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A new compact AMS system at Peking University
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 259 (2007) 23–26 www.elsevier.com/locate/nimb
A new compact AMS system at Peking University Kexin Liu a
a,*
, Xingfang Ding a, Dongpo Fu a, Yan Pan b, Xiaohong Wu b, Zhiyu Guo a, Liping Zhou c
Key Laboratory of Heavy Ion Physics, Ministry of Education and Institute of Heavy Ion Physics, School of Physics, Peking University, Beijing 100871, China b School of Archaeology and Museology, Peking University, Beijing 100871, China c Key Laboratory for Earth Surface Processes, Ministry of Education and College of Environmental Sciences, Peking University, Beijing 100871, China Available online 12 March 2007
Abstract A compact 14C AMS system manufactured by the National Electrostatics Corporation has been installed at the Institute of Heavy Ion Physics, Peking University. The system is based on a Model 1.5SDH-1 Pelletron accelerator with a maximum terminal voltage of 0.6 MV. The 14C measurement accuracy with this system is better than 0.4% and the machine background is lower than 0.03 pMC. The performance of the new system, especially the background and the d13C measurements, is presented. Several important applications are also described briefly. Ó 2007 Elsevier B.V. All rights reserved. PACS: 07.75. +h Keywords: AMS; Precision; Background; d13C measurement
1. Introduction The EN tandem based AMS system at Peking University has been in operation since 1992 and thousands of samples for many projects as well as a dating service have been measured. However the EN tandem is not dedicated to AMS; it is also used for other applications such as ion beam irradiation. To meet the increasing demand for 14C sample analyses from the Peking University and other research institutes in China, a compact 14C AMS system manufactured by the National Electrostatics Corporation based on the research of AMS laboratory at ETH, Zurich [1] was installed at the Institute of Heavy Ion Physics, Peking University in September of 2004. This machine is the fourth commercially delivered compact AMS system from NEC, the previous three similar systems are at the Univer-
*
Corresponding author. Tel.: +86 10 6275 8528; fax: +86 10 6275 1875. E-mail address: [email protected] (K. Liu).
0168-583X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.01.314
sity of Georgia, UC Irvine and Poznan radiocarbon laboratory [2–4].
2. Description of system The compact AMS system utilizes a NEC 40 sample Multi-Cathode SNICS ion source. The ion source housing has been modified to allow the future addition of the necessary equipment for gas sample cathodes. The fast switching injection system injects the carbon ions 12C , 13C , 14 C for 0.3, 1 and 100 ms, respectively, sequentially with a cycle repeat rate of 10 Hz. A Model 1.5SDH-1 Pelletron provides acceleration for the AMS system. The maximum terminal voltage of 0.6 MV can be reached and a terminal voltage of 0.46 MV is used during routing operation. Two turbo-molecular pumps are used to re-circulate the stripper gas. The high energy analyzing system includes a 90° double focusing magnet and a 90° electrostatic deflector. The final ion detector is a simple silicon surface barrier detector.
24
K. Liu et al. / Nucl. Instr. and Meth. in Phys. Res. B 259 (2007) 23–26
A computer is used for data acquisition and control of the system while another one is used for off-line data analysis. 3. Performance 3.1. Beam current and transmission 12
The C beam currents are about 50–80 lA from different kinds of cathodes. Typical operating parameters of the ion source are shown in Table 1. If we choose a very high cesium focus voltage (more than 3 kV) intense beam currents can be obtained, but the cesium beam spot at the surface of cathode is too big and a lot of aluminum is sputtered out and deposited on the ionizer. Therefore, the ion source has to be cleaned frequently. To solve this problem, we retracted the cathode disk back about 3 mm and used a cesium focus voltage of 2.5 kV, as shown in Table 1, to obtain a suitable beam current for routine measurements. At the terminal voltage of 0.46 MV, an ion beam transmission of 43% (HE/LE 12C current) is typical over a stripper gas pressure range of 14–16 lT. 3.2. Precision The uncertainty and reproducibility of the AMS system was tested by using samples of the standards OXI, OXII and ANU before routine measurements were undertaken. Three OXI standard samples together with background samples were used to test the system as the first sample wheel. Repeated measurements were carried out one day later with 2 OXIs, 2 OXIIs and 2 ANUs. Each sample was measured 6–8 times. Table 2 shows the 14C/12C ratios and uncertainties of those samples. The results clearly demonstrate that the measurement precision and reproducibility of our system are better than 0.3% for modern samples. The measured pMC values of OXII and ANU using OXI as primary standard are 150.05 and 134.15, in good agreement with the accepted values of 150.81 and 134.07. OX-II, ANU and IAEA reference samples are also used periodically as the secondary standards during routine measurements. The original stripper gas valve was sensitive to the ambient temperature and the stripper gas pressure changed periodically with the temperature of cooling water. However, this variation did not increase the scatter of the 14C/12C ratios significantly when the temperature of cooling water
Table 2 14 C/12C ratios of the test samples Sample
14
C/12C ratio
Uncertainty (%)
OXI (Cathode-1) OXI (Cathode-2) OXI (Cathode-3)
1.2086 1.2037 1.2034
0.172 0.289 0.335
OXI (Cathode-1) OXI (Cathode-2)
1.2069 1.2089
0.289 0.150
OXII (Cathode-1) OXII (Cathode-2)
1.5591 1.5615
0.137 0.178
ANU (Cathode-1) ANU (Cathode-2)
1.7831 1.7868
0.183 0.130
was controlled within ±2 °C. A new metering valve was delivered by NEC and installed. The new valve is less sensitive to temperature changes and the stripper gas pressure stability has been markedly improved. 3.3. Background During the design of our system, NEC accepted a suggestion from Dr. John Southon of the UC Irvine Keck AMS laboratory that the bias voltage on the vacuum chamber of injection magnet should be +200 V rather than zero when the 14C ions is injected to the accelerator. This has led to a marked improvement in the background for the compact AMS system. The machine background of our system is lower than 0.03 pMC (corresponding to a 14 C age of 65 ka BP) when measured using samples of alpha-graphite, which is regularly measured at the Kiel AMS laboratory. This machine background is comparable to machine background reached by AMS systems based on larger tandems [5]. The variation of the machine background with the stripper gas pressure has also been investigated and the results are shown in Fig. 1.
Table 1 Typical ion source parameters Source bias voltage Extractor voltage Cathode voltage Cesium focus Ionizer Cesium oven Line heater
35 (kV) 16 (kV) 6 (kV) 2.5 (kV) 22 (A) 43 (V)
2.0 (mA) 1.0 (mA) 1.0 (mA) 7.4 (V) 4.4 (A) 32 (A)
126.8 (W) 200 (°C)
Fig. 1. The relationship between machine background and stripper gas pressure.
K. Liu et al. / Nucl. Instr. and Meth. in Phys. Res. B 259 (2007) 23–26
Fig. 2. The difference of AMS d13C and MS d13C for oracle bone samples.
Fig. 3. The difference of d13C versus the difference of beam current.
4. Stable carbon isotope d13C measurement The d13C values based on the 13C/12C ratios measured by our compact AMS system are used for fractionation corrections of most unknown samples. The uncertainty of this AMS d13C is around 1.0&. Because the measurements of oracle bone samples under one of our projects need to be
25
high accuracy measurements, the d13C values were also measured by stable isotope MS with a resulting measurement uncertainty of less than 0.2&. A systematic difference was found between the AMS d13C and MS d13C; the mean value of the difference was about 2&. As shown in Fig. 2, the AMS d13C values are more negative than MS d13C values for most oracle bone samples. To ensure the reliability of the 14C ages of oracle bone samples, the reason of this difference and its effect on 14C ages was investigated and an experiment with ‘‘dilute’’ samples was undertaken. After a careful analysis of the measurement data of all the oracle bone samples, we found that there was an approximate linear relationship between the AMS d13C minus MS d13C difference and the difference between 13C beam current of unknown sample and the average 13C beam current of standard samples (Fig. 3). In most cases, the 13C beam current of unknown sample is lower than the average 13C beam current of standard samples and the AMS d13C of this unknown sample is more negative than its real value. To make this relationship clearer, a further experiment was designed. Some ‘‘dilute’’ samples (more iron powder added) were made of standard and unknown materials. These ‘‘dilute’’ samples were measured together with normal samples and the results are compared in Table 3. The AMS d13C values of ‘‘dilute’’ samples are obviously different with their real d13C values due to the much lower beam currents, but the corrected pMC values or 14C ages calculated with the 14C/12C ratios are almost the same with the normal samples. It is clear that there is a fractionation effect related to beam current in our compact AMS system. This effect causes an inaccuracy in AMS d13C measurements if the unknown sample has a beam current that is different from standard samples. However, the pMC values or 14C ages corrected with the AMS d13C are still reliable even if the beam current of the unknown sample is much lower than the standard. Therefore, we use the AMS d13C value rather than MS d13C value for d13C corrections to obtain reliable 14 C ages. Considering the imperfect linear relationship in Fig. 3, the uncertainty of the 14C ages should be enlarged according to the standard deviation of the linear relationship. The uncertainty of 14C ages obtained directly from the NEC off-line data analyzing software is smaller than the actual uncertainty.
Table 3 Comparison of AMS d13C values and pMC values or ages Sample
Beam current (nA)
OXI D-OXI-1 D-OXI-2 SA98168 D-SA98168 SA98230 D-SA98230 SA98207 D-SA98207
302 123 125 268 135 330 104 306 182
AMS d13C (&) 41.8 44.6 8.62 29.8 9.28 34.2 14.0 21.4
Real d13C (&) 19.0 19.0 19.0 7.02 7.02 9.31 9.31 9.84 9.84
pMC or age
Error
105.26 105.42 106.07 3000a 2999a 2973a 2943a 2952a 2918a
0.39% 0.35% 24 38 21 30 28 26
26
K. Liu et al. / Nucl. Instr. and Meth. in Phys. Res. B 259 (2007) 23–26
5. Applications More than 1500 samples have been measured with our new compact AMS system. Some important applications are listed below. The details of these application programs will be described elsewhere. (1) Oracle bones with inscriptions are very important for historical research of the Shang Dynasty of ancient China. Most of oracle bones were excavated around the Yinxu site in Henan province, which was the capital of late Shang Dynasty. The names of nine Kings of late Shang Dynasty have been found in the oracle inscriptions and their genealogy is known from the historical book Shiji written in Han Dynasty, about one thousand years after Shang. Therefore, it is possible to establish a chronology of late Shang Dynasty by dating the oracle bones inscribed with the Kings’ names and calibrating the ages using Bayesian methods. More than 60 Oracle bone samples have been measured with the new machine. To ensure reliability, each sample was divided and loaded into at least two cathodes; the measurement precisions are better than 0.3%. Together with the oracle bone samples measured using our EN based AMS system, more than 100 oracle bone samples have now been dated. The preliminary calibration results obtained with OxCal 3.9 shows that the calendar ages of these oracle bones range from 1260 BC to 1050 BC. The ages indicate that the end of Shang Dynasty should be about 1050 BC. This is consistent with 14C dating results of the serial bone samples from the Yinxu site and results from eclipse calculations based on the records in oracle bone inscriptions.
(2) Samples have been measured for a research program investigating the impact of ecological environment changes on the bird islands in the South China Sea in response to global environmental change. Timeframes have been obtained for sediment cores from several islands. (3) Bio-medical applications carried out with this new machine include a study of acrylamide adduction with biomacromolecules by AMS at environmental levels and studies on the in vivo incorporation and/ or adduction of nicotine with DNA. (4) We joined the Fifth International Radiocarbon Intercomparison organized by the IAEA and measured the first batch of samples. The samples were prepared at the Peking University and the Institute of Earth Environment in Xian separately. Acknowledgements We thank M. Sundquist, T. Hauser, R. Loger and G. Klody of NEC for their help with our compact AMS system, to T. Hauser and J. King for their hard work during installation of the system in our laboratory. We are also grateful to Dr. J. Southon of UC Irvine for his useful discussions and valuable suggestions. References [1] H.A. Synal, S. Jacob, M. Suter, Nucl. Instr. and Meth. B 172 (2000) 1. [2] M.L. Roberts, R.A. Culp, D.K. Dvoracek, et al., Nucl. Instr. and Meth. B 223–224 (2004) 1. [3] T. Goslar, J. Czernik, E. Goslar, Nucl. Instr. and Meth. B 223–224 (2004) 5. [4] Personal communication with J. Southon, UC Irvine, USA. [5] M. Schleicher, P.M. Grootes, M.J. Nadeau, A. Schoon, Radiocarbon 40 (1998) 85.
Nuclear Instruments and Methods in Physics Research B 259 (2007) 23–26 www.elsevier.com/locate/nimb
A new compact AMS system at Peking University Kexin Liu a
a,*
, Xingfang Ding a, Dongpo Fu a, Yan Pan b, Xiaohong Wu b, Zhiyu Guo a, Liping Zhou c
Key Laboratory of Heavy Ion Physics, Ministry of Education and Institute of Heavy Ion Physics, School of Physics, Peking University, Beijing 100871, China b School of Archaeology and Museology, Peking University, Beijing 100871, China c Key Laboratory for Earth Surface Processes, Ministry of Education and College of Environmental Sciences, Peking University, Beijing 100871, China Available online 12 March 2007
Abstract A compact 14C AMS system manufactured by the National Electrostatics Corporation has been installed at the Institute of Heavy Ion Physics, Peking University. The system is based on a Model 1.5SDH-1 Pelletron accelerator with a maximum terminal voltage of 0.6 MV. The 14C measurement accuracy with this system is better than 0.4% and the machine background is lower than 0.03 pMC. The performance of the new system, especially the background and the d13C measurements, is presented. Several important applications are also described briefly. Ó 2007 Elsevier B.V. All rights reserved. PACS: 07.75. +h Keywords: AMS; Precision; Background; d13C measurement
1. Introduction The EN tandem based AMS system at Peking University has been in operation since 1992 and thousands of samples for many projects as well as a dating service have been measured. However the EN tandem is not dedicated to AMS; it is also used for other applications such as ion beam irradiation. To meet the increasing demand for 14C sample analyses from the Peking University and other research institutes in China, a compact 14C AMS system manufactured by the National Electrostatics Corporation based on the research of AMS laboratory at ETH, Zurich [1] was installed at the Institute of Heavy Ion Physics, Peking University in September of 2004. This machine is the fourth commercially delivered compact AMS system from NEC, the previous three similar systems are at the Univer-
*
Corresponding author. Tel.: +86 10 6275 8528; fax: +86 10 6275 1875. E-mail address: [email protected] (K. Liu).
0168-583X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.01.314
sity of Georgia, UC Irvine and Poznan radiocarbon laboratory [2–4].
2. Description of system The compact AMS system utilizes a NEC 40 sample Multi-Cathode SNICS ion source. The ion source housing has been modified to allow the future addition of the necessary equipment for gas sample cathodes. The fast switching injection system injects the carbon ions 12C , 13C , 14 C for 0.3, 1 and 100 ms, respectively, sequentially with a cycle repeat rate of 10 Hz. A Model 1.5SDH-1 Pelletron provides acceleration for the AMS system. The maximum terminal voltage of 0.6 MV can be reached and a terminal voltage of 0.46 MV is used during routing operation. Two turbo-molecular pumps are used to re-circulate the stripper gas. The high energy analyzing system includes a 90° double focusing magnet and a 90° electrostatic deflector. The final ion detector is a simple silicon surface barrier detector.
24
K. Liu et al. / Nucl. Instr. and Meth. in Phys. Res. B 259 (2007) 23–26
A computer is used for data acquisition and control of the system while another one is used for off-line data analysis. 3. Performance 3.1. Beam current and transmission 12
The C beam currents are about 50–80 lA from different kinds of cathodes. Typical operating parameters of the ion source are shown in Table 1. If we choose a very high cesium focus voltage (more than 3 kV) intense beam currents can be obtained, but the cesium beam spot at the surface of cathode is too big and a lot of aluminum is sputtered out and deposited on the ionizer. Therefore, the ion source has to be cleaned frequently. To solve this problem, we retracted the cathode disk back about 3 mm and used a cesium focus voltage of 2.5 kV, as shown in Table 1, to obtain a suitable beam current for routine measurements. At the terminal voltage of 0.46 MV, an ion beam transmission of 43% (HE/LE 12C current) is typical over a stripper gas pressure range of 14–16 lT. 3.2. Precision The uncertainty and reproducibility of the AMS system was tested by using samples of the standards OXI, OXII and ANU before routine measurements were undertaken. Three OXI standard samples together with background samples were used to test the system as the first sample wheel. Repeated measurements were carried out one day later with 2 OXIs, 2 OXIIs and 2 ANUs. Each sample was measured 6–8 times. Table 2 shows the 14C/12C ratios and uncertainties of those samples. The results clearly demonstrate that the measurement precision and reproducibility of our system are better than 0.3% for modern samples. The measured pMC values of OXII and ANU using OXI as primary standard are 150.05 and 134.15, in good agreement with the accepted values of 150.81 and 134.07. OX-II, ANU and IAEA reference samples are also used periodically as the secondary standards during routine measurements. The original stripper gas valve was sensitive to the ambient temperature and the stripper gas pressure changed periodically with the temperature of cooling water. However, this variation did not increase the scatter of the 14C/12C ratios significantly when the temperature of cooling water
Table 2 14 C/12C ratios of the test samples Sample
14
C/12C ratio
Uncertainty (%)
OXI (Cathode-1) OXI (Cathode-2) OXI (Cathode-3)
1.2086 1.2037 1.2034
0.172 0.289 0.335
OXI (Cathode-1) OXI (Cathode-2)
1.2069 1.2089
0.289 0.150
OXII (Cathode-1) OXII (Cathode-2)
1.5591 1.5615
0.137 0.178
ANU (Cathode-1) ANU (Cathode-2)
1.7831 1.7868
0.183 0.130
was controlled within ±2 °C. A new metering valve was delivered by NEC and installed. The new valve is less sensitive to temperature changes and the stripper gas pressure stability has been markedly improved. 3.3. Background During the design of our system, NEC accepted a suggestion from Dr. John Southon of the UC Irvine Keck AMS laboratory that the bias voltage on the vacuum chamber of injection magnet should be +200 V rather than zero when the 14C ions is injected to the accelerator. This has led to a marked improvement in the background for the compact AMS system. The machine background of our system is lower than 0.03 pMC (corresponding to a 14 C age of 65 ka BP) when measured using samples of alpha-graphite, which is regularly measured at the Kiel AMS laboratory. This machine background is comparable to machine background reached by AMS systems based on larger tandems [5]. The variation of the machine background with the stripper gas pressure has also been investigated and the results are shown in Fig. 1.
Table 1 Typical ion source parameters Source bias voltage Extractor voltage Cathode voltage Cesium focus Ionizer Cesium oven Line heater
35 (kV) 16 (kV) 6 (kV) 2.5 (kV) 22 (A) 43 (V)
2.0 (mA) 1.0 (mA) 1.0 (mA) 7.4 (V) 4.4 (A) 32 (A)
126.8 (W) 200 (°C)
Fig. 1. The relationship between machine background and stripper gas pressure.
K. Liu et al. / Nucl. Instr. and Meth. in Phys. Res. B 259 (2007) 23–26
Fig. 2. The difference of AMS d13C and MS d13C for oracle bone samples.
Fig. 3. The difference of d13C versus the difference of beam current.
4. Stable carbon isotope d13C measurement The d13C values based on the 13C/12C ratios measured by our compact AMS system are used for fractionation corrections of most unknown samples. The uncertainty of this AMS d13C is around 1.0&. Because the measurements of oracle bone samples under one of our projects need to be
25
high accuracy measurements, the d13C values were also measured by stable isotope MS with a resulting measurement uncertainty of less than 0.2&. A systematic difference was found between the AMS d13C and MS d13C; the mean value of the difference was about 2&. As shown in Fig. 2, the AMS d13C values are more negative than MS d13C values for most oracle bone samples. To ensure the reliability of the 14C ages of oracle bone samples, the reason of this difference and its effect on 14C ages was investigated and an experiment with ‘‘dilute’’ samples was undertaken. After a careful analysis of the measurement data of all the oracle bone samples, we found that there was an approximate linear relationship between the AMS d13C minus MS d13C difference and the difference between 13C beam current of unknown sample and the average 13C beam current of standard samples (Fig. 3). In most cases, the 13C beam current of unknown sample is lower than the average 13C beam current of standard samples and the AMS d13C of this unknown sample is more negative than its real value. To make this relationship clearer, a further experiment was designed. Some ‘‘dilute’’ samples (more iron powder added) were made of standard and unknown materials. These ‘‘dilute’’ samples were measured together with normal samples and the results are compared in Table 3. The AMS d13C values of ‘‘dilute’’ samples are obviously different with their real d13C values due to the much lower beam currents, but the corrected pMC values or 14C ages calculated with the 14C/12C ratios are almost the same with the normal samples. It is clear that there is a fractionation effect related to beam current in our compact AMS system. This effect causes an inaccuracy in AMS d13C measurements if the unknown sample has a beam current that is different from standard samples. However, the pMC values or 14C ages corrected with the AMS d13C are still reliable even if the beam current of the unknown sample is much lower than the standard. Therefore, we use the AMS d13C value rather than MS d13C value for d13C corrections to obtain reliable 14 C ages. Considering the imperfect linear relationship in Fig. 3, the uncertainty of the 14C ages should be enlarged according to the standard deviation of the linear relationship. The uncertainty of 14C ages obtained directly from the NEC off-line data analyzing software is smaller than the actual uncertainty.
Table 3 Comparison of AMS d13C values and pMC values or ages Sample
Beam current (nA)
OXI D-OXI-1 D-OXI-2 SA98168 D-SA98168 SA98230 D-SA98230 SA98207 D-SA98207
302 123 125 268 135 330 104 306 182
AMS d13C (&) 41.8 44.6 8.62 29.8 9.28 34.2 14.0 21.4
Real d13C (&) 19.0 19.0 19.0 7.02 7.02 9.31 9.31 9.84 9.84
pMC or age
Error
105.26 105.42 106.07 3000a 2999a 2973a 2943a 2952a 2918a
0.39% 0.35% 24 38 21 30 28 26
26
K. Liu et al. / Nucl. Instr. and Meth. in Phys. Res. B 259 (2007) 23–26
5. Applications More than 1500 samples have been measured with our new compact AMS system. Some important applications are listed below. The details of these application programs will be described elsewhere. (1) Oracle bones with inscriptions are very important for historical research of the Shang Dynasty of ancient China. Most of oracle bones were excavated around the Yinxu site in Henan province, which was the capital of late Shang Dynasty. The names of nine Kings of late Shang Dynasty have been found in the oracle inscriptions and their genealogy is known from the historical book Shiji written in Han Dynasty, about one thousand years after Shang. Therefore, it is possible to establish a chronology of late Shang Dynasty by dating the oracle bones inscribed with the Kings’ names and calibrating the ages using Bayesian methods. More than 60 Oracle bone samples have been measured with the new machine. To ensure reliability, each sample was divided and loaded into at least two cathodes; the measurement precisions are better than 0.3%. Together with the oracle bone samples measured using our EN based AMS system, more than 100 oracle bone samples have now been dated. The preliminary calibration results obtained with OxCal 3.9 shows that the calendar ages of these oracle bones range from 1260 BC to 1050 BC. The ages indicate that the end of Shang Dynasty should be about 1050 BC. This is consistent with 14C dating results of the serial bone samples from the Yinxu site and results from eclipse calculations based on the records in oracle bone inscriptions.
(2) Samples have been measured for a research program investigating the impact of ecological environment changes on the bird islands in the South China Sea in response to global environmental change. Timeframes have been obtained for sediment cores from several islands. (3) Bio-medical applications carried out with this new machine include a study of acrylamide adduction with biomacromolecules by AMS at environmental levels and studies on the in vivo incorporation and/ or adduction of nicotine with DNA. (4) We joined the Fifth International Radiocarbon Intercomparison organized by the IAEA and measured the first batch of samples. The samples were prepared at the Peking University and the Institute of Earth Environment in Xian separately. Acknowledgements We thank M. Sundquist, T. Hauser, R. Loger and G. Klody of NEC for their help with our compact AMS system, to T. Hauser and J. King for their hard work during installation of the system in our laboratory. We are also grateful to Dr. J. Southon of UC Irvine for his useful discussions and valuable suggestions. References [1] H.A. Synal, S. Jacob, M. Suter, Nucl. Instr. and Meth. B 172 (2000) 1. [2] M.L. Roberts, R.A. Culp, D.K. Dvoracek, et al., Nucl. Instr. and Meth. B 223–224 (2004) 1. [3] T. Goslar, J. Czernik, E. Goslar, Nucl. Instr. and Meth. B 223–224 (2004) 5. [4] Personal communication with J. Southon, UC Irvine, USA. [5] M. Schleicher, P.M. Grootes, M.J. Nadeau, A. Schoon, Radiocarbon 40 (1998) 85.