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Accelerator Mass Spectrometry (AMS) 1977–1987
Nuclear Instruments and Methods in Physics Research B 268 (2010) xvii–xxii
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Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Accelerator Mass Spectrometry (AMS) 1977–1987 q H.E. Gove a, K.H. Purser b,c, A.E. Litherland d,* a
Department of Physics and Astronomy, University of Rochester, Bausch & Lomb Hall, P.O. Box 27017, 1500 Wilson Boulevard, Rochester 14627-0171, NY, USA Southern Cross Corporation, 49 Ledge Rd., Gloucester, MA 01930, USA c Twincreeks Technologies, #1 Industrial Park, Danvers, MA 01923, USA d Department of Physics, University of Toronto, 60 St. George St., Toronto Ontario, Canada M5S 1A7 b
a r t i c l e
i n f o
Article history: Available online 9 October 2009 Keywords: Accelerator Mass Spectrometry AMS conferences
a b s t r a c t The eleventh Accelerator Mass Spectrometry (AMS 11) Conference took place in September 2008, the Thirtieth Anniversary of the first Conference. That occurred in 1978 after discoveries with nuclear physics accelerators in 1977. Since the first Conference there have now been ten further conferences on the development and applications of what has become known as AMS. This is the accepted acronym for the use of accelerators, together with nuclear and atomic physics techniques, to enhance the performance of mass spectrometers for the detection and measurement of rare long-lived radioactive elements such as radiocarbon. This paper gives an outline of the events that led to the first conference together with a brief account of the first four conferences before the introduction of the second generation of accelerator mass spectrometers at AMS 5. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Year 2008 is the thirtieth anniversary of the first conference on ‘‘Radiocarbon Dating with Accelerators”, at the University of Rochester, USA. The first author was the chair of the group that organized the meeting. Later the University of Rochester published the proceedings [1]. Fig. 1 shows the cover page of the Proceedings that was designed by Gove. The drawings on the cover are imaginary combinations of cyclotrons and tandem accelerators, representing the fact that significant results from both types of accelerator laboratories were discussed at the first conference. These conferences have now become called Accelerator Mass Spectrometry (AMS) Conferences and this is the eleventh. In what follows we give a brief account of the initial discoveries that led to the first conference and some inevitably brief comments on the development of AMS from the first to the fourth conference. The first author was also chair of the fourth conference in 1987. Many developments after the fourth conference are not mentioned and it should not be forgotten that accurate radiocarbon dating using beta ray counting, rather than counting the radioactive atoms, has not been replaced but complemented significantly by q This paper was originally to be presented personally at AMS 11 by H.E. Gove. It was to have been on the first four AMS Conferences and a draft was written. However, the draft had to be finished electronically by the other authors after Harry Gove found that he could not continue the process. In general the original concept of the author, who died in February 2009, has been preserved. An obituary is on page N. * Corresponding author. Tel.: +1 416 978 3785; fax: +1 416 978 4711. E-mail address: [email protected] (A.E. Litherland).
0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.10.007
the powerful new mass spectrometric methods. At AMS 5 the second-generation radiocarbon dating tandem accelerator systems were introduced and smaller systems were invented later. The mass spectrometry of long-lived radioactive isotopes continues to develop vigorously. 2. The discovery of the instability of N AMS using negative atomic ions can be said to have begun 31 years ago. It was based upon some discoveries at the Nuclear Structure Laboratory of the University of Rochester in the United States and at McMaster University, in Canada. On May 18th 1977 it was found that, as hoped, the 14N negative ions were so unstable they did not interfere with the detection of 14C ions at natural abundance (14C/C 1012) and that the background to such measurements was very low. These experiments, at Rochester, were to test whether or not a small tandem could be designed for radiocarbon dating and then possibly built by the General Ionex Corporation (GIC) of which K.H. Purser was the president and founder at that time. In addition to observing 14C at natural abundances a surprisingly low background was observed at about 60,000 radiocarbon years before the present. This was in spite of a concern that machines used regularly to make nuclear reactions might contain 14 C contamination. Contemporary radiocarbon occurs at ratio of 14 C/C of about 1012 and so the discoveries had revolutionary implications for radiocarbon dating. As a result they were reported at the Second International Conference on Electrostatic Accelerator Technology on May 24th to 27th at Strasbourg, France and then reported in the New York Times on June 9th [2].
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Fig. 1. The front cover of the report on the first AMS conference proceedings.
The experiments at Rochester showed an apparent absence of N ions, an unexpectedly low ion source memory and low background of the then new high current Cs+ solid sample sputter ion source, which was very welcome news for later attempts to try radiocarbon dating using 14C ions. The low reactor grade graphite background implied the possibility that radiocarbon dating could be extended into the more distant past without the need to enrich the 14C isotope or that, with enrichment of the 14C, dating could be extended much further back in time. The low memory also implied that a sequence of samples including standards could be intercompared with an accuracy that would still have to be established. In early June 1977, 14C at a low level was also detected independently at McMaster University in Canada, although they were unable to confirm the actual numerical 14C/C ratio or the background level due to technical difficulties. However, this almost simultaneous detection of low-level 14C in two laboratories was dramatic confirmation of the long suspected instability of most of the excited states of the nitrogen negative atomic ion. The idea of atom counting instead of beta ray counting for radiocarbon dating [3] has been known since Zacharias suggested it to Libby in 1947, as was revealed at AMS 4 by Arnold, a student of Libby [4]. Since that time it has been advocated [5] a number of times and even attempted experimentally [6]. Many researchers in 1977 did not know that the negative ion idea for separating the 14C and 14N isobars had already been tried unsuccessfully [6] using the N 2 ion. Apparently a suitable ion source of C ions was not available, although the potential of that ion was realised. However, unlike the N ion, the N 2 ion is now known to have long-lived metastable states that were not discovered until 1997 [7]. With modern techniques the choice of CN must be well worth revisiting using mass spectrometry, as the ion has a much higher electron affinity than that of C. A bold idea to use a cyclotron and positive ions at high ion energy for radiocarbon dating was introduced by Muller in 1977 [8]. In this case the expected very large flux of 14N ions and the very rare 14C ions were to be separated by their differing ranges in matter. In addition, the resonant acceleration of ions by the cyclotron was expected to remove the interfering molecular isobars in this case. The suggestion was made after experiments to search for 14
integrally charged quarks had been completed [9] using a cyclotron and the approach was also based upon the earlier discoveries of Alvarez and Cornog in 1939 [10]. Some of the basic principles of AMS were laid down at that time, although the idea of carrying out the atomic isobar separation at the ion source and the complete molecular isobar destruction in a tandem accelerator were not then part of those principles. Negative ions were little known in 1939 and the tandem accelerator was not then available. However, the tandem accelerator, which exploits negative ions, had been invented by Bennett in 1935 and patented later [11]. It was re-invented independently by Alvarez again later in 1951 [12] and first operated for experiments at The High Voltage Engineering Corporation (HVEC) almost exactly 50 years ago. Later in 1977 [13,14] using modern tandems, some of the early exploratory experiments on the detection of 14C at Rochester and McMaster were also reported almost simultaneously. These results from both laboratories were discussed in some detail in early 1978 at the Conference at Rochester on ‘‘Radiocarbon Dating with Accelerators” that later became known as AMS 1 [1]. The first actual attempts at radiocarbon dating using negative ions and tandems and positive ions from a cyclotron were also reported. The discovery of the high degree of instability of the negative ion of nitrogen and the surprisingly low 14C background of tandem accelerators, however, resulted in their dominance for the first thirty years of AMS, although small negative ion cyclotrons [15], to exploit their potential higher mass resolution for molecular isobar removal, were developed after much research [16,17]. The greater suitability of the tandem as a component in mass spectrometry, and the removal of the main atomic isobar at the ion source, was decisive until recently, as discussed very briefly later.
3. The Rochester group The Rochester group was formed in April 1977 after Harry Gove had joined a discussion between Ken Purser and Ted Litherland (then a Professor of Physics at the University of Toronto). This discussion was on the degree of instability of N ions that would be needed for radiocarbon dating with a 1MV tandem. The removal of the molecular isobars, by charge changing from negative to positive ions at MeV energy, was not in question. Ken Purser had, at that time, applied for a patent on what was essentially the AMS of stable atoms in 1976 [18] using a tandem to remove the molecular ions from the mass spectrometry. At the initial meeting the question was how best to study the degree of instability of N at abundance ratios much less than 14N/12C < 1012. It was decided to try to find out just how unstable were those ions, or their excited states, using the well instrumented Rochester Tandem as a test bed, as that accelerator had a working Cs+ sputter ion source operating for some time. There was at that time no intention to use the large tandem for radiocarbon dating as that accelerator was used routinely for nuclear physics studies. The idea was to use it for finding out whether or not the instability of N was adequate for radiocarbon dating, before proposing a smaller tandem for funding. Here it is worth noting that the three of us were old acquaintances that had previously worked together and often had interacted professionally. Ted Litherland and Harry Gove had worked on cyclotrons for PhD theses at Liverpool and MIT, respectively, and then worked together at Chalk River, Canada on positive ions from electrostatic accelerators starting in 1953. They were not sorry to see the last of the early complicated cyclotrons, as the ions were difficult to inject for acceleration and hard to extract afterwards. Gove and Litherland then worked together for about ten years until 1964 at Chalk River. In the summer of 1958 they did research on the first electrostatic tandem accelerator, which had recently been re-in-
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vented by Alvarez [12]. The tandem built for the Atomic Energy of Canada at Chalk River but was then located at the factory of the High Voltage Engineering Corporation (HVEC) in Burlington, MA. Here it is worth noting that Robert J. Van de Graaff of HVEC, the inventor of the electrostatic accelerator, was enthusiastic about the tandem for several reasons, one being the possibility of bringing all the resources of the laboratory together at the ion source. For AMS this turned out to be an ideal arrangement, as was the ease with which the ions could enter and leave the tandem. Consequently this year is actually the Fiftieth Anniversary of the first tandem accelerator operation – an anniversary also worth noting. After the tandem had been installed at Chalk River, Canada, it was discovered later in 1960 [19] that the N ion could not be used in a tandem for nuclear physics studies, although the degree of instability was not measured. During the following year it was also discovered that strong C beams could be generated readily, from the charge-changing gas ion source of the tandem, solely from the memory effect of previously used CO2 gas. Harry Gove left the Chalk River Laboratories of the Atomic Energy of Canada in 1964 to found the Nuclear Structure Laboratory at Rochester University in New York State, USA. That laboratory was based upon the then largest tandem accelerator and for some time Ken Purser, a tandem expert from HVEC in Massachusetts, was also a Professor at the University. He left Rochester in 1975 after founding GIC, which made small tandems and the associated equipment for research and industry. The important Cs+ sputter ion source used at Rochester was, as you have heard earlier today from the talk by Ken Purser, developed starting in 1972 [20] by Roy Middleton and further developed at Rochester and manufactured by GIC in a productive collaboration. The Generating Voltmeter, which stabilized the tandem voltage independent of the strength of the ion beam, gave an initial brief advantage to the Rochester tandem, but was soon standard for nuclear physics and AMS. GIC, after the Rochester discovery, made the first five dedicated tandems for Radiocarbon Dating at the Universities of Arizona, Oxford, Toronto, Gif-sur-Yvette France and Nagoya Japan. Later, as US AMS, the staff from GIC made the first of the modern radiocarbon machines for the Woods Hole Laboratory. From 1974 onwards Ted Litherland was lecturing on radiocarbon dating, in a course on Physics and Archaeology at the University of Toronto. In 1976 Roelf Beukens, a young member of the Rochester group from Toronto had just completed a study of the proton capture by 14C at lower ion energies for his Ph.D. and also had recent experience at making 14C targets. One young member of the Rochester group, Charles Bennett, was also initially interested in the possibilities of using atom counting for radiocarbon dating a Stradivarius violin, only to be disappointed later by the problems of the radiocarbon calibration curve. Consequently the Rochester group was ideally poised to start the work on what later became part of AMS. The other members of the Rochester group, who are listed in Ref. [2], all contributed to the 24 h per day work and excitement of the discovery. In thinking back on the research during 1961 at Chalk River, Canada, it is now possible to say that maybe a golden opportunity to detect 14C there at natural abundance was missed, but with the ion sources then available no dating measurements would have been possible and years of research and development were necessary before the present form of the Cs+ sputter ion source was perfected. Also the degree of 14C contamination from nuclear reactions associated with the nuclear physics work at accelerators was unknown. Later during 1977 at Rochester we were also exceedingly lucky to be first to do the experiments. Our success was in part due to the reinvention of the tandem by Alvarez [12], the foresight of the Atomic Energy of Canada in accepting the proposal of HVEC to build a tandem for nuclear physics research and experience with tandem accelerators. The connection with industry (GIC) at
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Rochester was also invaluable and, of course, graduate students did most of the hard work. The timing of all the various experiments that led to the series of eleven AMS Conferences must be regarded, however, as simply chaotic, due to issues that were not understood at that time and the slow flow of information. Maybe that could not happen today as a result of the introduction of the worldwide web.
4. The first conference on AMS Year 2008 is the 30th anniversary of the first AMS Conference that was held at Rochester University in 1978. The title of the Conference was ‘‘Carbon Dating with Accelerators” and was not well chosen as early results for the important isotopes 10Be and 36Cl were also reported. After the second AMS Conference in 1981 at the Argonne National Laboratory (ANL), which was called at the time a ‘‘Symposium on Accelerator Mass Spectrometry” [21], the title became more simply an ‘‘International Conference on Accelerator Mass Spectrometry”. The first Conference is now referred to as AMS 1. The first AMS conference [1] contained papers on 14C dating from the Berkeley, Chalk River, McMaster and the Rochester Accelerator Laboratories and they demonstrated the great promise of radiocarbon dating using accelerators. However, although the Berkeley cyclotron used positive ions, tandems using negative ions soon became the accelerators of choice for AMS. The remarkable early results from Rochester, on the low level of background from reactor grade graphite, are shown in Fig. 2. The 12C and 13C peaks are associated with the destruction of molecular isobars and are easily be removed by the use of an electric analyser as they have different ion energies. The graphite background was later shown to be real [22] and corresponded to about 75,000 radiocarbon years. The origin of the small residual 14C background is still not fully understood but it later did not later prevent measurements down to a ratio of 14C/C 1018, using 100 enriched 13C in CO derived from natural gas as the source of carbon. This result was for the Borexino solar neutrino detector [23], which later was constructed and at 5 Mg was the largest scintillation counter built that could study radiocarbon. Even the shape of the beta ray spectrum could be studied and the low level of 14C level confirmed. It was the startling result from graphite that convinced the Rochester group to change the emphasis from doing basic measurements on the degree of instability of the nitrogen negative ion and the study of ions such as CHþþ 2 , to exploratory radiocarbon
Fig. 2. An early spectrum from the Rochester group showing the low intrinsic background from a graphite sample and a tandem accelerator.
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dating at Rochester. Initially the experiments at Rochester were basically to see if the nitrogen ion instability was enough for small 1MV tandem accelerators to be built by GIC for radiocarbon dating. was stable enough However, it was discovered that the ion 12 CHþþ 2 to be always present and so some time was spent studying it. The 13 CH++ ion was not observed. The 12 CHþþ ions could, of course, 2 interfere with the detection of the 14C++ ion in the final detector, as they are molecular isobars. It was then discovered that, by lowering the pressure of the gas in the Rochester Tandem stripping canal, the intensity of the CHþþ 2 ions was increased dramatically in an exponential manner, which facilitated their study. Some of them were observed to decay slowly in flight, thereby creating a continuum of CH+ and C+ ions that would be removed by the mass spectrometry. The presence of the CH+ and CHþ 2 ions were expected to prevent the use of singly charge ions for 14C measurements. The outlook then became bleak for the use of a small 1MV tandem for radiocarbon dating. The exponential destruction of both the molecules CH+ and CHþ 2 with stripping gas pressure was later observed at the Toronto IsoTrace Laboratory [24,25] at 0.7 MV and this information was later cleverly exploited at the Zurich AMS laboratory in an imaginative redesign [26] of a 0.6 MV tandem for AMS using singly charged ions and higher stripping gas pressure. Later the tandem voltage was lowered to 200 kV [27] and single ended accelerators were developed [28] to replace the tandems. The latter have their ion detectors at voltage. In both cases, higher than normal stripping canal pressures are employed to destroy the singly charged molecular anions and the high-energy mass spectrometry redesigned to accommodate the extra multiple scattering. Our failure to appreciate the significance of the exponential destruction of CHþþ 2 , observed first at Rochester, meant that we abandoned too soon the idea of a 1 MV tandem in 1978 and so missed a golden opportunity to create a really small machine. In the meantime using the normal stripping canal pressure it was shown at Oxford [29] that C+++ ions could be used for radiocarbon dating using a 2.5 MV tandem voltage. Triply charged ions from a tandem have since then been used widely for AMS in general as they have very low background from the tandem. The five 3 MV tandems designed and built for this purpose by GIC were put into operation in the early 1980s [30]. The first AMS conference also included a paper from the Rochester group on the measurement of 36Cl in a sample of CCl4 used in the duoplasmatron ion source of the tandem. It was discovered, as with the use of CO2 in a gas ion source at Chalk River in 1961, that the memory effect of some ion sources could be very large. As a result we switched to a Cs+ sputter ion source for all future 36Cl measurements, using neutron irradiated NaCl instead of gaseous CCl4 for tests. Then, as in the case of carbon, a low ion source memory was observed. 36Cl ions, at a level of 1011, were also readily observed in addition to a startlingly large number of 36 S ions. In this case, as in many other AMS measurements, the advantage of using higher ion energies for mass spectrometry, first stated clearly in 1939 by Alvarez and Cornog [10] and later in 1977 re-emphasized by Muller [8] was exploited. The separation of 36Cl and 36S was demonstrated using the separation of the isobars exploiting E and dE/dx measurements just has had been predicted [8], although this proved to be difficult to do with a cyclotron [15,21]. However, the first separation of the isobars 10B and 10Be by range was demonstrated convincingly by the first 10Be measurements using the Grenoble cyclotron [32]. Those data were also presented at AMS 1 [1]. The Rochester tandem accelerator benefited from the use of a generating voltmeter for stabilizing the terminal voltage and the provision of the latest model sputter ion source from GIC. These components it should be noted were designed for nuclear physics experiments. When replacements for them at Rochester were urgently required they were rapidly forthcoming from GIC, which
demonstrates the importance of collaborative research with industry. The other papers presented at AMS 1 were on imaginative proposals [1] for future work and the implications of the discoveries. Copies of the proceedings, of AMS 1 and AMS 2 at the Argonne National Laboratory ANL in 1981, are now becoming very rare and our copies are disintegrating with a half-life of a decade or so. Fortunately, all later proceedings are now available electronically, having been published by Nuclear Instruments and Methods in Physics Research B. In 1980 the Journal of Applied Radiation and Isotopes (JARI), published by Pergamon Press, presented a substantial silver medal to the leaders of the Rochester group for their outstanding contributions to AMS and in particular for the discovery of the degree of instability of the nitrogen negative ion. Fig. 3 shows the presentation of the medal. A brief resume of the next three conferences will now be given.
5. The second AMS conference AMS 2 was held at the Argonne National Laboratory (ANL) Chicago in 1981 and the proceedings [21] contain a unique and entertaining account of the very early days of AMS, during which Alvarez and a graduate student Cornog discovered natural 3He in helium [8]. The nuclei 3He and 3H had then recently been created by the early accelerator use of deuterons bombarding deuterons and the question of which was stable and which was radioactive was being asked. 3He++ was seen and some of the advantages of using a high-energy accelerator clearly stated and demonstrated. Isobar separation by complete stripping was accompanied by a range measurement.
Fig. 3. JARI award at the third conference on Electrostatic Accelerator Technology, April 13th to 16th 1981, Oak Ridge, Tennessee, USA.
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Many new AMS measurements were reported at AMS 2 and for example the 10Be concentration in ice versus depth in the Ross Ice Shelf was obtained by the Grenoble group, again using the Grenoble cyclotron [21]. The increase at about 700 m depth is probably associated with the Maunder minimum at about 1700 AD indicating the great value of AMS measurements that were previously not possible by mass spectrometry or by decay counting. At that meeting the Rochester group presented results of 14C measured in various wood samples, provided generously by Meyer Rubin, and compared them with those from the United States Geological Survey (USGS) decay counting measurements. The ages ranged from about 200 to 4000 years and the results were in agreement. In addition a result at 40,000 years was obtained, which again demonstrated the low background of the sputter ion source combined with the use of a tandem accelerator. A table in that paper listed the radio-nuclei measured up to 1981 at Rochester. These included 10 Be, 14C, 32Si, 36Cl and 129I. It also listed the limits for the detection of each. These ranged from 3 1016 for 14C/C and for 32Si/Si the limit was approximately 7 1012. A bold presentation [15] at AMS 2 was to use a small specially designed cyclotron (cyclotrino) exploiting 20–100 keV negative ions to measure 14C, 26Al using no final particle selection. This project evolved slowly and such a device is now in operation in China [16] for radiocarbon dating. Other papers in the AMS 2 proceedings included programs at existing accelerators at Grenoble and Orsay, Chalk River, Argonne, Pennsylvania, Zurich, the University of Washington, Saclay and Rehovat. Clearly many groups were keen on doing AMS measurements with applications to various fields. These fields included hydrology, 10Be in the environment and manganese nodules, cosmogenic radioactive nuclei, meteorites and geology. 6. The third AMS conference AMS 3 was held at Zurich as a major International Conference in 1984 and this time published in the readily available literature [31]. In the entertaining conference summary by Walter Henning, then at the Argonne National Laboratory, compared the change in the number of laboratories and 14C analyses with those mentioned at AMS 2. The number of Laboratories had grown from 16 to 24 and the number of radiocarbon measurements had grown to 1000, a remarkable increase in both cases. He also compared two facilities, GSI-UNILAC and the small cyclotron or cyclotrino. These differed in size by several orders of magnitude. The work of Raisbeck and Yiou on 10Be concentrations in marine sediments was noted too. A seminal paper on the introduction of catalytic cracking to produce superior graphite for the sputter ion source was given at AMS 3. This development by Vogel et al. [33] from McMaster University was subsequently widely copied and rapidly became the dominant target preparation method for radiocarbon dating by AMS. Contributions on 14C dating of archaeological and geological samples by Mook and by Taylor on the dating of bones and of 36 Cl in limestone and palaeontological samples by Kubik et al. were also singled out as highlights, as were the 14C measurements on foraminifera in ocean cores by Broecker. Papers on the detection of rare 205Pb created by solar neutrinos from 205Tl and on the half-life of 60Fe added to the interest at this third AMS conference. 7. The fourth conference on the tenth anniversary of AMS AMS 4 was held in 1987 at Niagara-on-the Lake, Ontario [34], partly in recognition of the 10th Anniversary of AMS using negative ions and the part played by both nearby Mc Master and Rochester in the early days. Visits to McMaster, Toronto and Rochester were
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part of the festivities, as were the exploratory talks on the potentially important role of lasers in separating isobars by M. Thonnard and W. Fairbank Jr. Gove, Litherland and Elmore edited the proceedings of the conference and as it was the Tenth Anniversary of the AMS series the front cover was designed with a stylized MP tandem as most of the papers now referred to the use of tandem accelerators. 32 laboratories were now engaged in AMS compared with 24 at AMS 3. At AMS 4 Professor W. S. A. Dale of the University of Western Ontario discussed the dating of the Shroud of Turin in a scholarly after dinner talk. The talk was on ‘‘The Shroud of Turin – Relic or Icon”, published in the proceedings [34], suggested from stylistic arguments that the linen cloth could be dated between 969AD and 1169AD and was an icon like many other objects. There was in addition a paper by H. E. Gove on a workshop proposing the correct procedure to be followed if there was a request for carbon dating the cloth. Ultimately three laboratories, Arizona, Oxford and Zurich, dated the cloth [35] and found it to be from 1325 ± 33AD, which is somewhat younger than predicted, although it could, of course, be a copy of one dating earlier. A book called ‘‘Relic, Icon or Hoax, Carbon Dating the Turin Shroud” was published later [36], as well as a more general book [37] on the development of AMS entitled ‘‘From Hiroshima to the Iceman, a Story of a Scientific Breakthrough”. There were many contributed papers. Applications to archaeology and ecology by F. Wendorf, R. Taylor and P. Gillespie were also reported and one on the indications for Pleistocene Man in Sardinia by H. Kofmeijr. The ages of some artefacts were reported by D. Donahue et al. of Arizona and by K. Kobayashi at Tokyo. There were also papers on ice core research by H. Oeschger and the in situ production of Cosmogenic Nuclei by D. Lal. Rather than mentioning the many other papers, in the fields of cosmochemistry, ocean and atmospheric sciences, hydrology and geology and nuclear physics, we will just note a most comprehensive and scholarly conference summary [38] by J.F. Sellschop that set a high standard for all others to follow. It was indeed a great first 10 years of AMS! In conclusion it is worth noting that many important developments in AMS occurred in the following twenty years, such as mature radiocarbon dating machines, better ion sources and detection systems. These developments have helped to make possible many of the imaginative researches in the Earth and Planetary Sciences often reported at these conferences. Further important developments are still taking place and many will be discussed at this AMS 11 conference. Negative ion methods may not be the only approach used in future for AMS but the tandem accelerator is likely to remain an important part of a very-low-background ion-specific detection system thereby keeping the A in AMS.
Acknowledgements We wish to thank our colleagues for their electronic help and advice in writing this paper and all the members of the Rochester group who contributed to a memorable week in May 1977. Richard Hyder, who made many contributions to AMS with tandem accelerators, also reminded us that this was also the 50th Anniversary of the operation of the first HVEC tandem accelerator. At least one of the early model tandems, an EN, has seen outstanding service at the Zurich Laboratories in AMS for the past 30 years, which was also celebrated recently.
References [1] H.E. Gove (Ed.), Proceedings of the First Conference on ‘‘Radiocarbon Dating with Accelerators”, University of Rochester, April 20 and 21, 1978. This is
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[17]
H.E. Gove et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) xvii–xxii available at the web site. K.H. Purser, R.B. Liebert, A.E. Litherland, R.P. Beukens, H.E. Gove, C.L. Bennett, M.R. Clover, W. Sondheim, An attempt to detect stable N ions from a sputter ion source and some implications of the results for the design of tandems for ultra-sensitive carbon analysis, Rev. Phys. Appl. 12 (October) (1977) 1487– 1492. and the New York Times of June 9th, 1977, article by B. Rensberger. W.F. Libby, Atmospheric 3He and radiocarbon from cosmic radiation, Phys. Rev. 69 (1946) 671–672. J.R. Arnold, Decay counting in the age of AMS, Nucl. Instr. and Meth. B29 (1987) 424–426. H. Oeschger, J. Houtermans, H. Loosli, M. Whalen, Comments on pages 471– 498, in: I.U. Olsen (Ed.), Proceedings of the 12th Nobel Symposium on Radiocarbon Variations and Absolute Chronometry, Uppsala, 1969, Wiley, New York, London, Sydney, 1970. See also: H.W. Wilson, Possibility of measurement of 14C by mass spectrometric techniques, in: R. Berger, H.E. Suess (Eds.), Proceedings of the Ninth International Conference on Radiocarbon Dating, Los Angeles and La Jolla, 1976, University of California Press, Berkeley, Los Angeles, London, 1979, pp. 238–245. R.W. Schnitzer, H. Aberth and M. Anbar, in: Proceedings of the 24th ASMS Annual Conference on Mass Spectrometry and Allied Topics, 1975, p. 479. See also M. Anbar, Mass Spectrometric Determination of Carbon 14, US Patent 3,885,155, May 20th, 1975. H. Gnaser, Formation of metastable N 2 and CO anions in sputtering, Phys. Rev. A56 (1997) R2518–R2521; R. Middleton, J. Klein, Production of metastable negative ions in a caesium sputter source: verification of the existence of N 2 and CO , Phys. Rev. A60 (1999) 3786–3799. R.A. Muller, Radiocarbon dating with a cyclotron, Science 196 (1977) 489–494. R.A. Muller, L.W. Alvarez, W.R. Holley, E.R. Stephenson, Quarks with unit charge: a search for anomalous hydrogen, Science 196 (1977) 521–552. L.W. Alvarez, R. Cornog, 3He in helium, Phys. Rev. 56 (1939) 379. W.H. Bennett, Tandem Patent, US 2,206,588, July 2nd, 1940. L.W. Alvarez, Energy doubling in dc accelerators, Rev. Sci. Instrum. 22 (1951) 705–706. D.E. Nelson, R.G. Korteling, W.R. Stott, Carbon-14 direct detection at natural concentrations, Science 198 (November) (1977) 507–508. C.L. Bennett, R.P. Beukens, M.R. Clover, H.E. Gove, R.B. Liebert, A.E. Litherland, K.H. Purser, W. Sondheim, Radiocarbon dating using electrostatic accelerators: negative ions provide the key, Science 198 (November) (1977) 508–510. R.A. Muller, P.P. Tans, T.S. Mast, J.J. Welch, Mass Spectrometry With a Very Small Cyclotron, vol. I, pp 342–345, see also papers leading up to K.J. Bertsche, C.A. Karadi, R.A. Muller, G.C. Paulson, Detection of radiocarbon in the cyclotrino, Nucl. Instr. and Meth. B52 (1990) 398–404 at AMS 5 and see also AMS 3 and 4. Mobai Chen et al., Minicyclotron (SMCAMS)-based accelerator mass spectrometry and real 14C measurements, Nucl. Instr. and Meth. B172 (2000) 193–200. at AMS 8. W.J. Zhou et al., Radiocarbon determinations using a minicyclotron: applications in archaeology, Nucl. Instr. and Meth. B172 (2000) 201–205. at AMS 8.
[18] K.H. Purser, An Ultra-Sensitive Mass Spectrometry Apparatus, 1977, US Patent No. 4,037,100. [19] J.A. Kuehner, E. Almqvist, D.A. Bromley, in: D.A. Bromley, E.W. Vogt (Eds.), Proceedings of the 1960 International Conference on Nuclear Structure at Kingston, Canada, University of Toronto Press, Toronto and North Holland Pub. Co., p. 280. See also Atomic Energy of Canada Limited, Progress Report of the Physics Division, PR-P-56 (1962) 2.7, Negative Ion Production Studies, A.E. Litherland. [20] K. Chapman (Ed.), Proceedings of the Symposium of North East Accelerator Personnel, at Florida State University, March 23rd and 24th, 1972, pp. 100–104. [21] W. Henning, W. Kutschera, R. Smitherand, J.L. Yntema (Eds.), Proceedings of the Second International Conference on AMS, May 11th–13th, 1981, Argonne National Laboratory, Physics Division, ANL/PHY–81-1. Available at. [22] R.P. Beukens, Radiocarbon accelerator mass spectrometry: background and contamination, Nucl. Instr. and Meth. B79 (1993) 620. [23] R.P. Beukens, H.E. Gove, A.E. Litherland, W.E. Kieser, X.-L. Zhao, The old carbon project: how old is old?, Nucl Instr. and Meth. B223–B224 (2004) 333–338. (24) H.W. Lee, A. Galindo-Uribarri, K.H. Chang, L.R. Kilius, A.E. Litherland, The CHþþ 2 molecule and radiocarbon dating by accelerator mass spectrometry, Nucl. Instr. and Meth. B5 (1984) 208–210. [25] A. Galindo-Uribarri, H.W. Lee, K.H. Chang, Beams of doubly ionized molecules from a tandem accelerator, J. Chem. Phys. 83 (1985) 3685–3693. [26] H.-A. Synal, S. Jacob, M. Suter, The PSI/ETH small radiocarbon system, Nucl. Instr. and Meth. B 172 (2000) 1–7. [27] H.-A. Synal, M. Stacker, M. Suter, MICADAS: a new compact radiocarbon AMS system, Nucl. Instr. and Meth. B 259 (2007) 7–13. [28] G. Skog, The single stage AMS machine at Lund University: status report, Nucl. Instr. and Meth. B 259 (2007) 1–6. [29] G. Doucas, E.F. Garman, H.R.M. Hyder, D. Sinclair, R.E.M. Hedges, N.R. White, Detection of 14C using a small van de Graaff accelerator, Nature 276 (1978) 253–255. [30] K.H. Purser, P.R. Handley, Ref. [1] (AMS 1), pp. 165–186, see also reference K.H. Purser, R.J. Schneider, J. McDobbs, R. Post, Ref. [21], p. 431. [31] W. Wolfli, H.A. Polach, H.H. Andersen (Eds.), Proceedings of the Third international Symposium on AMS, Nucl. Instr. and Meth. B5 (1984) 91–448. [32] G.M. Raisbeck, F. Yiou, M. Fruneau, J.M. Loiseaux, Beryllium-10 mass spectrometry with a cyclotron, Science 202 (1978) 215–217. [33] J.S. Vogel, J.R. Southon, D.E. Nelson, T.A. Brown, Performance of catalytically condensed carbon for use in accelerator mass spectrometry, Nucl. Instr. and Meth. B5 (1984) 289–293. [34] H.E. Gove, A.E. Litherland, D. Elmore (Eds.), Proceedings of the Fourth International Conference on AMS, Nucl. Instr. and Meth. B29 (1987) 1–455. [35] P. Damon et al., Nature 337 (1989) 302–304. [36] H.E. Gove, Relic, Icon or Hoax – The Dating of the Turin Shroud, Institute of Physics Publishing, Bristol and Philadelphia, 1996. [37] H.E. Gove, From Hiroshima to the Iceman: The Development and Applications of Accelerator Mass Spectrometry, Institute of Physics Publishing, Bristol and Philadelphia, 1999. [38] J.P.F. Sellschop, Ref. [27], pp. 439–445.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Accelerator Mass Spectrometry (AMS) 1977–1987 q H.E. Gove a, K.H. Purser b,c, A.E. Litherland d,* a
Department of Physics and Astronomy, University of Rochester, Bausch & Lomb Hall, P.O. Box 27017, 1500 Wilson Boulevard, Rochester 14627-0171, NY, USA Southern Cross Corporation, 49 Ledge Rd., Gloucester, MA 01930, USA c Twincreeks Technologies, #1 Industrial Park, Danvers, MA 01923, USA d Department of Physics, University of Toronto, 60 St. George St., Toronto Ontario, Canada M5S 1A7 b
a r t i c l e
i n f o
Article history: Available online 9 October 2009 Keywords: Accelerator Mass Spectrometry AMS conferences
a b s t r a c t The eleventh Accelerator Mass Spectrometry (AMS 11) Conference took place in September 2008, the Thirtieth Anniversary of the first Conference. That occurred in 1978 after discoveries with nuclear physics accelerators in 1977. Since the first Conference there have now been ten further conferences on the development and applications of what has become known as AMS. This is the accepted acronym for the use of accelerators, together with nuclear and atomic physics techniques, to enhance the performance of mass spectrometers for the detection and measurement of rare long-lived radioactive elements such as radiocarbon. This paper gives an outline of the events that led to the first conference together with a brief account of the first four conferences before the introduction of the second generation of accelerator mass spectrometers at AMS 5. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Year 2008 is the thirtieth anniversary of the first conference on ‘‘Radiocarbon Dating with Accelerators”, at the University of Rochester, USA. The first author was the chair of the group that organized the meeting. Later the University of Rochester published the proceedings [1]. Fig. 1 shows the cover page of the Proceedings that was designed by Gove. The drawings on the cover are imaginary combinations of cyclotrons and tandem accelerators, representing the fact that significant results from both types of accelerator laboratories were discussed at the first conference. These conferences have now become called Accelerator Mass Spectrometry (AMS) Conferences and this is the eleventh. In what follows we give a brief account of the initial discoveries that led to the first conference and some inevitably brief comments on the development of AMS from the first to the fourth conference. The first author was also chair of the fourth conference in 1987. Many developments after the fourth conference are not mentioned and it should not be forgotten that accurate radiocarbon dating using beta ray counting, rather than counting the radioactive atoms, has not been replaced but complemented significantly by q This paper was originally to be presented personally at AMS 11 by H.E. Gove. It was to have been on the first four AMS Conferences and a draft was written. However, the draft had to be finished electronically by the other authors after Harry Gove found that he could not continue the process. In general the original concept of the author, who died in February 2009, has been preserved. An obituary is on page N. * Corresponding author. Tel.: +1 416 978 3785; fax: +1 416 978 4711. E-mail address: [email protected] (A.E. Litherland).
0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.10.007
the powerful new mass spectrometric methods. At AMS 5 the second-generation radiocarbon dating tandem accelerator systems were introduced and smaller systems were invented later. The mass spectrometry of long-lived radioactive isotopes continues to develop vigorously. 2. The discovery of the instability of N AMS using negative atomic ions can be said to have begun 31 years ago. It was based upon some discoveries at the Nuclear Structure Laboratory of the University of Rochester in the United States and at McMaster University, in Canada. On May 18th 1977 it was found that, as hoped, the 14N negative ions were so unstable they did not interfere with the detection of 14C ions at natural abundance (14C/C 1012) and that the background to such measurements was very low. These experiments, at Rochester, were to test whether or not a small tandem could be designed for radiocarbon dating and then possibly built by the General Ionex Corporation (GIC) of which K.H. Purser was the president and founder at that time. In addition to observing 14C at natural abundances a surprisingly low background was observed at about 60,000 radiocarbon years before the present. This was in spite of a concern that machines used regularly to make nuclear reactions might contain 14 C contamination. Contemporary radiocarbon occurs at ratio of 14 C/C of about 1012 and so the discoveries had revolutionary implications for radiocarbon dating. As a result they were reported at the Second International Conference on Electrostatic Accelerator Technology on May 24th to 27th at Strasbourg, France and then reported in the New York Times on June 9th [2].
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Fig. 1. The front cover of the report on the first AMS conference proceedings.
The experiments at Rochester showed an apparent absence of N ions, an unexpectedly low ion source memory and low background of the then new high current Cs+ solid sample sputter ion source, which was very welcome news for later attempts to try radiocarbon dating using 14C ions. The low reactor grade graphite background implied the possibility that radiocarbon dating could be extended into the more distant past without the need to enrich the 14C isotope or that, with enrichment of the 14C, dating could be extended much further back in time. The low memory also implied that a sequence of samples including standards could be intercompared with an accuracy that would still have to be established. In early June 1977, 14C at a low level was also detected independently at McMaster University in Canada, although they were unable to confirm the actual numerical 14C/C ratio or the background level due to technical difficulties. However, this almost simultaneous detection of low-level 14C in two laboratories was dramatic confirmation of the long suspected instability of most of the excited states of the nitrogen negative atomic ion. The idea of atom counting instead of beta ray counting for radiocarbon dating [3] has been known since Zacharias suggested it to Libby in 1947, as was revealed at AMS 4 by Arnold, a student of Libby [4]. Since that time it has been advocated [5] a number of times and even attempted experimentally [6]. Many researchers in 1977 did not know that the negative ion idea for separating the 14C and 14N isobars had already been tried unsuccessfully [6] using the N 2 ion. Apparently a suitable ion source of C ions was not available, although the potential of that ion was realised. However, unlike the N ion, the N 2 ion is now known to have long-lived metastable states that were not discovered until 1997 [7]. With modern techniques the choice of CN must be well worth revisiting using mass spectrometry, as the ion has a much higher electron affinity than that of C. A bold idea to use a cyclotron and positive ions at high ion energy for radiocarbon dating was introduced by Muller in 1977 [8]. In this case the expected very large flux of 14N ions and the very rare 14C ions were to be separated by their differing ranges in matter. In addition, the resonant acceleration of ions by the cyclotron was expected to remove the interfering molecular isobars in this case. The suggestion was made after experiments to search for 14
integrally charged quarks had been completed [9] using a cyclotron and the approach was also based upon the earlier discoveries of Alvarez and Cornog in 1939 [10]. Some of the basic principles of AMS were laid down at that time, although the idea of carrying out the atomic isobar separation at the ion source and the complete molecular isobar destruction in a tandem accelerator were not then part of those principles. Negative ions were little known in 1939 and the tandem accelerator was not then available. However, the tandem accelerator, which exploits negative ions, had been invented by Bennett in 1935 and patented later [11]. It was re-invented independently by Alvarez again later in 1951 [12] and first operated for experiments at The High Voltage Engineering Corporation (HVEC) almost exactly 50 years ago. Later in 1977 [13,14] using modern tandems, some of the early exploratory experiments on the detection of 14C at Rochester and McMaster were also reported almost simultaneously. These results from both laboratories were discussed in some detail in early 1978 at the Conference at Rochester on ‘‘Radiocarbon Dating with Accelerators” that later became known as AMS 1 [1]. The first actual attempts at radiocarbon dating using negative ions and tandems and positive ions from a cyclotron were also reported. The discovery of the high degree of instability of the negative ion of nitrogen and the surprisingly low 14C background of tandem accelerators, however, resulted in their dominance for the first thirty years of AMS, although small negative ion cyclotrons [15], to exploit their potential higher mass resolution for molecular isobar removal, were developed after much research [16,17]. The greater suitability of the tandem as a component in mass spectrometry, and the removal of the main atomic isobar at the ion source, was decisive until recently, as discussed very briefly later.
3. The Rochester group The Rochester group was formed in April 1977 after Harry Gove had joined a discussion between Ken Purser and Ted Litherland (then a Professor of Physics at the University of Toronto). This discussion was on the degree of instability of N ions that would be needed for radiocarbon dating with a 1MV tandem. The removal of the molecular isobars, by charge changing from negative to positive ions at MeV energy, was not in question. Ken Purser had, at that time, applied for a patent on what was essentially the AMS of stable atoms in 1976 [18] using a tandem to remove the molecular ions from the mass spectrometry. At the initial meeting the question was how best to study the degree of instability of N at abundance ratios much less than 14N/12C < 1012. It was decided to try to find out just how unstable were those ions, or their excited states, using the well instrumented Rochester Tandem as a test bed, as that accelerator had a working Cs+ sputter ion source operating for some time. There was at that time no intention to use the large tandem for radiocarbon dating as that accelerator was used routinely for nuclear physics studies. The idea was to use it for finding out whether or not the instability of N was adequate for radiocarbon dating, before proposing a smaller tandem for funding. Here it is worth noting that the three of us were old acquaintances that had previously worked together and often had interacted professionally. Ted Litherland and Harry Gove had worked on cyclotrons for PhD theses at Liverpool and MIT, respectively, and then worked together at Chalk River, Canada on positive ions from electrostatic accelerators starting in 1953. They were not sorry to see the last of the early complicated cyclotrons, as the ions were difficult to inject for acceleration and hard to extract afterwards. Gove and Litherland then worked together for about ten years until 1964 at Chalk River. In the summer of 1958 they did research on the first electrostatic tandem accelerator, which had recently been re-in-
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vented by Alvarez [12]. The tandem built for the Atomic Energy of Canada at Chalk River but was then located at the factory of the High Voltage Engineering Corporation (HVEC) in Burlington, MA. Here it is worth noting that Robert J. Van de Graaff of HVEC, the inventor of the electrostatic accelerator, was enthusiastic about the tandem for several reasons, one being the possibility of bringing all the resources of the laboratory together at the ion source. For AMS this turned out to be an ideal arrangement, as was the ease with which the ions could enter and leave the tandem. Consequently this year is actually the Fiftieth Anniversary of the first tandem accelerator operation – an anniversary also worth noting. After the tandem had been installed at Chalk River, Canada, it was discovered later in 1960 [19] that the N ion could not be used in a tandem for nuclear physics studies, although the degree of instability was not measured. During the following year it was also discovered that strong C beams could be generated readily, from the charge-changing gas ion source of the tandem, solely from the memory effect of previously used CO2 gas. Harry Gove left the Chalk River Laboratories of the Atomic Energy of Canada in 1964 to found the Nuclear Structure Laboratory at Rochester University in New York State, USA. That laboratory was based upon the then largest tandem accelerator and for some time Ken Purser, a tandem expert from HVEC in Massachusetts, was also a Professor at the University. He left Rochester in 1975 after founding GIC, which made small tandems and the associated equipment for research and industry. The important Cs+ sputter ion source used at Rochester was, as you have heard earlier today from the talk by Ken Purser, developed starting in 1972 [20] by Roy Middleton and further developed at Rochester and manufactured by GIC in a productive collaboration. The Generating Voltmeter, which stabilized the tandem voltage independent of the strength of the ion beam, gave an initial brief advantage to the Rochester tandem, but was soon standard for nuclear physics and AMS. GIC, after the Rochester discovery, made the first five dedicated tandems for Radiocarbon Dating at the Universities of Arizona, Oxford, Toronto, Gif-sur-Yvette France and Nagoya Japan. Later, as US AMS, the staff from GIC made the first of the modern radiocarbon machines for the Woods Hole Laboratory. From 1974 onwards Ted Litherland was lecturing on radiocarbon dating, in a course on Physics and Archaeology at the University of Toronto. In 1976 Roelf Beukens, a young member of the Rochester group from Toronto had just completed a study of the proton capture by 14C at lower ion energies for his Ph.D. and also had recent experience at making 14C targets. One young member of the Rochester group, Charles Bennett, was also initially interested in the possibilities of using atom counting for radiocarbon dating a Stradivarius violin, only to be disappointed later by the problems of the radiocarbon calibration curve. Consequently the Rochester group was ideally poised to start the work on what later became part of AMS. The other members of the Rochester group, who are listed in Ref. [2], all contributed to the 24 h per day work and excitement of the discovery. In thinking back on the research during 1961 at Chalk River, Canada, it is now possible to say that maybe a golden opportunity to detect 14C there at natural abundance was missed, but with the ion sources then available no dating measurements would have been possible and years of research and development were necessary before the present form of the Cs+ sputter ion source was perfected. Also the degree of 14C contamination from nuclear reactions associated with the nuclear physics work at accelerators was unknown. Later during 1977 at Rochester we were also exceedingly lucky to be first to do the experiments. Our success was in part due to the reinvention of the tandem by Alvarez [12], the foresight of the Atomic Energy of Canada in accepting the proposal of HVEC to build a tandem for nuclear physics research and experience with tandem accelerators. The connection with industry (GIC) at
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Rochester was also invaluable and, of course, graduate students did most of the hard work. The timing of all the various experiments that led to the series of eleven AMS Conferences must be regarded, however, as simply chaotic, due to issues that were not understood at that time and the slow flow of information. Maybe that could not happen today as a result of the introduction of the worldwide web.
4. The first conference on AMS Year 2008 is the 30th anniversary of the first AMS Conference that was held at Rochester University in 1978. The title of the Conference was ‘‘Carbon Dating with Accelerators” and was not well chosen as early results for the important isotopes 10Be and 36Cl were also reported. After the second AMS Conference in 1981 at the Argonne National Laboratory (ANL), which was called at the time a ‘‘Symposium on Accelerator Mass Spectrometry” [21], the title became more simply an ‘‘International Conference on Accelerator Mass Spectrometry”. The first Conference is now referred to as AMS 1. The first AMS conference [1] contained papers on 14C dating from the Berkeley, Chalk River, McMaster and the Rochester Accelerator Laboratories and they demonstrated the great promise of radiocarbon dating using accelerators. However, although the Berkeley cyclotron used positive ions, tandems using negative ions soon became the accelerators of choice for AMS. The remarkable early results from Rochester, on the low level of background from reactor grade graphite, are shown in Fig. 2. The 12C and 13C peaks are associated with the destruction of molecular isobars and are easily be removed by the use of an electric analyser as they have different ion energies. The graphite background was later shown to be real [22] and corresponded to about 75,000 radiocarbon years. The origin of the small residual 14C background is still not fully understood but it later did not later prevent measurements down to a ratio of 14C/C 1018, using 100 enriched 13C in CO derived from natural gas as the source of carbon. This result was for the Borexino solar neutrino detector [23], which later was constructed and at 5 Mg was the largest scintillation counter built that could study radiocarbon. Even the shape of the beta ray spectrum could be studied and the low level of 14C level confirmed. It was the startling result from graphite that convinced the Rochester group to change the emphasis from doing basic measurements on the degree of instability of the nitrogen negative ion and the study of ions such as CHþþ 2 , to exploratory radiocarbon
Fig. 2. An early spectrum from the Rochester group showing the low intrinsic background from a graphite sample and a tandem accelerator.
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C
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dating at Rochester. Initially the experiments at Rochester were basically to see if the nitrogen ion instability was enough for small 1MV tandem accelerators to be built by GIC for radiocarbon dating. was stable enough However, it was discovered that the ion 12 CHþþ 2 to be always present and so some time was spent studying it. The 13 CH++ ion was not observed. The 12 CHþþ ions could, of course, 2 interfere with the detection of the 14C++ ion in the final detector, as they are molecular isobars. It was then discovered that, by lowering the pressure of the gas in the Rochester Tandem stripping canal, the intensity of the CHþþ 2 ions was increased dramatically in an exponential manner, which facilitated their study. Some of them were observed to decay slowly in flight, thereby creating a continuum of CH+ and C+ ions that would be removed by the mass spectrometry. The presence of the CH+ and CHþ 2 ions were expected to prevent the use of singly charge ions for 14C measurements. The outlook then became bleak for the use of a small 1MV tandem for radiocarbon dating. The exponential destruction of both the molecules CH+ and CHþ 2 with stripping gas pressure was later observed at the Toronto IsoTrace Laboratory [24,25] at 0.7 MV and this information was later cleverly exploited at the Zurich AMS laboratory in an imaginative redesign [26] of a 0.6 MV tandem for AMS using singly charged ions and higher stripping gas pressure. Later the tandem voltage was lowered to 200 kV [27] and single ended accelerators were developed [28] to replace the tandems. The latter have their ion detectors at voltage. In both cases, higher than normal stripping canal pressures are employed to destroy the singly charged molecular anions and the high-energy mass spectrometry redesigned to accommodate the extra multiple scattering. Our failure to appreciate the significance of the exponential destruction of CHþþ 2 , observed first at Rochester, meant that we abandoned too soon the idea of a 1 MV tandem in 1978 and so missed a golden opportunity to create a really small machine. In the meantime using the normal stripping canal pressure it was shown at Oxford [29] that C+++ ions could be used for radiocarbon dating using a 2.5 MV tandem voltage. Triply charged ions from a tandem have since then been used widely for AMS in general as they have very low background from the tandem. The five 3 MV tandems designed and built for this purpose by GIC were put into operation in the early 1980s [30]. The first AMS conference also included a paper from the Rochester group on the measurement of 36Cl in a sample of CCl4 used in the duoplasmatron ion source of the tandem. It was discovered, as with the use of CO2 in a gas ion source at Chalk River in 1961, that the memory effect of some ion sources could be very large. As a result we switched to a Cs+ sputter ion source for all future 36Cl measurements, using neutron irradiated NaCl instead of gaseous CCl4 for tests. Then, as in the case of carbon, a low ion source memory was observed. 36Cl ions, at a level of 1011, were also readily observed in addition to a startlingly large number of 36 S ions. In this case, as in many other AMS measurements, the advantage of using higher ion energies for mass spectrometry, first stated clearly in 1939 by Alvarez and Cornog [10] and later in 1977 re-emphasized by Muller [8] was exploited. The separation of 36Cl and 36S was demonstrated using the separation of the isobars exploiting E and dE/dx measurements just has had been predicted [8], although this proved to be difficult to do with a cyclotron [15,21]. However, the first separation of the isobars 10B and 10Be by range was demonstrated convincingly by the first 10Be measurements using the Grenoble cyclotron [32]. Those data were also presented at AMS 1 [1]. The Rochester tandem accelerator benefited from the use of a generating voltmeter for stabilizing the terminal voltage and the provision of the latest model sputter ion source from GIC. These components it should be noted were designed for nuclear physics experiments. When replacements for them at Rochester were urgently required they were rapidly forthcoming from GIC, which
demonstrates the importance of collaborative research with industry. The other papers presented at AMS 1 were on imaginative proposals [1] for future work and the implications of the discoveries. Copies of the proceedings, of AMS 1 and AMS 2 at the Argonne National Laboratory ANL in 1981, are now becoming very rare and our copies are disintegrating with a half-life of a decade or so. Fortunately, all later proceedings are now available electronically, having been published by Nuclear Instruments and Methods in Physics Research B. In 1980 the Journal of Applied Radiation and Isotopes (JARI), published by Pergamon Press, presented a substantial silver medal to the leaders of the Rochester group for their outstanding contributions to AMS and in particular for the discovery of the degree of instability of the nitrogen negative ion. Fig. 3 shows the presentation of the medal. A brief resume of the next three conferences will now be given.
5. The second AMS conference AMS 2 was held at the Argonne National Laboratory (ANL) Chicago in 1981 and the proceedings [21] contain a unique and entertaining account of the very early days of AMS, during which Alvarez and a graduate student Cornog discovered natural 3He in helium [8]. The nuclei 3He and 3H had then recently been created by the early accelerator use of deuterons bombarding deuterons and the question of which was stable and which was radioactive was being asked. 3He++ was seen and some of the advantages of using a high-energy accelerator clearly stated and demonstrated. Isobar separation by complete stripping was accompanied by a range measurement.
Fig. 3. JARI award at the third conference on Electrostatic Accelerator Technology, April 13th to 16th 1981, Oak Ridge, Tennessee, USA.
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Many new AMS measurements were reported at AMS 2 and for example the 10Be concentration in ice versus depth in the Ross Ice Shelf was obtained by the Grenoble group, again using the Grenoble cyclotron [21]. The increase at about 700 m depth is probably associated with the Maunder minimum at about 1700 AD indicating the great value of AMS measurements that were previously not possible by mass spectrometry or by decay counting. At that meeting the Rochester group presented results of 14C measured in various wood samples, provided generously by Meyer Rubin, and compared them with those from the United States Geological Survey (USGS) decay counting measurements. The ages ranged from about 200 to 4000 years and the results were in agreement. In addition a result at 40,000 years was obtained, which again demonstrated the low background of the sputter ion source combined with the use of a tandem accelerator. A table in that paper listed the radio-nuclei measured up to 1981 at Rochester. These included 10 Be, 14C, 32Si, 36Cl and 129I. It also listed the limits for the detection of each. These ranged from 3 1016 for 14C/C and for 32Si/Si the limit was approximately 7 1012. A bold presentation [15] at AMS 2 was to use a small specially designed cyclotron (cyclotrino) exploiting 20–100 keV negative ions to measure 14C, 26Al using no final particle selection. This project evolved slowly and such a device is now in operation in China [16] for radiocarbon dating. Other papers in the AMS 2 proceedings included programs at existing accelerators at Grenoble and Orsay, Chalk River, Argonne, Pennsylvania, Zurich, the University of Washington, Saclay and Rehovat. Clearly many groups were keen on doing AMS measurements with applications to various fields. These fields included hydrology, 10Be in the environment and manganese nodules, cosmogenic radioactive nuclei, meteorites and geology. 6. The third AMS conference AMS 3 was held at Zurich as a major International Conference in 1984 and this time published in the readily available literature [31]. In the entertaining conference summary by Walter Henning, then at the Argonne National Laboratory, compared the change in the number of laboratories and 14C analyses with those mentioned at AMS 2. The number of Laboratories had grown from 16 to 24 and the number of radiocarbon measurements had grown to 1000, a remarkable increase in both cases. He also compared two facilities, GSI-UNILAC and the small cyclotron or cyclotrino. These differed in size by several orders of magnitude. The work of Raisbeck and Yiou on 10Be concentrations in marine sediments was noted too. A seminal paper on the introduction of catalytic cracking to produce superior graphite for the sputter ion source was given at AMS 3. This development by Vogel et al. [33] from McMaster University was subsequently widely copied and rapidly became the dominant target preparation method for radiocarbon dating by AMS. Contributions on 14C dating of archaeological and geological samples by Mook and by Taylor on the dating of bones and of 36 Cl in limestone and palaeontological samples by Kubik et al. were also singled out as highlights, as were the 14C measurements on foraminifera in ocean cores by Broecker. Papers on the detection of rare 205Pb created by solar neutrinos from 205Tl and on the half-life of 60Fe added to the interest at this third AMS conference. 7. The fourth conference on the tenth anniversary of AMS AMS 4 was held in 1987 at Niagara-on-the Lake, Ontario [34], partly in recognition of the 10th Anniversary of AMS using negative ions and the part played by both nearby Mc Master and Rochester in the early days. Visits to McMaster, Toronto and Rochester were
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part of the festivities, as were the exploratory talks on the potentially important role of lasers in separating isobars by M. Thonnard and W. Fairbank Jr. Gove, Litherland and Elmore edited the proceedings of the conference and as it was the Tenth Anniversary of the AMS series the front cover was designed with a stylized MP tandem as most of the papers now referred to the use of tandem accelerators. 32 laboratories were now engaged in AMS compared with 24 at AMS 3. At AMS 4 Professor W. S. A. Dale of the University of Western Ontario discussed the dating of the Shroud of Turin in a scholarly after dinner talk. The talk was on ‘‘The Shroud of Turin – Relic or Icon”, published in the proceedings [34], suggested from stylistic arguments that the linen cloth could be dated between 969AD and 1169AD and was an icon like many other objects. There was in addition a paper by H. E. Gove on a workshop proposing the correct procedure to be followed if there was a request for carbon dating the cloth. Ultimately three laboratories, Arizona, Oxford and Zurich, dated the cloth [35] and found it to be from 1325 ± 33AD, which is somewhat younger than predicted, although it could, of course, be a copy of one dating earlier. A book called ‘‘Relic, Icon or Hoax, Carbon Dating the Turin Shroud” was published later [36], as well as a more general book [37] on the development of AMS entitled ‘‘From Hiroshima to the Iceman, a Story of a Scientific Breakthrough”. There were many contributed papers. Applications to archaeology and ecology by F. Wendorf, R. Taylor and P. Gillespie were also reported and one on the indications for Pleistocene Man in Sardinia by H. Kofmeijr. The ages of some artefacts were reported by D. Donahue et al. of Arizona and by K. Kobayashi at Tokyo. There were also papers on ice core research by H. Oeschger and the in situ production of Cosmogenic Nuclei by D. Lal. Rather than mentioning the many other papers, in the fields of cosmochemistry, ocean and atmospheric sciences, hydrology and geology and nuclear physics, we will just note a most comprehensive and scholarly conference summary [38] by J.F. Sellschop that set a high standard for all others to follow. It was indeed a great first 10 years of AMS! In conclusion it is worth noting that many important developments in AMS occurred in the following twenty years, such as mature radiocarbon dating machines, better ion sources and detection systems. These developments have helped to make possible many of the imaginative researches in the Earth and Planetary Sciences often reported at these conferences. Further important developments are still taking place and many will be discussed at this AMS 11 conference. Negative ion methods may not be the only approach used in future for AMS but the tandem accelerator is likely to remain an important part of a very-low-background ion-specific detection system thereby keeping the A in AMS.
Acknowledgements We wish to thank our colleagues for their electronic help and advice in writing this paper and all the members of the Rochester group who contributed to a memorable week in May 1977. Richard Hyder, who made many contributions to AMS with tandem accelerators, also reminded us that this was also the 50th Anniversary of the operation of the first HVEC tandem accelerator. At least one of the early model tandems, an EN, has seen outstanding service at the Zurich Laboratories in AMS for the past 30 years, which was also celebrated recently.
References [1] H.E. Gove (Ed.), Proceedings of the First Conference on ‘‘Radiocarbon Dating with Accelerators”, University of Rochester, April 20 and 21, 1978. This is
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[2]
[3] [4] [5]
[6]
[7]
[8] [9] [10] [11] [12] [13] [14]
[15]
[16]
[17]
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[18] K.H. Purser, An Ultra-Sensitive Mass Spectrometry Apparatus, 1977, US Patent No. 4,037,100. [19] J.A. Kuehner, E. Almqvist, D.A. Bromley, in: D.A. Bromley, E.W. Vogt (Eds.), Proceedings of the 1960 International Conference on Nuclear Structure at Kingston, Canada, University of Toronto Press, Toronto and North Holland Pub. Co., p. 280. See also Atomic Energy of Canada Limited, Progress Report of the Physics Division, PR-P-56 (1962) 2.7, Negative Ion Production Studies, A.E. Litherland. [20] K. Chapman (Ed.), Proceedings of the Symposium of North East Accelerator Personnel, at Florida State University, March 23rd and 24th, 1972, pp. 100–104. [21] W. Henning, W. Kutschera, R. Smitherand, J.L. Yntema (Eds.), Proceedings of the Second International Conference on AMS, May 11th–13th, 1981, Argonne National Laboratory, Physics Division, ANL/PHY–81-1. Available at