- Email: [email protected]
New concepts for radiocarbon detection systems
Nuclear Instruments and Methods in Physics Research B 161±163 (2000) 29±36
www.elsevier.nl/locate/nimb
New concepts for radiocarbon detection systems H.-A. Synal *, S. Jacob, M. Suter Paul Scherrer Institute, c/o Inst. fur Teilchenphysik, F-13, ETH H onggerberg, Building HPK, CH-8093 Z urich, Switzerland
Abstract A new method to dissociate molecules in low charge states (1 and 2 ) makes it possible to develop small and compact radiocarbon detection systems. The concept will have strong impact on radiocarbon dating as well as on biomedical applications. The status of projects and experiments in connection with molecular destruction methods is summarized. The requirements for dedicated, very small radiocarbon dating facilities are discussed. Our now operational, compact 500 kV radiocarbon AMS system is described in detail. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 07.75; 41.85 Keywords: Accelerator mass spectrometry; Radiocarbon dating; Molecular destruction
1. Introduction Accelerator mass spectrometry (AMS) is now established as a versatile tool for the detection of long-lived radionuclides at natural isotopic concentrations, such as 10 Be, 14 C, 26 Al, 36 Cl, 41 Ca, 53 Mn, 59 Ni, 60 Fe and 129 I. Among these radionuclides, radiocarbon plays the primary role due to its geochemical behavior and its special role in the biosphere. In particular, radiocarbon dating has become an essential technique in archaeology and in many ®elds of environmental research. The extensive range of radiocarbon applications has continually increased the worldwide demand for radiocarbon analyses and today several 10 000
*
Corresponding author. Tel.: +41-1-633-2027; fax: +41-1633-1067. E-mail address: [email protected] (H.-A. Synal).
samples are analyzed per year. Radiocarbon AMS systems, which are able to provide high precision measurements, are commercially available [1,2], and several so-called dedicated AMS systems have been set up during the past few years. However, these systems are based on relatively large tandem accelerators (2.5 and 5 MV, respectively) for which the investment and operating costs are high. In addition, the complexity of these systems require a sophisticated environment, which, in general, cannot be provided directly by the user community. This is one of the main reasons, why dedicated radiocarbon AMS centers have established in the past. Many of them oer analyses on a commercial basis to meet the existing demand. Nevertheless, the analysis costs are high, and for many projects limited funds will impede a more extensive use of the AMS technique. This is especially important for biomedical applications. The great potential of using the AMS in connection
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 8 8 1 - 2
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H.-A. Synal et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 29±36
with radiocarbon tracer studies has been demonstrated, and it has been shown that AMS provides adequate sensitivity and precision. But systematic investigations require large numbers of samples, ecient sample preparation and short turnover times. And here lie the limitations of the available AMS facilities. Therefore, it would be highly desirable to have analytic instrumentation suitable to be operated directly by the users and under operating condition of a standard laboratory or even in clinical environment. 2. New concepts of radiocarbon detection systems The unique sensitivity of AMS is possible because molecules interfering in the analysis are destroyed during the stripping process in the tandem accelerator and the resulting molecular fragments can be removed eciently from the selected radionuclide beam. To charge exchange negative ions in a tandem accelerator, thin carbon foils (3 lg/ cm2 ) or dierentially pumped gas cells (i.e. Ar gas at 1 lg/cm2 ) are used. The dissociation of molecular ions is essential to achieve unambiguous identi®cation of the radionuclide of interest. It has been known for many years and it has been a golden rule of AMS that small molecules in charge state of 3 or higher are unstable against the Coulomb repulsion force. Therefore, present day AMS systems are based on large accelerators of at least 2.5 MV terminal voltage in order to have sucient stripping yields for the required charge states of 3 and higher. Recent interest in the use of AMS in biomedical research and the increasing demand of radiocarbon dating analyses have initiated experimental investigations into the destruction of molecular ions in charge state 1 and 2 . Dissociation of 12 CH2 2 molecules has been investigated at 1 MeV beam energy in single and double stripper foils [3]. The results of this study proved that foil stripping can provide the required detection sensitivity for biomedical AMS. Interfering molecules can also be destroyed in collisions with a stripper gas. A certain probability of breaking up molecular ions exist, even if only a single electron is knocked o in a charge changing collision. Initial experiments on
the destruction of CH2 and CH2 2 molecules have been performed by the AMS group in Toronto [4,5]. Experiments at the AMS facility of the Paul Scherrer Institute and the ETH (PSI/ETH) [6] with 1 and 2 molecular ions have shown that the molecular component at mass 14 can be suppressed by as much as 11 orders of magnitude. This is sucient to reach background levels suitable for radiocarbon dating. Fig. 1 shows the decrease of the counting rate of 1 mass 14 ions as a function of stripper thickness. At higher densities an exponential decrease of the counting rate is observed. In this region of gas density, the crosssection of the molecular break-up reaction can be computed directly from the slope of the curve. The cross-section for the dissociation of 1 molecules
Fig. 1. Counting rate of 13 CH ions as a function of stripper gas density at 510 keV beam energy. Using an additional stripper foil at the image point of the HE-magnet, the remaining molecules were dissociated and molecular fragments of a selected electric rigidity were measured with a silicon solid-state detector.
H.-A. Synal et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 29±36
of mass 14 at 510 keV beam energy in Ar gas is about 8 10ÿ16 cm2 . 3. Systems for biomedical applications Triggered by the commercial prospective of biomedical applications, two companies, High Voltage Engineering (HVEE, [1]) and Newton Scienti®c [7] have started to develop dedicated AMS systems for biomedical research. Both systems are designed to use ions in charge state 2 . Molecular destruction is achieved by stripping negatively charged ions at energies of about 1 MeV in carbon stripping foil. The HVEE system has a very compact design [8]. Floorspace of only 2:25 m 1:25 m is required for the installation. The compact design is possible, because a folded tandem accelerator is used. At the high voltage platform an electrostatic de¯ector bends the ions by 180°. In a stripping foil in front of the de¯ector the negative ions charge exchange whereby molecules are destroyed. On the high energy end, a magnetic spectrometer is used to eliminate the molecular fragments. Final particle identi®cation is made with a gas ionization detector. First results of the system, presented at the 15th International Conference on Application of Accelerators in Research and Industry, Denton, Texas, 1998, show that 14 C can be identi®ed and isotopic ratios of about 2.5 pMC can be measured. The overall accuracy of the system is quoted to be in the range of 2% [9]. The 14 C system of Newton Scienti®c is based on a linear tandem accelerator of 1 MV terminal voltage [10]. A magnetic spectrometer at the low energy side and a combination of an electrostatic de¯ector and a magnetic spectrometer on the high energy side are used to mass analyze the ion beam. Similar to the HVEE system, a foil stripper at the high voltage terminal is used to destroy the molecular beam component. The system is not yet fully operational. As a special feature of the system a gas chromatograph is connected to the ion source and the sample material is fed as gas directly into the sputter ion source. This should simplify the sample preparation procedure signi®cantly. However, uncertainties associated with
31
cross talk between dierent samples and with contamination of the system from samples of high 14 C concentrations have to be investigated carefully. A research group at the Shanghai Institute of Nuclear research has developed a small cyclotron (diameter 1.5 m) for the detection of radiocarbon [11]. In the early years of AMS, measurements using a cyclotron were made by Muller [12] in 1977. In contrast to tandem AMS, where molecular interference are destroyed during the stripping process, a cyclotron based system uses the high mass resolution of the cyclotron itself to resolve interference from molecular ions of mass 14. It was demonstrated that an overall mass resolution of 3600 is sucient to separate 14 C from 13 CH molecules [13]. For contemporary 14 C/12 C ratios and for blank samples counting rates of 5 and 0.2 cps, respectively, were measured resulting in an abundance sensitivity of about 10ÿ14 . Besides unambiguous 14 C detection, the normalization of 14 C events to the 12 C current extracted from the ion source is dicult with a cyclotron system. To switch between ion beams of dierent isotopes, the cyclotron has to be re-tuned. This will limit the accuracy of the device. At present, an overall accuracy of 1±2% is quoted by the authors [13]. This will enable biomedical applications, but it is not sucient to perform high precision radiocarbon dating. 4. A system for radiocarbon dating At PSI/ETH the feasibility of high precision radiocarbon dating has been investigated [14]. In collaboration with National Electrostatics Corporation (NEC, [2]) an instrument was developed to demonstrate the potential of radiocarbon dating at low energies and to study the relevant processes of molecular destruction [15]. The small and compact AMS system is based on a 500 kV Pelletron accelerator. It operates with ions in charge state 1 . To charge-exchange the injected negative ions, argon gas is used as stripper medium. The gas is re-circulated with two 150 l/s turbo molecular pumps. Negative ions are produced with a commercial MC SNICS cesium sputter ion
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source [16]. The ion beam extracted from the source is mass analyzed in a double focusing spectrometer magnet. A fast beam switching system is used to inject the three isotopes of carbon into the accelerator in a fast sequence (12 C:20 ls; 13 C:200 ls; 12.5 Hz repetition rate) [17]. One oaxis Faraday cup is used to measure the 13 C, 12 CH and 12 C currents at the low energy end of the accelerator. During injection of mass 14, the Faraday cup measures the current of the mass 13 beam (13 C and 12 CH), while during the pulsed injection of mass 13 ions, the 12 C beam current is detected. A terminal voltage of maximal 500 kV is generated with the Pelletron accelerator. Ions emerging from the accelerator are mass-analyzed by a combination of a magnetic and an electrostatic sector ®eld. Both devices have stigmatic focusing properties and have the same bending radius (75 cm). The combination of the two elements forms an achromatic beam transport system. The analyzing magnet directly follows the accelerator and the center of the stripper tube is imaged without an additional lens. In the focal plane of the magnet, Faraday cups for the detection of the stable isotope beams are placed. Additional to the 12 C and
the 13 C cups, a Faraday cup is placed to measure C ions, which are molecular fragments of the injected 13 CH molecules. This current is used to monitor the molecular component (13 CHÿ ) of the ion beam from the ion source at mass 14. Finally, the mass and energy ®ltered ions are detected with a silicon solid-state detector. Fig. 2 shows a photograph of the system, whose overall dimensions are 4 6 m2 . The system has been designed as an experimental platform to evaluate the potential of the new principle of molecular destruction, to investigate the relevant processes of charge changing and molecular dissociation and to demonstrate the possibility of radiocarbon dating at low energies. 13
4.1. Performance The system is operational since July 1998 and the ®rst measurements were made to demonstrate the potential for radiocarbon dating. To study the molecular suppression capability of the system, samples of standard reference material and processed blanks were measured at 970 keV beam energy. At a gas density of about 2 lg/cm2 of argon, the molecular component at mass 14 is
Fig. 2. The PSI/ETH compact AMS system for radiocarbon dating. The system is based on a 500 kV Pelletron accelerator. At the high energy side, an achromatic mass spectrometer performs separation of the 14 C ions.
H.-A. Synal et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 29±36
reduced by more than 10 orders of magnitude. Processed blank samples having molecular 13 CH beams of less than 10ÿ4 compared to the extracted negative 12 C current gave background values of 2± 8 ´ 10ÿ15 . Similar isotopic ratios were measured for the same blank material at the large PSI/ETH radiocarbon system [18], where ions of 26 MeV in charge state 4 are used. Fig. 3(a) shows the spectra obtained for the dierent samples. From the oxalic acid standard a clear peak of 14 C is observed. Except for individual events at the very low energies, the spectrum is free of background events. Events from the blank sample cluster in the region of the 14 C peak. By integrating over the 14 C
Fig. 3. Energy spectra of standard and blank samples measured using charge state 1 (a), and 2 (b), at 460 kV terminal voltage. Both measurements were taken at a stripper density of 2 lg/ cm2 . The 14 C events detected from the blank sample correspond to an isotopic ratio of 2 ´ 10ÿ15 . 14 C2 detection is heavily impeded by molecular fragments of 7 Liÿ 2 molecules. At a ®nal beam energy of 1430 keV the counting rate of two 7 Li ions reaching the detector simultaneously is ten times higher than the 14 C counting rate from a modern sample.
33
peak area an isotopic ratio of the blank sample of 2:4 10ÿ15 can be calculated. This corresponds to a radiocarbon age of about 48 000 yr. Measurements have also been made with ions in charge state 2 . At a terminal voltage of 460 kV the resulting beam energy is 1430 keV. The detection of 14 C2 ions is interfered by fragments of the 7 Liÿ 2 molecule, which is injected into the accelerator simultaneously with 14 C. During the charge exchange process these molecules dissociate, but the molecular fragments 7 Li have the same magnetic and electric rigidities as the 14 C2 ions and cannot be rejected by the mass spectrometer. If both molecular fragments leave the stripper in charge state 1 , they can reach the ®nal detector simultaneously. These twin events cannot be resolved by a total energy measurement in the detector alone. 7 Li molecular fragments, for which only one part is in charge state 1 , generate events at half of the energy of the 14 C events. The ratio of single to double 7 Li events is constant at a given beam energy and depends on the charge state distribution and ion beam transmission. This makes a background correction for 7 Li possible. Fig. 3(b) shows the spectra of the same standard and blank samples as in Fig. 3(a), but now measured using ions in charge state 2 . The spectra for both, standard and blank were taken at the same accumulated charge and were normalized to the rate of single 7 Li events. The dierence of both spectra gives the contribution of the real 14 C events. At a beam energy of 510 keV, which is reached at the stripper channel, the 1 charge state is most abundant and the lithium background is approximately ten times higher than the 14 C counting rate of a modern sample. At higher energies the situation improves, but the lithium background is a serious limitation for radiocarbon dating, if 14 C detection is performed with 2 ions. 4.2. Precision
14
C measurements
Radiocarbon dating requires high precision C/12 C isotope ratio measurements. State of the art AMS systems are able to provide data with an overall accuracy of 0.2±0.5%. To be competitive with existing high precision systems, the new PSI/ ETH system should reach this range. In order to 14
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test the performance measurements of oxalic acid standard material have been made. Seven CO2 samples have been reduced to graphite using a standard preparation technique. Each graphite sample (2 mg) was subdivided into two fractions (A/B). The fractions were pressed into individual cathodes suitable for AMS analysis. The analyses were performed with average 12 Cÿ currents of 20 lA extracted from the ion source. Each sample was measured in three individual measurement runs, which were subdivided into 10 cycles of 20 s of measurement time. The overall uncertainty of each sample due to counting alone, was of the order of 0.5%. Averaging all results, the mean 14 C/ 12 C ratio has a standard deviation of 0.5% and an error of 0.13% for the mean value. This compares nicely with the internal error computed from counting statistics alone (0.13% and 0.5% overall statistical error and statistical error of the three individual runs of one cathode, respectively). The standard deviation of the 13 C/12 C measurements is about 1&. The average values of the two dierent fractions A and B agree in both the 13 C/12 C and 14 C/12 C isotopic ratios. These results show that carbon isotopic ratio measurements at the 2±3& level can be performed with this compact AMS system. Moreover, the system may have the potential for even better performance, but this has to be demonstrated in routine radiocarbon dating measurements. 4.3. Optimum operation conditions High precision measurements of isotopic ratios C/12 C and 13 C/12 C require the same optimal beam transport through the whole system for all three carbon isotopes. The most critical part is the injection into the tandem accelerator. Using the fast beam sequencing technique, the ion beams of the stable carbon isotopes 13 C and 12 C are injected into the accelerator in very short beam pulses. To avoid isotope speci®c beam losses, all three beams have to be matched as close as possible. Another important constraint is the necessity for the highest possible beam transmission, which is determined by the yield of the selected charge state and by the beam losses caused by an increasing phase space due to angular straggling. To
investigate the optimal operating conditions, yields of the dierent charge states have been measured for 12 C in the energy range of 200±600 keV for dierent stripper densities. The measurements were performed with the stripper of the acceleration system. The ion beam of a certain charge state coming out of the accelerator was measured at dierent stripper densities in a Faraday cup after the magnet following the accelerator. The stripper density was calculated from the pressure measured inside the stripper housing, the gas feeding line of the stripper tube and from the conductance of the stripper tube. The phase space of the ion beam and the dimensions of the stripper tube de®ne the optical transmission of ions. Due to interactions of ions with the stripper gas, phase space increases causing a linear decrease of the beam transmission with stripper gas density. Fig. 4 shows measured transmission curves of 12 C ions of charge state 1 , 2 and 3 as function of stripper gas density at 470 keV beam energy. Charge exchange processes will lead to an equilibrium charge state distribution as soon as sucient stripper thickness is reached. Consequently, the beam losses observed as function of stripper density can be attributed to angular straggling of the ions. In order to extract the equilibrium yield of a given charge state, the slope of the transmission curve is extrapolated to zero stripper density. In this way, the yields of the charge states 1 , 2 and 3 have been estimated. A signi®cant fraction of the injected ion beam is
14
Fig. 4. Transmission of 12 C ions in charge states 1 , 2 and 3 at 470 keV beam energy in argon stripper gas. Due to increasing angular straggling with stripper density, a decrease of transmission is observed. Their equilibrium yields, T0 q , were calculated from the linear slope a of the transmission curves.
H.-A. Synal et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 29±36
being neutralized during the stripping process and cannot be detected with the mass spectrometer at the high energy end of the accelerator. To determine the fraction of neutral ions, the sum of all measured yields of the positive charge states has been subtracted from the beam intensity injected into the accelerator. The energy dependence of the stripping yield data is shown in Fig. 5 in the energy range from 200 to 600 keV together with semi-empirical model curves of Sayer [19]. For the model a Gaussian distribution of the dierent charge states yields at a ®xed energy is assumed. In order to ®t the experimental data, the width of this distribution r, an oset to the modelÕs mean charge at a ®xed energy d and the ion optical transmission (T) have been varied. A best ®t is obtained for r 0:73, oset d 0:03 and an ion optical transmission T 97%. However, the model assumption of a Gaussian charge state distribution, and the ion optical transmission of less than 100% will limit the accuracy of absolute values of the charge state yields. Thus, the given values can be regarded only as lower limits. From 250 to 550 keV the yield of the 1 charge state is more than 50%, while the cross-section for molecular dissociation is high enough to destroy the molecular component at stripper densities of about 2 lg/cm2 . The main limitation comes from the increasing angular straggling with decreasing beam energy. Calculations of angular straggling using the model and the associated tables derived
Fig. 5. Charge state distribution of 12 C ions in argon stripper gas as a function of ion energy under equilibrium conditions. Together with experimental data points model calculations are shown.
35
Fig. 6. Angular straggling as function of beam energy according to calculations using the model of Sigmund and Winterbon [20]. Half angles of acceptance for beam fractions of 68%, 80% and 95% are shown.
by Sigmund and Winterbon [20] result in large mean straggling angles below beam energies of less than 400 keV (Fig. 6). In order to avoid signi®cant beam losses, a large acceptance of the analyzing system following the accelerator is necessary. This has to be taken into account in the design of future commercial AMS radiocarbon dating systems.
5. Conclusion During the last years, large eorts have been made to extend the AMS technique to an energy range which permits the construction of small and compact systems. It has been shown that interfering molecules can be destroyed eciently, even if ions in low charge states (1 and 2 ) are used. Since also high yields for these charge states can be reached at low beam energies, large accelerators are not needed anymore for modern radiocarbon detection systems. When using 1 ions the interference of molecular fragments (7 Li ) can be avoided and unambiguous 14 C detection is possible. At the compact PSI/ETH AMS system, which has been developed in collaboration with NEC, it has been demonstrated that radiocarbon dating is possible at beam energies of less than 1 MeV. Both the required abundance sensitivity and the overall accuracy have been reached. Systematic investigations of the charge exchange processes and of
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H.-A. Synal et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 29±36
the molecular dissociation at low beam energies are in progress. The data available at present indicate that the new technique has the potential to be used at acceleration voltages, which can be provided by commercially available high-voltage power supplies. Systems for biomedical applications and for radiocarbon dating are now commercially available. These new developments will in¯uence the future use of AMS. In particular, in the case of radiocarbon dating, a more widespread use of the technique can be predicted. Acknowledgements The authors would like to thank B.J. Hughey and D.J. Mous for information on the status of their respective biomedical AMS projects. Special thanks go to J.B. Schroeder and to National Electrostatics Corporation for their valuable contribution to the PSI/ETH small AMS system. We thank P.W. Kubik for his comments on this manuscript, and Georges Bonani and Irka Hajdas for providing the 14 C test samples. References [1] HVEE, Amersfoort, The Netherlands. [2] NEC, Middleton, WI, USA. [3] B.J. Hughey, X.L. Zhao, W.E. Kieser, A.E. Litherland, Nucl. Instr. and Meth. B 123 (1997) 186.
[4] H.W. Lee, Ph.D. Thesis, University of Toronto, 1988. [5] H.W. Lee, K.H. Chang, L.R. Kilius, A.E. Litherland, Nucl. Instr. and Meth. B 5 (1984) 208. [6] M. Suter, S. Jacob, H.-A. Synal, Nucl. Instr. and Meth. B 123 (1997) 148. [7] NSI, Newton Scienti®c Cambridge, MA, USA. [8] D.J. Mous, W. Fokker, R. vanden Broek, R.B. Koopmans, Nucl. Instr. and Meth. B 123 (1997) 159. [9] D.J. Mous, W. Fokker, R. vanden Broek, R.B. Koopmans, AIP Conf. Proc. 475 (1999) 657. [10] B.J. Hughey, R.E. Shefer, P.L. Skipper, S.R. Tannenbaum, J.S. Wishnok, Nucl. Instr. and Meth. B 123 (1997) 153. [11] M. Chen, S.-L. Xu, G.-S. Chen, L.-G. Shen, Y.-J. Zhang, X.-S. Lu, W.-Y. Zhang, Y.-X. Zhang, Z.-K. Zhong, Nucl. Instr. and Meth. B 92 (1994) 213. [12] R.A. Muller, Science 196 (1977). [13] M. Chen, G. Chen, S. Xu, L. Shen, D. Li, Y. Zhang, P. Gong, Y. Zhang, W. Zhou, Nucl. Instr. and Meth. B 123 (1997) 102. [14] M. Suter, Nucl. Instr. and Meth. B 139 (1998) 150. [15] M. Suter, S. Jacob, J.B. Schroeder, H.-A. Synal, AIP Conf. Proc. 475 (1999) 665. [16] G.A. Norton, J.E. Raatz, R.D. Rathmell, in: Proceeding of the Symposium of Northeastern accelerator Personnel (SNEAP), Santa Fe, World Scienti®c, Singapore, 1992, p. 295. [17] R. Balzer, M. Suter, G. Bonani, W. W ol¯i, Nucl. Instr. and Meth. B 5 (1984) 242. [18] H.-A. Synal, B. Bonani, M. D obeli, R.M. Ender, P. Gartenmann, P.W. Kubik, Ch. Schnabel, M. Suter, Nucl. Instr. and Meth. B 123 (1997) 62. [19] R.O. Sayer, Revue de Physique Appliquee 12 (1977) 1543. [20] P. Sigmund, K.H. Winterbon, Nucl. Instr. and Meth. 119 (1974) 541.
www.elsevier.nl/locate/nimb
New concepts for radiocarbon detection systems H.-A. Synal *, S. Jacob, M. Suter Paul Scherrer Institute, c/o Inst. fur Teilchenphysik, F-13, ETH H onggerberg, Building HPK, CH-8093 Z urich, Switzerland
Abstract A new method to dissociate molecules in low charge states (1 and 2 ) makes it possible to develop small and compact radiocarbon detection systems. The concept will have strong impact on radiocarbon dating as well as on biomedical applications. The status of projects and experiments in connection with molecular destruction methods is summarized. The requirements for dedicated, very small radiocarbon dating facilities are discussed. Our now operational, compact 500 kV radiocarbon AMS system is described in detail. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 07.75; 41.85 Keywords: Accelerator mass spectrometry; Radiocarbon dating; Molecular destruction
1. Introduction Accelerator mass spectrometry (AMS) is now established as a versatile tool for the detection of long-lived radionuclides at natural isotopic concentrations, such as 10 Be, 14 C, 26 Al, 36 Cl, 41 Ca, 53 Mn, 59 Ni, 60 Fe and 129 I. Among these radionuclides, radiocarbon plays the primary role due to its geochemical behavior and its special role in the biosphere. In particular, radiocarbon dating has become an essential technique in archaeology and in many ®elds of environmental research. The extensive range of radiocarbon applications has continually increased the worldwide demand for radiocarbon analyses and today several 10 000
*
Corresponding author. Tel.: +41-1-633-2027; fax: +41-1633-1067. E-mail address: [email protected] (H.-A. Synal).
samples are analyzed per year. Radiocarbon AMS systems, which are able to provide high precision measurements, are commercially available [1,2], and several so-called dedicated AMS systems have been set up during the past few years. However, these systems are based on relatively large tandem accelerators (2.5 and 5 MV, respectively) for which the investment and operating costs are high. In addition, the complexity of these systems require a sophisticated environment, which, in general, cannot be provided directly by the user community. This is one of the main reasons, why dedicated radiocarbon AMS centers have established in the past. Many of them oer analyses on a commercial basis to meet the existing demand. Nevertheless, the analysis costs are high, and for many projects limited funds will impede a more extensive use of the AMS technique. This is especially important for biomedical applications. The great potential of using the AMS in connection
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 8 8 1 - 2
30
H.-A. Synal et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 29±36
with radiocarbon tracer studies has been demonstrated, and it has been shown that AMS provides adequate sensitivity and precision. But systematic investigations require large numbers of samples, ecient sample preparation and short turnover times. And here lie the limitations of the available AMS facilities. Therefore, it would be highly desirable to have analytic instrumentation suitable to be operated directly by the users and under operating condition of a standard laboratory or even in clinical environment. 2. New concepts of radiocarbon detection systems The unique sensitivity of AMS is possible because molecules interfering in the analysis are destroyed during the stripping process in the tandem accelerator and the resulting molecular fragments can be removed eciently from the selected radionuclide beam. To charge exchange negative ions in a tandem accelerator, thin carbon foils (3 lg/ cm2 ) or dierentially pumped gas cells (i.e. Ar gas at 1 lg/cm2 ) are used. The dissociation of molecular ions is essential to achieve unambiguous identi®cation of the radionuclide of interest. It has been known for many years and it has been a golden rule of AMS that small molecules in charge state of 3 or higher are unstable against the Coulomb repulsion force. Therefore, present day AMS systems are based on large accelerators of at least 2.5 MV terminal voltage in order to have sucient stripping yields for the required charge states of 3 and higher. Recent interest in the use of AMS in biomedical research and the increasing demand of radiocarbon dating analyses have initiated experimental investigations into the destruction of molecular ions in charge state 1 and 2 . Dissociation of 12 CH2 2 molecules has been investigated at 1 MeV beam energy in single and double stripper foils [3]. The results of this study proved that foil stripping can provide the required detection sensitivity for biomedical AMS. Interfering molecules can also be destroyed in collisions with a stripper gas. A certain probability of breaking up molecular ions exist, even if only a single electron is knocked o in a charge changing collision. Initial experiments on
the destruction of CH2 and CH2 2 molecules have been performed by the AMS group in Toronto [4,5]. Experiments at the AMS facility of the Paul Scherrer Institute and the ETH (PSI/ETH) [6] with 1 and 2 molecular ions have shown that the molecular component at mass 14 can be suppressed by as much as 11 orders of magnitude. This is sucient to reach background levels suitable for radiocarbon dating. Fig. 1 shows the decrease of the counting rate of 1 mass 14 ions as a function of stripper thickness. At higher densities an exponential decrease of the counting rate is observed. In this region of gas density, the crosssection of the molecular break-up reaction can be computed directly from the slope of the curve. The cross-section for the dissociation of 1 molecules
Fig. 1. Counting rate of 13 CH ions as a function of stripper gas density at 510 keV beam energy. Using an additional stripper foil at the image point of the HE-magnet, the remaining molecules were dissociated and molecular fragments of a selected electric rigidity were measured with a silicon solid-state detector.
H.-A. Synal et al. / Nucl. Instr. and Meth. in Phys. Res. B 161±163 (2000) 29±36
of mass 14 at 510 keV beam energy in Ar gas is about 8 10ÿ16 cm2 . 3. Systems for biomedical applications Triggered by the commercial prospective of biomedical applications, two companies, High Voltage Engineering (HVEE, [1]) and Newton Scienti®c [7] have started to develop dedicated AMS systems for biomedical research. Both systems are designed to use ions in charge state 2 . Molecular destruction is achieved by stripping negatively charged ions at energies of about 1 MeV in carbon stripping foil. The HVEE system has a very compact design [8]. Floorspace of only 2:25 m 1:25 m is required for the installation. The compact design is possible, because a folded tandem accelerator is used. At the high voltage platform an electrostatic de¯ector bends the ions by 180°. In a stripping foil in front of the de¯ector the negative ions charge exchange whereby molecules are destroyed. On the high energy end, a magnetic spectrometer is used to eliminate the molecular fragments. Final particle identi®cation is made with a gas ionization detector. First results of the system, presented at the 15th International Conference on Application of Accelerators in Research and Industry, Denton, Texas, 1998, show that 14 C can be identi®ed and isotopic ratios of about 2.5 pMC can be measured. The overall accuracy of the system is quoted to be in the range of 2% [9]. The 14 C system of Newton Scienti®c is based on a linear tandem accelerator of 1 MV terminal voltage [10]. A magnetic spectrometer at the low energy side and a combination of an electrostatic de¯ector and a magnetic spectrometer on the high energy side are used to mass analyze the ion beam. Similar to the HVEE system, a foil stripper at the high voltage terminal is used to destroy the molecular beam component. The system is not yet fully operational. As a special feature of the system a gas chromatograph is connected to the ion source and the sample material is fed as gas directly into the sputter ion source. This should simplify the sample preparation procedure signi®cantly. However, uncertainties associated with
31
cross talk between dierent samples and with contamination of the system from samples of high 14 C concentrations have to be investigated carefully. A research group at the Shanghai Institute of Nuclear research has developed a small cyclotron (diameter 1.5 m) for the detection of radiocarbon [11]. In the early years of AMS, measurements using a cyclotron were made by Muller [12] in 1977. In contrast to tandem AMS, where molecular interference are destroyed during the stripping process, a cyclotron based system uses the high mass resolution of the cyclotron itself to resolve interference from molecular ions of mass 14. It was demonstrated that an overall mass resolution of 3600 is sucient to separate 14 C from 13 CH molecules [13]. For contemporary 14 C/12 C ratios and for blank samples counting rates of 5 and 0.2 cps, respectively, were measured resulting in an abundance sensitivity of about 10ÿ14 . Besides unambiguous 14 C detection, the normalization of 14 C events to the 12 C current extracted from the ion source is dicult with a cyclotron system. To switch between ion beams of dierent isotopes, the cyclotron has to be re-tuned. This will limit the accuracy of the device. At present, an overall accuracy of 1±2% is quoted by the authors [13]. This will enable biomedical applications, but it is not sucient to perform high precision radiocarbon dating. 4. A system for radiocarbon dating At PSI/ETH the feasibility of high precision radiocarbon dating has been investigated [14]. In collaboration with National Electrostatics Corporation (NEC, [2]) an instrument was developed to demonstrate the potential of radiocarbon dating at low energies and to study the relevant processes of molecular destruction [15]. The small and compact AMS system is based on a 500 kV Pelletron accelerator. It operates with ions in charge state 1 . To charge-exchange the injected negative ions, argon gas is used as stripper medium. The gas is re-circulated with two 150 l/s turbo molecular pumps. Negative ions are produced with a commercial MC SNICS cesium sputter ion
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source [16]. The ion beam extracted from the source is mass analyzed in a double focusing spectrometer magnet. A fast beam switching system is used to inject the three isotopes of carbon into the accelerator in a fast sequence (12 C:20 ls; 13 C:200 ls; 12.5 Hz repetition rate) [17]. One oaxis Faraday cup is used to measure the 13 C, 12 CH and 12 C currents at the low energy end of the accelerator. During injection of mass 14, the Faraday cup measures the current of the mass 13 beam (13 C and 12 CH), while during the pulsed injection of mass 13 ions, the 12 C beam current is detected. A terminal voltage of maximal 500 kV is generated with the Pelletron accelerator. Ions emerging from the accelerator are mass-analyzed by a combination of a magnetic and an electrostatic sector ®eld. Both devices have stigmatic focusing properties and have the same bending radius (75 cm). The combination of the two elements forms an achromatic beam transport system. The analyzing magnet directly follows the accelerator and the center of the stripper tube is imaged without an additional lens. In the focal plane of the magnet, Faraday cups for the detection of the stable isotope beams are placed. Additional to the 12 C and
the 13 C cups, a Faraday cup is placed to measure C ions, which are molecular fragments of the injected 13 CH molecules. This current is used to monitor the molecular component (13 CHÿ ) of the ion beam from the ion source at mass 14. Finally, the mass and energy ®ltered ions are detected with a silicon solid-state detector. Fig. 2 shows a photograph of the system, whose overall dimensions are 4 6 m2 . The system has been designed as an experimental platform to evaluate the potential of the new principle of molecular destruction, to investigate the relevant processes of charge changing and molecular dissociation and to demonstrate the possibility of radiocarbon dating at low energies. 13
4.1. Performance The system is operational since July 1998 and the ®rst measurements were made to demonstrate the potential for radiocarbon dating. To study the molecular suppression capability of the system, samples of standard reference material and processed blanks were measured at 970 keV beam energy. At a gas density of about 2 lg/cm2 of argon, the molecular component at mass 14 is
Fig. 2. The PSI/ETH compact AMS system for radiocarbon dating. The system is based on a 500 kV Pelletron accelerator. At the high energy side, an achromatic mass spectrometer performs separation of the 14 C ions.
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reduced by more than 10 orders of magnitude. Processed blank samples having molecular 13 CH beams of less than 10ÿ4 compared to the extracted negative 12 C current gave background values of 2± 8 ´ 10ÿ15 . Similar isotopic ratios were measured for the same blank material at the large PSI/ETH radiocarbon system [18], where ions of 26 MeV in charge state 4 are used. Fig. 3(a) shows the spectra obtained for the dierent samples. From the oxalic acid standard a clear peak of 14 C is observed. Except for individual events at the very low energies, the spectrum is free of background events. Events from the blank sample cluster in the region of the 14 C peak. By integrating over the 14 C
Fig. 3. Energy spectra of standard and blank samples measured using charge state 1 (a), and 2 (b), at 460 kV terminal voltage. Both measurements were taken at a stripper density of 2 lg/ cm2 . The 14 C events detected from the blank sample correspond to an isotopic ratio of 2 ´ 10ÿ15 . 14 C2 detection is heavily impeded by molecular fragments of 7 Liÿ 2 molecules. At a ®nal beam energy of 1430 keV the counting rate of two 7 Li ions reaching the detector simultaneously is ten times higher than the 14 C counting rate from a modern sample.
33
peak area an isotopic ratio of the blank sample of 2:4 10ÿ15 can be calculated. This corresponds to a radiocarbon age of about 48 000 yr. Measurements have also been made with ions in charge state 2 . At a terminal voltage of 460 kV the resulting beam energy is 1430 keV. The detection of 14 C2 ions is interfered by fragments of the 7 Liÿ 2 molecule, which is injected into the accelerator simultaneously with 14 C. During the charge exchange process these molecules dissociate, but the molecular fragments 7 Li have the same magnetic and electric rigidities as the 14 C2 ions and cannot be rejected by the mass spectrometer. If both molecular fragments leave the stripper in charge state 1 , they can reach the ®nal detector simultaneously. These twin events cannot be resolved by a total energy measurement in the detector alone. 7 Li molecular fragments, for which only one part is in charge state 1 , generate events at half of the energy of the 14 C events. The ratio of single to double 7 Li events is constant at a given beam energy and depends on the charge state distribution and ion beam transmission. This makes a background correction for 7 Li possible. Fig. 3(b) shows the spectra of the same standard and blank samples as in Fig. 3(a), but now measured using ions in charge state 2 . The spectra for both, standard and blank were taken at the same accumulated charge and were normalized to the rate of single 7 Li events. The dierence of both spectra gives the contribution of the real 14 C events. At a beam energy of 510 keV, which is reached at the stripper channel, the 1 charge state is most abundant and the lithium background is approximately ten times higher than the 14 C counting rate of a modern sample. At higher energies the situation improves, but the lithium background is a serious limitation for radiocarbon dating, if 14 C detection is performed with 2 ions. 4.2. Precision
14
C measurements
Radiocarbon dating requires high precision C/12 C isotope ratio measurements. State of the art AMS systems are able to provide data with an overall accuracy of 0.2±0.5%. To be competitive with existing high precision systems, the new PSI/ ETH system should reach this range. In order to 14
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test the performance measurements of oxalic acid standard material have been made. Seven CO2 samples have been reduced to graphite using a standard preparation technique. Each graphite sample (2 mg) was subdivided into two fractions (A/B). The fractions were pressed into individual cathodes suitable for AMS analysis. The analyses were performed with average 12 Cÿ currents of 20 lA extracted from the ion source. Each sample was measured in three individual measurement runs, which were subdivided into 10 cycles of 20 s of measurement time. The overall uncertainty of each sample due to counting alone, was of the order of 0.5%. Averaging all results, the mean 14 C/ 12 C ratio has a standard deviation of 0.5% and an error of 0.13% for the mean value. This compares nicely with the internal error computed from counting statistics alone (0.13% and 0.5% overall statistical error and statistical error of the three individual runs of one cathode, respectively). The standard deviation of the 13 C/12 C measurements is about 1&. The average values of the two dierent fractions A and B agree in both the 13 C/12 C and 14 C/12 C isotopic ratios. These results show that carbon isotopic ratio measurements at the 2±3& level can be performed with this compact AMS system. Moreover, the system may have the potential for even better performance, but this has to be demonstrated in routine radiocarbon dating measurements. 4.3. Optimum operation conditions High precision measurements of isotopic ratios C/12 C and 13 C/12 C require the same optimal beam transport through the whole system for all three carbon isotopes. The most critical part is the injection into the tandem accelerator. Using the fast beam sequencing technique, the ion beams of the stable carbon isotopes 13 C and 12 C are injected into the accelerator in very short beam pulses. To avoid isotope speci®c beam losses, all three beams have to be matched as close as possible. Another important constraint is the necessity for the highest possible beam transmission, which is determined by the yield of the selected charge state and by the beam losses caused by an increasing phase space due to angular straggling. To
investigate the optimal operating conditions, yields of the dierent charge states have been measured for 12 C in the energy range of 200±600 keV for dierent stripper densities. The measurements were performed with the stripper of the acceleration system. The ion beam of a certain charge state coming out of the accelerator was measured at dierent stripper densities in a Faraday cup after the magnet following the accelerator. The stripper density was calculated from the pressure measured inside the stripper housing, the gas feeding line of the stripper tube and from the conductance of the stripper tube. The phase space of the ion beam and the dimensions of the stripper tube de®ne the optical transmission of ions. Due to interactions of ions with the stripper gas, phase space increases causing a linear decrease of the beam transmission with stripper gas density. Fig. 4 shows measured transmission curves of 12 C ions of charge state 1 , 2 and 3 as function of stripper gas density at 470 keV beam energy. Charge exchange processes will lead to an equilibrium charge state distribution as soon as sucient stripper thickness is reached. Consequently, the beam losses observed as function of stripper density can be attributed to angular straggling of the ions. In order to extract the equilibrium yield of a given charge state, the slope of the transmission curve is extrapolated to zero stripper density. In this way, the yields of the charge states 1 , 2 and 3 have been estimated. A signi®cant fraction of the injected ion beam is
14
Fig. 4. Transmission of 12 C ions in charge states 1 , 2 and 3 at 470 keV beam energy in argon stripper gas. Due to increasing angular straggling with stripper density, a decrease of transmission is observed. Their equilibrium yields, T0 q , were calculated from the linear slope a of the transmission curves.
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being neutralized during the stripping process and cannot be detected with the mass spectrometer at the high energy end of the accelerator. To determine the fraction of neutral ions, the sum of all measured yields of the positive charge states has been subtracted from the beam intensity injected into the accelerator. The energy dependence of the stripping yield data is shown in Fig. 5 in the energy range from 200 to 600 keV together with semi-empirical model curves of Sayer [19]. For the model a Gaussian distribution of the dierent charge states yields at a ®xed energy is assumed. In order to ®t the experimental data, the width of this distribution r, an oset to the modelÕs mean charge at a ®xed energy d and the ion optical transmission (T) have been varied. A best ®t is obtained for r 0:73, oset d 0:03 and an ion optical transmission T 97%. However, the model assumption of a Gaussian charge state distribution, and the ion optical transmission of less than 100% will limit the accuracy of absolute values of the charge state yields. Thus, the given values can be regarded only as lower limits. From 250 to 550 keV the yield of the 1 charge state is more than 50%, while the cross-section for molecular dissociation is high enough to destroy the molecular component at stripper densities of about 2 lg/cm2 . The main limitation comes from the increasing angular straggling with decreasing beam energy. Calculations of angular straggling using the model and the associated tables derived
Fig. 5. Charge state distribution of 12 C ions in argon stripper gas as a function of ion energy under equilibrium conditions. Together with experimental data points model calculations are shown.
35
Fig. 6. Angular straggling as function of beam energy according to calculations using the model of Sigmund and Winterbon [20]. Half angles of acceptance for beam fractions of 68%, 80% and 95% are shown.
by Sigmund and Winterbon [20] result in large mean straggling angles below beam energies of less than 400 keV (Fig. 6). In order to avoid signi®cant beam losses, a large acceptance of the analyzing system following the accelerator is necessary. This has to be taken into account in the design of future commercial AMS radiocarbon dating systems.
5. Conclusion During the last years, large eorts have been made to extend the AMS technique to an energy range which permits the construction of small and compact systems. It has been shown that interfering molecules can be destroyed eciently, even if ions in low charge states (1 and 2 ) are used. Since also high yields for these charge states can be reached at low beam energies, large accelerators are not needed anymore for modern radiocarbon detection systems. When using 1 ions the interference of molecular fragments (7 Li ) can be avoided and unambiguous 14 C detection is possible. At the compact PSI/ETH AMS system, which has been developed in collaboration with NEC, it has been demonstrated that radiocarbon dating is possible at beam energies of less than 1 MeV. Both the required abundance sensitivity and the overall accuracy have been reached. Systematic investigations of the charge exchange processes and of
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the molecular dissociation at low beam energies are in progress. The data available at present indicate that the new technique has the potential to be used at acceleration voltages, which can be provided by commercially available high-voltage power supplies. Systems for biomedical applications and for radiocarbon dating are now commercially available. These new developments will in¯uence the future use of AMS. In particular, in the case of radiocarbon dating, a more widespread use of the technique can be predicted. Acknowledgements The authors would like to thank B.J. Hughey and D.J. Mous for information on the status of their respective biomedical AMS projects. Special thanks go to J.B. Schroeder and to National Electrostatics Corporation for their valuable contribution to the PSI/ETH small AMS system. We thank P.W. Kubik for his comments on this manuscript, and Georges Bonani and Irka Hajdas for providing the 14 C test samples. References [1] HVEE, Amersfoort, The Netherlands. [2] NEC, Middleton, WI, USA. [3] B.J. Hughey, X.L. Zhao, W.E. Kieser, A.E. Litherland, Nucl. Instr. and Meth. B 123 (1997) 186.
[4] H.W. Lee, Ph.D. Thesis, University of Toronto, 1988. [5] H.W. Lee, K.H. Chang, L.R. Kilius, A.E. Litherland, Nucl. Instr. and Meth. B 5 (1984) 208. [6] M. Suter, S. Jacob, H.-A. Synal, Nucl. Instr. and Meth. B 123 (1997) 148. [7] NSI, Newton Scienti®c Cambridge, MA, USA. [8] D.J. Mous, W. Fokker, R. vanden Broek, R.B. Koopmans, Nucl. Instr. and Meth. B 123 (1997) 159. [9] D.J. Mous, W. Fokker, R. vanden Broek, R.B. Koopmans, AIP Conf. Proc. 475 (1999) 657. [10] B.J. Hughey, R.E. Shefer, P.L. Skipper, S.R. Tannenbaum, J.S. Wishnok, Nucl. Instr. and Meth. B 123 (1997) 153. [11] M. Chen, S.-L. Xu, G.-S. Chen, L.-G. Shen, Y.-J. Zhang, X.-S. Lu, W.-Y. Zhang, Y.-X. Zhang, Z.-K. Zhong, Nucl. Instr. and Meth. B 92 (1994) 213. [12] R.A. Muller, Science 196 (1977). [13] M. Chen, G. Chen, S. Xu, L. Shen, D. Li, Y. Zhang, P. Gong, Y. Zhang, W. Zhou, Nucl. Instr. and Meth. B 123 (1997) 102. [14] M. Suter, Nucl. Instr. and Meth. B 139 (1998) 150. [15] M. Suter, S. Jacob, J.B. Schroeder, H.-A. Synal, AIP Conf. Proc. 475 (1999) 665. [16] G.A. Norton, J.E. Raatz, R.D. Rathmell, in: Proceeding of the Symposium of Northeastern accelerator Personnel (SNEAP), Santa Fe, World Scienti®c, Singapore, 1992, p. 295. [17] R. Balzer, M. Suter, G. Bonani, W. W ol¯i, Nucl. Instr. and Meth. B 5 (1984) 242. [18] H.-A. Synal, B. Bonani, M. D obeli, R.M. Ender, P. Gartenmann, P.W. Kubik, Ch. Schnabel, M. Suter, Nucl. Instr. and Meth. B 123 (1997) 62. [19] R.O. Sayer, Revue de Physique Appliquee 12 (1977) 1543. [20] P. Sigmund, K.H. Winterbon, Nucl. Instr. and Meth. 119 (1974) 541.