- Email: [email protected]
Initial measurements with the SUERC accelerator mass spectrometer
Nuclear Instruments and Methods in Physics Research B 223–224 (2004) 195–198 www.elsevier.com/locate/nimb
Initial measurements with the SUERC accelerator mass spectrometer Stewart Freeman a,*, Sheng Xu a, Christoph Schnabel a, Andrew Dougans a, Andrew Tait a, Richard Kitchen b, George Klody b, Roger Loger b, Tom Pollock b, James Schroeder b, Mark Sundquist b a
Scottish Universities Environmental Research Centre, Scottish Enterprise Technology Park, East Kilbride G75 0QF, UK b National Electrostatics Corporation, Middleton, WI, USA
Abstract 10 Be, 14 C, 36 Cl and 129 I test measurements have been made with a new Pelletron-based accelerator mass spectrometer operating at up to 5.2 MV. All ion detection was with a versatile gas ionization detector. Low-background radiocarbon measurements with 2% scatter of identical samples was performed with both spectrometer ion sources. 10 Be/Be backgrounds of 3 · 1015 were achieved using a gas cell adjoining the detector for 10 B suppression. High samplethroughput Cl AMS with 36 Cl/Cl backgrounds of 4 · 1015 was accomplished. 2004 Elsevier B.V. All rights reserved.
PACS: 07.75.+h; 7.77.ka Keywords: Accelerator mass spectrometry; AMS
1. Introduction A new National Electrostatics Corporation (NEC) 5 MV accelerator mass spectrometer is being commissioned at the Scottish Universities Environmental Research Centre (SUERC) [1]. 10 Be, 14 C, 36 Cl and 129 I test measurements have been made according to procurement arrangements. Accelerator mass spectrometry of radiocarbon and other species with 5 MV NEC machines elsewhere is already routine, and at SUERC 14 C AMS performance is good as well. In
contrast, Be AMS and Cl AMS is subject to atomic isobaric interference and in overcoming these at SUERC there is some novelty: this is the first deployment of a new particle detector intended for all-species AMS; environmental-level Cl AMS is achieved at relatively low ion energies without a specialised gas filled magnet detector. Brief tests were sufficient to confirm the capability for 129 I3þ detection by energy loss ion identification, and are not considered further here.
2. Ion measurement *
Corresponding author. Tel.: +44-1355-270-187; fax: +441355-229-898. E-mail address: [email protected] (S. Freeman).
The gas ionization detector features five dE=dx anodes and a sixth signal corresponding to ion
0168-583X/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2004.04.040
196
S. Freeman et al. / Nucl. Instr. and Meth. in Phys. Res. B 223–224 (2004) 195–198
3. C AMS
total energy is obtained from the cathode plate. The pulses are filtered with single channels analysers (SCA) and signal analogue-to-digital conversion is triggered by up to fivefold coincidence. By summing signal values in software further synthetic parameters are made available too. For ion counting software gates can be set on parameter histograms and on two-dimensional plots of one parameter verses another featuring non-rectilinear ‘banana’ gating and colour-coded signal frequency. Plots can be made conditional on others so that gates are cascaded; to facilitate gate setting plots can show histograms of both all signals and of only those signals passing other gates. A special software gate records signals from a pulser applied to the detector signal preamplifiers for on-line deadtime corrections to the measured ion/current ratios. Conveniently the pulser also independently triggers the analogue-to-digital converters, with SCA-matching delay, so that pulser signals need not fall within all hardware gating. The supervising computer program measures samples in groups. Each measurement of each sample is made to counting statistics and other limits prescribed for that sample. Single measurements of each sample are made in turn and the group sequence repeated as necessary. Sample data acquisition is considered complete and the sample removed from the sequence according to an algorithm, for instance when a measure of the scatter of a minimum number of measurements achieves another threshold prescribed for that sample. Ratios are normalised offline.
The SUERC spectrometer is equipped with two ion sources. One source (S1) can accommodate up to 134 samples and is intended for the routine measurement of all species. The other 40-sample source (S2) is for radiocarbon measurements with either graphite or CO2 samples, although the latter sample form is not considered here. Good spectrometer 14 C performance, seemingly limited by the error in the on-line measured d13 C, was achieved with both ion sources. Table 1 summarises measurements of IAEA standard materials and other bulk materials routinely prepared by the radiocarbon laboratories at SUERC. Using S2 up to 100 lA 12 C could be generated by adjusting ion source parameters, although accelerator ion transmission tended to be less with the largest beams. However, this did not obviously otherwise affect the data. Using S1 in its initial set-up performance was similar except that maximum currents were about 40 lA 12 C . Subsequently currents of over 100 lA have been produced by withdrawing the sample wheel to increase the separation of sample and ioniser, as has been done at other laboratories operating similar ion sources.
4. Be AMS The detector gas and window are typically P10 and 5 lm Mylar foil. For Be AMS a 5 lm Havar window mounted in a KF gasket is installed before
Table 1 Summary of SUERC accelerator mass spectrometer test performance Species
Ion source
Ion current
Terminal potential
Stripper
Transmission
Scattera
Backgroundb
C4þ C4þ 10 Be3þ 36 Cl7þ
S1 S2 S1 S1
30 lA C 80 lA C 3 lA BeO 20 lA 35 Cl
4.5 MV 4.5 MV 5 MV 5.2 MV
Ar Ar Ar 1.5 lg/cm2 C
57% 50% 23% 27%
0.2% 0.2% 1% 5%
65 kyrs 65 kyrs 3 · 1015 Be 4 · 1015 Cl
14
14
a The average standard deviation of normalised appropriately precise measurements of triplicate samples of common material with species isotope ratio of about 1 · 1012 . b Instrument background upper limits based upon C, Be and Cl measurements respectively of Alfa Aesar graphite powder 100 mesh, BeO from formerly available Merck BeSO4 and Weeks Island halite AgCl.
S. Freeman et al. / Nucl. Instr. and Meth. in Phys. Res. B 223–224 (2004) 195–198
of the detector and Ar added to the inter-window space. This gas cell preferentially slows the 10 B interference. However, the lowest 10 Be backgrounds have been achieved not by completely stopping the 10 B ions in the cell but instead by permitting some into the detector such that 10 Be ions have sufficient range to produce two anode signals for gating on. The resulting 10 B versus 10 Be separation is good and the background correspondingly low as per Table 1; we have managed to well reproduce measurements at other laboratories of several materials used to indicate background. Ideally this resolution would be achievable at lower terminal potential and to this end we also intend to experiment with thinner Havar [2].
5. Cl AMS Useful 36 Cl7þ versus 36 S7þ resolution is possible with the SUERC 5 MV spectrometer provided that the ions deposit sufficient energy in the detector and ion energy straggling is small. This requires very thin stripper foils and detector windows. Also, to date the accelerator has been operated at 5.2 MV for 41.7 MeV Cl7þ ion energy. This requires conditioning to even higher voltages for stable running. We use collodion backed 1.5 lg/cm2 carbon stripper foils (ACF-Metals, Tucson, AZ, USA). Charge-changing is efficient and accelerator ion transmission correspondingly high, as per Table 1. The foils are sufficiently long-lived, each being usable for about a day; systematic foil thickening can be accounted for by normalizing measurements of unknowns to those of standards close in time. Only 1.7% accelerator transmission is achieved with alternative gas stripping. 1.5 lm Mylar detector windows proved sufficiently thin for useful Cl versus S separation but we find resolution even better with 10 mm square 100 nm thin silicon nitride membrane windows (Silson Ltd., Northampton, UK). The detector gas used is low pressure propane. The pressure is varied about 20 Torr so that the Cl and S ion energy distributions coincide at the short anode in the detector middle. This provides for obvious Cl
197
versus S separation at the first and last anodes and, by gating upon these, further obvious additional separation in parameters based on the remaining two anodes. Whilst the resulting background is low it may yet be improved upon by further segmenting anodes and using anode signal timing to reject scattered ions. Even thinner windows are potentially available too. Measurement precision may improve as we become experienced in the pressing and handling of the Cl sample cathodes. Be and C samples are loaded and pressed into hollow sample holders from the back of the sample holders. This achieves a consistent front surface that is sputtered. In contrast, Cl samples are front-pressed into a bed of AgBr previously front-pressed into large 6 mm internal diameter Cu cathodes. Encouragingly measurements of longer-sputtered identical samples tend to show less scatter, presumably because the more a sample is ‘burnt in’ the less they are sensitive to surface pressing artefacts.
6. Conclusions The SUERC spectrometer is certainly a good instrument for Be and C AMS where sample preparatory chemistry rather than the instrument itself may be limiting. Brief I AMS has been promising too. Despite its relatively low achievable terminal potential for Cl AMS the machine seemingly will also well measure Cl, albeit subject to a background that is to be investigated further. Further experience will determine whether routine 5.2 MV operation is practical or whether the eventual measurement background at lower terminal potentials is acceptable and easier lower voltage operation therefore preferable. Encouragingly, though, the Cl performance has been achieved without compromising measurement rate by efficiently foil-stripping to Cl7þ . Analysis of mg graphite samples from the SUERC-hosted radiocarbon laboratories is now routine. Routine Be, Cl and other species AMS awaits the completion of SUERC chemistry laboratories; the Be, Cl and I measurements to date have been of dilution series of respective NIST standard solutions or materials donated by the
198
S. Freeman et al. / Nucl. Instr. and Meth. in Phys. Res. B 223–224 (2004) 195–198
AMS community. We look forward to also establishing the instruments capabilities for 26 Al and 41 Ca measurement.
Southon, UC Irvine; John Vogel and Tom Brown, LLNL; Robin Golser, VERA; Shigeru Itoh, JNCTONO; Juerg Beer, EAWAG; and Marc Caffee, Purdue. This work is supported by NERCadministered JIF award GR3/J001.
Acknowledgements We are grateful to colleagues at the SUERChosted Glasgow University and NERC radiocarbon laboratories and to William Phillips, Edinburgh University, for furnishing measurement materials. The following also kindly provided advice and/or material assistance: Keith Fifield, ANU; Tim Jull amd colleagues, Arizona; Peter Kubik, Arno Synal and Martin Suter, ETH; John
References [1] S. Freeman, P. Bishop, C. Bryant, G. Cook, A. Fallick, D. Harkness, S. Metcalfe, M. Scott, R. Scott, M. Summerfield, Nucl. Instr. and Meth. B, these Proceedings. doi:10.1016/ j.nimb.2004.04.010. [2] S. Itoh, M. Abe, M. Watanabe, S. Nakai, H. Touyama, S. Xu, Nucl. Instr. and Meth. B, these Proceedings. doi:10.1016/j.nimb.2004.04.023.
Initial measurements with the SUERC accelerator mass spectrometer Stewart Freeman a,*, Sheng Xu a, Christoph Schnabel a, Andrew Dougans a, Andrew Tait a, Richard Kitchen b, George Klody b, Roger Loger b, Tom Pollock b, James Schroeder b, Mark Sundquist b a
Scottish Universities Environmental Research Centre, Scottish Enterprise Technology Park, East Kilbride G75 0QF, UK b National Electrostatics Corporation, Middleton, WI, USA
Abstract 10 Be, 14 C, 36 Cl and 129 I test measurements have been made with a new Pelletron-based accelerator mass spectrometer operating at up to 5.2 MV. All ion detection was with a versatile gas ionization detector. Low-background radiocarbon measurements with 2% scatter of identical samples was performed with both spectrometer ion sources. 10 Be/Be backgrounds of 3 · 1015 were achieved using a gas cell adjoining the detector for 10 B suppression. High samplethroughput Cl AMS with 36 Cl/Cl backgrounds of 4 · 1015 was accomplished. 2004 Elsevier B.V. All rights reserved.
PACS: 07.75.+h; 7.77.ka Keywords: Accelerator mass spectrometry; AMS
1. Introduction A new National Electrostatics Corporation (NEC) 5 MV accelerator mass spectrometer is being commissioned at the Scottish Universities Environmental Research Centre (SUERC) [1]. 10 Be, 14 C, 36 Cl and 129 I test measurements have been made according to procurement arrangements. Accelerator mass spectrometry of radiocarbon and other species with 5 MV NEC machines elsewhere is already routine, and at SUERC 14 C AMS performance is good as well. In
contrast, Be AMS and Cl AMS is subject to atomic isobaric interference and in overcoming these at SUERC there is some novelty: this is the first deployment of a new particle detector intended for all-species AMS; environmental-level Cl AMS is achieved at relatively low ion energies without a specialised gas filled magnet detector. Brief tests were sufficient to confirm the capability for 129 I3þ detection by energy loss ion identification, and are not considered further here.
2. Ion measurement *
Corresponding author. Tel.: +44-1355-270-187; fax: +441355-229-898. E-mail address: [email protected] (S. Freeman).
The gas ionization detector features five dE=dx anodes and a sixth signal corresponding to ion
0168-583X/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2004.04.040
196
S. Freeman et al. / Nucl. Instr. and Meth. in Phys. Res. B 223–224 (2004) 195–198
3. C AMS
total energy is obtained from the cathode plate. The pulses are filtered with single channels analysers (SCA) and signal analogue-to-digital conversion is triggered by up to fivefold coincidence. By summing signal values in software further synthetic parameters are made available too. For ion counting software gates can be set on parameter histograms and on two-dimensional plots of one parameter verses another featuring non-rectilinear ‘banana’ gating and colour-coded signal frequency. Plots can be made conditional on others so that gates are cascaded; to facilitate gate setting plots can show histograms of both all signals and of only those signals passing other gates. A special software gate records signals from a pulser applied to the detector signal preamplifiers for on-line deadtime corrections to the measured ion/current ratios. Conveniently the pulser also independently triggers the analogue-to-digital converters, with SCA-matching delay, so that pulser signals need not fall within all hardware gating. The supervising computer program measures samples in groups. Each measurement of each sample is made to counting statistics and other limits prescribed for that sample. Single measurements of each sample are made in turn and the group sequence repeated as necessary. Sample data acquisition is considered complete and the sample removed from the sequence according to an algorithm, for instance when a measure of the scatter of a minimum number of measurements achieves another threshold prescribed for that sample. Ratios are normalised offline.
The SUERC spectrometer is equipped with two ion sources. One source (S1) can accommodate up to 134 samples and is intended for the routine measurement of all species. The other 40-sample source (S2) is for radiocarbon measurements with either graphite or CO2 samples, although the latter sample form is not considered here. Good spectrometer 14 C performance, seemingly limited by the error in the on-line measured d13 C, was achieved with both ion sources. Table 1 summarises measurements of IAEA standard materials and other bulk materials routinely prepared by the radiocarbon laboratories at SUERC. Using S2 up to 100 lA 12 C could be generated by adjusting ion source parameters, although accelerator ion transmission tended to be less with the largest beams. However, this did not obviously otherwise affect the data. Using S1 in its initial set-up performance was similar except that maximum currents were about 40 lA 12 C . Subsequently currents of over 100 lA have been produced by withdrawing the sample wheel to increase the separation of sample and ioniser, as has been done at other laboratories operating similar ion sources.
4. Be AMS The detector gas and window are typically P10 and 5 lm Mylar foil. For Be AMS a 5 lm Havar window mounted in a KF gasket is installed before
Table 1 Summary of SUERC accelerator mass spectrometer test performance Species
Ion source
Ion current
Terminal potential
Stripper
Transmission
Scattera
Backgroundb
C4þ C4þ 10 Be3þ 36 Cl7þ
S1 S2 S1 S1
30 lA C 80 lA C 3 lA BeO 20 lA 35 Cl
4.5 MV 4.5 MV 5 MV 5.2 MV
Ar Ar Ar 1.5 lg/cm2 C
57% 50% 23% 27%
0.2% 0.2% 1% 5%
65 kyrs 65 kyrs 3 · 1015 Be 4 · 1015 Cl
14
14
a The average standard deviation of normalised appropriately precise measurements of triplicate samples of common material with species isotope ratio of about 1 · 1012 . b Instrument background upper limits based upon C, Be and Cl measurements respectively of Alfa Aesar graphite powder 100 mesh, BeO from formerly available Merck BeSO4 and Weeks Island halite AgCl.
S. Freeman et al. / Nucl. Instr. and Meth. in Phys. Res. B 223–224 (2004) 195–198
of the detector and Ar added to the inter-window space. This gas cell preferentially slows the 10 B interference. However, the lowest 10 Be backgrounds have been achieved not by completely stopping the 10 B ions in the cell but instead by permitting some into the detector such that 10 Be ions have sufficient range to produce two anode signals for gating on. The resulting 10 B versus 10 Be separation is good and the background correspondingly low as per Table 1; we have managed to well reproduce measurements at other laboratories of several materials used to indicate background. Ideally this resolution would be achievable at lower terminal potential and to this end we also intend to experiment with thinner Havar [2].
5. Cl AMS Useful 36 Cl7þ versus 36 S7þ resolution is possible with the SUERC 5 MV spectrometer provided that the ions deposit sufficient energy in the detector and ion energy straggling is small. This requires very thin stripper foils and detector windows. Also, to date the accelerator has been operated at 5.2 MV for 41.7 MeV Cl7þ ion energy. This requires conditioning to even higher voltages for stable running. We use collodion backed 1.5 lg/cm2 carbon stripper foils (ACF-Metals, Tucson, AZ, USA). Charge-changing is efficient and accelerator ion transmission correspondingly high, as per Table 1. The foils are sufficiently long-lived, each being usable for about a day; systematic foil thickening can be accounted for by normalizing measurements of unknowns to those of standards close in time. Only 1.7% accelerator transmission is achieved with alternative gas stripping. 1.5 lm Mylar detector windows proved sufficiently thin for useful Cl versus S separation but we find resolution even better with 10 mm square 100 nm thin silicon nitride membrane windows (Silson Ltd., Northampton, UK). The detector gas used is low pressure propane. The pressure is varied about 20 Torr so that the Cl and S ion energy distributions coincide at the short anode in the detector middle. This provides for obvious Cl
197
versus S separation at the first and last anodes and, by gating upon these, further obvious additional separation in parameters based on the remaining two anodes. Whilst the resulting background is low it may yet be improved upon by further segmenting anodes and using anode signal timing to reject scattered ions. Even thinner windows are potentially available too. Measurement precision may improve as we become experienced in the pressing and handling of the Cl sample cathodes. Be and C samples are loaded and pressed into hollow sample holders from the back of the sample holders. This achieves a consistent front surface that is sputtered. In contrast, Cl samples are front-pressed into a bed of AgBr previously front-pressed into large 6 mm internal diameter Cu cathodes. Encouragingly measurements of longer-sputtered identical samples tend to show less scatter, presumably because the more a sample is ‘burnt in’ the less they are sensitive to surface pressing artefacts.
6. Conclusions The SUERC spectrometer is certainly a good instrument for Be and C AMS where sample preparatory chemistry rather than the instrument itself may be limiting. Brief I AMS has been promising too. Despite its relatively low achievable terminal potential for Cl AMS the machine seemingly will also well measure Cl, albeit subject to a background that is to be investigated further. Further experience will determine whether routine 5.2 MV operation is practical or whether the eventual measurement background at lower terminal potentials is acceptable and easier lower voltage operation therefore preferable. Encouragingly, though, the Cl performance has been achieved without compromising measurement rate by efficiently foil-stripping to Cl7þ . Analysis of mg graphite samples from the SUERC-hosted radiocarbon laboratories is now routine. Routine Be, Cl and other species AMS awaits the completion of SUERC chemistry laboratories; the Be, Cl and I measurements to date have been of dilution series of respective NIST standard solutions or materials donated by the
198
S. Freeman et al. / Nucl. Instr. and Meth. in Phys. Res. B 223–224 (2004) 195–198
AMS community. We look forward to also establishing the instruments capabilities for 26 Al and 41 Ca measurement.
Southon, UC Irvine; John Vogel and Tom Brown, LLNL; Robin Golser, VERA; Shigeru Itoh, JNCTONO; Juerg Beer, EAWAG; and Marc Caffee, Purdue. This work is supported by NERCadministered JIF award GR3/J001.
Acknowledgements We are grateful to colleagues at the SUERChosted Glasgow University and NERC radiocarbon laboratories and to William Phillips, Edinburgh University, for furnishing measurement materials. The following also kindly provided advice and/or material assistance: Keith Fifield, ANU; Tim Jull amd colleagues, Arizona; Peter Kubik, Arno Synal and Martin Suter, ETH; John
References [1] S. Freeman, P. Bishop, C. Bryant, G. Cook, A. Fallick, D. Harkness, S. Metcalfe, M. Scott, R. Scott, M. Summerfield, Nucl. Instr. and Meth. B, these Proceedings. doi:10.1016/ j.nimb.2004.04.010. [2] S. Itoh, M. Abe, M. Watanabe, S. Nakai, H. Touyama, S. Xu, Nucl. Instr. and Meth. B, these Proceedings. doi:10.1016/j.nimb.2004.04.023.