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Interview of Peter Hirsch by Stephen Case on 1981 January 16, Niels Bohr Library & Archives, American Institute of Physics, College Park, MD USA, www.aip.org/history-programs/niels-bohr-library/oral-histories/31625
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Topics discussed include: his family background and childhood; his interest in diffraction, crystallography, and x-ray monochrometers; working with Will Taylor, Woody Bragg, and Noel Keller; and the Institute of Metals conferences.
I know you went to secondary school in Chelsea but I don’t really know anything else about your childhood, what your parents did and such.
Well, I was born in Germany, in Berlin, in 1925. My father was in the clothing trade. My parents were divorced in 1934, and my mother remarried immediately. My father died in 1936. My step-father was a dental surgeon, and we emigrated just before the outbreak of war. We were Jews, and my brother came over first, in the late summer of 1938. My mother came over late in 1938. I came over on the 1st of January 1939. My step-father finally followed in the early part of 1939.
You came over relatively late really, compared to lots of people who came over.
Yes. I was a young boy. When I arrived here I was just about 14.
It must have been a great shock to you, coming to England, a great change.
Oh yes, the whole period was one of considerable turmoil and upheaval.
And were you aware at that age of what was going on?
Yes, to a considerable extent. There was a period, particularly in the later part of 1938 when my step-father and I were the only ones left of the family; my brother and my mother had by that time gone. It was about the time of the Kristallnacht, and it was a rather unpleasant period. The person who was responsible actually for getting us out of Germany was my mother. She worked exceedingly hard. She was responsible for getting my brother out in the first place. She then came over herself and had to find work to be allowed to stay in England. She got herself a job as a domestic servant. She then persuaded somebody to act as a guarantor for me, and I came over on one of the Kinder transports (they were groups of children who were accepted in the country), and on arrival I was looked after by the family of this guarantor. And it was my mother who was working, of course, for the family, as a domestic servant. I started going to school within about two weeks of arrival. Like throwing me in at the deep-end.
How was your English?
Practically non-existent. The sort of thing that you remember is, of course, that people played tricks on you. There was one particular school teacher who went by the name of Boozer Brown. I didn’t know what Boozer was. They made me go to him and address him by that name. “Excuse me, Mr. Boozer Brown.” My punishment was writing several thousand lines.
That kind of story was rife in grammar school when I was there.
Anyway, it was the right thing to do, because by getting pitched in at the deep end like that, it was the best way of learning a language and getting used to the new country.
Yes. Was there any particular reason why your family decided to come to England rather than anywhere else?
I suppose I was too young to really know that. I suppose my mother had tried all sorts of methods of getting out, but she did have friends in England. There was a man by the name of Lytton who was an ophthalmic surgeon and he had emigrated earlier, and she had contacts with him. I think that contact helped to establish the method by which we got out at all. And the first thing was to get my brother out. My mother was told about a scheme run by HMV (later EMI) at Hayes (Middlesex), under which science/engineering students who had good reason to emigrate were given Technician/Laboratory Assistant jobs at EMI. A guaranteed job was the condition for obtaining a visa. My brother was 4½ years older than I and had started at the Technische Hochschule in Berlin to study physics/engineering but was not allowed to carry on. He then applied for and got a job at EMI in Hayes through the scheme I mentioned earlier. So I think the answer to your question is, yes, there was a potential contact which was helpful. Once my brother was out, into England, then it was clear the direction in which we were going to go. Since you want a bit of background on this, the next thing that happened was that my mother got the Lyttons to send a telegram to my mother, saying that my brother was very seriously ill, and that she had to come over. Under those conditions you were able to get a visa, a temporary visitor visa for about a week, on compassionate grounds. Otherwise, to get an entry visa into a country was incredibly difficult because of strict quotas. But armed with a telegram saying “Your son is desperately ill.” she managed to get a temporary visa to come over. Once having come over here, there was a Jewish Refugee organization, centered on Woburn House in Holborn, London, which did a marvelous job helping refugees to stay on. If you wanted to stay, you had to get a job, and they fixed her up with a job as a domestic with a family in Chelsea. She had a very hard time. And then, again through the Woburn House organization, she found a guarantor for me, so that I could come out with one of these groups of children. We came over on the S. S. WASHINGTON. Finally she persuaded the same guarantor to give my step-father a job in his taxi firm, so that he could come over, which he did in the spring of 1939. Anyway, so the answer to your question, why did we come to England, is, I think, personal contacts through particular people.
You say your brother was a physicist. Did he influence you at all in your early interests?
Yes, when I finished school. I went to Sloane School (a London County Council Secondary School in Chelsea) when I first came to England, and Sloane School was evacuated during the war to a small place called Addlestone in Surrey. I don’t think I really had any very decided leanings towards one subject or another. Unlike my brother, who at the age of five had already shown leanings towards the science side by building tremendous little airplanes and God knows what. I’m afraid I was never like that. Academically, I was a bit of a swat and got good results, and then the question was, what was I going to do in the 6th form? If you became a scientist, at least there was a possibility of getting a job as a scientist. At least that was the notion in those days; to some extent it still applies. And so I took my Higher School Certificate on the science side. Then if you ask me, why physics, which is what I went into, yes, I think my brother was an influence; I followed in his footsteps.
So then the route was mapped out, in a way.
In a way, yes. I’m afraid I wasn’t one of these people who knew from the year dot that that’s what he was going to do. I rather drifted into it. By these external close influences, if you like. I did get an exhibition to St. Catharine’s College in Cambridge. In those days, St. Catharine’s and Selwyn in Cambridge were the only two colleges which had entrance examinations in February or March of the second year in the 6th form. The situation at Sloane School was such that, as far as I was concerned, I wasn’t going to stay three years in the 6th. That’s why they put me in for this exam at St. Catharine’s and I got an exhibition. I was very lucky. So that was the next step mapped out.
What do you remember of science at school? Did you have the classic sort of teacher-demonstration kind of lessons? Or were you allowed to do much practical work yourself?
The facilities at Addleston, where the school was evacuated, were exceedingly limited. Teaching tended to be through demonstrations of experiments, with audience participation.
So you went to University, would it have been 1943?
How did you settle at the university, first time away from home, apart from when you first came to England?
I found the first year rather difficult, I must admit. I was rather lonely and I was an unsociable sort anyway. But the second and third years became much easier, and I enjoyed it. I think I enjoyed the third year even more. I read the Natural Sciences Tripos, specializing in Physics in Part II (3rd year). I wasn’t a sporting type, so I didn’t have the benefit of an obvious fellowship with others with common interests, nor had I learned any musical instrument or anything like that, so there wasn’t an obvious niche for me. In retrospect, this is one of the things which people who go to boarding school benefit from, of course. They usually have acquired some other skills, whether on the sporting field or in other ways. We didn’t really have the same opportunities at a school like Sloane. No doubt it’s different now. There were of course other influences at the time, due to the war, and some of us felt rather awful in not contributing ourselves towards the war effort. I remember, I think it was at the end of Part I in the second year that there were a number of us who were looking into the possibility of joining the armed forces. I don’t remember the details now, it’s such a long time ago, but there was at that time a definite policy of making sure that there would be a sufficient number of fully trained scientists and engineers. I can’t remember whether I was directed or not, but at any rate I was encouraged to finish my degree.
Yes, it was a very definite policy. The lessons were learned, I think, in the First World War. What particular lecture courses stand out in your undergraduate course?
That’s again very difficult to say. I think one of the things which I remember is Alex Wood’s course on sound or acoustics. And they were really fabulous, with lots of experimental demonstrations. I suppose that must have had an influence on me, maybe in getting me interested in wave propagation. I did also take Mineralogy, as part of my Part I Natural Sciences Tripos. In those days you had to take three experimental subjects, and apart from physics and chemistry, I took mineralogy. You could take it as a half or a full subject. I took it as a full subject, first in Prelims, in the first year, and then in Part I in the second year. That course did make a tremendous impression on me. There is no doubt about that, and I’m sure it was instrumental in charting out the direction in which I was going to go. That course was extremely influential in the teaching of crystallographers and in developing crystallography in the UK. I think you may hear the same from other people, other crystallographers. It was run by Norman Henry, W. A. Wooster and Robert Evans — at least during my period. I suppose Henry Lipson may have been involved, in establishing it. He must have been in Cambridge, in the Cavendish, but before my time. It was a very good course. It gave me a very good grounding in crystallography, and an appreciation of crystals which I suppose I’ve never lost, and symmetry. And amongst other things, they already used an overhead projector, believe it or not, in 1943. We used to call it the Belshazzar for some reason I’m not clear about. But it’s amazing to me that it’s taken, what, almost 20 years before that technology had actually taken off elsewhere.
I was teaching physics a few years ago, and when I first started teaching, my head of department told me, just a few years before, which would be late sixties, early seventies, they had to get a special grant to actually take an overhead projector for a trial period and that would have been late sixties.
So it’s taken an enormous time for this thing to become generally accepted. But in the Mineralogy Course in Cambridge they used it all the time. They were very advanced, in this. As to other influences which might be material, when I took Part II, I was very interested in diffraction. I remember that I just got very interested in it, and learned the Fresnel diffraction theory in great detail and wrote a comprehensive answer to a question in my final Part II examination. So the love for diffraction was already established. Now, exactly where that has arisen, I’m not sure at this time. I’m sorry that my memory is so bad. I certainly remember Alex Wood as being a marvelous teacher in Part I on the acoustic side. I remember Ratcliff being an incredibly good teacher on the electricity and magnetism side (also in Part I). You went away after the lecture thinking you’ve understood every single word of it until, when you tried to do some tutorial or supervision work, you found that there were lots of gaps that you didn’t actually understand. But he was a magnificent lecturer. But I can’t now remember who actually lectured on diffraction in Part I Physics. You would think it was Bragg. But I’m not sure that it was. But the mineralogy course included indexing of x-ray diffraction patterns, and generally crystallography, which I found exciting. Certainly I was very interested in diffraction. So there were two influences, the crystallography course, and actually taking X-ray diffraction patterns at some stage during Part II physics. So my love for crystals was established by mineralogy, and diffraction somehow or other in the physics course.
Were there any particular books you started turning to as your interests in these areas developed?
Not really. But I liked the books by Jenkins and White on Optics, and referred also to R. W. Wood. The book by Henry, Lipson and Wooster on X-ray diffraction from crystals was important for the mineralogy course. As a development of the diffraction interest, I took a special course in Part II (you could take a series of optional courses) given by Will Taylor, who was head of the crystallography department, on diffraction from imperfect crystals, in particular, diffraction effects from crystals with modulations in composition and lattice parameter, giving side bands at the diffraction spots. That was an important further step down the road. Will Taylor’s lectures had a great influence. I also had another special option on tensor notation, and tensor properties of crystals. This reflected the important influence of the mineralogy course. I had various supervisors during my three year undergraduate course, some of them very stimulating. Freddie Dainton (later Lord Dainton) was one of them, actually. He was a Physical Chemist and supervised me in chemistry. I remember him trying to explain to me what activation energy was. An important influence was Charles Smith, usually called Cyril Smith, who was a metal physicist, and he worked in Orowan’s group. He worked on creep.
The experimental work at the undergraduate level — I remember you saying you weren’t a very practical person in your childhood. How did you get into experimental work? It was obviously a direction you were going in the future.
I don’t think I was especially good at experimental work, but my practical work was adequate.
Do you remember many of your fellow students, people who stood out, people you were friendly with, people who were later close colleagues?
One of the things that happened during my college days, and made my undergraduate days very happy was the friends that I made in St. Catharine’s, through joining a society that calls itself the Three Arts Club, which was started by a group of young people from Sutton Coldfield. Many of them attended the local King Edward’s school. They had common interest in the Arts and also in hiking in the country side. They were a delightful crowd, and there were two or three of them who happened to be in Cambridge at the same time. One of them was Wally Eldred, who is a metallurgist who is now at Windscale. He was one of the people who was responsible together with Jack Harris at the Berkeley Nuclear Labs, in devising means of extending the lifetime of fuel elements in civil nuclear reactors, for which they got a Royal Society medal, only very recently, a year or two ago, and he was also made a CBE. Another friend is John Newton, who I think is now Professor of Nuclear Physics somewhere in Australia, and he’s very well known. They were both at St. Catharine’s and there was a third called Donald Cruxton who actually died young, rather sadly. And there were many more friends that one made through them including girlfriends. And that really made by college life much more interesting. Another person I was very friendly with during my undergraduate and also research student days was Bobbie Burns, who was a physical chemist. He worked in Physical Chemistry, and was one of Freddie Dainton’s research students. But Bobbie was also an undergraduate at St. Catharine’s and we were contemporaries. We were very friendly, and he is now at Harwell, working on radiolysis.
I asked you about your friends and also about people who stood out in your early days at college academically.
Bobbie Burns did academically. Then of course, the fellows at college, Professor Steers (Professor of Geography) was my tutor. He was a very fine man, an important figure in his field. He used to run music evenings; he played records, and you were invited to listen. The only trouble was, he never told you what the music was. He expected you to know the pieces he played. And Sydney Smith who was a zoologist, still there, I think, is a great pianist. One of the things I remember about him was, playing Mussorgsky’s “Pictures at an Exhibition” on the piano. It was a tremendous performance. And there were many other fine people. Of my contemporaries, Burns certainly stood out academically, very strong.
What strikes me about the time of your undergraduate days is that it did occur at a time when pure science really rose in status, and people began to care much more about the contribution that science could make to everyday life, to the economy and elsewhere. Were you ever aware of that?
I think it was too early for that. I think we were much more influenced by the war as undergraduates. I didn’t graduate until 1946. There were slightly older people at Cambridge who finished their degree in two years, and as I said, quite a number of us at the end of Part I (1945), felt that we should leave and join the war effort. The whole period was overshadowed by the war. One was aware of what scientists did for radar, but that was about all. But mostly we spent our time making the best of the opportunities at Cambridge. I joined the Senior Training Corps. We did some army drill and fire watching.
One particular point, when we talked to Professor Sondheimer who was at university one or two years ahead of you, he said an overriding memory he had was that, of a lot of his fellow students, many disappeared across to the States, obviously to work on the Manhattan Project, but at the time, he said no-one’s (?) reaction was very strange because he said at the time he wasn’t sure why they were going, security was very tight. Were you aware of this?
I wasn’t aware of this going on at all. We were just aware of the importance of science in connection with the war effort, but in connection with radar rather than with anything else. I should add that the lectures we attended were given partly by the permanent Cambridge staff, and partly by others temporarily transferred to Cambridge. These included Zetler and Schnetzler from Southampton who taught us electronics. There were also quite a lot of people evacuated from London University.
Getting towards the end of your university days, you must have begun to think about what you would be doing next.
By this time the war was over, and in those days people looked at doing research as a privilege and as an academic achievement to be accepted as a research student. I think it was rather different from what it is these days, when in many cases you wonder if it isn’t just another job for three years, towards a further qualification. Of course the number of people who were actually doing research was a smaller proportion than it is today. The market was quite different.
Had you considered any alternatives if you weren’t able to get into research?
I’m sure I did, but I can’t remember what they were. I don’t think that I ever got as far as going to interviews.
So how did you actually go about getting a research position?
That was very interesting. It’s another one of these accidents of fate. Cyril (Charles) Smith worked in Orowan’s research group in the Cavendish Laboratory, which was called the Metal Physics group. He suggested that if I wanted to do research, to visit Orowan and to see if I could get into his group. In those days the most popular group in the Cavendish was the low temperature physics group in the Mond Laboratory. The field was at its peak, with David Shoenberg, Allen, Pippard, and others with high reputations. I was fascinated by adiabatic demagnetization and getting down to temperatures less than 1 degree K. It was an exciting time for low temperature research, and everybody wanted to get into the Mond Laboratory. My position was that because I was not British I was not eligible for a DSIR grant, and that was one difficulty. I tried to get into the Mond, and talked to Shoenberg, but was told the laboratory was full. I went to see Orowan in my third year as an undergraduate, and he gave me what subsequently transpired to be a Ph.D. viva. I do remember this vividly. I just sat there and said “Excuse me, sir, but we haven’t done that yet.” It was a fairly disastrous interview and at the end of it Orowan told me to come back after the Part II Physics examination. In the end I did not do that. Instead I saw W.H. Taylor in Crystallography because of my interest in that field. At that time Sir Lawrence Bragg, the Head of the Cavendish, suggested a project on X-ray diffraction from cold worked metals, but he wanted Will Taylor to supervise the project in the Crystallography Group. Will Taylor put this project to me, and he also offered to arrange from the British Iron and Steel Research Association (BISRA) a research support grant which I seem to remember, was about £180 a year. I also got a little bit of money on a scholarship which I got through an organization for refugees run by Mrs. Burkill. She was the wife of an eminent mathematician (Dr. Burkill, FRS) at Peterhouse. So that clinched it. Will Taylor was able to offer me this BISRA grant, and on this particular research project. I think if it had been possible I would have gone into low temperature physics as my first choice, like everybody else.
Why did it have such status at that time?
It was the period in which people were really getting down to lower and lower temperatures, and seeing the quantum effects coming in and the breakdown of classical physics. It fascinated us.
So it was a real frontier area.
Yes, it was a frontier area, very much so. You know, things change. At that time it was the most exciting field to get into. Of course, later on the most exciting topic of the day changed to Radio Astronomy. But, having said that, my second love was crystallography and diffraction. I don’t know if I would have preferred Orowan’s or Will Taylor’s group, but as it turned out, Will Taylor’s group was probably right for me. I’ve never regretted it. I was very lucky to get in. I owe a lot to Will Taylor, there is no doubt about it. He supported my work and got me funds and he provided some of the stimulation in the first place, in his undergraduate lectures.
Yes. Let me make one point. I was very interested, certainly, that you had a BISRA grant because I spoke to three people so far, and it wasn’t BISRA, in the other two cases, but I’d never realized that research associations did fund and were involved in so much pure research at universities.
They were in those days. I think nowadays it’s different. In this department, I am sure Professor Hume-Rothery, my predecessor here, had BISRA grants for students. But I think when I came in 1966 the number of bursaries BISRA gave had dropped. There was also support from the British Cast Iron Research Association. I think the most recent support we’ve had from a research association was from the Cement and Concrete Research Association. But we have had much support from the CEGB and UKAEA. The trend has changed more towards applied research with aims specific to a particular Industry. Thus, there was a shift towards Industry direct. But in the earlier days, you’re absolutely right, BISRA and other Research Associations were important sources of support for the universities.
So you were given this project to do by W. H. Taylor.
Yes, it was Bragg’s project, and as was usual in his case, the question to be answered and the way to answer it were clear and simple. That was one of the great things that I remember about Bragg, the clarity of his lectures and of the way he discussed research problems. He had a pictorial mind, and I suppose that’s why he was such an outstanding crystallographer. Everything was described in simple pictorial terms.
Sort of geometric, three dimensional?
Yes. Not in abstract mathematical, but in simple pictorial terms. The project was conceived because there was a controversy around that time, in the early forties, between Lipson and his group in Manchester and W. A. Wood — who worked in Australia — about the origin of the broadening of X-ray diffraction lines in cold work metals. Around 1939, Bragg had made two contributions to Metal Physics. First, the Bubble model of dislocations, which is typical of Bragg, a model of beautiful simplicity, absolute genius. Second, he considered crystals as mosaic structures, consisting of slightly mis-orientated microcrystals with boundaries between them made up of arrays of dislocation lines. He proposed that the strength of metals was controlled by the size of these crystals. The smaller the microcrystals, the stronger the metal. So his explanation of the effect of cold work, which increases strength, was that the original grains break up into smaller ones; a typically clear and simple picture. He now linked that up with the controversy in the interpretation of X-ray diffraction lines from cold work metals. Lipson said it was due to strain, i.e. the original crystals were bent and strained causing changes in lattice plane spacing, and thereby line broadening. On the other hand, Wood proposed that the crystals break up into small microcrystals, and that you get line broadening due to the small particle size effect. So Bragg’s idea was simplicity itself. Suppose you make the X-ray beam diameter smaller and smaller; if the material consists of tiny little crystals, when you irradiate sufficiently small numbers of them instead of getting a continuous diffraction ring you get spotty rings, each spot coming from a microcrystal. All you have to do is to use a very fine X-ray beam, a micro-beam, and a high intensity X-ray tube to give spots from the individual microcrystals in a reasonable time. There existed already a method of determining the particle size or crystal size from the number of spots on the diffraction rings. That was worked out by a Russian called Shdanov. So I joined Noel Kellar who had already come to the Department of Crystallography in the Cavendish in 1946, some months before I arrived. He had come back from war service, and was several years older than I was. This was a period when there were a lot of mature people who came back to study as undergraduates or graduates. Noel Kellar had done his Physics degree at Reading University. I think it was actually an external degree at London University in those days. He came to Cambridge to do a PhD with Will Taylor, and I joined them. He was the leader of the team. He had been in the Navy. Our project was split up. He was in charge of the design of the rotating anode high intensity X-ray tube, and my job was to design and get built the micro-beam camera. The collimator consisted of fine glass capillaries, down to a few microns in internal diameter, and they could be aligned in precision holders, which could also be moved along a rail. The camera also had specimen and photographic film holders. The camera was a precision instrument made in the Crystallography workshop by an outstanding instrument maker, called Charles Chapman. The X-ray tube was to be a fine focus tube to give high brightness. To achieve this the electron beam in the X-ray tube had to be focused by an electrostatic electron lens system. We got help from Professor Oakley in the Engineering Laboratory who headed a group working on electron lenses. We used an electrolytic trough to trace out the potential lines, in order to get the best geometry for focusing the beam. It was a big project. The rotating anode was eight inches in diameter, and it took about two years before we got the whole X-ray tube working. We were frustrated that it took so long. In the meantime we did two other things. Since the camera was ready before the rotating anode X-ray tube, we put the camera on to an ordinary X-ray tube and ran it for an exposure time of 125 hours, with a specimen of beaten aluminum. At the same time we worked on an X-ray monochrometer. We did some experiments on the intensity of the X-rays reflected from what are called concentrating X-ray monochrometers. The idea of a concentrating monochrometer was first introduced by Fankuchen in the United States, who actually spent some time in Cambridge, with Bernal. This was before I joined the Cavendish. It was Robert Evans in the Mineralogy Department who suggested the project of actually looking at the efficiency of X-ray monochrometers cut at different angles. It worked by cutting the crystal asymmetrically at some angle, such that the beam coming off is actually narrower than the beam going in. And Robert Evans, being a crystallographer, immediately devised a method of doing it as a function of the angle between the reflecting plane and the surface, by rotating the crystal about an axis not normal to the surface, but normal to the reflecting plane. This is obvious to someone who can think in three dimensions and is familiar with stereograms. We published the results in a paper by Evans, Hirsch and Kellar in Acta Crystallography.
That was in 1948?
Yes. It must be in the list of references.
Yes. So why did you get involved in this work on the monochrometer?
Robert Evans knew Noel Kellar, and I got involved because Noel Kellar got involved. Anyway, it had already been discussed when I arrived, although nothing had been done.
So, back to your main project.
In fact the long exposure X-ray diffraction photographs did show spotty rings from cold worked aluminum, which turned out to be the ideal material. They were the first successful pictures taken by this technique. The results were published in a letter to Nature in 1950 by Kellar, Hirsch and Thorp. From the number of spots on the rings a sub-grain size of 2?m was deduced. Subsequently the technique was used by others to look at other materials. Peter Gay and Tony Kelly joined us, and there were papers with Gay and Kelly in 1953 in Acta Crystallographica.
Is Kelly Vice Chancellor of Kent?
He is Vice Chancellor at Surrey University. By this time Noel Kellar had been killed tragically in a boating accident in Holland in 1948 while we were attending a crystallographic conference in Delft. It was very tragic.
That must have been very hard at the time, because he’d been your main collaborator.
Oh it was absolutely shattering. You may not realize it but I actually married his widow in 1959. Anyway it was a shattering experience. We carried on, and then other people joined the group. John Thorp must have joined us first; he had major responsibility for the Rotating Anode X-ray tube and took over from Noel Kellar. John Thorp is now at Durham University. That little group used the micro-beam technique, and got results from it. John Thorp was much involved in getting the equipment going. Peter Gay and Tony Kelly then used it.
May I ask a few general questions at this point? Can you think of any particular papers and ideas you were becoming interested in during this period?
We were becoming more and more interested in dislocation theory. Cottrell’s book on dislocations and Plastic Flow in Crystals was very influential. My guess is that was 1953. You could look that up.
Yes, I will.
The other was W. T. Read’s book on Dislocations in Crystals. Those were the two books which were really rather influential, but Cottrell’s book more than any other. It was really the first book on dislocations in crystals which related them to mechanical properties. Read’s book was much more on dislocation theory. Cottrell’s book was much more useful to us, as we were supposed to be looking at work hardening and relating properties to structure. So those ideas on interpreting plastic properties in terms of dislocations were very influential. However, as you have probably realized, my research interest lines were developing along two directions. My involvement in the work on the concentrating monochrometer was an accident. I’m a great believer in accidents. It was that particular experiment which led me to look at the dynamical theory of X-ray diffraction, and for me, Zachariasen’s book on X-ray diffraction was very influential. If you ask crystallographers about Zachariasen’s book on X-ray diffraction, many of them will say “Who?” The reason for that is, it is not the kind of a book that a conventional X-ray crystallographer interested in structure analysis would read. It really dealt with the dynamical theory of X-ray diffraction, and to me it was a great influence, because it was THE book which really dealt in considerable detail with the basic theory, including absorption for perfect crystals in the Bragg and Laue cases, and there was quite a bit of material which was actually new, in the sense of a development. It had an important influence also later on. Now, there’s another thing I should tell you about, if you want some background. I tended not to read the literature until I had to. I think that was one of my shortcomings. I tended not to read prior literature until I got a result. I’m now referring to the old days when I was much more immersed personally in research than I am now. The way I operated is that you do something, following your ideas, and then you find out what the background knowledge is. To me, reading books and reading papers only assumes some real meaning when I actually need to know. It’s a shortcoming, I know that, but there it is.
Do you think that kind of approach can give you the greatest potential for what you call accidents, because you’re not so preconditioned?
That is my rationalization for not reading. I’m a great believer in the British system of getting research students to do original research at the age of 21, without having read a great deal or having been exposed to a large number of postgraduate courses. There are advantages and disadvantages, but a great advantage is that they have no preconceived ideas, and they will go and do an experiment before they actually know, A, that other people say it’s impossible and B, that they can find out things by accident which otherwise they wouldn’t have done. I think one thing I have learned, also, over the years, is that actually if you don’t do anything, you’ll never discover anything, but if you do something, you’ve got a chance of finding something, which may not however be what you set out to find. I have been putting an exaggerated point of view, of course. These days, before you actually think about a research project, you ask yourself very, very carefully what the point of it is, and whether you are going to get a result and so on. But I’m still in favor of doing speculative research problems, on the grounds that what you think of, what you hope for, may not actually come about, but something else may. It doesn’t always, but sometimes it does.
Shall we move on in the direction of moving dislocations?
One of the problems was that before we could use the rotating anode X-ray tube we had these tremendously long exposure times, using ordinary X-ray tubes. The other problems were that we wanted to achieve smaller beam diameters, and we couldn’t resolve the spots on diffraction rings from cold-worked copper and gold. The question was whether we could do better using electron rather than X-ray diffraction. The idea of using electron diffraction was influenced by a Physical Society conference I attended in Bristol in 1947, on the Strength of Solids. It was an important conference at which many new interesting results were reported, and in particular Heidenreich and Shockley presented their paper on dissociation of dislocations in face centered cubic crystals. But more relevant to this particular issue is that Heidenreich also reported an experiment of examining the surface of a deformed crystal in an electron microscope. From that paper it became clear that there was a diffraction facility in the microscope. I’ve forgotten the details, but it was clear that you could get diffraction patterns from very small areas. Some years later Heidenreich published (J. Appl. Phys. 20, (1949)) the results of his famous experiment on looking at the structure of cold-worked aluminum by transmission through thin foils in an electron microscope in which he saw the small sub-grains which we had also inferred by micro beam X-ray diffraction about the same time (Kellar, Hirsch and Thorp, Nature 165,(1950)). I remember being incredibly depressed by the fact that Heidendreich just pressed a button and got images in ten-second exposures, whereas we had exposures of 100 hours for our diffraction patterns (10 hours once the Rotating Anode X-ray tube worked). That had a lasting influence on me. It was a very influential paper. As far as I was concerned I realized, probably subconsciously, that this signaled the end of the application of the X-ray micro beam technique to the study of deformed metals. That happened long before we actually gave it up. The rationale of continuing with the micro beam studies after Heidenreich’s experiment was, of course, that Heidenreich was looking at thin foils, while we were looking at bulk material. Before Heidenreich’s transmission electron microscope studies we explored the possibility of using electron diffraction patterns from very small areas in an electron microscope. At that time Jim Menter, who was a research student in Bowden’s laboratory in the Cavendish, was operating a Metropolitan Vickers electron microscope with diffraction facilities. We used this to get transmission electron diffraction patterns of beaten gold foil. Jim Menter suggested taking images, and did so. I still have one of the pictures of beaten gold foil. The specimen has to be very thin; that was one of the difficulties. But we knew we could get very thin beaten gold foil, and succeeded in getting transmission electron diffraction patterns. We wrote a paper called “The Structure of Cold Work Gold, 1, a study by electron diffraction”. It was a paper authored by myself, Tony Kelly and Jim Menter (Proc. Phys. Soc. B68 (1955)). This paper did not include the micrographs. This was to be the subject of Part II of this study.
There’s one thing I wasn’t sure about. How long after you actually wrote that was that paper published? Or after the actual work was done? How much gap was there between getting the results and finishing the paper?
It was probably 2 -3 years. It was in Tony Kelly’s period in our group and Tony Kelly was only there for about three years, I think. He joined us in 1950, and the work was probably done in 1952 and/or 1953. It was published in 1955. The interpretation of the micrographs took a considerable amount of time. It is quite interesting that we started off as crystallographers. Crystallographers are programmed to do everything by diffraction. We took very small steps, very small steps. You might have thought, if you knew about Heidenreich’s experiments at that time, and we did, why didn’t you do transmission electron microscopy straight away? Why did we have to wait for Jim Menter to say “Let’s take a micrograph”? Why did we bother to take diffraction patterns? The reason was that we had background expertise in diffraction but not in microscopy. As it was, when we actually saw the micrographs, we couldn’t understand them, we couldn’t interpret them.
These were the contrast fringes.
Yes, the fringes, or more accurately straight bands of contrast. The interpretation of the micrographs was to be the subject of another publication. It was never published.
Because it took you a while to actually begin to understand what was happening?
There’s one other point, I think. How did you know Menter?
He was a research student in the Cavendish. I think we were aware of what was going on in Bowden’s group. We probably found out that they had acquired a Metropolitan Vickers EM3 electron microscope. It may have been that we actually got to know Jim mainly through this. But you also knew people because you were demonstrating with them in practical classes and so on. Theoretically, of course, there was this great tea room in the Austin Wing of the Cavendish where everybody went and had tea, and you were supposed to talk to everybody else. That was a fine principle. In practice, actually what happened was that the crystallography group and the other groups had their own table so it didn’t actually work that well. But nevertheless I think the tradition did help to find out what others were doing. There were also links through friends and social activities. At that time there were many other crystallographers in the department — one of them was Ian Nichol who is now Registrar at Cambridge, and Jimmy Nelson who went into BCURA, British Coal Utilization Research Association, and he worked on coal. They knew Jim Menter and I think we must have met socially. As research students we may also have met while demonstrating in practical classes. The influence of an Australian, Mervyn Patterson, was quite important in the interpretation of the diffraction patterns from beaten gold foil. By coincidence he was working in Orowan’s group on an X-ray diffraction experiment on deformed crystals, and he developed a theory of diffraction from stacking faults. We were interested in this and you can trace the reason for the interest back to Will Taylor’s lectures that I mentioned as an undergraduate course. It helped us, of course, greatly in the interpretation of the streaking we observed on the diffraction patterns of beaten gold foil. The streaks were indicative of the presence of stacking faults and/or thin twins. A.J.C. Wilson’s Methuen monograph on X-ray Optics, which included diffraction from crystals with faulted-structures, was also useful. Now what happened was that, being an X-ray diffraction man basically, I thought the contrast effects — the straight bands of contrast that we saw on the micrographs of gold foil are due to a change in phase when the electron beam passes across a stacking fault. That was the first step towards an idea that individual dislocations may be visible in thin foils by transmission in an electron microscope. The second step was based on Heidenreich and Shockley’s paper on dissociated dislocations in f.c.c. crystals. According to that paper dislocations are dissociated into two partial dislocations, bounding a stacking fault ribbon. Given that the bands of contrast on the micrographs of beaten gold foil could be due to stacking faults, this suggested that dislocations might be visible through stacking fault contrast. That was the basic idea. However, rather than trying to interpret the rather complicated images of beaten gold foil, we thought the sensible thing to do is to study some simple material where you could hope to see individual dislocations.
So you moved to aluminum?
We moved to aluminum, because we knew we could get transmission through thin foils because Heidenreich had done it. What we wanted to do is to anneal the aluminum foils and in that way to produce specimens with small numbers of dislocations which we then hoped to see individually. That was the idea of the experiment. The electron microscopists told us it was impossible to do these experiments because for transmission the thickness of the specimens had to be very small. We estimated the thickness of the electron transparent specimens of beaten gold foil to be about 1000Å. The electron microscope establishment considered this to be impossible. In retrospect, I think, it must have been clear from Heidenreich’s experiments on extinction contours in foils of aluminum that in fact he was getting transmission through a thousand angstroms or more. Anyhow, the electron microscopists’ views were based on experience with essentially amorphous or highly polycrystalline thin foils, while we were looking at single crystals. At the back of my mind I thought that for single crystals in certain orientations, there might be enhanced transmission due to the Borrmann effect which is known for X-ray diffraction. I was familiar with this phenomenon of the anomalous transmission of X-rays through crystals, because through my work on the concentrating monochrometer I became interested in and studied the dynamical theory of X-ray diffraction, and that led me to the Borrmann effect. Although at this time there was no evidence for an equivalent effect with electrons, this gave me some justification to proceeding with the experiments. Mike Whelan joined me as a research student in 1954. About that time Castaing in France had used the transmission technique to look at GP (Guinier-Preston) zones in aluminum 4% copper alloys. The main problem in those days was to make thin enough specimens. He developed an ion bombardment technique, and succeeded in getting pictures of GP zones. The main focus at that time was however the development of the replica technique for the study of metallurgical structures. By this technique the structure at the surface was replicated. Slip steps were revealed directly by the surface topography, different phases by suitable differential etchants. By using this technique one could not see directly the structures inside the metal. Anyway, Mike Whelan joined me as a research student in 1954 to try to reveal dislocations directly by the diffraction mechanism in transmission microscopy of thin foils. This is an example of how I operated, in that I didn’t actually work out the theory of this before we actually tried the experiment. Mike spent the first year trying to get the ion bombardment thinning method going. That turned out to be a complete waste of time. Then he tried thinning Al foils by etching. In October 1955 Mike got transmission micrographs of both aluminum and gold foils with a new microscope, the Siemens Elmiskop, which had been acquired by Dr. Cosslett, the Head of the Electron Microscopy Group in the Cavendish. That was another fortunate accident. We were clear that anybody could have done this experiment. It was luck that the Elmiskop I arrived in Cambridge just at the right time, when we were ready to do these experiments. Bob Horne, who was an expert electron microscopist with interests in biological specimens, was in charge of the microscope, and in setting up the operating conditions. Anyway, Mike got these pictures in October ’55 from aluminum and gold, and the pictures from aluminum consisted of individual short lines in linear arrays, at the boundaries between neighboring sub-grains. Mike determined the mis-orientations by diffraction methods and found good agreement between the spacing between the lines and Frank’s formula for dislocation arrays. However as crystallographers we immediately realized the lines could be Moiré pattern fringes due to overlapping crystals, slightly mis-orientated relative to one another. Furthermore, the periodicity of fringes under many situations would be the same as the periodicity expected from dislocation arrays. So we were not quite sure whether these lines really were dislocations. Now the microsope was designed for biologists. It had a double condenser lens which was primarily used for high resolution microscopy work. And although the machine was able to produce selected area diffraction patterns, it turned out because of the design you couldn’t use the double condenser lens in that mode of operation. And since we switched from microscopy to diffraction in our work, Bob Horne set up the operating conditions without the double condenser lens. As crystallographers we insisted on having a picture and a diffraction pattern. Then one day, however, Bob Horne set it up in the high resolution mode with the double condenser system, which meant higher brightness. He pulled out the condenser aperture which increases brightness further, and then the short lines started moving. It was a quite unexpected fluke. So the lines started running around, and from there on everything became clear.
Who observed them moving first, then?
Bob Horne and Mike Whelan. I was never allowed to operate the microscope and I have never taken an electron micrograph. Most people don’t actually realize that. The only thing I ever did on an electron microscope that was allowed by my research student Mike Whelan, was to use the control to move a specimen and to tilt it using the stereo control. That was all. I was never allowed to operate the microscope.
So you were actually called in later, when they discovered this.
Yes. It was Mike who made the first observations. Mike and Bob Horne worked together and I was called in to see the motion.
What was your reaction at the time then when they came and told you?
Well, fantastic! We saw these things moving around. There were incredible pieces of luck. It turned out, quite unexpectedly, that the dislocations left traces of their paths. For example, you could see clear examples of cross-slip. As far as we were concerned, everything became obvious then. But there was an immediate problem of interpretation of the traces. It took a little bit of convincing people that in fact these were slip traces. We looked at these things in a completely different way from what other electron microscopists were used to doing from surface observations. It was completely different. For example, you knew from diffraction the orientation of the crystal, and from the direction of the slip traces what the corresponding crystallographic slip plane was. Therefore you could derive the angle between the normal to the slip plane and the foil normal, and from the width of the slip trace the foil thickness, which came out as about 1000 angstroms. It took some doing to persuade the electron microscopists. They were surprised that at that thickness the foils were transparent. We showed the moving dislocations to G.I. Taylor who was incredibly tickled — I think we were very lucky that he was still alive and we could show it to him. Mott was also pleased. Neville Mott has this story which he told on numerous occasions, that Mike comes to see him and says “Prof, come and see moving dislocations”, and on another occasion he says “Prof, come and see moving dislocations”, and he finds actually it was a celebration because I got married. But the odd thing is, I’m sure my memory’s right on this, that Neville was actually somewhat skeptical at first when he saw it. It wasn’t as obvious to him as it was to us. But the incredible thing, what was so lucky, was that everything fell out immediately. For example the cross-slip process. There was a dislocation bowing effect, the dislocations were pinned at the surfaces and bowed out before they could move, i.e. line tension effects. These things were in the books. And what I think this technique did was that it persuaded the metallurgists that these dislocations were real, not just figments of the imagination of solid state physicists. It was a case of seeing is believing. We made a film of moving dislocations in Al by photographing the image on the screen in the microscope with a ciné camera outside one of the viewing windows of the microscope.
So in that sense, you were bridging the gap between solid state theoreticians and metallurgists?
Oh no doubt. To look back at some of the papers during this period might be quite interesting from an historical point of view. This happened in 1956, and if you look back at Institute of Metals conferences a few years before that, you’d be surprised at the language in which mechanical properties are actually discussed. I’d be hard put to it now to put my finger on the appropriate references, but there were Institute of Metals conferences in the late forties and early fifties, e.g. papers by M.K. Allen (of the National Physical Laboratory) in which dislocations did not feature or were not accepted. There were some metallurgists of course, like Cottrell, whose advances were based on the application of dislocation theory. But there’s no doubt about it, that this technique that let you see the dislocations did a lot to bridge the gap, and provide support for those metallurgists like Cottrell who had accepted dislocation theory and had applied it. I remember soon after giving a paper and showing the film of moving dislocations at MIT, probably in 1957, Burt Warren, who was a crystallographer of very great repute for his work on amorphous silica, that the film had convinced him. And from that time onwards developments were rapid. Having got the experiment to work, we then developed the theory to try to understand why we actually could see the dislocations. I explain here what happened with Walter Bollmann. He developed the technique to see dislocations by transmission electron microscopy nearly at the same time and I think quite independently. I wrote to Walter, and asked him for his history, and I wrote it down in the contribution to the history of electron diffraction.
Yes, the contribution to the history of electron diffraction.
I wrote to Walter and his letter traces in fact how he got to doing this experiment. I always thought that Walter’s imaginative step, so to speak, was very much greater than ours was, because I believe Walter was a nuclear physicist who went to Battelle to work on metallurgical problems, quite different from his previous experience. It turns out from Walter’s letter that the original suggestion came from Dr. Siegfried, the Head of the Metallurgy Section, who had in turn been influenced by a paper by Castaing on TEM of aluminum alloys. Walter’s key contribution to further development was that he devised the electro polishing technique for producing thin foils, which was crucial I think in most further developments. The etching technique that we used was very crude. We had followed Heidenreich but this thinning technique went nowhere. The development that made it possible from then on was Walter Bollmann’s electro-polishing technique, which could be applied eventually to many different metals and alloys. What amazed all of us was actually the quality of Walter’s pictures, because he got them on an old Philips microscope console, which was quite remarkable. However, he couldn’t see any movement of dislocations. I made a comment about whether these experiments were done independently. I’m quite sure that Walter Bollmann’s experiments were done absolutely independently. I think the only thing where there might be some doubt is whether Walter’s interpretation of what he actually saw was quite independent of what we had done. This is because the timing of a visit by Professor Crussard, who had visited us and had seen the moving dislocations, and had subsequently visited Bollmann. Would you like to have Bollmann’s letter?
Walter’s account of it is quite interesting. As I remember it, he thought about the interpretation of his observations one week-end and came in on Monday and decided that the short lines he was seeing were dislocations, and on the following day Crussard arrived at the lab. In my mind it’s quite unimportant, because there’s no doubt that Walter had made the observations independently, with the intention of seeing dislocations individually. Whether in fact there was any interaction in interpretation at this time, I don’t think matters. As I said, his key contribution to further development to the technique, was actually the development of the electro-polishing technique. He didn’t really make any contribution to the interpretation of the contrast effects, or to the developments of the kinematical and dynamical theories of contrast. My original idea of why we should see dislocations in aluminum, i.e. due to the effect of the stacking fault ribbon in a dissociated dislocation, was erroneous because in aluminum the dislocations are very narrow because of the relatively high stacking fault energy and they are effectively dissociated. The reason why we saw dislocations in Al was because of the lattice bending. We were of course aware of the strain field surrounding the dislocation. But while we were aware of the fact that strains around dislocations would give image contrast, I did not know how to calculate it at that time. As you probably realize, we did calculate the image contrast after we had done the experiments. My thinking before the experiments was influenced by Darwin’s theory of X-ray diffraction in crystals and I knew that if there was lattice bending, extinction would be reduced and one might therefore see something, but I didn’t have any idea how to calculate it. But although the proposed contrast mechanism from stacking faults was not applicable to dislocations in Al, it is the correct mechanism for contrast from stacking faults. But dislocations are usually observed by the effect of lattice bending. Thus, the original mechanism for seeing dislocations turned out not to be the important one. Walter’s idea of why he saw dislocations was because of the local changes in the lattice parameters, which is not correct. Having seen the dislocations we then developed the contrast theory, first of all, for stacking faults. Mike Whelan developed the dynamical theory of contrast from stacking faults. A kinematical theory was worked out independently by himself and me. But the dynamical theory was his, and that is why the papers by Whelan and Hirsch, ’57, a and b, have got the authors in that order. I consider that Mike Whelan’s dynamical theory of contrast from stacking faults was a most important contribution, because it is the first time that anybody had calculated the contrast from a defect using dynamical theory. It was a big advance from Heidenreich’s work, in which he had worked out the contrast effects from perfect crystals due to changes in thickness, or orientation. This was really based on McGillivray’s work in 1940. Those were influential papers, and of course also Bethe’s original paper. What Mike had done was to develop a dynamical theory of contrast for a defect, and it was the first calculation of its kind. The basic physical mechanism giving contrast in this theory is the phase shift across a stacking fault, which was my original contrast mechanism. We independently worked out the simpler kinematical theory of contrast from a stacking fault based on the same mechanism. My other contribution was the column approximation, probably in discussion with Mike. It is a very simple idea. The image of a crystal is a projection along the beam direction, and the contrast at a point in the image is due to what is diffracted from a column along the beam direction passing through that point. That was considered to be a very narrow column. I went on to develop the kinematical theory of contrast from dislocations (due to the lattice bending), using amplitude phase diagrams, and involving Bessel type functions. Archie Howie joined us as a research student in 1958, and he also contributed to the development of this theory. Unlike the case for contrast from stacking faults the phase shift varies continuously with depth in a given column, depending on the position of the column relative to the dislocation core. The theory was first published as a brief joint paper (Hirsch, Howie and Whelan) at a conference in 1959, but the complete work was published in Phil. Trans. Roy. Soc. not till 1960. By that time Howie and Whelan had developed their dynamical theory of contrast from dislocations, which superseded it. I remember when the reprints of the Phil. Trans. Roy. Soc. paper arrived, Archie Howie wrote on a reprint for me “Of historical interest only.”