Radiocarbon dating in Cambridge: some personal recollections
A Worm's Eye View of the Early Days, by E. H. Willis
- Harry Godwin
- Bill Libby
- Alfred Maddock
- Nick Shackleton
- Richard West
- Hessel de Vries
- The first counter
- The first dates
- Bomb radiocarbon
- Stratospheric radiocarbon sampling
It is nearly fifty years now since Willard Libby's concept of Radiocarbon
Dating spread like wildfire and captured the imagination of every archaeologist
and Quaternary geologist world-wide. It was the 'brave new world', 'the
new frontier', and every other clichZˇ one can think of - so if one word
could be used to describe it, it would be 'excitement'. It promised to create
an absolute chronology where speculation had been rife; it promised to vindicate
imaginative theories and their champions; and it threatened the cherished
beliefs of distinguished authorities which, through much repetition, had
been endowed with gospel-like qualities.
Generally speaking, it was the sounder and more confident heads in the community who sought grants to start the early Radiocarbon Laboratories. It was customary to then prevail on some well known physics or chemistry professor to supply the technical expertise, and he in turn would select some unsuspecting graduate student to build it. Graduate students were among the few at that time who could even spell the magic words 'nucleonics', 'electronics' and 'vacuum lines' born of hush-hush wartime technologies.
In many ways the era was analogous to today's computer explosion where fortune favours the young and the brave. One may detect the deliberate use of the word 'build', and not 'operate' for, as we shall see, few had discovered the magic formula for making it work. It was in this firmament of hope compounded by confusion that rewarding careers began and lifelong friendships were forged.
1952It was thus in early 1952 that Harry Godwin, who had recently formed the University Sub-Department of Quaternary Research in the Botany School, applied for a grant from the Nuffield Foundation of eight thousand pounds over five years to create the Cambridge Laboratory. Harry secured the enthusiastic advice of Alfred Maddock, a radiochemist by avocation and a walking encyclopaedia by nature, to steer the technical side. I had met Alfred at a Radiation Chemistry Conference in Leeds in April of 1952, and was as impressed with his capacity then as I am forty-four years later. Alfred had spent the War working on secret atomic matters in Canada, and his moment of glory had come when he recovered the entire stock of Canada's plutonium from the sawn-up pieces of a laboratory bench top - how it got there in the first place is a matter which the faithful never discuss.
The next step of the two Wise Men was to recruit the 'Third Man': the graduate student who would toil away like a troglodyte in the basement to make it all happen. A basement was thought to the most prudent place because the Botany School floors were no match for the five tons of metal required to shield the radiocarbon detection system, or counter. The coal was thoughtfully removed and the freshly painted cell was readied for the third man, me, and I started work on 1st December, 1952. Of the generous Nuffeld Foundation grant, £475 per annum were to be my share, quarterly in arrears.
This then is a tale of the early days of the Cambridge Radiocarbon Laboratory from the 'worm's eye view'. But this worm, even in his wildest dreams, could not begin to envisage that these beginnings would lead directly to arms control talks and a treaty with the Soviets in Moscow, or defending a $3.8 Billion energy budget before the United States Congress. But that is another tale.
The camaraderie was enormous - I was fortunate to belong to two cultures, the Quaternary Group under Harry Godwin in the Botany School and the Radiochemistry Group under Alfred Maddock in the Chemistry Department. We were nothing if not cosmopolitan: South Africa, Australia, New Zealand, Finland, America, Canada, Brazil, Argentina - we even had some fellow all the way from Yorkshire. These were people who worked together and did things together. One of these was a particularly gifted fellow student called Richard West with whom the term brotherhood has taken on a lifelong meaning.
Richard and his cohorts gave me my first taste of inter-disciplinary science. Prior to this time a geophysicist, for instance, was looked upon as an indifferent geologist and a lousy physicist who had taken a soft option. Today, such interdisciplinary research is not only taken for granted, but has proven to be the vital synergism for some spectacular advances.
The legendary Bill Libby became a firm supporter of our laboratory, and a personal mentor until he died. I used to stay at his house in Santa Monica, watching the sun go down over the Pacific and arguing furiously well into the night with generous doses of libation. He was always churning with ideas which flowed from him like an open fire hydrant. One of the better ones was of course Radiocarbon Dating itself, and thoroughly deserved the Nobel Prize. It was a beautiful piece of intuitive deductive thinking, backed up by the barest minimum of experimental effort to demonstrate its validity. I have been impressed how frequently this seems to happen in science, where the genius rank prove the point and the next wave improves the technique and fills in the detail. Other of Libby's ideas were sometimes more entertaining than practical, and picking the winners was quite impossible. As far as the experimental aspect of radiocarbon was concerned, he was almost antediluvian. He was wedded to the screen wall Geiger counter, a cumbersome, inefficient, but quite workable device. But Libby had proved the practicality of his hypothesis, and he was forever proud of the bright young people who had followed in his wake.
Alfred Maddock had already come to the conclusion that, since radiocarbon was a weak Beta emitter, gas proportional counting was the way to go. Acetylene was to be the preferred counting gas for two reasons; it had good counting characteristics and the two atoms of carbon in each molecule got twice as much carbon per litre into the counter. It had a very unfortunate quality which I demonstrated conclusively - it made a very big bang. Why I was not fried right then remains a mystery. The most obvious gas to have used was carbon dioxide. After all, it did not go bang, was a direct product of combustion, and the chemistry was straightforward. It too had an unfortunate quality - nobody knew how to make it work as a counting gas. A research student in the Cavendish, working under no less an authority than Otto Frisch, had just written a thesis showing conclusively that it was electronegative, and his case was amply supported in the literature.
To confound the sceptics, at the end of 1953 there appeared an obscure letter to the Editor in the journal 'Physica' (XIX, p.987, 1953 ) by Hessel de Vries and his student G. W. Barendsen entitled Radiocarbon Dating by a Proportional Counter filled with Carbon Dioxide - someone had done it! Having been forewarned of this impending publication by a visiting Dutchman in September, I wrote to de Vries in Groningen, Holland, to ask if I might visit him, and to learn at first hand how the miracle had been wrought. For a research student to travel to another country at that time was a privilege indeed, and certainly not by air even at cheap off-peak rates. De Vries extended not only a generous welcome, but gave enthusiastic and continuing support to the Cambridge Laboratory until we were all saddened by his untimely death in the early sixties.
De Vries is one of the unsung heroes of the Radiocarbon Story. The answer to the carbon dioxide riddle was simplicity itself - it was extremely sensitive to the presence of electronegative impurities, such as sulphur dioxide, and you had to purify the gas to better than one part in ten million, and only then did you obtain decent counting characteristics. The voltage required was very high since the gas was going to be used at three atmospheres pressure to get as much carbon into the counter as possible - but at last it was proven that it could be done! It was interesting that even in 1955 Bill Libby confided to me in a corridor that he doubted it would work!
Meanwhile, the international brotherhood of radiocarbon troglodytes and their handlers was growing yearly. We had our first European-only get together in Copenhagen in 1954 in which we shared our frustrations without much to show in the way of fulfilment. This was followed by a more ecumenical meeting we hosted in Cambridge in 1955, to which we graciously invited Americans. At this a few tantalising dates were banded about, but the focus was still mainly on the techniques. If the tone was more upbeat than the year before then it still seemed that our guiding philosophy was "to travel expectantly is better than to arrive".
1956The next year, 1956, saw the first world-wide conference in Andover, Massachusetts. Harry and I travelled by ship of course because only rich Americans flew. We were lustily greeted from the deck above by the ever enthusiastic Hessel de Vries who talked Radiocarbon non stop from one side of the Atlantic to the other - we were exhausted by the time we reached New York. It was at the Andover Conference that we gave our first Cambridge dates, and it proved a good story.
The conference also provided a significant milestone; it was the first time in which the fundamentals of the carbon exchange reservoirs were discussed by geochemists and oceanographers using the radiocarbon specific activity of sea water to determine the rate of turnover of the oceans. Thirty or more years later, this was to become a critical research tool in the building of models to determine the rate of global carbon dioxide mixing, and its implications for global warming. Successive years brought more such conferences each with increased sophistication as the raw research students of yesterday now were becoming authorities in their own right in increasingly wider fields, and with graduate students of their own.
Our first counter in Cambridge was a real beauty. It worked perfectly with excellent plateaux and devoid of the outgassing problems that plagued some later designs. It had one unfortunate shortcoming. In the rush to use radioactivity for nigh on everything from people to peanuts, radioactive tracers such as cobalt 60 were inserted into steel furnace liners to detect wear.
It was our misfortune to have our counter made from a batch of this steel with a minute trace of Co 60. The original clue came from a small but recognisable plateau at a slightly lower voltage than the main plateau. Our supposition was confirmed when de Vries lent us his original proportional counter to test in our system. It showed that there was nothing untoward with our system that a Co 60 free counter wouldn't cure! It was not enough to render the counter useless, but it meant a higher background than we would have liked and which we subsequently obtained. With the upcoming conference in Andover in October 1956 looming, not to mention an impending Ph.D. thesis, we made the conscious decision not to let the best become the enemy of the good.
We produced a series of dates which subsequently were shown to be remarkably reproducible when repeated later with our improved apparatus. It is interesting that although the precision of the measurements increased with improved techniques, particularly lower backgrounds, the overall accuracy of the method still rested on some untested fundamental assumptions. These included the constancy of the contemporary specific radiocarbon activity of the atmosphere over time, measured in disintegration's per minute per gram of carbon. We were to embark on an experiment to test this fundamental assumption, and it taught me the critical scientific lesson of never confusing precision with accuracy, because one can be totally wrong, with even great precision.
1958Henrik Tauber of Copenhagen, Karl Otto Munnich of Heidelberg, and I were at a Conference in Hamburg in 1958. Hessel de Vries was scheduled to give a paper on possible variations in the atmospheric radiocarbon content with time based on work with an oak from the Spessart Forest. In truth, I had not the foggiest notion what he was talking about. Hessel, the man who could never stay around for a few days when there were more urgent things to do, didn't want to wait to give his paper so he asked me to give it in his place. I protested, but to no avail and I gave the speech with much consultation with my two friends. All three of us became intrigued by the line of de Vries' reasoning and the implications of his results.
On our return home, I wrote to de Vries to ask if we could follow in his footsteps and organise a three way experiment between Cambridge, Heidelberg and Copenhagen. Far from objecting, he warmly endorsed the idea, and we set to work. Fortunately, we had at hand a large cheese-shaped segment of a giant sequoia tree which had adorned the entrance lobby to the Botany School for years; one of those objects one passes each day without seeing. This was duly drilled from the back at fifty year intervals as measured by the tree rings. The shavings from each interval were carefully collected and divided into four parts, one for each of the three laboratories and one set for reference. Although it posed a big work load, each interval was to be measured by two of the labs as a cross check. In this way we were able to cover the whole 1300 year range of the samples from the sequoia.
The results were given at a Conference in Groningen, Holland the following year, 1959. They showed that there had indeed been variations over the past 1500 years as de Vries had predicted, but they had generally been positive with respect to the present day specific activity of radiocarbon. This was conclusive proof that the radiocarbon method was not as absolute as we would have liked and expected.
Although the experiment was a great success, the result proved somewhat misleading since subsequent experiments with the much longer lived bristle cone pine showed that the variation started to swing from positive to negative before Zero AD by appreciable percentage points. This would have the effect of making the actual age of a sample older than the 'radiocarbon age' would indicate, and more than the counting error by itself would imply. In hindsight, when one considers the enormous upheaval of the ocean circulation system which could conceivably have taken place when a hundred or more metres of melted ice were returned to the oceans after the last Ice Age, it was optimistic at best to postulate the absolute stability of the radiocarbon content of the atmosphere as the basic radiocarbon dating theory demanded.
In these days of almost trouble free electronics and fancy desk top computers, it is difficult to imagine a situation where you had to fight your electronics each and every day. Vacuum tubes always needed replacement, capacitors sprung leaks, and resistances burned out. We bought some fancy shiny equipment from the Atomic Energy jokers at Harwell on the assumption that they knew how to make these things. We were dreadfully wrong, and we ended up eventually virtually making our own, often using the same shiny boxes with winking lights - when one showed off one's apparatus with pride, it was distressing that it was invariably the winking lights which most impressed visitors! One could not obtain commercially, or even build oneself, an electronic means of making a high voltage power supply for the counter. We needed a long-lived supply of about eight thousand volts, stable and spike free. This was accomplished by buying literally hundreds of deaf aid batteries, stringing them together like sausages, and immersing them in ceresin wax. I had a certain interest in the lethality of this contraption, and laced it liberally with mega-ohm resistors. However, on applying the high voltage to the counter, I still had to stand on a rubber mat and discharge myself to a piece of metal with an audible spark coming from my finger. Surprisingly, this proved to be the only trouble free part of the apparatus for many years.
Our first sample for dating was one from the excavations at Starr Carr. It was designated Q-14. The 'Q' designation was adopted because Libby had already bagged 'C' for Chicago, and we felt that 'Q' for Quaternary would be an appropriate trade-mark. The sample had already been dated by Libby in Chicago as C 353 by the older Geiger counter method. It had also been dated with reference to the Swedish varve chronology, for which I came to have a healthy respect. On opening the sealed bag returned by Libby after he had performed his dating, we were chagrined to find that it contained a mixture of sample, wood shavings and match-sticks, the ultimate sample contamination horror. When we sorted this mess out to our satisfaction, we achieved our very first date of 7600 BC + 210, in good agreement both with Libby and the varve chronology. It was an immensely proud moment, and in the ensuing euphoria Harry resorted to the telephone, as distinct from writing postcards, to tell everyone - a rare display of extravagance!
When the electronics were behaving, we tried to run a sample a night for 1000 minutes duration. I can't begin to describe for you the sheer excitement Harry and I shared as we would stay late at night trying to figure out what the date of that evening's sample might be - this was followed by the anticlimactic experience of pedalling the seven miles over the Fen to Landbeach, where I then lived in a frigid sixteenth century Rectory. One such evening vigil occurred when a piece of a longbow from Somerset appeared to be giving an age consistent with the Neolithic - could the longbow be that old? Indeed it was, much to the consternation of some archaeologists whose views often proved more colourful than they were factual.
Proving our archaeological colleagues plain wrong seemed to be becoming our unfortunate lot, and we became worried about how many friends we were on the point of losing. Our later high points included our first Royal Society paper, and a splendiferous Conversazione there replete with evening dress - a total novelty for me. It was too bad that we returned home in torrential rain to find my newly built little house in Girton taking in water at the front door and letting it out at the back.
I had the opportunity recently to look at a school textbook used by my grandson. There I was astonished to find in a graphically coloured text details of the chronology of the climatic history following the shrinking of the world's ice sheets at the end of the last glaciation. It seemed odd to me that what was totally unknown to us when we set up the Radiocarbon Laboratory in Cambridge is now accepted as fact to the point of being commonplace.
Our goal at the onset of our dating program had been clear. We were first and foremost exploring the relationship between pollen zones established in the British Isles with the strikingly parallel, but by no means identical, vegetational changes in NW Europe and elsewhere. While it was presumed that the vegetational changes were driven by climatic changes, no case could be made to suppose synchroneity between such parallel systems. The first focus of interest was the period when the climatic amelioration after the last Ice Age received a transient setback to cold conditions in what had been termed the Late-Glacial Period. Our series of dates at three sites in the British Isles amply confirmed the length of this transient, and furthermore established that it occurred in the British Isles concurrently with the rest of Europe between ten and eleven thousand years ago. The most astonishing thing about this renewed cold period of several hundred years is the remarkable severity and speed of its onset - perhaps in as little as a man's lifespan. The change was enough to transform open parkland with luxuriant lake flora back to treeless tundra conditions. I often reflect, when I listen to the debate on global warming, what profound and sudden changes in climate nature can do all by itself without the humble intervention of man.
The Post-glacial Period, or Holocene, proved equally fascinating. It coincided with the emergence of Man in western Europe, and particularly with the opening of the Neolithic. This, we showed, had occurred earlier than had previously been thought, and coincided with the intriguing and precipitous drop in the elm population as manifested by the pollen diagrams. Theories abound for this occurrence, but it was as if someone had blown a whistle and elms, and to a lesser extent the lime, virtually disappeared around Europe about 5500 years ago. Why the Neolithic culture appears to have blossomed over wide areas of NW Europe about the same time as the elm decline has always intrigued me. I am sure that by now others have tied the bow neatly on this one. Our most illustrative dating series on this subject was done with Alan Smith at Fallahogy, Ireland, where pollen records showed the forest clearings of Neolithic man just after the elm decline.
A near catastrophe about 1958 gave rise to an unexpected blessing. An American visiting Professor, whose name I have conveniently and mercifully forgotten, decided that we were behind the times in the Botany School, and that in this modern age every self respecting plant physiology group should be working with carbon-14 as a tracer. This had been tacitly banned in the Botany School by mutual consent in deference to the possibility of our being put out of business by contamination. To our dismay the dreaded isotope had already been ordered and used somewhat wantonly on three floors. Harry Godwin, who as I recall was on the University General Board of the Faculties at the time, managed to get the University to move us to some prefab huts behind a building in Station Road. They also provided enough money to outfit us in splendid style in a fully-fitted laboratory, so we not only escaped ruin but benefited considerably. I don't think Harry's position on the General Board hurt us at all! I was joined in turn by two wonderful assistants who became firm friends, Richard Burleigh and Gerald Sutton. Their devotion to their tasks was quite superb, and I gratefully acknowledge my debt to each of them.
The late fifties were overshadowed by the increasing fallout problem from atmospheric nuclear tests, which included bomb-produced radiocarbon. The Institute for Agricultural Botany provided annual samples of oats from 1953 onwards, so we were able to reconstruct the rising level of radiocarbon with each year. At 32% above normal levels, radiocarbon was giving increasing cause for concern in the public's mind as a possible fallout health hazard because it is an integral part of all body tissue, including the all important genetic tissue and the DNA molecule which had just been discovered in the Cavendish.
At this point I must digress. The New York Times had picked up on our work, and given it a full feature article. Unfortunately, in doing so they had transposed the 'o' in oats for a 'c'. The 'Churchman' of St. Petersburg, Florida, saw this as an opportunity to rail against vivisectionists because I had done experiments on successive crops of cats! I had the honour of sharing the same page and pillory with John F Kennedy, who was being castigated for running for the US Presidency as a Catholic, and therefore equally untrustworthy.
Monitoring the food chain thus became a matter of much attention, and thus participation in a public health issue became a natural adjunct to our normal dating program. Our samples bordered on the gruesome at times with several from car accidents, but the one nearest to home was hair from my own neck gleaned from the barber's floor which proved eleven percent elevated. Fortunately, my procreative years proved to have been behind me.
Partly as a result of public pressure world-wide, both the Americans and the Soviets agreed to a moratorium on atmospheric testing. But in 1961, the Soviets broke it with a sixty megaton atmospheric burst over Novaya Zemlya, the biggest on record In late 1961, Alfred and I went to the Ministry of Defence in London, and had the temerity to ask for the services of an RAF bomber to sample the stratosphere for radiocarbon. To our great surprise, they said "Yes".
A specially equipped Canberra Bomber had been outfitted with wingtip ducts used to sample the British series of atmospheric nuclear tests at a place in the Pacific called, incongruously enough, Christmas Island. This plane was put at our disposal for a stratospheric flight once a month. We trapped the carbon dioxide in the stratosphere using a molecular sieve which was exposed to the outside air at the correct altitude. Recovering the gas from the sieve proved quite easy in a high temperature vacuum furnace, and we never had the slightest hint of contamination to affect our dating, even though we measured activity of more than ten times normal after the big Soviet explosion. This high activity proved to have one beneficial outcome for it provided a unique tracer spike with which to follow the decrease in atmospheric radiocarbon activity as it gradually migrated to the troposphere, and thence to the upper levels of the oceans. Radiocarbon from bombs thus became an invaluable tool in constructing atmospheric turnover models and troposphere / upper ocean carbon dioxide exchange rates. Again, these carbon dioxide exchange rates proved crucial to modelling the rate at which fossil fuel produced carbon entered the oceans from the atmosphere with its implications for global warming trends.
Pursuing the idea of developing Quaternary Research beyond radiocarbon and pollen analysis, we applied for and obtained a grant to start an oxygen 16 / 18 ratio unit to work on palaeotemperatures in the manner pioneered by Emiliani at Miami and Ericsson at Lamont. This was an exciting new venture, and in my turn I set about enticing a graduate student to build it just as Alfred and Harry had done with me twelve years previously. How the wheel, or the worm, had turned! We got a young Nick Shackleton, straight from Part II in the Cavendish, to come and work with us: and the rest is history. Splendid history. I have continued to take vicarious pride in his achievements over the years, and I am glad to have been there at least as a midwife at the project's birth. About this time, I saw new horizons and new challenges awaiting me, and fortified with the skills and wide interests bestowed upon me by Radiocarbon Dating, I set sail for new ventures in America. They were eminently successful, but as I said earlier, that is another tale. Suffice to say, the creation of the Radiocarbon Dating Laboratory started a splendid adventure, and it still has a very special place in my heart.
E. H. Willis
Arlington, Virginia 22 September 1996