VERA is a new centre for accelerator mass spectrometry (AMS) at the University of Vienna, operated by the Institute for Radium Research and Nuclear Physics. In 1992, the creation of such an interdisciplinary research facility was recommended by the Committee for Restructuring Physics in Austria (gesamt-österreichische Strukturkommission für Physik), and financed by the Austrian Federal Ministry of Science and Research.
The VERA facility, which is based on a 3-million-volt Pelletron tandem accelerator, is located close to the Physics Department of the University of Vienna in the so-called "Kavalierstrakt" at Währingerstrasse 17, A-1090 Wien. The building is a small two-storied Palais from the last century under landmark protection. The interior of the Kavalierstrakt was completely reconstructed to generate a modern laboratory for AMS, but at the same time efforts were made of preserving as much as possible of the original features of the building. Fig. 3 shows a floor plan of the Kavalierstrakt. The total space available (~600 m2) was divided into 4 areas of roughly equal proportions: accelerator, shops, sample preparation, and offices including a seminar room. The S-shaped accelerator room was formed by removing the walls between three adjacent rooms. An additional beamline system (probably for heavy isotope detection) is planned for the future. A relatively large fraction of the space available in the building was reserved for sample preparation, based on the experience from other AMS laboratories that this can easily become a bottleneck for an efficient sample through-put. The reconstruction of the building and the refurbishing of the rooms were completed in 1995.
The VERA AMS system was ordered in 1994 from National Electrostatics Corporation in Wisconsin USA, with a total cost of approximately two million Dollars. The system arrived in Vienna in the fall of 1995, and met specifications during test operations in March 1996. It is currently being brought into fully computerised operation for the measurement of 14C and other long-lived radionuclides.
The name of the facility, Vienna Environmental Research Accelerator, indicates its primary mission. The AMS method is well suited for this task by being capable of tracing long-lived natural and artificial radionuclides in the atmosphere, biosphere (living matter), hydrosphere (rain, rivers, lakes, groundwater, oceans), cryosphere (polar ice sheets, glaciers), lithosphere (solid part of earth's surface), and technosphere (man-made things). All of these domains have some influence on our environment, e.g. the climate (Graedel und Crutzen, 1994).
Long-lived radionuclides are useful for environmental research since many of them are produced by cosmic-ray interaction in the atmosphere (Lal and Peters, 1967), thus becoming tracers of the various pathways interconnecting the domains mentioned above. In most cases, however, they cannot be measured at natural levels through radioactive decay counting, particularly for small samples in the milligram range which typically contain only a relatively small number of radionuclide atoms (105 to 108). There are simply too few decays in any reasonable measuring time. With AMS, the radionuclides are measured by direct atom counting rather than by observing their infrequent decays. This makes it possible to measure important radionuclides such as 10Be (t½= 1.6x106 a), 14C (5730 a), 26Al (7.2x105 a), 36Cl (3.01x105 a), 41Ca (1.04x105 a), 129I (1.6x107 a), in many terrestrial and extraterrestrial materials (Elmore and Phillips, 1987; Kutschera, 1993). Quantitative results are obtained by measuring the extremely small ratio of the rare radioisotope to a stable isotope, which lie in the range of 10-10 to 10-16.
Artificial radionuclides can also be useful as tracers in the environment, particularly in those cases where the input conditions (amount, location and time) are reasonably well known. An example is the proposed use of 129I, released in nuclear fuel reprocessing, as an oceanographic tracer (Yiou et al., 1994). The concentration from artificial radionuclides can be well above natural levels, allowing one to study processes in the environment over a large dynamic range.
The heart of the VERA facility is the AMS system built by National Electrostatics Corporation in Wisconsin, USA. Fig. 4 displays the system, with short descriptions of its main components.
Samples to be investigated for their radionuclide content (typically a few milligrams of material) are loaded in a caesium-beam sputter source which can hold up to 40 samples. For 14C measurements the sample material is solid carbon mixed with some catalyst used in the graphitisation process (see section 3.4.3). Negative ions of carbon, C-, are extracted from the source, preaccelerated to about 70 keV. and selected according to their energy with an electrostatic analyser. They then pass through a high-resolution injector magnet for a first mass analysis. The magnet vacuum chamber is electrically insulated and can be biased in short sequence (milliseconds) with voltages in such a way as to give the different carbon isotopes, 12C-, 13C- and 14C-, energies which result in the same magnetic rigidity. This means that all three carbon isotopes can be injected in fast sequence through the accelerator system, which adds to the overall precision of isotope ratio measurements by smoothing out small but finite instabilities of the system. Movable offset Faraday cups monitor the ion current of the stable isotopes 12C- and 13C- at the exit of the injector magnet, whenever the radionuclide 14C- is sent into the accelerator.
The negative ions are accelerated to the terminal of the tandem accelerator which can be operated at a positive voltage of up to 3 million volts (MV). At the terminal, the ions pass through a gas canal or a thin foil, where several electrons are stripped off. The ions therefore change their polarity from negative to positive, and are thus accelerated again by the positive terminal voltage (tandem principle). An important function of the stripper is the dissociation of 12CH2- and 13CH- molecular ions, which are injected into the accelerator together with the 14C-, but with billion times higher intensities than 14C. Fortunately, nitrogen cannot form stable negative ions, therefore the otherwise overwhelming background from 14N (78% of the atmosphere is nitrogen) is virtually absent. The destruction of hydrocarbon molecules can be accomplished by stripping off a sufficient number of electrons to break the molecular bond. It has been recognised early in the development of AMS, that the selection of 14C3+ ions at the high energy end guarantees the molecular dissociation. For a high yield of the 3+ charge state fraction in the stripping process, a terminal voltage between 2 and 3 MV is needed. At VERA, the typical terminal voltage for 14C measurements is 2.7 MV.
The high-energy analysing magnet is set to select 14C3+ ions on its central orbit, and removes most of the molecular break-up products. The stable carbon isotopes 12C3+ and 13C3+ are more strongly bent and are monitored in offset Faraday cups through ion current measurements. The 14C3+ ions are transported further on and are cleaned from residual molecular fragments by a Wien filter which selects ions of a particular velocity. After this final purification, the 14C3+ ions are stopped in a Si detector where the total energy and the counting rate is measured. 14C/12C and 14C/13C ratios are measured by comparing the 14C counting rate with the ion currents for 12C3+ and 13C3+ ions measured in the offset Faraday cups at the exit of the high-energy magnet. For a reliable determination of 14C/12C ratios (typically in the range from 10-12 to 10-14), these measurements are performed relative to calibration samples of precisely known 14C/12C ratios, loaded in the ion source together with the unknown samples. The entire operation of the AMS facility is computer controlled, and it is anticipated that 14C/12C ratio measurements with a precision of 0.5% can be performed on a "routine" basis. In special cases, given enough time and material for multiple sample measurements, a higher precision my be achievable.
The principle of AMS measurements described above for 14C can be applied for many radionuclides. However, a very important feature for a sensitive detection is the instability of negative ions of certain stable isobars. As mentioned above, 14N does not form stable negative ions and therefore does not interfere with a 14C detection. The relative mass difference between 14N and 14C would be far too small (10-5) to allow for a mass separation in the analysing magnets. This is true for any radionuclide-stable isobar pair, which therefore both arrive at the final detector system. Without the negative ion discrimination, the stable isobar interference can be very severe. Fortunately, the detection of two other important radionuclides, 26Al and 129I, is also greatly alleviated by the non-existence of 26Mg- and 129Xe- ions. For 10Be, which is of interest for VERA but is accompanied by a strong isobaric background of stable 10B, other means of separation (Raisbeck et al., 1987) will be used. It is envisioned that one of the major technical development programs at VERA will be the investigation of new techniques to solve the isobar separation problem for hitherto little used radionuclides.
The bending power of the magnetic elements of VERA (see Fig. 4) were chosen such as to transport even the heaviest long-lived radionuclide of interest (244Pu5+ ions). Recently, it has been shown (Fifield et al., 1996) that actinides can well be measured with AMS at energies available at VERA using a suitable detector system for these heavy ions.
Although the primary mission of VERA is environmental research, the AMS technique offers opportunities in other fields such as biomedical research (Vogel and Turteltaub, 1994), which may also be pursued. In general, the research program will be determined by the truly interdisciplinary character of AMS. Among other things, the specific directions being pursued will strongly depend on interactions with people from other fields who are interested to apply this unique tool for their own research.
In order to determine a correct radiocarbon age, it is essential to know the isotope ratio of the stable carbon isotopes, 13C/12C, for the material to be dated. This measurement provides the necessary information to correct for mass fractionation effects of 14C with respect to 12C due to mass-dependent processes in the various pathways of carbon. The mass fractionation is measured through the d13C value, which is defined as
d 13C = [13C/12C)sample - (13C/12C)PDB standard ] / [(13C/12C)PDB standard]
and is usually expressed in per mil (‰).
For dating purposes, mass fraction means that materials of the same age and feeding from the same 14C reservoir, may still lead to differences in measured age if not corrected for differences in mass fractionation. For example, C3 plants (95% of all plants on earth) acquire d13C values between -31 and -24‰ , whereas C4 plants (e.g. sugar cane, maize) have d13C values between -15 and -10‰ (Schmidt et al., 1993). In general, d13C values vary between 0 and -30‰, which translates into variations twice as large for the 14C/12C ratio. The latter variation corresponds to an age range of approximately 500 years (Stuiver and Polach, 1977).
In addition of being a necessary correction to determine an accurate radiocarbon age, d13C can help to identify the origin of the material to be dated. In general, high-precision stable isotope ratio measurements are widely used as fingerprints for environmental processes and can be very useful to solve a complex dating problem. For example, besides d13C, d15N measurements help to identify bone materials suitable for radiocarbon dating since bone collagen contains nitrogen in the form of a variety of amino acids.
From the above it is quite evident that for high-quality 14C dating one needs the capability for high-precision stable isotope ratio measurements. The precision needed for these measurements (in the range of 0.01‰) can best be met with commercially available stable isotope mass spectrometers. Since VERA does not have such a mass spectrometer, we propose to buy one which is most suitable in conjunction with AMS measurements.
Mass spectrometers for stable isotopes are used for many applications in research and industry, and therefore various commercially available systems exist. In our effort to find a system most suitable in connection with AMS, we consulted several AMS laboratories (at the Universities of Arizona, Groningen, Kiel, Oxford) which have extensive experience with such equipment. From these discussions and from studying systems available on the market, the following guiding principles for the purchase of a new system evolved:
The basic mass spectrometer must be capable of performing high-precision stable isotope ratio measurements to determine d13C for correcting mass fractionation effects for 14C dating and for identifying the origin of the sample material. The possibility to perform stable isotope ratio measurements for the determination of d15N, d18O, and d34S should also exist.
Peripherals of the mass spectrometer must allow one to perform semi-automated preparation of multiple samples for both stable isotope and AMS measurements.
The system must be capable of handling original sample material in both solid or gaseous form.
The possibility for integrating additional features in the future (in a modular form) should exist.
Based on these conditions we have chosen a Stable Isotope Ratio Mass Spectrometer with peripherals shown schematically in Fig. 5. As the major peripherals we have chosen a Dual Microinlet for analysing individual CO2 samples with respect to a reference standard material for highest-precision measurements, and a C, N-Elemental Analyser for automatic combustion of material to CO2 from up to 50 samples. The CO2 passes through a Sample Splitter. The bulk of the CO2 will be transferred to quartz tubes of a cryo-trapping system (to be built in-house). This CO2 will be used for subsequent graphitisation (see section 3.4.3.) and 14C analysis with VERA. A small portion of the CO2 is transferred to the mass spectrometer via a Continuous-Flow Interface. For comparison with standards, a Dual Reference Gas Injector (CO2, N2) is also connected to the Continuous-Flow Interface.
We envision that such a system will very well meet the requirement for high-precision and high-throughput measurements of both d13C determinations with the Stable Isotope Mass Spectrometer and 14C/12C ratio measurements with VERA. Thus, it will have a large impact on the quality of 14C dates produced at the VERA lab.
The main advantage of AMS is the small amount of carbon (~1 mg) needed for a 14C age determination. This is thousand times less than the amount used for conventional beta counting. Correspondingly, much less original sample material is required for AMS. However, handling small amounts of sample material increases the danger of contamination. Since already minute additions of non-genuine carbon can lead to wrong results, the entire sample preparation has to be performed with utmost care, which is time consuming.
Sample preparation proceeds in general in three steps: chemical treatment, combustion to CO2, and graphitisation to elementary carbon.
The chemical treatment depends on the kind of material from which one is trying to extract the carbon. It also depends on the condition of the original sample material (e.g. state of preservation). In addition, a number of preparatory steps have to be taken to clean chemicals and equipment used for preparation, which are not described here in detail.
The most common pre-treatment procedure to remove contaminants from the sample is the so-called ABA (acid-base-acid) method. After mechanical cleaning, most terrestrial samples (charcoal, textiles, etc.) are treated with dilute HCl to remove non-contemporaneous carbonates, which may be added to the sample as a consequence of groundwater percolation. Humic acids - the degradation products of plant material - are very mobile and can also be incorporated into the sample. They are extracted from the sample with dilute NaOH. In the final step another acid wash of the sample is necessary to ensure that CO2 possibly added during the alkali treatment is removed (see e.g. Bonani et al., 1994). For sufficiently large charcoal samples, identification of the wood species will be attempted by an experienced botanist.
The outer layer of carbonate materials (e.g. shells) may contain secondary carbonates. After a mechanical cleaning , the secondary carbonates are removed by etching the sample with dilute HCl which leads to a loss of sample material of up to 50%.
A large number of
samples to be dated in this research project will be bone samples.
A review of the current
pre-treatment strategies of bone for radiocarbon dating by AMS is given
by Hedges
and Van Klinken, 1992 and will be shortly summarised here. Dating of
bone is of great importance, because bones may be found very often in sites
of archaeological interest and may also give valuable archaeological information
(e.g. animal species, cut marks etc.). Bones occur frequently in relatively
large pieces and are then less mobile than charcoal. The difficulty in
the chemical treatment, especially of diagenetically changed bone samples,
lies in the extraction of compounds containing indigenous carbon only.
They should be available in a sufficient amount, and should not contain
any exogenous carbon. Carbonate fractions from bone cannot be used for
dating, because an exchange with the environment is possible after death.
For bones in a good preservation state it is customary to separate the
bone collagen, and convert it into gelatine as a further purification step.
In principle, even more elaborate procedures have been considered by some
laboratories (e.g. Oxford) to ensure the use of collagen genuine to bone.
For the characterisation of bone samples (state of preservation, degree
of contamination) different chemical analyses can be performed: The total
collagen content is determined via the N content of the sample and low
values will indicate poor preservation. C/N ratio values much larger than
4 are measured for samples containing large amounts of exogenous carbon
(e.g. humics) or for samples with a high degree of diagenesis of the collagen.
Infrared spectra of the collagen are helpful for the identification of
a few percent of contaminations with preservatives and humics. Anomalous
d13C
values indicate gross contamination, and changes of the d13C
values can be used to screen the removal of the contaminant by the chosen
chemical treatment steps. The amino acid composition of the collagen can
also be used for determining the preservation state of a sample.
For most bones samples - up to a loss of 95% of their initial protein - gelatine extraction yields reliable radiocarbon dates. If a bone sample is identified as critical the preparation methods have to be more elaborate. It is even possible to separate amino acids specific to the proteins in bone collagen (e.g. hydroxyproline) by high performance liquid chromatography (HPLC), and to use only carbon from these aminoacids for 14C dating. However, every additional preparatory step increases the possibility of contamination and isotope fractionation, thus complicating the determination of systematic errors. After discussions with colleagues experienced in bone dating, it was decided that the very elaborate procedure of separating and dating specific amino acids in bone collagen will not be implemented at this point.
As an additional information on the condition of the bone material to be dated, microscopic inspections of thin sections will be performed. These sections will be taken from the immediate vicinity of the samples used for 14C measurements. The thin section method is increasingly used by anthropologists to assess stages of bone decomposition (Grupe and Dreses-Werringloer, 1993; Grupe, G., Dreses-Werringloer U., and Parsche F., 1993)
A semi-automatic collagen extraction system has been developed by the Oxford group (Law and Hedges, 1989) enabling the extraction and purification of bone collagen to the gelatinisation step in an automatic way. A similar system is requested in this proposal, because it provides a consistent sample treatment and lies on line with the intend to process larger amounts of samples.
At present, the suitably cleaned organic sample material is transferred into quartz tubes together with pre-treated CuO (as an oxidizing agent) and some silver wire, evacuated and sealed by fusing the quartz tube. Sample tubes are then heated for two hours to 900 oC, which converts organic carbon completely to CO2.
In future, when the elemental analyser combined with a stable isotope ratio mass spectrometer (EA-IRMS) is available (section 3.3.1), the combustion step of the samples will be performed in an automated fashion. Liquid and solid samples will be introduced into ultra pure tin capsules and up to 50 individual samples may be loaded on the autosampler of the elemental analyser. Samples are sequentially dropped into the combustion furnace and at the same time oxygen is injected into the He carrier gas. Complete sample combustion takes place at 1800°C. Oxides of nitrogen are reduced to N2 in the reduction furnace, water is removed chemically prior to the separation of N2 and CO2 by a small gas chromatography-column. After passing the thermal conductivity detector for quantitative C and N measurements, CO2 will be delivered for both 14C and d13C measurements.
Pyrolysis of carbonates to CO2 would also include the combustion of non-carbonate impurities (Hedges et al., 1992). Therefore the EA-IRMS system, which is used for samples with high organic content, cannot be used for carbonates. In this case the CO2 samples have to be prepared manually.
Precleaned (etched) carbonate samples are dissolved in 85% or 100% H3PO4. The evolved CO2 is collected and split into two parts. The bulk will be used for the determination of the radiocarbon age and cryogenically transferred to the graphitisation line. A small amount of the CO2 gas will be will be injected into the mass spectrometer via the dual Microinlet system for the d13C measurement (see Fig. 5).
For the AMS measurement with VERA the sample material has to be solid carbon, which is loaded into the Cs-beam sputter source for the production of the C-beam. Therefore it is necessary to convert CO2 to elementary carbon. The CO2 produced in the combustion step is cryogenically transferred to a reactor vessel, where it is reduced to elementary carbon with H2 at 580 oC using Co as catalyst. This step takes about 6 to 7 hours. The resulting mixture of C and Co is pressed into 1-mm holes of Al sample holders. Up to 40 sample holders can be loaded into the sputter source. (Vogel et al., 1994)
Currently, a graphitisation system with two reactor vessels is in use. For the proposed project, a dedicated graphitisation system with eight reactor vessels is requested, in order to obtain the required sample throughput.
In order to match the sample preparation capacity with the potentially large sample throughput of VERA for 14C measurements, the following measures are requested:
full-time dedicated person (chemist) for sample preparation. This person will be trained to perform all steps necessary for extracting carbon from the original sample material and to prepare it for the AMS measurements of 14C with VERA.
dedicated graphitisation line with eight reactor vessels for parallel operation to increase the throughput.
semi-automatic collagen extraction system (Law and Hedges, 1989) providing a consistent sample treatment. This lies on line with the intend to process larger amounts of samples.
dedicated stable
isotope ratio mass spectrometer with suitable peripherals. This system
(section 3.3.1) provides utmost versatility for 14C dating in
combination with
the VERA AMS system.