Foreword to the Collected Works of Robert Hofstadter
University of Texas
"... time does not flow on emptily;
it brings, and takes, and leaves behind."
- Wilhelm von Humboldt
As I write this Foreword to the collected papers of one of my closest and dearest friends, Robert Hofstadter, my mind is filled with a wide panorama of wonderful recollections coupled with a profound feeling of sadness. Our lives were intertwined over a very long period of time, extending from 1931 until November 1990, during which we were close friends and scientific colleagues.
Bob had a deep yearning for knowledge and a constant striving toward creative achievement. Throughout his life, he deeply loved nature - a fact that I learned early on and came to appreciate even more as I saw Bob in later years at his ranch near Flournoy, California. As a youth he collected insects, leaves, and rocks, and developed a great interest in photography, which he pursued much of his life. One of Bob's passions was music, both classical and jazz, which he truly enjoyed over the years. Perhaps above all he was a gentle and modest man with great integrity and a lifelong appreciation of humor and good fun.
For many years I encouraged Bob to write about his life and experiences, but he always demurred on the grounds that he couldn't write very well and that he had nothing all that interesting to say. Of course he was wrong on both counts. To compensate in a small way for his not having written any memoirs, I will myself touch on some personal recollections. Indeed, I have often felt that the way we do our work is perhaps more important in a sense than the specific accomplishments themselves.
* * *
I met Bob Hofstadter in 1931, when he entered the City College of New York. I had started the year before. As it turned out, it was a great event in my life to have met Bob, and over the years, the members of his family. We both majored in physics, and took modern physics from Henry Semat and theoretical physics from Charles Aloysius Corcoran. A highlight in the CCNY physics curriculum was studying with Simon ("Si") Sonkin, who later joined the High Energy Physics Laboratory at Stanford, and with Mark Zemansky, who was a wonderful pedagogue and who became very well known, especially for his text on thermodynamics. Both of these professors made deep impressions on us in our formative years and became our good friends later in life.
I might comment briefly on the extremely high quality of undergraduate education at CCNY during those days. There was an excellent student body at the college, including many of the best students from the New York City high schools, who for economic reasons had little choice but to go to the free city colleges, since at that time there were very few opportunities elsewhere. With this high quality of students, you can well imagine the level and intensity of discussions that constantly took place in the basement alcoves, ranging from the most academic and esoteric topics to the most pragmatic and political ones. At the time, City College was one of the most outstanding educational institutions in the nation, and over the years many of its graduates went on to distinguish themselves in diverse ways.
Upon graduation, Bob went to Princeton University. I followed a year later, after spending an academic year at CCNY as a teaching fellow in the Physics Department. On my arrival at Princeton in the fall of 1936, Bob immediately introduced me to Edward Condon, his research supervisor at the time and a brilliant theoretical physicist. Condon strongly influenced our research interests, and as a consequence Bob and I both wound up working on infrared molecular spectra in the basement of Palmer Physical Laboratory. Bob concentrated on the spectrum of formic acid, HCOOH, and its deuterated form, HCOOD, while together he and I studied the absorption spectra of acetic acid, CH3COOH, and its deuterated form, CH3COOD, to determine the hydrogen-bond strength for the latter molecule.
What is perhaps more interesting is that by applying Badger's rule to the frequencies observed in the infrared absorption spectra of the light and heavy acetic-acid molecules, we were able to estimate the lengths of the two bonds flanking the hydrogen atom in the dimer of acetic acid. We published our finding in the Physical Review and the Journal of Chemical Physics in 1938, and this determination was cited as the definitive value in Linus Pauling's classic opus The Nature of the Chemical Bond. Although this was but a very small type of recognition (it was mostly in a footnote), it was one Bob and I always cherished.
In any case, while Bob did his research on the formic acids, I worked mainly on determining the bond strengths of various deuterated forms of the fatty-acid series of molecules in the vapor phase. All of this kept us working in the bowels of Palmer Physical Laboratory almost entirely at night, since the construction of the cyclotron generated electrical power surges that disturbed the sensitive infrared recording mechanism almost all day long. In those days we would pull a string to elevate the shutter and allow the infrared beam to do its stuff. We took galvanometer readings in the darkened laboratory by watching the light spot slowly travel across the translucent scale and come to a shaky rest. The work was fascinating, since in those early days, molecular spectroscopy was one of the best ways to check the validity of quantum mechanics.
During one of these late-night sessions at Palmer Laboratory,
Bob and I were doing a vacuum preparation of a deuterated fatty-acid
molecule. At some point, Bob, who was crouching down at the
apparatus, called to me in a tired voice, asking what day of the
week it was. When I replied that it was Friday, he said, "Gee,
how past this feek has wassed!" I have always cherished this
triple spoonerism, the only one I have ever come across, and
often quoted it to Bob in later years to our mutual amusement.
In those days, we had great fun telling jokes and limericks. Bob used to urge me on, and often suggested that I should go on stage as a comedian. I would retort that I would gladly do so, provided he would agree to be my manager - and he always said he would. But in fact, we never did it, and I think it turned out for the best that we both continued our scientific careers instead.
It was mainly during our graduate days at Princeton that Bob introduced me to the wonders and pleasures of jazz. He was an avid collector of records, and as time went on he built up a magnificent jazz collection. It was in Bob's rooms at the graduate college that I learned about Louis Armstrong, Benny Goodman, Billie Holiday, Ella Fitzgerald, Clyde McCoy, and many others. It is a joy to remember our visits to Harlem to hear some of the jazz greats at the Apollo Theater during the 1930's.
At Princeton, Bob and I learned how to blow glass, as well as how to make drawings for apparatus to be built in the glass-blowing shop by Leigh Harris, a wonderful man, and by the people in the machine shop, which included Billy Duryea. Billy was a Scot, a small man with a high-pitched voice, who taught us much about shop practice. He often said, "This is a fine sketch, but I will have to put the last screw in place from the inside!" Bob and I used to watch Billy with wonder as he walked onto the large liquid-air compressor, moving his arm in synchrony with an oil cup and lubricating the machine while the flywheel and other large parts were in motion. Chester Grove, another member of the staff, was in charge of the instrument room. He fascinated us when he made various types of waxes out in the open behind Palmer Physical Laboratory. Over and over, he would skillfully beat out the flames from his preparations, and always wound up producing excellent high-vacuum waxes. During this period, Bob and I learned a great deal about high-quality work. Over the years, we frequently talked about these wonderful people, whom we always remembered with great respect and affection.
Bob and I shared many teachers at CCNY and Princeton. Aside from Condon, certain of our professors in graduate school stand out, like Edwin P. Adams, one of the last great classical physicists, H. P. Robertson, famous relativity theorist, and Eugene Wigner, whose undergraduate course in theoretical physics was finally the one course we took with him that we truly understood. There were also many great physicists who visited Princeton during our days there, including P. A. M. Dirac, Niels Bohr, Wolfgang Pauli, Enrico Fermi, R. H. Fowler, J. H. van Vleck, and J. C. Slater. We were fascinated by the great astronomer Henry Norris Russell, when for example he gave an informal discussion one evening of some of the latest findings in astronomy at the Princeton Graduate College, and we were especially intrigued by Einstein, who was at the Institute for Advanced Study, and who often came over to the Physics Department to attend colloquia, in many of which he made comments. Bob used to say that although he had never dared to speak a word to Einstein, he had many times stood "within spitting distance" of the great man at the coffees held before colloquia. Over the years, Bob and I continued to be amused by the fact that, during those early days when we were budding physicists, we naïvely supposed that all physicists were like these greats.
* * *
Bob and I were eager young scientists anxious to swallow the world. Thus we thought it would be interesting to do more than just our dissertation research, and that led us into the realm of photoconductivity, which came as a result of Bob's having spent a summer at General Electric in Schenectady with researchers doing solid-state studies. We built a grating spectrometer in which the exit slit at the sample and the grating were both fixed at opposite ends of a diameter on the Rowland circle, with the quartz-mercury arc source moving on the circle to sweep the spectrum across the exit slit. The new mercury arc lamp generated a high level of ozone, which seemed to be giving us sore throats. In an attempt to prevent this, Bob and I used a rather primitive technique. We got dirt on our hands by rubbing them on the very dirty laboratory floor, and then we rubbed the dirt on our hands onto the surface of the quartz lamp, carefully avoiding its window. This burned the dirt into the quartz-lamp walls, thus absorbing the ultraviolet light, which seemed to resolve our minor health problem.
At some point, we decided to investigate the photoconductivity of a natural crystal of willemite (zinc orthosilicate) containing a manganese impurity. We obtained a small single crystal, beautiful and perfect, from Dr. Pugh, the curator of the mineral collection at the Museum of Natural History in New York City. We spent a long time grinding and polishing this lovely small crystal at Guyot Hall, the geology building. Late one night, we were washing it in one of the very old large sinks when one of us let it fall (over the years we each thought the other had dropped it, but now I believe I was the culprit). The crystal bounced around in the sink and then, to our horror, slid down the drain in slow motion while we looked on helplessly. We borrowed a large Stilson wrench, but our muscles were not strong enough to budge the big nut at the bottom of the very old trap. Fortunately, a big and muscular geology student who happened to be around late that night did the trick, and out came decades of detritus. And lo and behold, there was our precious bit of crystal!
This recovery was fortunate because we went on to carry out a very successful and interesting study of the photoconductivity of willemite. What is especially interesting is that we would cool the crystal down to liquid-air temperatures and irradiate it with 2537Å ultraviolet light from a mercury source. Then, with the light off, we would allow the crystal to warm up to room temperature. During the warm-up period, we found that the dark current increased from a very low value to huge levels, as indicated by the galvanometer spot drifting across the room many times after zeroing. This was a very unexpected effect, and we didn't know what to make of it for a while.
In the fall semester of 1939, Bob left Princeton for the University of Pennsylvania where, first as a Harrison Fellow and then as a temporary instructor of physics, he helped to construct a large van de Graaff machine for nuclear-physics research. There Bob made numerous friends, including Don Bayley and Leonard Schiff. Leonard and Bob collaborated on some research, which marked the beginning of a very close and lasting friendship.
I remained at Princeton and continued our photoconductivity studies. Since Penn and Princeton were only about an hour apart by train, Bob often returned to Princeton to work with me. We were particularly interested in explaining the anomalously high dark current we had observed, and we had long and sometimes heated discussions during which we considered the possibility of explaining it by spurious electrode and dirt effects, among many others. In the end, though, we agreed that the effect was real, and that it indicated the existence of trapping states just below the conduction band, from which electrons trapped during irradiation were released during the warming process. This was the first time such states had been revealed by observation. We published our discovery in a letter to the editor in Physical Review, but strangely, we never wrote and published a full paper on the subject.
Through Bob, I learned of an opportunity at the Moore School of Electrical Engineering at the University of Pennsylvania, and went there as a research assistant to work on the Bush Differential Analyzer during the academic year 1940-41. However, the next year, at the request of some of our former City College teachers, especially Si Sonkin, we both ended up as temporary physics instructors back at CCNY.
In late 1941, just before the United States entered World War
II, Bob decided to leave his teaching post at CCNY to join the
group at the National Bureau of Standards working on the
proximity fuze for bombs. Again as a result of information from
him, I joined a similar group working on the proximity fuze for
naval anti-aircraft gunfire at the Department of Terrestrial
Magnetism of the Carnegie Institution of Washington, which
subsequently became the Applied Physics Laboratory of The Johns
There were problems at the Bureau of Standards, and so when Bob was recruited by Don Bayley to join Norden Laboratories in New York City in 1943, he accepted the offer. Bob worked at Norden for three years, during which time he helped develop an automatic altitude sensor that enabled planes to fly at very low altitudes avoiding conventional detection devices. To check out the device, he himself was involved in test-flying, which was hair-raising and very dangerous, and left him with respect and admiration for test pilots and later for astronauts in the world's space programs.
Once the war was over, Bob wanted to return to a university position and in 1946, he accepted an offer to become an assistant professor of physics at Princeton University. During the next few years, he did precision measurements of the Compton effect, and carried out research on crystal conduction counters, scintillation counters, and the detection and measurement of gamma rays and their energies. In 1948, Bob discovered that thallium-activated sodium iodide [NaI(Tl)] made an excellent scintillation counter, and in 1950, he and Jack McIntyre showed how NaI(Tl) could be used as a spectrometer. I will discuss this contribution in detail later.
In June of 1990, I visited Bob in his home in California, and on a couple of very pleasant afternoons we sat in the patio and reminisced about the old days. I taped the conversation, and listening to that tape recently has been a source of delight to me. The following is a snippet of our conversation that pertains to our Princeton days.
Herman: I remember Condon lecturing about the stimulated emission of light. You know, there were the Einstein coefficients... If we'd been smart, at that time, we would've understood how to make a laser.
Hofstadter: Look, Einstein was pretty smart -
Herman: You better believe it!
Hofstadter: And he knew all about stimulated emission, and he didn't invent the laser!
Herman: Well, technologically, we weren't ready.
Hofstadter: The laser invention came out of the maser, which came from microwaves... I don't understand why somebody didn't invent the laser before that.
Herman: But you can say that about virtually anything.
Hofstadter: You know, it's interesting - I gave a course in my first year teaching at Princeton, which was 1946. I was asked to give a course - a graduate course - in optics. Can you imagine that?
Herman: No, I can't. That is an extremely difficult, formal, sophisticated subject.
Hofstadter: And dumb as I was, I picked Born's Optik - in German! I think I have the original copy here.
Herman: Oh, my! And you were struggling through that...
Hofstadter: Yeah - the German, and the physics -
Herman: - and the mathematics!
Hofstadter: Right. The big thing I remember from that course was that I had a lot of expression from the few students in the class, saying, in effect, "What's interesting about optics ? Optics is all finished!" Of course I didn't know anything about a laser or anything, but I remember telling them, "Look, this is such a beautiful subject that there must be other beautiful things coming out of it."
Hofstadter: I didn't know what they possibly could be, but the subject was such a beautiful one...
I think this fragment of our conversation captures in a nutshell a cornerstone of Bob's philosophy, which was that beauty and simplicity are at the core of all great science. And the discovery of the laser proved that his intuition about optics was right.
* * *
In 1950, Bob left Princeton and went to Stanford University to become an associate professor, at the invitation of Leonard Schiff and Felix Bloch, who became lifelong friends and cherished colleagues. It was at this point that Bob began his outstanding and so well-known and recognized work on high-energy electron-scattering studies of the charge and magnetic-moment distributions in nuclei. I well remember how impressed I was during the early 1950's when I heard from Bob about his plans to build a spectrometer to study the electromagnetic properties of nuclei by means of electron scattering. It was fascinating to be in on the beginning of his thinking, and to hear about his obtaining a naval gunmount, which had been used to swivel a piece of artillery with a barrel five inches in diameter, to carry the magnetic spectrometer.
On the completion of the apparatus, Bob undertook a systematic program of study of elastic and inelastic scattering of high-energy electrons by atomic nuclei. In this lengthy project, he collaborated closely with many graduate students and research associates who came to the High Energy Physics Laboratory (usually known as "HEPL") from all over America and many countries around the world. At the same time, he was also studying high-energy electromagnetic showers, as well as developing scintillation counters for X-rays and neutrons.
Our patio conversation in 1990 also touched on those early Stanford days quite a bit. Here is another small part of it, showing how Bob felt back then about his research.
Herman: When you went to Stanford, very shortly thereafter, I remember your talking to me about the high-energy electron-scattering work. Let's see, that was in the early fifties. I was still at the Applied Physics Laboratory at Johns Hopkins, and you would come to Washington, because you were talking to the Navy about getting a five-inch gunmount.
Hofstadter: Yeah, right.
Herman: Now, you went to Stanford in nineteen-
Hofstadter: Fifty. September 1950.
Herman: And when did you get the gunmount?
Hofstadter: Oh, very shortly afterwards - '51 or '52. And installed it with all the magnets in '53, and we had results in '53. I had two graduate students then - Dick Helm and Harry Fechter.
Herman: If I recall correctly, the first experiments you did, you did scattering from many nuclei.
Hofstadter: Yeah. We started with lead, because it had a high Z2, and so it should have had a high cross-section, but its form factor was so small that hydrogen, with Z=1, had a bigger cross-section! We realized that in the process. Nobody had ever pointed that out to us - they didn't know what the size of the form factor would be! And we went down the periodic system, but not doing everything, not by any means. We ran into beryllium, and there we discovered the inelastic scattering, which was really beautiful. Then we did helium - the alpha particle - and we could also see that very well, and then we did the proton... One of the things I wanted to say was, when we did that early work - it's a funny thing, but I was more thrilled by seeing the structure in the alpha particle than by almost anything else, because Rutherford used the alpha particle in figuring out the scheme of construction of the atom! Carbon was also very interesting to me, because it's so connected with life. Those were really exciting days, long time ago...
Bob often expressed great admiration for Ernest Rutherford, the pioneering British experimental physicist whose scattering of alpha particles by gold atoms first revealed the existence of the atomic nucleus, in 1911. These were among the earliest scattering experiments ever conducted in physics, and they set a model for physicists to follow for many decades thereafter. Bob's remark that he was thrilled at being able to probe the very particle that Rutherford used as his probe is a humble and poetic way of giving tribute to his great predecessor. It also strikes me as very characteristic of Bob's personality.
Over the years, Bob doggedly pursued the elastic scattering of fast electrons to its logical limits, and in the end his investigations revealed the charge and magnetic-moment distributions inside ever-smaller atomic nuclei, including the alpha particle, the triton and deuteron, and ultimately, the proton and neutron. It was a great achievement, showing for the first time that the proton and the neutron are not point particles, and therefore possess structure.
It is interesting at this juncture to recall Bob's reflections on how all this began. He recounted to me on several occasions over the years that when he left Princeton to drive across the country in the summer of 1950, he had only a vague idea of what research he might do. He felt sure that he wanted to use NaI(Tl) in high-energy physics in some manner. On reaching St. Louis, Bob and his family stopped to visit the family of Eugene Feenberg. Eugene asked Bob what he was planning to work on, and suggested carrying out some diffraction experiments - which, of course, meant scattering. For the rest of the trip, Bob thought about this, and by the time he arrived at Stanford, he had come to the conclusion that he would need a magnetic spectrometer and something like a gunmount. His original idea of using sodium iodide went onto the back burner, but was not forgotten. Indeed, it was resurrected a decade and a half later.
Bob told me that it took him many months to realize that Leonard Schiff had written a HEPL report about the high-energy electron-scattering problem, and had started to carry out Born-approximation calculations. Subsequently, Schiff suggested the problem to Marshall Rosenbluth, who developed the scattering formula now known by his name, which played a central role in Bob's research. Once Bob understood how closely Schiff's theoretical work dovetailed with his own experimental goals, consultations with Schiff became frequent and important. These anecdotal stories, which Bob told me toward the end of his life, reflect Bob's deep honesty and desire to be meticulous about scientific history and truth.
One of the great joys for Bob at Stanford was the chance to interact on a daily basis with Felix Bloch. Not only were the two extremely close friends, they also saw many issues in physics in the same way. Though primarily a theoretician, Felix had also done some experimental work, and so he greatly appreciated Bob's research and made many insightful suggestions. The Bloch and Hofstadter families often got together, and in the mid- and late 1950's, they spent some wonderful skiing vacations together at the Sugar Bowl, in the Sierras.
The Schiff, Bloch, and Hofstadter families were part of a close-knit group of Stanford faculty families most of whom met originally at Stanford Village in the early 1950's, and whose friendship continued to grow and deepen over the next four decades. Among these very close friends were Dan and Mildred Mendelowitz, Marvin and Leah Chodorow, Bill and Adelaide Iverson, and Fred and Sally Pindar.
* * *
An amusing and interesting set of experiences that Bob and I shared was the following. One day in 1957, Bob phoned me at work - the General Motors Research Laboratories in Warren, Michigan - to say that four Soviet scientists were coming to the International Conference on Nuclear Sizes and Density Distribution to be held at Stanford, and to ask if I, a bilingual in English and Russian from early childhood, would meet them on their arrival in New York City at Idlewild (now Kennedy) airport, and escort them to Stanford. The prospect delighted me, and when the time came in mid-December of that year, I flew to Idlewild and met the group, which consisted of Dmitrij Ivanovich Blokhintsev, Venedikt Petrovich Dzhelepov, Sergej Yakovlevich Nikitin, and Lev Borisovich Okun. From there the five of us flew to San Francisco - a trip of over twelve hours in a vibrating tri-tailed propeller-driven Constellation aircraft. We were met at the airport by Bob and a group of people from Stanford and Berkeley, as well as by numerous newspaper reporters. The overall scientific interaction with these Soviet scientists was excellent. On the human side, we were quite surprised by their exceptional interest in obtaining a mimeograph machine and, of all things, bathroom scales. In honor of these foreign visitors, Bob arranged an elegant Chinese banquet at Ming's restaurant in Palo Alto, but unfortunately, the Russians, used to eating boiled meat, boiled potatoes, and boiled vegetables day in and day out, seemed less than enthusiastic.
It was at this nuclear meeting that I met Geoffrey Ravenhall, who with Don Yennie had developed a beautiful, accurate phase-shift calculation for electron scattering. This led to a wonderful collaboration of many years, including Bob and Geoff and, at the GM Research Labs, Bunny Clark (now a professor of physics at The Ohio State University) and me. We performed many analyses of electron-scattering data obtained by the Stanford group, especially for the calcium nucleus, which revealed a "hole" - that is, a reduction in the charge density at the nucleus' center.
All of this work led to many trips for Bob and me back and forth between Stanford and the GM Labs. Very late one night in early November of 1961, Bob arrived at my home in Royal Oak, Michigan. By the time we had done some chatting, it was in the wee hours of the morning. We had hardly fallen asleep when we were awakened by a telephone call from Bob's wife, Nancy. My wife Helen, who had met Bob and me at the same time during 1939, and I were very worried to see Bob's face grow pale as he took the call sitting on the edge of our bed. But it all suddenly turned to excitement and deep joy when we learned that Nancy had been telling Bob he had just been awarded the Nobel Prize in Physics for his beautiful work. I was invited by Professor Oskar Klein to attend the Nobel ceremony in Stockholm that year. It was a great experience to be present at this high moment in the life of one of my closest friends.
* * *
Sometime during the year 1954, in a small informal meeting in the living room of a colleague, Bob suggested the rather outlandish idea (for those days) that Stanford might undertake to construct a one-mile-long linear electron accelerator. The others present were enthusiastic, and the idea was dubbed "Project M," for "monster." It soon became even more of a monster as its proposed dimensions grew from one mile in length to two, and the number of people involved mushroomed. Eventually known as "SLAC" (Stanford Linear Accelerator Center), this pioneering accelerator was built during the mid-1960's, and had its first runs in about 1967, which soon led to the discovery of partons and in 1974, to the "November Revolution", which provided a solid basis for the quark picture of hadrons. Bob was always proud of having been the person from whose mind that then-surrealistic vision had sprung.
In 1960, a fresh young English Ph.D. from the University of Leeds named Barrie Hughes wrote to Bob in the hope of obtaining a research post in Bob's lab. Bob was apparently sufficiently impressed to make an offer to Hughes sight unseen, and so in 1961, Barrie arrived at HEPL and began working with Bob on electron scattering and related projects. Thus began a very fruitful and multifaceted collaboration, as Barrie continued to work with Bob to the very end of both of their lives, about 30 years later.
In the late 1960's and thereafter, Bob and Barrie explored new detectors for high-energy physics, which led, among other things, to the development, at HEPL and SLAC, of the "Crystal Ball," a spherical detector made of 732 crystals of NaI(Tl). With the Crystal Ball, fundamental discoveries were made at SLAC on the spectrometry of charmonium and upsilonium, the new mesons containing charmed and bottom quarks. These successes were, in effect, Bob's realization of the dream he had had, when driving out to California in 1950, of somehow using NaI(Tl) in particle physics.
Bob had always been interested in astronomy, and in the mid-sixties, when satellites became available for scientific research, he started thinking about using new types of detectors in space. In 1966, he and Don Aitken, who had received his Ph.D. doing electron scattering under Bob, wrote a grant proposal to NASA to do X-ray astronomy aboard a satellite. Specifically, the idea they proposed was to use some of their newly-developed types of X-ray detectors to scan the sky for X-ray sources emitting rapidly time-varying signals, down to the level of a millisecond. One of their main targets of interest was the Crab Nebula. Unfortunately, this proposal was turned down by NASA. By now, of course, everyone has heard of pulsars, which were discovered in 1968 by a ground-based radio-astronomy group in England. Needless to say, the Crab Nebula is a pulsar in the X-ray region of the spectrum, and was exactly the kind of thing their proposal had suggested looking for. Bob and Don always felt disappointed that the chance to make this beautiful and profound discovery had been denied to them by the administrative decision of a granting agency.
Although regrettable, this incident didn't kill Bob's interest in doing astrophysics. In fact, in 1968, he launched one of his longest research projects - namely, gamma-ray astronomy in space. Once again, Barrie Hughes was an important collaborator, along with Carl Fichtel at Goddard Space Center and Klaus Pinkau in Germany. The first instrument they proposed was intended to be part of the HEAO (High-Energy Astronomy Observatory) satellite series. It was originally scheduled to be carried into orbit in the mid- or late 1970's aboard a Viking rocket, but when the Viking program ran into troubles, the whole gamma-ray experiment was put off. Luckily, NASA's space-shuttle program gave gamma-ray astronomy in space another chance. The original HEAO gamma-ray project was expanded and became known as EGRET (standing for "Energetic Gamma-Ray Experiment Telescope"), one of four main instruments comprising NASA's ambitious Gamma-Ray Observatory (GRO) satellite.
Unfortunately, the launch of GRO was also delayed many times, not least because of the Challenger disaster. Finally, in April of 1991, GRO was launched aboard the space shuttle Atlantis. The launch was perfect, but unfortunately it took place just a few months after the deaths of both Bob and Barrie Hughes. Several members of Bob's family, as well as Barrie's wife Sylvia, were present at Cape Canaveral to see the liftoff. After working for some twenty-two years on gamma-ray astronomy projects in space, Bob and Barrie had greatly looked forward to that big day and even more, of course, to the scientific rewards to follow. It is deeply regrettable that neither of them lived to see this landmark in their scientific efforts come to fruition.
* * *
In the 1970's, Bob's scientific interests branched out even further, and in particular he started thinking about ways of using fundamental physics for social good. He became Executive Scientist of KMS Fusion, Inc., a small Ann-Arbor-based company founded by Keeve M. ("Kip") Siegel, which was working on laser fusion. Bob was very attracted to the idea of helping to develop this powerful, safe, non-polluting, and virtually inexhaustible source of energy.
In early 1974, KMSF carried out the world's first successful laser-induced fusion in a deuterium-tritium pellet, the evidence for which was provided by neutron-sensitive nuclear-emulsion detectors developed by Bob, and which he had arranged for KMSF to obtain. For a while in the mid-70's, KMSF was indisputably the most advanced laser-fusion laboratory in the world. Unfortunately, Kip Siegel died suddenly in 1975 and the company started running into heavy opposition from large federal weapons laboratories that didn't want competition from a civilian project, especially an upstart little company like KMSF. At this time there were many people in both government and the scientific community who were extremely opposed to such a fundamental and important energy program being in private hands and possibly creating difficulties with patent ownership. To keep going, KMSF needed outside funding, and Bob spent a great deal of his time and energy lobbying congress and talking to other potential funding sources. At the same time, he was deeply involved in the scientific and engineering problems of laser fusion, concentrating on ways to improve neutron detection and to analyze the extremely short-lived implosion of the pellets.
Sadly, despite the long and valiant efforts of Bob and Henry Gomberg, the company's chairman, KMSF was unable to get enough outside support to remain a dynamic and viable laboratory, and around 1980 Bob and Henry both resigned. In the subsequent years, the ideas that were pioneered at KMSF have been vindicated by being copied elsewhere, but there remains a long way to go in the field. It is unfortunate that this pioneering scientific research by KMSF was terminated.
Another new research direction for Bob in the 1970's was the use of physics - especially particle and radiation detectors - in medicine. Bob was among the first to propose the idea now known as positron-emission tomography (PET). The idea was to place a group of NaI(Tl) crystals around the head or torso of a person who had been injected with a positron-emitting radioactive substance. The radioactive substance would be taken up selectively by certain tissues, and the emitted positrons would instantly annihilate with electrons to produce a pair of gamma rays that would be detected by the scintillators. This would permit precise monitoring of activities inside the brain or other parts of the body. Like the X-ray astronomy proposal, this was one of Bob's foresightful ideas the value of which only a few other people recognized, and despite his attempts to get such a project going at the Stanford Medical School, it never made it. Nowadays, PET scans are important research tools in medicine and biology.
Characteristically, Bob rebounded from this setback with a lively interest in a new idea for coronary angiography proposed by his old collaborator Barrie Hughes together with Edward Rubenstein, a colleague from the Medical School. The idea was to get detailed pictures of coronary arteries without invasive means such as arterial catheterization. The key notion was to exploit synchrotron radiation - the highly focused beam of X-rays produced at SLAC by electrons in storage rings - from which a particular wavelength could be selected by Bragg reflection. A patient would be injected with a contrast agent such as iodine, which, passing through the bloodstream, would absorb X-rays of that wavelength. This would contrast with the surrounding areas of the body, through which the X-rays would pass relatively unabsorbed. Thus, using solid-state X-ray detectors, a clear image could be obtained of the coronary arteries, traditionally one of the hardest regions of the body to image. Bob lived to witness definite successes of this project on both animal and human patients, and the new method is likely to become an important alternative to arterial catheterization in the next few years. It is especially suitable for long-term repeated use on the same individual, owing to its relative safety. It is ironic that Bob himself had to undergo arterial catheterization in the last few years of his life, before the non-invasive device on which he had worked so hard was ready to be used on humans.
In the mid-1980's, Bob lovingly put together a richly visual lecture that he gave in many places and on many occasions, each time updating it, including new slides whenever possible, and tailoring it to local purposes. (He also turned it into an article, reprinted herein.) Titled "Cross Strands Linking Physics and Medicine," this article detailed the intertwined relationships of these two fundamental disciplines over the course of history. An annual lecture series honoring Bob's memory has been established in the Stanford University Physics Department, and, reflecting this long-standing interest of Bob's, one major focus of the series will be the multifarious and deep connections between physics and medicine.
* * *
As I remarked earlier, I especially wish to address Bob's contributions in the area of scintillation detection for which, in my opinion, he never received proper formal recognition from the nuclear-physics community. This outstanding body of research is most unusual in that Bob not only discovered the excellent scintillation properties of NaI(Tl), but also developed the device, did fundamental physics research with it, and over a period of more than forty years saw his work exploited in virtually every field of science, ranging from physics to medicine and geological exploration.
Here are some of the details. In 1948, Bob discovered that an inorganic thallium-activated sodium iodide scintillator has superior gamma-detection efficiency. He also showed that it has a very high photoelectric and pair-production yield, allowing gamma-ray spectroscopy to be carried out. These properties of NaI(Tl) have led to its use in a large number of very significant experiments.
At the Scintillation and Semiconductor Symposium held in December 1974, Bob gave a talk called "Twenty-Five Years of Scintillation Counting," which is reprinted in this collection. In this address, he reviewed in a very interesting fashion the early developments that led him to the discovery of the thallium-activated sodium iodide crystal detector. Bob always laid great stress on the importance of the work of Hartmut Kallmann, who developed the very first scintillation counters, based on naphthalene. Although Bob's own work superseded Kallmann's, he always expressed great admiration for Kallmann. It is significant that Kallmann and Hofstadter each discovered the scintillation principle by design and not by chance. Let me describe Bob's own process of discovery in a little more detail.
When Bob joined the Princeton physics faculty in 1946, he started his research on scintillation materials. I recall that he had heard about Kallmann's work with naphthalene in Germany from a lecture that Martin Deutsch had given in the United States. Bob resurrected the old grating spectrometer that he and I had used in our photoconductivity experiments during our graduate-school days some seven years earlier. The one-meter concave grating was still there, and amazingly, the telescoping tubing and other parts of the spectrometer had also been preserved. Bob exposed all sorts of materials to the ultraviolet lines that were brought to a focus at the exit slit of the spectrometer. He then checked the materials that exhibited fluorescence for scintillation, but many of these showed little or no effect. At this point, he remembered the fluorescence work at the General Electric laboratory in Schenectady, where he had spent a summer. He wrote to Frank Quinlan, technician to Fred Seitz, and asked for samples of fluorescent substances, including KI(Tl) (thallium-activated potassium iodide), with which they were then working. When he received them, he put the various crystals or powders together with some naphthalene on a photographic plate, and irradiated them for half an hour with a one-millicurie radium source he borrowed from Professor Alan Shenstone, one of the world's experts on atomic spectra, as well as one of our teachers at Princeton. When the plate was developed, the area under the KI(Tl) was ten to twenty times blacker than all the rest.
Bob immediately sent off for some pure sodium iodide, and obtained a quantity of thallium iodide as well from Fort Belvoir, Virginia, where research on crystal counters was being conducted. He placed some NaI together with, as he put it, a "pinch" of thallium iodide in a crucible, and simply torched the powder. When it cooled down, he placed a small amount of the resulting glaze on a photographic plate, together with naphthalene, and repeated the irradiation experiment with the radium source. The response of the NaI(Tl), Bob told me, was tremendous. He knew then that he had found a wonderful scintillation material.
He proceeded to make some crystals of NaI(Tl) by torching the powders in vacuum in a quartz tube. This tube, containing the first good sample, is now on exhibit at the Smithsonian Institution in Washington, and has been for some years. Bob used this first sample, together with a 931 photomultiplier, and compared its response with that of naphthalene. In the first paper he published in Physical Review in 1948, the photograph showing the responses was reproduced upside down, so it was difficult for readers to interpret the results. Fortunately, the spectral energy response to the radium source was given also as a function of bias on the discriminator, so it was clear that NaI(Tl) was by far the better scintillator. Bob also determined that the rise time of the NaI(Tl) sample was about a quarter of a microsecond.
It is a remarkable fact that for over forty years, no scintillator superior to NaI(Tl) has been discovered. It is relatively rare that a new device or technique remains important for so long a period of time. Generally speaking, new techniques disappear in a rather small number of years as improvements or substitutions are made. This has not been the fate, however, of the sodium iodide scintillator. At this time there appears to be nothing else in sight that is likely to match it with respect to size and luminous efficiency. For example, bismuth germanate, another useful scintillator of high density, has a luminous efficiency of only about a tenth that of NaI(Tl).
The sodium iodide scintillator has been employed in a series of highly significant experiments in physics and astronomy, and is the gamma-ray detector of choice for most applications in nuclear medicine. What follows is a list of some of the important physical experiments that were done with NaI(Tl), and in one case with CsI(Tl), whose scintillation properties were also discovered by Bob.
* Martin Deutsch used NaI(Tl) in his discovery of positronium by detecting the gamma rays emitted in e+e- annihilation (1954).
* The very important verification of John Wheeler's prediction of mu-mesic atoms was made in 1953 by Fitch and Rainwater using NaI(Tl) in spectral determinations of discrete gamma-ray lines.
* There is also the remarkable discovery by Wu, Ambler, et al in 1957 that the quantum-mechanical property of parity is not conserved in weak interactions. In the experimental demonstration of this phenomenon, the NaI(Tl) scintillation counter was used in a fundamental way.
* In one of the most significant discoveries of modern physics, Rudolf Mössbauer showed with conceptually primitive means that gamma rays could be emitted from nuclei in an essentially recoilless manner. He employed a NaI(Tl) scintillation counter in an important and essential way in making this discovery (1958).
* In the field of general relativity, Einstein's predictions based on the equivalence principle led Pound and Rebka to search for a shift in the wavelength of a gamma-ray line due to the difference in gravitational potential at the sites of emission and absorption of the quantum. They used a NaI(Tl) scintillation counter in conjunction with the Mössbauer effect for the detection of the gamma-ray shift (1960).
* In 1973, simultaneous observations of gamma-ray bursts by several Vela-type satellites in varying positions around the globe signaled that powerful gamma-emitting events were taking place, probably outside the solar system. Each Vela satellite contained six CsI scintillation counters, which detected the arrival of the burst of gamma rays.
* In quantum chromodynamics, the Crystal Ball apparatus, described above, demonstrated the existence of excited chi states in charmonium, and clearly separated the three P states as well as yielding the values of their energies. The Crystal Ball has also been used to provide candidate states for the existence of a combination of two gluons, commonly called a "glueball" or "gluonium." Incidentally, Bob was a member of the team that made these discoveries.
* Discoveries made with COS-B, the European gamma-ray telescope, and other gamma-ray telescopes, all using NaI(Tl) and CsI(Tl), have shown the existence of over twenty-five localized sources of high-energy gamma rays from our own galaxy and from one intense gamma-ray source outside our own galaxy, which is, incidentally, a quasar.
In addition to the above highly important examples of work in physics and astronomy, NaI(Tl) scintillation counters in medicine and in geophysical and environmental science have yielded many further important results for society. In nuclear medicine, the Anger camera using NaI(Tl) scintillation counters enables physicians to see where certain radioactive tracers, such as Tc99, settle in the body, and thus to tell whether an organ is healthy or diseased. There are now whole-body imagers and various systems for tomography based on NaI(Tl) scintillators. It is fair to say, as Bob's angiography colleague Edward Rubenstein once did, that the sodium iodide scintillator is "the central device that made possible the use of isotopes in biological and medical research."
In geophysics and environmental science, the NaI(Tl) scintillator has been employed in measuring natural terrestrial-surface radioactivity, in making subsurface borehole explorations to gain information on geological structure, and in many other ways. In the environmental domain, it has been used in low-level radioassay, in gamma-ray spectrometry in the nuclear industry for isotope monitoring, in detection of fission-product radionuclides in reactor coolant, in snow-density measurements in hydrology, and in many other ways. The number of uses of NaI(Tl) in nuclear-physics experiments and applications is well-nigh countless.
As I mentioned above, the recitation of significant basic experiments using the sodium iodide scintillation counter as well as its applied use covers the forty-five-year period stretching from Bob's discovery of it all the way to the present. It is remarkable that this 1948 discovery continues to be used in novel ways by the scientific and technical communities to this day, and that Bob continued to develop and use both NaI(Tl) and CsI(Tl) scintillation detectors during the many years since his discovery.
I should like to note one other aspect of Bob's research, which, although not directly associated with the NaI(Tl) discovery, has had a significant and profound effect on the field of nuclear physics. I refer to the inelastic scattering of electrons from nuclei and the very lucid revelation of fundamentally new information on the excited states of nuclei, including magnetic effects. While Bob is primarily known for his work on the elastic scattering of electrons from the ground states of nuclei and nucleons, the independent studies of inelastic scattering provided a new tool for the understanding of nuclear structure.
In later years, a continuation of this line of investigation took place at SLAC, and led a team headed by Henry Kendall, Jerome Friedman, and Richard Taylor to the discovery of the quark structure of the proton. It is of interest that both Kendall and Friedman had worked as research associates with Bob at the High Energy Physics Laboratory in the late 1950's. For their discovery, Kendall, Friedman, and Taylor were awarded the 1990 Nobel Prize only a couple of weeks before Bob died. Bob was very pleased by this award.
* * *
To return to the human side, I owe a profound debt to Bob, from whom I learned a great deal about science and many other aspects of life. I owe much to him for my development, and it is a joy to recall all of the wonderful and exciting interactions that we had for so many decades of our lives.
Besides sharing scientific work, Bob and I shared many other joys. I recall with much pleasure visiting the ranch in Flournoy, California, which he and Nancy bought in 1963. There they tended cattle, and enjoyed the peace and beauty of the rolling hills, the wildflowers, and views of the mountains to the west. From the very top of the highest hill on the ranch, there was even a view of Mount Shasta, 150 miles to the north, and Mount Lassen, to the east. Bob and Nancy were very fond of their neighbors Elwyn and Darlene Wolcott, and took great pleasure in hearing Elwyn's colorful tales of country life. They also owned some small olive orchards in the nearby town of Corning, which were taken care of by a wizened and gnarled old fellow by the name of Jerry Hawkins, who lived in a shed together with a dozen or more assorted dogs, all of whom he loved. One time I remarked to Bob, "You know, Jerry looks exactly like an olive tree himself!" Bob found this comparison charming and funny, and often repeated it with delight.
Something that gave Bob great pleasure was wine. He especially
loved Californian and French wines, but over the years he came to
savor Chilean wines, Australian wines, and many others. However,
his attitude was anything but snobbish; indeed, he took
tremendous pleasure in discovering inexpensive good wines. One
time, there was a wine-tasting evening at the Stanford Faculty
Club to which Bob went with his daughter Laura. It turned out
that there was to be a wine-identification contest, and so, just
for fun, Bob joined in. Although he didn't fancy himself a wine
expert, to his astonishment, Bob was the only person to get all
ten samples right, for which performance he was awarded a couple
of bottles of wine. It always made him chuckle to recall this
I have attempted in this essay to present a sense of Bob Hofstadter's gentle and modest personality, and also a sense of how far and wide his creative mind ranged over many diverse scientific areas. Unlike many scientists in our modern socio-technical culture, Bob persisted in doing creative scientific work over his entire lifetime and actually to the very end of his life. He never rested on his laurels, but just kept on creating a marvelous body of technical work. In addition, he was always deeply concerned about helping his colleagues, particularly in trying to see that good work was properly recognized.
Now, in my late years, Bob lives on in my mind and heart, as he does in many others', and continues to shape my thoughts and feelings. I am deeply grateful for his profound influence on my life. At the close of this celebration of the life of a world-renowned scientist and fine human being, I am moved to recall the following quotation from Alexey Tolstoy, a distant relative of Leo Tolstoy: "A great man's passage in history is not marked by two dates - that of his birth and that of his death - but by only one: the date of his birth."
Finally, I would like to thank Nancy, Doug, and Laura Hofstadter for giving me the opportunity to express my admiration and love for Bob Hofstadter in this Foreword. The totality of Bob's works, to be deposited in various institutions of learning and research around the world, will serve as an outstanding contribution to scholarship as well as a testament to Bob's creative genius.
- Robert Herman
Austin, Texas, 1993.
Note added in 2000: At the time when he wrote this Foreword, Robert Herman, a lifelong friend and professional colleague of Robert Hofstadter, was L. P. Gilvin Centennial Professor Emeritus at the University of Texas at Austin. Bob Herman died in 1997.