 decaying in two different channels, namely:

Vo --+ p + n + neutral particle

+ Vo --+ n + n + n

This example illustrates clearly the kind of difficulties cosmic-ray physicists were constantly meeting. To a large extent, the problems of interpretation lay in the sometimes very broad error margins of the mass values and in the often rather vaguely known decay schemes. 90

While the information about the neutral particles increased rapidly, little was known about the charged particles, and it was therefore not possible to draw definite conclusions about their mass, disintegration scheme and lifetime. The reason for this lay in the fact that the ratio of detected charged to neutral V-particles was roughly 1:6. Moreover only positively charged V-particles had been found until then. The mass of these charged V-particles was determined to be roughly 1100 me but with error-margins of 20-30\. This indicated at least that these particles were 0 certainly not the charged counterparts of the V1 • On the other hand, some charged V-events seemed to contain pi-mesons or mu-mesons in their decay products and would therefore be similar to the decay products of the K-particles Cx and K), to which we will come later. Until 1952 this was all the information which could be gained on charged V-particles, whose status was still in doubt at the time.91 43

0 By 1952 the two-body decay of the V2 could be regarded as the

most probable one and so these two particles (V1 ° and v2 °> were definitely established. No other new neutral unstable particles were found in the period up to 1952.

In 1952 the Manchester Group found the first example of a negatively charged V-particle decaying into a neutral V-particle and a meson 92

0 v- --+ v1 or 2 +light meson

As there was no evidence as yet for a charged V-particle of mass greater than the proton it was assumed that the neutral V-particle in the decay 0. 0 would be a v But in fact this was not the case, it was a V I and this 2 1 event represents the first example of a 'charged hyperon' : , as we now call it.

IV.3.2. The heavy mesons

In January 1949, the Bristol Group published a photograph of another new type of particle. A heavy particle (lighter than a proton) of mass roughly 1000 m came to rest and emitted three particles, a negative e pi-meson and two other mesons. This photograph was only possible because of the progress made in emulsion techniques. (In the meantime electron sensitive emulsions had been developed, with which any particles of charge one could be recorded, even if it was moving with the velocity of light.) This particle was called the I-meson, being the first example of what we now call a Kw 3decay. 93

In 1950 a second I-event was found and so this particle's existence could be regarded as established.

The number of charged particles was still increasing. In 1951 O'Ceallaigh identified the first K-meson, as he called it, in the photo- 44

graphic emulsion while studying the decay electrons from mu-mesons. In fact he measured two different mass values, one 1320 and one 1125 m . But as e the mass values were so close he supposed that there was only one new particle decaying in a mu-meson and unobserved neutral particles.94

Investigating the K-decay closer, it was discovered that there were in fact two different decay modes and thus it was supposed that they came from two different particles with mass values close to each other. The particle having a three-body decay involving a mu-meson kept the name K-meson (nowadays called Kµ 3), the other, having a pi-meson among the decay products, was named x-meson, nowadays called Kw 2.95

An additional heavy charged particle was found in 1951 and 1952 by a group at M.I.T. using multiplate cloud chambers. It was named the S-particle and had a mass of roughly 1400 m .96 e

Let us stop here and have a look at what has happened. From 1947 onwards the contribution of cosmic-ray physicists to finding new particles had been enormous. A completely new, complex world, full of unexpected features had been revealed. It had become clear that particle physics had not stopped with the discovery of the pi-meson; this had only been a start. What we have presented here is only a small part of the work done, only the outstanding and well-known results reported. The cosmic-ray physicists had, within five years, more than doubled the number of particles, although we should state here that at the time different decay modes of the same particle were often taken to be different particles.

In conclusion we would say that in the early fifties it had become clear that the mass spectrum of elementary particles was highly complex and that only a part of it had been established at the time.

Before we look at the steps taken to impose order on this chaos of information (at the Bagneres-de-Bigorre Conference in 1953), we will present a short list of the new particles and some of their known features: 45

mass [m ] Mode of decay present date of e name detection

- v 0 2250 (2174.5) v 0 p + 1T Ao 1951 1 1 -- + - v 0 800 (970) v 0 1T + 1T K o 1947 2 2 - s v: 1000 v: - ? + ? K+ 1947 - - 0 - ? (n,µ) v (2585.7) v - v1 or 2 + meson - 1952 + + - 1 969 1 - 1T + 1T + 1T Kn3 1949 + K 1300 (962) K -- µ + ? + ? Kµ3 1951 x 1100 x -- 11 + ? Kw2 1952 s 1400 s -- meson (µI 1T) + Kµ2 1952 ? + (?)

TABLE 3. LIST OF V-PARTICLES AND HEAVY HESONS KNOWN IN 1952 97

(The mass values in brackets are the present values.)

IV.3.3. The Bagneres-de-Biqorre Conference (1953)

From 6-12 July 1953 the 'International Conference on Cosmic Rays' took place in Bagneres-de-Bigorre. Bagneres-de-Bigorre is a small village near the Pie du Midi. It has an astronomical observatory and the chambers of the Manchester Group and the Ecole Polytechnique were installed there. The conference lasted six days and its aim was to draw as coherent a picture as possible of all these new particles. 46

The reason for our choosing this conference in particular is that it was a very big international conference and therefore enables us to get an impression of the situation in particle physics in the early fifties seen from the point of view of cosmic-ray physics. More than 70 talks were given in the six days of the conference, and a considerable amount of detailed information on all the new particles found in cosmic radiation was presented.

In the preceding section, we briefly looked at the various particles found before 1952. As we are concerned rather with the overall development of the field of elementary particle physics, than with a detailed knowledge of those particles, we do not want to go into the scientific details of the talks presented. It is much more interesting for us to see that here, for example, the first efforts were made to build up a general vocabulary. The talk by B. Rossi on the last day was devoted to summarizing the experimental results presented during the preceding days and he also presented the 'nomenclature more frequently used during the conference'. The particles were divided according to their mass in L(light)-, K(mass between pi-meson and proton)- and H(mass between neutron and deuteron; hyperons) -particles. The events were divided into two categories, namely V- and S-events. Finally the use of small Greek letters f or mesons an d capital. ones for h yperons was also suggested . 98 Th' is nomenclature and division was kept as a general rule for quite some time. What is noteworthy is not simply the way this division was made or that it was kept for quite a while, but the fact that it was made at all. This was a sign, firstly, that the field of elementary particles had grown in its scientific content to the extent that some order needed to be imposed and, secondly, that the community of scientists working in the field in many different places had grown to such a size that if international collaboration and communication in particle physics were to be possible, a common vocabulary had to be found.

However, on this same occasion, when cosmic-ray physicists de­ monstrated their dominant contribution to elementary particle physics, 47

M. Schein also reported on the plans for 'the artificial production of v0 by 227 MeV w-mesons generated in the Chicago Cyclotron' (which did not succeed). 99 In 1951 and 1952 both the Chicago and the Pittsburgh synchro­ cyclotrons had been completed and, with energies of respectively 450 MeV and 440 MeV protons, they were able to produce pi-mesons of higher energy than before. The Brookhaven Cosmotron with 3 GeV energy was completed in 1952 and soon promised to deliver interesting results from a very high energy range.

In fact, in the mid-fifties cosmic-ray physics had to some extent reached its zenith in regard to its contribution to particle physics, as many physicists realized. The accelerators were slowly but surely taking over the leading role in the field of elementary particles. Let us thus conclude this chapter· with a description of the situation as presented by Leprince-Ringuet in his closing speech at the conference:100

'Pour terminer en quelques mots, on peut sans doute dire que l'avenir du rayonnement cosmique, dans le domaine de la physique nucleaire, depend des machines, de la facon dont le 'strong focusing' se developpe plus ou moins vite, mais c'est probablement une vue seulement partielle. Nous avons l'exclusivite des phenomenes, bien rares il est vrai, dont les energies sont beaucoup plus grandes et nous serons un peu a l'abri, dans la mesure ou nos techniques seront plus developpees, de la montee tres rapide de l'energie dans les machines.'

V. The theoreticians: from the first Shelter Island Conference to the Rochester Conferences

Remembering what was said in the first chapter about the status of theoretical physics in 1946, we will now continue our study, covering the period up to the end of 1952. This was, as we will see a very eventful time in theoretical physics. Many questions remained unsolved, and many more were arising with the new experimental results. However, it was also the time of 48

the first successes and thus also the time when the self-confidence of the theoreticians increased. In this chapter we will concentrate on the de­ velopments in theoretical physics with respect to elementary particle physics. We will therefore not cover topics which belong to nuclear physics, for example, models of the nucleus (shell model, compound nucleus model, ... ), a field in which a lot of very important work had been done around 1950. 101

To understand these developments better, we will study the three Shelter Island Conferences102 and the first three Rochester Conferences in detail. While we are aware that there were many other very important conferences in this field, we want to restrict ourselves to these. The reasons for the choice are, firstly, that the Shelter Island Conferences were the only ones so strictly oriented towards theoretical work and secondly, that we have here a series of conferences. Thus it is possible for us to get a better grasp of the evolution of theoretical physics during these years. The Rochester Conferences thereafter revealed the changes theoretical physics was undergoing in the early fifties, particularily the establishment of a very close collaboration between theoreticians and experimentalists.

The initial idea of holding the Shelter Island Conferences - as they were later called - came from Duncan Macinnes. He was a physical chemist at the Rockefeller Institute and his idea was that the National Academy of Sciences should sponsor several small conferences on various subjects. He therefore contacted K. K. Darrow, Secretary of the American Physical Society. The idea was to have something like the Solvay Congress, so as to gather a rather select audience and to publish the discussions later. This was the proposal Darrow presented to Wolfgang Pauli, who was at that time at the Institute for Advanced Study in Princeton. Pauli, however, was more in favour of something on a larger scale, rather like the big conference held in Cambridge in 1946, as he was convinced that this would be necessary to re-establish communal values in physics after the war. However, it was finally decided to hold a small conference, to be as informal as possible, and without proceedings. The aim was to convene a group consisting 49

mainly of young physicists, the number of participants being limited to roughly 25-30. The date was fixed for 2-4 June 1947 on Shelter Island (near Long Island, New York).

This was a purely American conference and the list of participants is remarkable. Nearly everybody who played an important role in theoretical physics and was in America at the time was present, as a complete list of t h e part1c1pants. . revea 1s: 103

Chairman: K. K. Darrow Discussion Leaders: H. A. Kramers J. R. Oppenheimer v. F. Weisskopf Participants: H. A. Bethe A. Pais D. Bohm L. Pauling G. Breit I.I. Rabi E. Fermi J. Schwinger H. Feschbach R. Serber R. P. Feynman E. Teller w. E. Lamb G. E. Uhlenbeck D. A. Mac Innes J. A. Wheeler R. E. Marshak B. Rossi J. von Neumann J. H. Van Vleck A. Nordsieck

In this list two things are worth noting: first, only a few experimen­ talists were invited and then only those who were at the time doing experiments of immediate importance to theoreticians, namely people like Lamb, Rossi and Rabi. Secondly, no less than four of the theoreticians present later won the Nobel prize: Bethe, Feynman, Lamb, and Schwinger.

The theme of the conference was 'The Foundations of Quantum Mechanics'. Three physicists had been asked to prepare general papers for discuss1on:. . Kramers, Oppen h e1mer . an d Weiss . k op f . 104

Weisskopf proposed to divide the discussion into three main parts: 50

A. The difficulties of Quantum Electrodynamics (QED) B. The difficulties of nuclear and meson phenomena C. The planning of experiments with high energy particles

In the section on QED, the self-energy problem, vacuum polarization, high energy limits to QED and the infra-red catastrophe were supposed to be dis­ cussed. Section B was to comprise an overview of the actual attempts to formulate meson theories and their representation of nuclear forces, and also a discussion of the experiment of Conversi et al. It is interesting to see that, regarding part C, Weisskopf mentioned that it 'could become the most useful part of the conference. A number of very powerful accelerators in the energy region of 200-300 MeV are near completion, and it is time to inaugurate a systematic program of research, with experiments which can be reliably interpreted.' He thus stressed the importance of close interaction between theory and experiment.

In his outline of topics for discussion, Oppenheimer concentrated more on the theoretical interpretation of cosmic-ray results. His aim was to find out if it would be possible to reconcile 'on the basis of usual quantum mechnanical formalism the high rate of production of mesons in the upper atmosphere with the small interactions which these mesons subsequently manifest in traversing matter.' He went on to say that 'to date no com­ pletely satisfactory understanding of this discrepancy exists, nor is it clear to what extent it indicates a breakdown in the customary formalism of quantum mechanics.' He thus suggested that topics like the theory of multiple production should be studied in greater detail.

We can therefore see that both Oppenheimer and Weisskopf showed considerable interest in current cosmic-ray work. In particular, the work done by Conversi et al. in Italy which had shown that the mesotron was definitely not a strongly interacting particle (for details see Chapter IV) was creating a lot of interest in the theoretical physics community. There were already results available which pointed out various discrepancies, but which also suggested a solution to the problem, like the work by Weisskopf done early in 1947, but this was not enough to settle the matter. 105 51

Kramers, on the other hand, entirely devoted his general outline to QED and gave a review of the problems which had occurred from its inception in 1927. He stressed two main problems:

the treatment of the behaviour of an electron with experimental mass in its interaction with the electromagnetic field, and the unsatisfactory feature in the relativity treatment of free particles.

From these three papers we can see the topics which were supposed to be discussed at the conference. Besides the experiment of Piccioni et al. on the decay of the mesotron, that of Lamb and Retherford on the fine structure of the 2s - 2p levels of hydrogen played a key role. The latter experiment had been reported at the end of April 1947, thus only very shortly before the conference (and so was not reflected in any of the discussion pro­ posals). It was the first indication that, contrary to Dirac's theory, the 2s 112 state was higher than the 2p 112 . 106 It should also be mentioned, that

the detection of the pi-meson by Powell in May 1947 was not known by the time the Shelter Island Conference took place.

The direct, and very well-known consequences of these three days of discussion in June 1947 were Bethe's non-relativistic Lamb shift calcu­ lation and Bethe and Marshak's work on the two meson hypothesis. 107

These new developments in theoretical physics and its first successes were a major stimulus for the new interest of theoreticians in quantum electrodynamical problems. In December 1947 Schwinger submitted a paper to the Physical Review on the quantum electrodynamical correction of the electron's magnetic moment, in which he already clearly spoke of the renormalization of the electron charge and mass. This was one of the first steps towards a renormalized relativistic invariant version of quantum e l ectro d ynam1cs.. 108

J. Robert Oppenheimer was the driving force for the second Shelter 52

Island Conference. Financial support again came from the National Academy of Sciences. Although the first conference had been such a success, it was decided not to let the number of participants grow. This time the place was Pocono Manor and the meeting was held from 30 March to 2 April 1948. The number of participants was 28, which now included some foreign guests who were visiting the United States at the time. The participants included P. A. M. Dirac, Aage and Niels Bohr, Walter Beitler, Gregor Wentzel and Eugene Wigner.

The discussion was mainly about the recently discovered new particle, the pi-meson, the precise measurements on atomic energy levels and about new developments in QED. The number of known particles had been increased by two - the positive and negative pi-mesons found in 1947 by C. F. Powell in cosmic radiation. These particles were the answer to the questions regarding the existence of two different mesons and their discovery also ended the search for the long predicted Yukawa particle.

But, as Feynman described in his report on the Pocono Conference, 'the most exciting event of the year was that heavy mesons [•-mesons] could be made by the great 184-inch cyclotron at Berkeley. This means that they can be studied under controlled conditions and in large numbers. •109 Now very accurate investigations on the subject could start.

Although enthusiastic about the progress in experimental physics, Feynman was rather pessimistic about the chances of resolving some of the theoretical problems with the description of these new facts. He summarized these feelings in a review article on the conference, saying that 'the theoretical physicists admitted that they were unable to bring appreciable order into the picture, and certainly not to predict what kind of particles would be discovered next, or any new properties for particles already discovered. The future of these problems lies almost completely in the hands

, . I 110 o f t h e exper1menta 1ists .

Although the theoreticians were still struggling with questions like meson theory, the law of nuclear forces etc., their progress in 53

formulating QED was considerable. At the Pocono Conference, i.e. in April 1948, both Schwinger and Feynman presented calculational schemes in QED which were relativistically invariant. Schwinger's presentation being a success, 111 Feynman's was not rea 11 y accepted at the time. . It was a more intuitive presentation and one could say that it was to some extent the origin of his diagrammatic approach. As Dyson described it at a conference in 1949, Feynman's approach was 'a critical re-examination of the basic principles of electrodynamics' and he was trying to 'construct a framework into which different theories can be made to fit.' 112 As with the first Shelter Island Conference, the results were only visible afterwards. After Pocono Feynman wrote his two crucial papers on 'Theory of the Positrons' and 'Space Time Approach to Quantum Electrodynamics' .113

Let us conclude our survey of this second conference with the words of Feynman written shortly after the conference in 1948, which give a rather good picture of the situation:

'The conference showed that just as we were apparently closing one door, that of the physics of electrons and photons, another was being opened wide by the experimenters, that of high-energy physics. The remarkable richness of new particles and phenomena presents a challenge and a promise that the problems of physics will not be all solved . ,114 f or a very 1ong time to come.

We want to make two remarks here which are not directly connected with the Shelter Island Conferences.

Immediately after this conference, Oppenheimer received a letter from S. Tomonaga informing him that in Japan extensive work had already been done for several years on the topic of a relativistically invariant formu­ lation of quantum field theory. 115 54

The second remark is about work done in the field of weak interac­ tions. It was also about the middle of 1948 that several papers appeared stating that the coupling constant of radioactive beta decay was of the same order of magnitude as the one for disintegration and mu capture. An Italian physicist, G. Puppi, concluded in an article written late in 1948 'that the Fermi constants between Dirac particles are equal.' This was the first essay to formulate the 'universal Fermi interaction'. We can see that in the late forties more and more ideas were formulated with the aim of bringing a certain order into the dense information provided by the exper1menta. l'ists. 116

A year later the third and last conference of this series took place. It was held from 11-14 April 1949 at Oldstone-on-the-Hudson. Robert Oppenheimer was again chairman. 24 participants were present, mainly those who had attended in previous years. Hideki Yukawa was invited as a guest.

The topic of this conference was 'Fundamental Physics', and sub­ jects like mesons, field theory, the relations of the elementary particles to nuclear forces and cosmic rays were discussed. Both Richard Feynman and Freeman Dyson, 117 the latter attending this series of conferences for the first time, presented their calculational schemes in quantum electrodynamics. It was clear at that time that QED in this renormalized version would be quite satisfactory. It was a big success and the credit for it must be shared by Dyson, Feynman, Schwinger and Tomonaga. For these reasons, it is also understandable that the centre of interest started to shift towards pi-meson physics, where there remained a huge number of open questions. 55

Oppenheimer wrote the following on the conference:

'The two years since the first conference have marked some changes in the state of fundamental physics, in large part a consequence of our meetings. The prob­ lems of electrodynamics which appeared so insoluble at our first meeting, and which began to yield during the following year, have now reached a certain solution; and it is possible, though in these matters prediction is hazardous, that the subject will remain closed for some time, pending the accumulation of new physical experience from domains at present only barely accessible. The study of mesons and of nuclear structures has also made great strides; but in this domain we have learned more and more convincingly that we are still far from a description which is either logical, consistent or in accord with experience., I 11 8

This then was the final impression after the last of the three conferences. A great deal had been achieved and this fact encouraged the theoreticians to continue on their difficult path. The three Shelter Island conferences were the precursors of the big Rochester conferences. They were small, intimate, elitist and mainly theoretical. The latter conferences were different in many respects, partly because a whole new era in physics had really started. Apart from the size and the choice of much broader topics, the major dif­ ference lay in the fact that they were attended by equal numbers of theoreticians and experimentalists. Robert Marshak, the father of these conferences, described the situation as follows:

'The performance of several important experiments on these accelerators, including the artificial production of pions for the first time in 1948, and the anticipation of many more to come had 56

persuaded me that a new series of conferences should be inaugurated in which experimentalists would be given 'equality' with the theorists.'119

The First Rochester Conference took place in ~- It lasted only one day, was a purely American conference attended by some 50 participants. A group of Rochester industries provided the financial support.

As no proceedings of this first, very short, and rather informal meeting were published, we do not know much about the contents. All we know is that there were three sessions of presentations by invited speakers on nucleon elastic scattering and meson production by nucleons and photons.

The Second Rochester Conference was held at a rather exciting time, from 11-12 January 1952. 120 By then many new results had been obtained particularly in cosmic-ray physics but also using the accelerators built since the war. Roughly 90 participants attended the meeting, all from America.

The talks were grouped around three topics:

Interaction of w-mesons with complex nuclei Interaction of w-mesons with hydrogen and deuterium Megalomorphs (V,v,K,T) 12oa

The first two topics occupied half a day each, the third the whole of the second day.

Of special interest among the talks on the first day was the report by Herbert Anderson on the 'Absorption of w+ and w mesons in hydro­ gen and deuterium' and the 'Discussion of these results' by Enrico Fermi. 57

This experiment had recently been carried out with the Chicago machine and + + 0 had shown that the cross sections o(w +p-+ n + p), o(w + p-+ n + n) and o(w + p-+ n + p) had the relative ratio of 9:2:1. Fermi explained this exciting result as follows:

'If one assumes charge independence, i. E. that the Isospin is a good quantum number, the two possible isospins, namely I=3/2 and I=1/2 scatter independently. If more­ over, one assumes that the isospin I=1/2 does not scatter at all one gets the ratio 9:2:1. [ ... ]This conclusion is independent of angular momentum, spin correlation, or anything else. One can therefore interpret the experimen­ tal results by postulating the existence of a broad resonance level I=3/2 in the band of energy 100-200 MeV, with the consequence that practically all the scattering comes through I=3/2 in this energy region. •121

Theoretical work on this topic was presented by Keith A. Brueckner who explained the result in terms of the first nucleon resonant state, with isospin I=3/2 and angular momentum J=3/2, known as the (3,3) pion-nucleon resonance. (In fact in the mid-forties Pauli had already elaborated similar ideas on behalf of cosmic-ray observations.)

This was the start of a lot of work on the concept of isospin invariance. . o f pion-nuc . 1 eon interactions. . . 122

In general, one thing should be noted about this conference. There was a very strong interaction between the experimentalists and the theorists. Each experimental presentation was followed by a theoretical discussion, asking more detailed questions and trying to give possible approaches to the particular problem.

The whole of the second day was devoted to megalomorphs (V,v,K,t), and was used to discuss the new heavy unstable particles detected in cosmic 58

radiation. At this session the experimental results of different groups working with cosmic radiation were presented and discussed. Reference was also made to the experiment which tried to produce these V-particles with the Chicago cyclotron.

Of greater interest to us is the talk by A. Pais called 'An ordering principle for megalomorphian zoology' and the introduction to the t h eoretica. 1 d"iscussion . b y J. Ro b ert Oppen h eimer. . 123

Oppenheimer described the general situation as it had emerged at the conference. The main questions concerned the production of particles and explanations for their lifetimes and specifically what sort of interaction was involved in all these processes. Pais followed with his talk. 'The following is an attempt,' he began, 'to decouple the production of these heavy particles from their decay, talking for the moment only of the observed species.' He tried to find an ordering by assigning to the protons, 1 2 4 neutrons, Vs, vs and vs a 'mass number' as indicated in the following table:

'Elementary Particle' Families

N0 (nucleons) w0 (pions) e

0 0 t + (p=No +, n=No o) ( 1f =w o ' 'I =v o-)

N1 (heavy V particles) w1 (light v particles) µ

( V o =N o . Vt =N t ) 1 1 I 1

( N2 ? ) ( ' resonance ' in ? ? -nucleon scattering) 59

He then continued by introducing a matrix of interaction of the form (NiNjnk) were i,j, and k should be either all equal to zero or to one. Pais proposed a selection rule which, though it did not follow from present theories, was not in contradiction with the experiments, namely '(N.N.nk) l J can only be strong if i+j+k is even.' This was one of the first attempts to find an order in the chaos of new particles, to be able to understand the various reactions and to find common features for the different particles. Towards the end of his talk, he once again referred to his table remarking 'that he would like to look at this schematization as the unfolding of an ordering in which one talks about families of elementary particles [our emphasis] rather than elementary particles themselves.'

We wish to conclude our review of this conference with a remark by Robert Marshak made about this time:

'Experimentalists were unfolding a strange new world on the subnuclear level, suggestive theoretical phrases were coined ('live parent', 'heavy brother') and the science of high-energy physics was entering a per10. d o f tremen d ous vita . l'ity. ' 125

With the Third Rochester Conference which took place eleven months later, from 18-20 December 1952 the developmental process in this series of conferences can be regarded as complete. 126 While the Second Rochester Conference was still called a 'Conference on Meson Physics', the third was already called 'High Energy Nuclear Physics' conference. This was not all that had changed. The number of participants had grown to 140 and for the first time foreign guests were also present - from Great Britain, France, Italy, Australia, Holland, Japan and several other countries. The conference had become international. The financing had also undergone some change. For the first time the National Science Foundation was co-sponsor and so support came from a government agency. 60

Let us now have a brief look at the scientific contents of the conference. In these three days seven sessions were held covering the whole breadth of high-energy physics at the time. These sessions were on charge independence and the saturation of nuclear forces, pion production and pion­ nucleon scattering, V-particles, superheavy mesons, theoretical approaches to the pion problem, pion-nucleon phase shifts, megalomorphs, theoretical calculations and experimental physics. On this occasion, quite a large number of the presentations were based on experiments with the accelerators performed in America since the war. Of course, new information still came from cosmic-ray physics, although it was diminishing steadily.

To conclude, let us summarize in a few words what we have des­ cribed in this chapter. We have seen the theoreticians in a rather confused position at the beginning of the Shelter Island Conferences. We have watched them struggle - successfully - for a final version of quantum electro­ dynamics. Being able to formulate QED reassured both them and the whole physics community. It was now much more realistic to imagine a possible solution of the many other questions demanding attention.

However, this is only part of the picture. We have also seen the impact of the change in experimental particle physics on the work of the theoreticians. While they had small elitist conferences like Shelter Island in the late forties, theoreticians switched over to work of another kind in the fifties. The conferences were large and were held in conjunction with the experimentalists, who were delivering an increasing number of results with the newly completed accelerators.

VI. Concluding remarks

So far we have restricted ourselves to giving detailed, separate reviews of accelerator programmes, and theoretical and experimental physics. We now wish to unify these items of information to form a more coherent picture. In this way we will be able to see how high-energy physics slowly developed into an autonomous field of investigation and to throw light on the attraction of this field for scientists. 61

To this end, we want to go back to 1946 and to try to draw a rough outline of the mainstream of the various parallel developments in the field of particle physics.

In the first chapter of this paper we studied the different parts of physics which dealt in one way or another with the basic concepts of the constitution of matter, considering the particles it comprises and the forces between them. The basic attitudes towards these elementary particles in the mid-forties fell into two main categories:

First, it was rather easy to be satisfied with the particles already discovered and not to believe in additional ones. Indeed it was asked why further particles should exist. They were not 'needed', there was no part of a theory which made sense of them.

The second attitude was generally not to ask fundamental existential questions, but 'simply' to collect information, out of which, one hoped, a certain system, an ordering principle would ultimately emerge.

Of course, the search for information on elementary particles took different paths in cosmic-ray physics, experimental nuclear physics and theoretical physics. By 1946 the experimental data on them came almost entirely from cosmic radiation. This kind of research was 'little science' in the narrowest sense. The daily work was done in small groups, the instrumentation was relatively simple and, as a consequence, not expensive. But, as we have shown in detail, this approach had primarily three weak points: The measurements were inaccurate, the results were difficult to reproduce and the theoretical background necessary for the interpretation was rather weak. Although all these factors caused difficulties, the most basic handicap was that of low reproducibility, with all its consequences.

Regarding the state of nuclear physics in 1946, the picture is completely different to that of cosmic-ray physics. Nuclear physics had 62

advanced to the forefront of research through its crucial role in the Second World War. Not only had the image and the prestige attached to it changed, but also the size of the projects, the project costs and the instrumentation at its disposal. Things that were impossible before the war had become possible. Seen from that aspect even the many years lost for basic research due to the war seemed to lose importance, particularly in America and in the Great Britain.

On the technological side, the principle fo phase stability (developed by Veksler and McMillan) made a huge accelerator programme possible, and the building of machines which would be big enough to produce artificially particles previously found only in cosmic radiation.

The theoreticians were in a rather weak position at the time. They saw themselves faced with many open questions and again and again the experimental results raised new ones. Those who were often expected to be the backbone of the research field found themselves unable to offer solutions, to give help or to survey the field in a comprehensive way and this was a constant source of concern for them.

At the same time the information in nuclear physics had not only become much more detailed, but it had also considerably broadened, so that the need for specialization began to crystallize. There were two possible directions, which later in the fifties would become clearly distinct, namely nuclear physics (as we now understand it) and high-energy physics.

This was the situation in 1946. We will now turn our attention to 1948, the year in which the situation changed drastically.

In America and in Great Britain nuclear physics was in a prosperous state. Brookhaven had been founded. The Atomic Energy Commission had decided to sponsor two enormous proton synchrotrons, one of 6 GeV at Berkeley and one of 3 GeV at Brookhaven. In addition, a variety of machines in the several hundred MeV-range were built, in particular six synchro­ cyclotrons and five electron synchrotrons. In Europe the activities in these 63

energy-regions were restricted to Great Britain. The rest of Europe also had an atomic energy programme, but only in the low energy region and not comparable in size to the British or American projects.

In the field of cosmic-ray physics considerable development had also taken place. First, Powell had finally found the pi-meson in cosmic radiation. Secondly, Rochester and Butler had detected the first two examples of V-particles in their cloud chamber. These two events had serious consequences and proved to be essential for the basic concepts in particle physics. The discovery of the pi-meson had the indirect consequence that the mu-meson lost its 'raison d'etre'. The discovery of V-particles was a clear sign that more of these particles could be expected at higher energies.

In 1948, the theoreticians were already on the way to improving the situation. The first two Shelter Island conferences had already hinted that the efforts to formulate quantum electrodynamics in a relativistically invariant way would lead to success. This success would strengthen the confidence of the theoretical physics community in its ability to tackle the numerous other questions in meson physics.

However, possibly the most central and crucial event occured in March 1948. Using the recently finished 184-inch synchro-cyclotron at Berkeley, the first mesons were artificially produced. In fact, this was the point where for the first time cosmic-ray physics and nuclear physics 'met' on the scientific level. It rapidly became clear that particle physics had now entered a completely new era and that basic research could be approached from a completely different angle. By using an accelerator, the disad­ vantages of cosmic-ray work would vanish. The hope of doing experiments under controlled conditions had become a reality - the entire approach to particle physics research had thus begun to change.

However, this was only a first step. It was still not possible to define clearly, for example, the term 'particle physicist'. Was he a cosmic­ ray physicist who dealt with questions relating to the constitution of cosmic radiation or was he a nuclear physicist, working with a machine of 64

sufficient energy to produce cosmic-ray particles? In other words, for the time being, the field of fundamental particles was tackeled by two groups in parallel. Both of them offered particular advantages, the nuclear physicists the reliability of their machine, the cosmic-ray physicists the high energies of their particles.

Finally, let us move on to the early fifties. In this period the tasks of nuclear physics and cosmic-ray physics with respect to their contribution to 'particle physics' were becoming more clearly defined. In general terms, one could say that it was at this time that work in the entire field began to be reconsidered. Terms like 'nuclear physics' were beginning to be too broad, too unspecific, and thus new terms like 'low energy nuclear physics' or 'classical nuclear physics' and 'high energy nuclear physics' began to be used in books, in articles and at conferences. The field was about to be reorganized. In the United States, nine accelerators (by 1952) already had energies high enough to produce pi-mesons and a great deal of very essential pi- and mu-meson physics was carried out there. These accelerators clearly demonstrated their advantages: more accurate results, a more rapid method of working and well-aimed experiments. The Cosmotron at Brookhaven and the Bevatron at Berkeley were on the way. The big machines were expected to be finished in 1952 and 1954 respectively, and were designed to take over another part of the cosmic-ray work.

Meanwhile, the cosmic-ray physicists had found a large number of new particles in the GeV region and were busy trying to determine their lifetimes, decay modes and other features. The information had grown to such an extent that it became necessary to draw up the first common nomenclature, since this part of physics needed its own particular vocabulary. Nevertheless, the cosmic-ray physicists were obliged to accept that this part of their work would soon also be done by accelerators.

On the theoretical side, the work on quantum electrodynamics had achieved success. The first ideas of ordering principles and selection rules, etc. were developed and a major effort was made to impose an order on the chaos of information. It was hoped that by understanding these 65

elementary particles and their interactions one could perhaps also find answers to open questions at the atomic level.

As we have seen, elementary particle physics had thus evolved out of two different disiplines, that of nuclear physics and that of cosmic-ray physics, incorporating the basic research tools (i.e. accelerators) and the research style of nuclear physics on the one hand, and the research topics and the tools (i.e. detection devices) of cosmic-ray physics on the other. It had also become clear that if one wished to make further progress along the exciting and challenging road indicated by the cosmic-ray physicists, one had to build accelerators with ever higher energies.

To conclude, one can say that elementary particle physics (high-energy physics) had taken shape as an autonomous field of physics in the early fifties and had become a field of high prestige and attraction, to considerable extent because of the fundamental nature of its research objectives. 66

N 0 T E S

A list of the books and articles referred to in these notes is given in the bibliography which follows them.

1. Both in 1980 and 1982 conferences had been held dealing with the history of 'elementary particle physics' from the thirties up to the forties and fifties respectively and are a rich source of general in­ formation: Brown & Hoddeson (1983) and Colloquium-Paris (1982). For a short overview of elementary particle physics in the forties and early fifties see also Kayas (1982), 141-161.

2. Two clarifying remarks should be made: a - In fact there is no clear dividing line between high-energy physics and nuclear physics. There is intermediate energy nuclear physics between the two, i.e. working with energies high enough to create pi-mesons, but the investigations are largely undertaken from a nuclear physics point of view.

b - As the deviding line between high-energy physics and nuclear physics is the energy from which meson physics is feasible. The experimental threshold energies for the production of pi-mesons lie at roughly 160 MeV, when bombarding a carbon target with protons, and at roughly 260 MeV when using alpha particles as projectiles. These given energies are, as stated, threshold energies and therefore only a negligible amount of pi-mesons is produced. To do meson physics effectively, the energy should be roughly above 250 MeV for protons and 350 MeV for alpha particles (when using a carbon target). See Thorndike (1952) and note 76 (fourth paragraph). Therefore the 300 MeV represent something like an average value. 67

3. Conference-Cambridge (1946)

4. The following books were consulted for some general information on the status of cosmic-ray physics in the forties: Janossy (1948); Montgomery (1949); Heisenberg (1946). For a more historical approach see Sekido & Elliot (1982); Amaldi (1979); Amaldi (1972); Cassidy (1981); Stuewer (1979).

5. Yukawa (1935)

6. J. G. Wilson, 'Problems concerning the measurement of mass of mesons', in Conference-Cambridge (1946), 73-74.

7. L. Leprince-Ringuet, 'Mass measurements of mesons by the method of elastic collision', in Conference-Cambridge (1946), 46.

8. W. Beitler, 'Cosmic-ray mesons and meson theory', in Symposium-Bristol (1948), 120.

9. Early in 1937, both Anderson and Neddermayer (1937) and Street and Stevenson (1937a) and (1937b), reported on the experimental evidence for the existence of particles less massive than protons, but more penetrating than electrons, obeying the Bethe-Beitler theory. The first real measurment of the mass of this particle is published in Anderson & Neddermayer (1938). A few months later in June, 1937 these observations were connected by several physicists with Yukawa's theoretical prediction of a particle of roughly 200 m mass. e

Stueckelberg (1937) mentioned right in the beginning of his paper that in fact Yukawa had predicted a particle similar to the one found in cosmic radiation. He concluded: 'it seems highly probable that Street and Stevenson, and Neddermayer and Anderson have actually discovered a new elementary particle, which has been predicted by theory.' Also Yukawa (1937) himself drew the conclusion that the particle found in cosmic radiation would be the one predicted by him. 68

Oppenheimer and Serber's (1937) conclusion is slightly different. On the other hand they expressed the hope that one could bring the mass of this new particle 'into connection with the length which plays so fundamental a part in the structure of nuclei: the 'size' of the proton and neutron: the range of nuclear forces.' But since the authors doubted that the theory of Yukawa was the final answer to the question of strong interactions, they were reluctant to connect this cosmic-ray particle definitely to Yukawa's predicted particle. Nevertheless, they also expected the cosmic-ray meson to be the one mediating the nuclear forces, and that it would therefore be strongly interacting.

10. Yukawa (1949), 130.

11. The lifetime of the mesotron was one of the features which did not fit into the theoretical predictions. The decay time of the mesotron had been first measured in 1940 and the value (confirmed throughout the following years) was 2x10- 6 sec. Yukawa's theoretical prediction had been 2x10 -7 sec., Thus there was a disagreement by an order of magnitude. Moreover, the cross-section of scattering of cosmic-ray mesons by nuclei was much smaller than theoretically predicted.

12. J. G. Wilson, 'Problems concerning the measurement of mass of mesons', in Conference-Cambridge (1946), 73.

13. For a general historical overview of the field of nuclear physics see Weiner (1972).

14. The titles of the four talks were: 'Production of low-energy neutrons by filtering through graphite', 'The bragg reflection of neutrons by a single crystal', 'Reflection of neutrons on mirrors', and 'Phase of neutron scattering'.

15. The title of Niels Bohr's talk was one of the very few occasions (at this time) When the term 'elementary particle pyhsics' was used. 69

16. L. Rosenfeld, 'The two-nucleon problem', in Conference-Cambridge (1946), 133.

17. Heisenberg (1943).

18. M. Born, 'Relativistic quantum mechanics and the principle of reci­ procity', in Conference-Cambridge (1946), 14.

19. L. Rosenfeld, 'The two-nucleon problem', in Conference-Cambridge (1946), 142.

20. Weiner (1972), 79.

21. We wish to mention here a few of the many of books and articles dealing with the role of physics during the war and the influence this had on the way science was practised thereafter: Kevles (1978), Snow (1982), De Solla Price (1963) E. L. Goldwasser, 'How Little Science Became Big Science in the U.S.A.', in Colloquium-Paris (1982), 345-354.

22. In January 1939 the famous paper on the production of an isotope of barium by neutron bombardment of uranium, by 0. Hahn and F. Strassmann, was published in Naturwissenschaften. These results were interpreted by Otto Frisch and Lise Meitner as the fission of uranium atoms upon impact by neutrons.

By the end of 1939, a lot of investigations on the question of fission had already been undertaken and the results had been published. The December 1939 issue of The Review of Modern Physics contained an overview article reviewing all the work done.

The final stage of the investigation into the possibility of using this knowledge to construct a bomb was a paper by Peierls and Frisch, in which they clarify two major points. First, they pointed out that it is 70

U-235 which fissions and secondly that it should be feasible to separate the necessary amount of U-235 from U-238. It was esitmated that several kilos of U-235 were required to build a bomb, i.e. to achieve the critical mass (Kevles (1978), 325).

23. Some books and articles dealing with the history of Los Alamos: Frisch (1979), Hawkins (1983) and Truslow & Smith (1983), S. R. Weart, 'The Road to Los Alamos', in Colloquium-Paris (1982), 301-313.

24. AIP-Report of the Director (1944), 217.

25. Conference-Philadelphia (1944).

26. Ibid, 318.

27. Ibid, 319.

28. Ibid, 322.

29. Some books and articles used as general background literature for this chapter: Livingston & Blewett (1962), Livingston (1969), Wilson (1958), Wilson (1981)

30. McMillan (1945); Verksler (1945)

31. Bethe & Rose (1937): The article begins: 'It is the purpose of this note to show that a very serious difficulty will arise when the attempt is made to accelerate ions in the cyclotron to higher energies than obtained thus far. This difficulty is due to the relativistic change of mass which has the effect of destroying either resonance or focusing.' In the article, the inevitable problems of using cyclotrons for acceleration of relativistic particles were clearly demonstrated. Briefly, they were: the mass m of a particle, which is moving with a 71

velocity v, increases by the factor 1l1- v2 /c2 0 (c =velocity of light) As the frequency of a particle revolving in a magnetic field is

Q B w = m

(Q = charge of the particle, B = magnetic field strength, w = fre­ quency) it decreases by the same factor as the mass increases. A numeric example: A proton of 25 MeV energy has a velocity of 0.23 times that of light which causes a mass increase of roughly 3%. This value represents roughly the upper limit of what could be tolerated.

31a.As electrons have a very small rest mass they already reach 99% of the velocity of light at 3 MeV. Since the injection energy into a syn­ chrotron is generally higher than this, the additional relativistic mass increase which occurs with further acceleration is negligible and so the accelerating frequency need not be varied.

32. To be able to calculate theoretically the threshold for the production of a particle one has to be clear about at least two factors the reaction in which the particle is produced and the mass values of the particles involved.

Since for cosmic-ray particles neither the mass values nor the kind of reaction in which those particles were produced, were exactly known, it proved very complicated to lay down the design energies for acce­ lerators. 72

A short example for illustation: When the threshold energy for mesotron production was calculated in 1946 two assumptions were made: 1 - the mesotron is the Yukawa meson; 2 - mesons are produced in pairs.

Both asumptions were wrong. On the one hand the mass value used should have been bigger, while on the other hand pi-mesons were not produced in pairs. Fortunately, therefore, these two 'errors' had 'cancelled' themselves out.

33. Ramsey (1966)

34. The nine universities which joined together to establish Brookhaven were: Columbia, Cornell, Harvard, Johns Hopkins, Massachusetts Insti­ tute of Technology, University of Pennsylvania, Princeton, Rochester and Yale.

35. See Initial Program Report of Associated Universities INC. Brookhaven National Laboratory, December 1946, p.6-9; (CERN), 10064, CHS/ARCH/5/7.

36. Il1iQ., Appendix B, Part II.

37. See Scientific Progress Report. July-December 1947, BNL, p.12-19; (CERN), 10064, CHS/ARCH/5/7.

38. See Scientific Progress Report July 1, 1947, BNL; (CERN), 10064, CHS/ARCH/5/7.

39. The information on the history of Radiation Laboratory is taken from Heilbron, Seidel & Wheaton (1981) and Seidel (1983), unless otherwise stated. 73

40. In 1940 Lawrence asked the Rockefeller foundation to sponsor the construction of a 184" cyclotron for the purpose of studying mesons (the mesotron). He suggested that the energy aimed for should be 150 MeV. Apparently he seemed not to be worried by or to be aware of the basic problems which would arise when attempts were made to produce such highly energetic particles in a conventional cyclotron (see note 31). Indeed the huge magnet was manufactured, but was used as an isotope separator throughout the war.

41. See Lawrence (1948).

42. For a short history of MIT's Radiation Laboratory see DuBridge (1946)

43. A short comparison of proton-beam intensities (for the machines built around 1950):

linear accelerator 0.4 µA synchro-cyclotron 1 µA (only for the internal beam) proton synchrotron 10- 4µA

Therefore, a proton linear accelerator could be a real alternative to the proton synchrotron from the point of view of beam intensity. Synchro-cyclotrons cannot compete with those two machines since a) they are limited in energy to roughly 1 GeV owing to financial considerations and b) the beam intensity goes down considerably when the attempt is made to extract the beam out of the machine.

A consequence of the success of the linear accelerator was the desire to build even larger linear accelerators (Mark I and II) to produce uranium-333 or plutonium. Mark I was constructed and operated from 1952 for roughly a year. The machine was 60 feet long, producing 50 µA of 30 MeV deuterons. Mark II which was planned to be a quarter of a mile in length was never built (see 'A Neutron Foundry', in Heilbron, Seidel & Wheaton (1981), 66-71). 74

44. Brobeck (1948): This is the first published design for the 10 GeV machine.

45. In fact, two preliminary designs were prepared in parallel, one for a 2.5 GeV and one for a 10 GeV machine. Both of them were submitted to the AEC. See Scientific Progress Report January-Julv 1948, BNL, p.15; (CERN), 10064, CHS/ARCH/5/7.

46. Ibid I 16.

47. For a description of the Cosmotron see Livingston et al. (1950) and Blewett (1956), 52-58.

48. Information on the Bevatron is to be found in Smith (1951) and Blewett (1956), 59-63.

49. The information for this table is taken from Accelerators (1948-1950) and (1951), Livingston & Blewett (1962) and Thorndike (1952).

50. See Harwell (1946-1951), 10.

51. Skinner (1948); Harwell (1946-1951), 12-14 and 34-42.

52. The information on the Harwell machines is taken from: Harwell (1946-1951), 43-55 and Gowing (1974), 203-259.

53. In Harwell (1945-1951), 49-50 we find the following remark about the 110" synchro-cyclotron: 'At the full energy of 180 MeV it should be possible to produce mesons[ ... ]. However, the yield of mesons at that energy is known to be small and it is not intended to spend a large proportion of operating time on meson experiments.'

For a short review of the Harwell programme in the early fifties see Pickavance & Cassels (1952). 75

54. In fact Pickavance and Adams would have liked to increase the Harwell synchro-cyclotron to 150" diameter (corresponding to a proton energy of about 300 MeV). Their machine would have then been well above the threshold for meson production. Gowing (1974), 258.

55. The information on the five machines built at universities is taken from Gowing (1974), 224-226.

56. No exact figure for the envisaged energy is known to us.

57. Oliphant's plan to build a ring-shaped proton-accelerator is frequently referred to in literature on accelerators, but details are never given. See, for example, Livingston (1969), 50-51.

58. For further and more technical information on this machine see Hibbard (1950) and Blewett (1956), 48-51.

59. Livingston & Blewett (1962), 400-401.

60. A general review on the situation in atomic energy all over the world by 1952 is to be found in Atomic Energy (1952). In this article the programmes of various countries are presented and the basic inf or­ ma tion for this chapter has been derived from it. Additional infor­ mation is indicated at the appropriate place. The exact energies for the accelerators are taken from Accelerators (1948), (1948-1950) and (1951).

61. As we will not deal with the reactor-programmes pursued in Europe, we wish briefly to mention the main activities (Weinberg (1954) and Atomic Energy (1952)): Sweden's first reactor went into operation by the end of 1953. It was of the research type, fuelled by uranium and air-cooled using heavy water as moderator. 76

Norway completed its first pile, a heavy water reactor built at Kjeller in 1950. Because of the lack of uranium, this project was converted into a Dutch-Norwegian venture, with the Netherlands providing the uranium. (see Randers (1953)).

In France in the mid-December 1948, the 5-kW reactor ZOE at Chatillon went critical for the first time. The second reactor P-2 (1.500 kW) followed then by October 1952.

62. Casimir (1983), 190.

63. M. E. Regenstreif, Rapport sur men yoyage en Suisse, 28-30 juin 1951, NS/MEM0/7984, UNESCO archives.

64. McReynolds (1955)

65. For a short overview of the French activities in nuclear physics after the war see D. Pestre, Les attitudes francaises face au projet de labo­ ratoire europeen de recherches nucleaire (1949-1954), CHS-15, August 1984, p14-17.

66. M. E. Regenstreif, Rapport sur mon voyage en Belgique 4-7 juillet 112..1, NS/MEM0/240168, UNESCO archives.

67. Courant, Livingston & Snyder (1952).

68. Christofilos (1950)

69. The value for the aperture is taken from Courant, Livingston & Snyder (1952), the financial estimate from Report to the Secretary, covering visit to USA by Goward, Wioderoe and Dahl in August 1952, CERN-PS/S4,

(CERN) I 20551.

70. Major books and review articles written on meson physics around 1950: Thorndike (1952), Marshak (1952), Powell (1950), Snyder (1949). 71. Congr~s-Bagn~res-de-Bigorre (1953) Two conferences of the same kind had been held in 1947 and 1950. The proceedings of these conferences represented a very valuable source of information, as they give an impression of the development in cosmic-ray physics at this period. Symposium-Krakow (1947) and Conference-Bombay (1950).

72. Conversi, Pancini & Picioni (1947) 0. Piccioni, 'The observation of the leptonic nature of the 'mesotron' by Conversi, Pancini, and Piccioni', in Brown & Hoddeson (1983), 222-241.

73. Before the experiment of Conversi et al., two Japanese theoreticians Sakata and Inoue (1946) published a paper in which they clearly doubted the fact that the Yukawa meson was to be identified with the mesotron. They suggested that the mesotron would be a decay product of the Yukawa meson. (This paper remained more or less unknown in America and Europe.)

74. Rotblat (1950).

75. Lattes et al. (1947): This publication contains the first two pi-mu decay events. Six months later quite a lot of information had been collected on the pi-mesons (Lattes, Occhialini & Powell (1947)). The main conclusions drawn were the following: 1.- There are two mesons the pi- and the mu-meson. 2.- The pi-meson decays into a mu-meson and a unique neutral particle. 3.- Pi-mesons have strong interaction and are produced in nuclear interaction. 4.- They decay quickly into a mu-meson, which has very weak interaction and represents the major component of cosmic radiation at sea level. 5.- Pi-mesons when stopped produce stars, mu-mesons do not. 78

7-6. Gardener & Lattes (1948): Meson tracks were found in photographic emulsions exposed near carbon and other targets bombarded by 380 MeV alpha rays. About 2/3 of the tracks ended in stars (thus were sigma mesons). A meson intensity of roughly 108 times that available in cos­ mic rays was obtained. The mass of the meson was measured to be 313!._16m . e

(As with photographic emulsions one cannot decide about the sign of the particle, the particles were classified according to their reaction at the end of their range: pi-mesons decay into mu-mesons, sigma-mesons produce stars. These sigma- mesons were in fact negative pi-mesons.)

In the same year a paper by Occhialini and Powell (1948) reported the measurements of small angle scattering of pi- and sigma-mesons in photographic emulsions and showed that they have a mass identical with those produced by Lattes and Gardener. This publication shows that it was not obvious for all physicists that the artificially produced mesons were identical to those produced in cosmic radiation.

Our second remark concerns the strong energy dependence of pi-meson production. This is illustrated nicely in the following paragraph to be found in Heilbron, Seidel & Wheaton (1981), 57: 'They found no pions when they sent 165 MeV protons against their carbon target; the yield at 200 MeV was but one percent, and that at 300 MeV less than half of the yield at 345 MeV. For alpha particles, the number of pions observed fell by two thirds as the energy of projection declined from 380 to 342 MeV, and to fewer than seven events (less than one percent of the maximum) at 260 MeV.

We want to list here some reactions in which pi-mesons are produced:

+ p + p p + n + 1T -- 0 p + p -- p + p + 1T p + n- p + p + 1T 79

77. The physicists at Berkeley had already tried to detect pi-mesons produced in their synchro-cyclotron by the help of photographic plates before the arrival of Lattes. But the fact that they were not able to develop the plates in the way needed made them fail.

78. For details on the Shelter Island Conferences see Chapter V: The theo­ reticians: from the first Shelter Island Conference to the Rochester Conference.

79. R. Serber, 'Artificial Mesons', in Conseil-Solvay (1948), 89-110.

Some brief information on the Solvay congresses: they were the most significant and important meetings of the European theoretical physics community during the pre-war period. The 8th Solvay Congress planned for 1939 was not held, because of the political situation in Europe and so the one from 22-29 October 1933 in Brussels under the title 'Structures et Proprietes des Noyaux Atomiques' had been the last before the war.

After the war the Solvay congresses lost their special position among the conferences in theoretical physics. Many of the leading European theoreticians had left Europe in the thirties and early forties. Other conferences, like the Shelter Island Conference started to play a rather central role.

The first post-war Solvay Congress was held from 27 September to 2 October 1948 in Brussels under the title 'Les Particules Elementaires'.

80. Heilbron, Seidel & Wheaton (1981), 57.

81. C. F. Powell, 'Properties of then- and µ-mesons of Cosmic Radiation', in Symposium-Bristol (1948), 87. 80

82. C. F. Powell, 'Introduction', in Symposium-Bristol (1948).

83. In September 1949 Bjorklund, Crandall, Moyer and York (1950) at Berkeley had observed high energetic photons coming from a target bombarded by protons from the 184" synchro-cyclotron (proton energies higher than 175 MeV). They suspected that the decaying particle producing these photons would be a neutral pi-meson, but they could not make the necessary confirming measurements because of the concrete shielding of the synchro-cyclotron.

In April 1950 a series of experiments was completed with the electron­ synchrotron which enabled Steinberger, Panofsky and Steller (1950) to announce that of the neutral pi-meson had definitely been found.

84. A short example of the advantages of using accelerators: With the Berkeley synchro-cyclotron it became possible to measure the pi/mu mass ratio with sufficient precision to identify the neutral recoil particle with a neutrino. Hitherto, the ratio had been measured too high and it had therefore been supposed that the neutral particle had a considerable mass.

85. The data for this table are taken from Thorndike (1952). In 1952 one thought of having only one kind of neutrino for both elec­ trons and mu-mesons.

86. Leprince-Ringuet & Lheritier (1944).

87. For an overview on the V-particles see Rochester (1951) and Rochester & Butler (1953). 81

88. Rochester & Butler (1947) Before the detection of these V-particles by the British physicists, there had already been some sporadic hints about their existence, but their importance had not been realized. To illustrate the difficulty of this kind of work, we should mention that the two photographs by Rochester and Butler of the V-particles were found out of 5000.

89. Seriff et al. (1950) 34 V-shaped tracks had been found in 11,000 photographs taken. 30 were neutral V-particles and only 4 were charged ones.

90. A review of the information on the neutral V-particles by 1951 can be found in Butler (1952). The two publications suggesting the existence of two neutral V-particles of different mass and decay schemes are: Armenteros et al. (1951a) and Armenteros et al. (1951b).

91. Rochester & Butler (1953); see also the remarks by O'Ceallaigh note 94.

92. Armenteros et al. (1952): 21 charged V-decays had been observed, but amoung those there was only one single photograph of the type des­ cribed. This particle was for a while in the literature referred to as

'cascade particle' since it decayed into a proton via a V1 °.

93. Detection of the tau meson: Brown et al. (1949) For further information see Fowler et al. (1951).

94. O'Ceallaigh (1951) The author remarked in this paper that 'the K-particles are about 1.3 times more massive than the T-particles, but the inaccuracies in the experimental value are such that the possibility cannot be excluded that they represent alternative modes of decay of particles of the same type. Further, they may also be of the same type as the unstable

charged particles of Rochester and Butler [v2°] which are observed to decay in flight.' 82

95. These experiments which led to the discovery that the K-meson was in fact two particles the K-and the x-meson, were carried out throughout 1952. 60 photographs were evaluated for this purpose. The results are presented in M. Menon and C. O'Ceallaigh, 'Observations on the mass and energy of secondary particles produced in the decay of heavy mesons', in Congres-Bagneres-de-Bigorre (1953), 118-124.

96. Bridge & Annis (1951); Annis et al. (1952).

97. The information for this table is taken from R. E. Marshak, 'Particle Physics in rapid transition: 1947-1952', Brown & Hoddeson (1983), 397, Rochester & Butler (1953), 403 and from the original papers cited above.

98. 'Texte de la conference du Prof. B. Rossi a la seance de cloture' in Congres-Bagneres-de-Bigorre (1953), 259-269. This nomenclature was largely spread out by physicists from all coun­ tries through various publications after the conference.

99. M. Schein, 'On the artificial production of v0 -particles by 227 MeV pi-mesons generated in the Chicago Cyclotron', in Congres-Bagneres-de­ Bigorre (1953), 166-168.

100. 'Discours de cloture par L. Leprince-Ringuet' in Congres-Bagneres-de­ Bigorre (1953), 288.

101. A very good insight into the status of theoretical nuclear pyhsics in low-energy region around 1950 can be gained from Blatt & Weisskopf {1952). For a short review on the status of meson theory see Bethe {1954). 83

102. The information on the three Shelter Island Conferences are taken from: Schweber (1984), R. E. Marshak, 'Particle Physics in rapid transition: 1947-1952', and J. Schwinger, 'Renormalization theory of quantum electrodynamics: an individual view', both in Brown & Hoddeson (1983), 376-401 and 329-353.

103. The list of participants is taken from Schweber (1984), 141.

104. Since there were no conference proceedings published, we have to take as basic information the discussion papers presented by Kramers, Oppenheimer and Weisskopf. They are entirely reprinted in Schweber (1984), 151-157 and give us an impression of the various aspects worked on in theoretical physics at that time.

105. Below a short account is given of some of the reactions of the theore­ ticians to the experiment by Conversi et al.:

A theoretical analysis of the Conversi et al. experiment by Fermi, Teller & Weisskopf (1947) led to the conclusion that the time of capture from the lowest orbit of carbon was in disagreement with theoretical estimates by at least 10 orders of magnitude.

Pontecorvo (1947), for example, suggested that because of its weak interaction the mesotron could not transmit the beta decay. He con­ cluded that the meson has spin 1/2 and is absorbed by the nucleus, emitting a neutrino. This formulation was the first of its kind.

Weisskopf (1947) presented a solution to the difficulty of reconciling the high rate of production of mesons with their subsequent weak interaction with matter. He postulated that the primary cosmic-ray proton would convert a normal nucleon (which is in an 'air' nucleus) into an 'excited' nucleon, which would be capable of emitting mesons. 84

106. Lamb & Retherford (1947): This experiment, which represents the most precise test of QED theory, had only become possible through the great war-time advances in microwave techniques in the vicinity of the three centimetre wavelength.

107. Bethe (1847); Bethe & Marshak (1947). Both papers referred to the fact that these results grew out of the ex­ tensive discussions at the Shelter Island Conference.

In the paper by Bethe and Marshak, footnote 4 is of particular interest: 'This paper [by Powell et al. on the detection of the pi­ meson] arrived in the United States shortly after the two meson hypothesis was presented at the Shelter Island conference.' That means that the work by Conversi, Pancini and Piccioni had already stimulated this kind of work. See also Weisskopf (1947).

The basic idea of Bethe and Marshak was that two kinds of mesons exist, with different masses. The heavy one was supposed to be produced with large cross-section in the upper atmosphere and to be responsible for the nuclear forces. The light one was regarded as a decay product of the heavy one, and it was assumed that as they are observed at sea level, they interact weakly with matter. The authors then attempt to work out ideas about lifetime, spin and other features on the basis of the then known experimental evidence.

108. Schwinger (1948a)

109. Feynman (1948a), 8.

110. lb.i..Q., 9.

111. In the course of 1948 and 1949 Schwinger wrote four fundamental papers on QED: Schwinger (1948b), (1949a), (1949b) and (1949c).

112. Dyson (1949a) 85

113. Feynman (1949a) and Feynman (1949b). Before these two papers Feynman (1948b) had presented his space-time approach to non-relativistic quantum mechanics.

114. Feynman (1948a), 10.

115. See for example: Tomonaga (1946); Tomonaga (1948) For an overview see: J. Schwinger, 'Two shakers of physics: memorial lecture for Sin-itiro Tomonaga', in Brown & Hoddeson (1983), 354-375.

116. Pontecorvo (1947), Klein (1948), Puppi (1948).

117. Dyson had sent already in October 1948 a paper to the Physical Review presenting 'a unified development of the subject of quantum electro­ dynamics, [ ... ], embodying the main features of the Tomonaga-Schwinger and of the Feynman radiation theory.' But he carried his work further in discussing higher order radiative corrections and vacuum polari­ zation phenomena. Dyson always stressed 'that the advantages of the Feynman theory [were] simplicity and ease of application, while those of Tomonaga and Schwinger [were] generality and theoretical completeness'. For Dyson's contribution to QED see also Dyson (1949c).

118. Schweber (1984), 150.

119. Marshak (1970), 93.

120. Conference-Rochester 2 (1952).

120a.Megalomorph: An expression for the new elementary particles used by Fermi, referring to the variety of different forms in which these particles occur.

121. Conference-Rochester 2 (1952), 26. 86

122. The results of this experiment on the pion cross sections were used as an argument to fix the energy for the CERN synchro-cyclotron at 600 MeV. At this energy it would also be possible to measure these very fundamental cross sections at higher energies. See U. Mersits, 'Construction of the CERN Synchro-cyclotron (1952-1957)', CHS-13, August 1984, p.5.

123. Conference-Rochester 2 (1952), 87-93.

124. The particles to which the symbol v was attributed are those charged and neutral V-particles with smaller mass (see Table 3). They had

generally been referred to a V+ and v2 °. The big Vs stand for the heavy charged V-particle found by the Manchester Group in 1952 and the V 0 . 1

125. Marshak (1970), 93.

126. Conference-Rochester 3 (1953) From the title of the Rochester Conferences and from the way the time was divided between theoreticians and experimentalists (both cosmic-ray physicists and thos working with machines), one can follow the changes in this field of physics over the years. The second Rochester Conference was called 'Conference on Meson Physics' and the cosmic-ray physicists were well represented on the experimental side. The following conference was called 'High Energy Nuclear Physics' and this designation was kept until the 8th Rochester Conference held in 1958 at CERN under the title 'high-energy physics'. It was not only the title that changed. At the fourth conference in 1954, the part of cosmic-ray physics had already shrunk to one out of five sessions and on the sixth conference, R. B. Leighton remarked that 'next year those people still studying strange particles using cosmic rays had better hold a rump session of the Rochester Conference somewhere else. '(Brown & Hoddeson (1983), 399). 87

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