- Email: [email protected]
The National Ocean Sciences AMS facility at Woods Hole Oceanographic Institution
278
Nuclear Instruments and Methods in Physics Research B52 (1990) 278-284 North-Holland
The National Ocean Sciences AMS Facility at Woods Hole Oceanographic Institution G.A. Jones, A.P. McNichol,
K.F. von Reden
and R.J. Schneider
Department of Geology and Geophysics, Woo& Hole Oceanographic Insrituion.
Woods Hole, Massachuserrs 02543. USA
A new and highly automated AMS facility for producin and analyzing graphite samples from seawater and other marine sources is nearing completion. Simultaneous measurements of “C. ? ‘C and 14C isotopes is planned with a throughput of up to 4500 samples per year.
1. Introduction In the spring of 1987 the United States National Science Foundation (NSF) issued a Request For Proposals (RFP) for establishing a national accelerator facility dedicated largely to the mission of the ocean sciences community. This RFP was precipitated as a result of the rapidly increasing demand for accelerator radiocarbon analyses within the ocean sciences community, increasing delays in obtaining results from existing facilities due to these new demands and the requirement for an estimated 3-4000 additional radiocarbon analyses per year through the 1990s for the two international projects WOCE (World Ocean Circulation Experiment) and GOFS (Global Ocean Flux Study). In July 1988 it was announced that the Woods Hole Oceanographic Institution (WHOI) was the successful requestor for establishing such a facility. Funding for the $5 million, three-year project began in January 1989. The facility as we conceived it in 1987 would have to be able to meet the following criteria to be successful: (1) process up to 4500 samples per year, (2) routinely achieve precisions of 3-5 per mil for seawater samples of near modern age, and (3) be able to distribute the results of these analyses rapidly to the scientific community. Successful implementation of these goals requires a new accelerator design; a commitment to automation at virtually all stages of sample preparation, analysis and data reduction; and access to facilities designed specifically for efficient AMS research. The Woods Hole Oceanographic Institution committed approximately 85% of the funds required to build an 864 m2 building designed specifically for, and dedicated exclusively to accelerator mass spectrometry. The remaining 15% of the building funds were supplied as part of the above-mentioned NSF funding. Construction of this building was begun in July 1989 and com0168-583X/90/$03.50
pleted in May 1990. The construction of a new-generation 3 MV tandem accelerator was contracted with US-AMS Corporation in February 1989 and delivery was initiated in September 1990. The details of the new capabilities of this high precision/high throughput machine are covered elsewhere within this volume [1,2]. We anticipate that we will be in an initial operational mode in early 1991. This will be in time to process water samples from the first Pacific leg of the WOCE project, scheduled to begin in March 1991. Once operational we anticipate the breakdown of analyses to be approximately 70% for global programs such as WOCE and GOFS; 20% for marine geology, paleoceanography and related subjects; and 10% for a wide range of miscellaneous radiocarbon applications. The facility will operate with a staff of ten (fig. 1). We started with a staff of two in January 1989 and presently (September 1990) consist of eight. Scientific and operational input is received from an intemal-toWHO1 advisory board consisting of seven members serving staggered three-year terms. In addition there is a joint WHOI-NSF appointed external national/intemational advisory board consisting of six members also with staggered three-year terms.
2. Facility The new AMS facility occupies an 864 m2 addition to the Institution’s existing McLean Laboratory. The floor plan of this addition is shown in fig. 2. In addition to the accelerator room, sample preparation, stable isotope mass spectrometer and engineering laboratories are included. In designing these laboratories, we attempted to utilize knowledge gained from experience with existing facilities. We have installed separate circuits for the various parts of the AMS system, with grounds retum-
0 1990 - Elsevier Science Publishers B.V. (North-Holland)
G.A. Jones et al. / The National
Ocean Sciences AMS
Facility
279
.-l,.r~+giq&q Fig. 1. Personnel wire diagram and relationships with WHO1 and National Advisory Boards.
ing to a common point at the transformer. This will avoid many of the common electrical ground-loop problems. The 0.64 m thick floor for the AMS system was poured separately from the rest of the building, and planed to within 8 ~ll~~lof being flat and level. It is 11 m wide by 17 m long. A gas handling system, including compressor, dryer, filters and two 1000 1 storage tanks are installed for the SF, insulating gas. All vacuum pump exhausts are ducted outside of the building. Special efforts were made to keep the sample preparation laboratory clean, with sealed windows, restricted entry with particle retention floor mats, filtered air and lowparticle generating ceiling tiles and furniture. It is located adjacent to the AMS laboratory, so that samples are not carried down a hall, or into another building. Sedimentology and beta-decay radiocarbon laboratories are located in the adjacent building.
2. Sample preparation The 92 mz sample preparation laboratory is designed to prepare graphite targets from the carbon in seawater, carbonate shells and organic matter. Large numbers of seawater samples, collected in the sampler shown in fig. 3, are expected beginning in the first quarter of 1991 and continuing throughout the 1990s. We expect to receive samples in 500 ml ground-glass stoppered bot-
tles. The water samples will be poisoned when collected to inhibit the microbial oxidation of organic matter. A portion of the water (300-400 ml) will be transferred to a stripping vessel and acidified to facilitate stripping the inorganic carbon as CO,. Using the line shown in fig. 4, CO, is entrained in a helium stream bubbled through the water, then condensed out and separated from the water vapor by a series of cold traps. Although the amount of CO, contained in seawater can vary with its origin, generally at least a millimole of carbon is contained in 500 ml of seawater. The CO, stripped from the water is measured and divided into aliquots which are transferred to the graphite preparation line. Several procedures have been developed for the conversion of CO, to graphite [3-61. Many of these have specific advantages/disadvantages regarding their ease of modification to the automation requirements of the National Ocean Sciences AMS Facility. We are presently evaluating the modifications required to each method to preserve sample quality and integrity while at the same time allowing the preparation laboratory to produce up to 60 targets per day. An example of one such extraction line, modified from ref. [3], that we have built and are testing is shown in fig. 5. Samples of 80-120 ymol carbon are reduced in reactors of approximately 6 ml volume and samples of 120-280 umol carbon are reduced in 11 ml volume II. NEW & FUTURE FACILITIES
G.A. Jones et al. / The National Ocean Sciences AMS Facility
(G)
(E)
(F)
(A)
(A)
Accelerator
(B)
SampldTerget
(Cl
Stable
(0)
Engineering
Spsctrometer
Preparation Spectrometer
(E)
Office Storage
Room
(G)
Utility
Roam
l
HP 9000 Work St&ion
l
Pwsonal computer
l
HP 3852 Oata Aquisith&ntml
l
HP StarLAN 10 N&n+ HP
Room
Laboratory Room
Laboratory
(F)
a
McLeen
Mess
Mess
hit
ti
Rwitable Optical Oisk Orive
Leboretory
Fig. 2. Floor plan of the National Ocean Sciences AMS Facility. The longest dimension is 37 m.
reactors. Briefly, the reaction occurs at high temperatures (600-650 o C) over a Co catalyst in the presence of a stoichiometric excess of H,. Water vapor produced during the reaction is trapped in a cold finger because it can slow down or inhibit the formation of graphite. The reaction takes 4-5 hours to go to completion. Although
the relative pressures are the same, the reaction is faster in the larger volume reactors. Currently, targets with a C/Co molar ratio of 0.8-1.5 are prepared. At the end of this process, the graphite-cobalt mixture is pressed mechanically into a solid pellet, which becomes the sputter target for the AMS ion source.
G.A. Jones ei al. / The National Ocean Sciences AMS Facility
3. Automation We have set up an automated process controller (Hewlett Packard 3852A) to operate valves and ovens, and record temperatures and pressures on a prototype
(6
281
sample preparation line (see above). We are using software originally developed for accelerator lab controls (Continuous Electron Beam Accelerator Facility, Newport News, Virginia) which allows graphics setup of the process on a screen prior to actual interfacing. It
210 CM FT.-IO IN.)
Fig. 3. Seawater sampler designed by Battelle Inc. for the WOCE project. This instrument is designed to collect up to 36 samples of 7 1 each per each lowering. This device will be used on all WOCE voyages throughout the 1990s. II. NEW & FUTURE
FACILITIES
282
G.A. Jones et al. / The National Ocean Sciences AMS Facility
Fig. 4. Extraction of CO, from seawater samples. The stripping vessel is used to bubble helium through the water, which carries CO2 with it. The gas is quantified before reduction.
runs on a UNIX workstation (HP 360). At present we have 40 pneumatically actuated glass valves (Vacutap) installed. Once we have finished evaluating the different sample preparation procedures, we will adapt one for our automation processes. Several parts of this process will utilize robotics, and we are investigating currently several laboratory and industrial style robots. The loading of sample bottles and the transfer of seawater to the stripper lines at the beginning, and the handling of
vscuw h/y i-
catalysts and graphites are the most cost effective applications. We are setting up a barcode system to track samples from the water bottle to the ion source.
4. AMS and MS systems The AMS instrument has been described in detail elsewhere [1,2,7] and is shown in fig. 6. It was conceived as an improvement over the original generation of AMS
Pin? ----_
II-11
Stainless Steel Line
/ L
Kwifold Atkhmt
Port %nifold Attachsent Pert
Fig. 5. Schematic of one of the graphitization processes being tested for automation. A measured amount of CC& is introduced into a quartz tube containing a quantity of catalyst. The reactors operate at 650°C with hydrogen to remove the oxygen, as the graphite deposits on the catalyst. After Vogel et al. [3].
283
Fig. 6. System built by US-AMS for Woods Hole Oceanographic Institution. This system contains dual 60.sample cartrlges with high current hemispherical ion sources, recombinator for simultaneous injection of the three carbon isotopes [12,13,14], and an enlarged accelerator tube and stripper canal.
instruments (at Oxford, Arizona, Toronto, Nagoya and
Gif-stir-Yvette) built by General Ionex Corporation in the early 1980s. Simultaneous acceleration and detection of all three carbon isotopes, rather than sequential meas~ements, should reduce machine fractionation. The accelerator column uses 32 cm diameter accelerator tubes, rather than 22 cm as in the earlier instruments. The column is under longitudinal compression and carefully voltage-graded. A larger, 1 cm diameter X 100 cm long argon gas stripping canal with pressure readout is provided. The c~ent-car~g capacity is 100 PA of carbon ions, as compared to 20 JJA for the presently operating systems. This is especially important when using the recombinator-type injector. The first ion source has been operational since March 1990 and we have analyzed “C beams from a few different target materials produced in our prototype grap~~tion lines. Spectroscopic graphite yields currents in excess of 100 PA, and the graphite-cobalt mixture yields currents about half as large. The cesium sputtering beam from the hemispherical ionizer focuses to less than 0.5 mm in diameter. We are planning to use 2 mm diameter targets. They are pressed into the ends of cartridges which are inserted into the 60-sample wheel of the ion source. If the targets yield 60 pA current for 30 min each, then 0.3% counting statistics can be obtained for samples of modem 14C activity. In this mode each full target wheel
would take 30 hours to run. The entire system has been designed to meet a minimum specification of 0.5% accuracy for carbon 14/12 ratios after any fractionation corrections are determined from the 13/12 ratios. A VG Prism stable isotope ratio mass spectrometer (VG ISOTECH) has been installed in the new facility. It is capable of measuring the carbon 13/12 and oxygen 18/16 ratios to better than 0.01 parts per mil. We intend to use this instrument for baseline measurements to check performance of the 13/12 measuring capability of the AMS inst~ment, and for stand-alone i3C and I80 measurements of scientific interest. Data from both of these instruments will be archived on an optical disk. All labs and offices are interconnected via a Hewlett Packard Starlan-10 network, which incorporates our HP360 workstations, PCs and the HP650A optical disk. We will have an interco~~tion to the WHOINET institution-DDE network, hence to the outside world. Authorized users will be able to obtain sample status and analysis results from an electronic bulletin board.
The authors would like to acknowledge the great amount of help from our research assistants Alan R. II. NEW & FUTURE FACILITIES
G.A. Jones et al. / The National Ocean Sciences AMS Facility
284
Gagnon and Gregory J. Cohen, as well as secretary C. Annie
[3] J.S. Vogel, J.R. Southon
Kauffman. [4] [5] [6]
References [l] K.H. Purser and T.H. Smick, these Proceedings Nucl. Instr. and Meth. B52 (1990) 263. (21 A.E. Litherland and K.H. Purser, ibid., p. 424.
(AMS
5)
[7]
and D.E. Nelson, Nucl. Instr. and Meth. B29 (1987) 50. P.J. Slota, Jr., A.J.T. Jull, T.W. Linick and L.J. Toolin, Radiocarbon 29 (1987) 303. D.M. Gurfinkel, Radiocarbon 29 (1987) 335. D.C. Lowe and W.J. Judd, Nucl. Instr. and Meth. B28 (1987) 113. K.H. Purser, T. Snuck, A.E. Litherland, R.P. Beukens, W.E. Kieser and L.R. Kilius, Nucl. Instr. and Meth. B35 (1988) 284.
Nuclear Instruments and Methods in Physics Research B52 (1990) 278-284 North-Holland
The National Ocean Sciences AMS Facility at Woods Hole Oceanographic Institution G.A. Jones, A.P. McNichol,
K.F. von Reden
and R.J. Schneider
Department of Geology and Geophysics, Woo& Hole Oceanographic Insrituion.
Woods Hole, Massachuserrs 02543. USA
A new and highly automated AMS facility for producin and analyzing graphite samples from seawater and other marine sources is nearing completion. Simultaneous measurements of “C. ? ‘C and 14C isotopes is planned with a throughput of up to 4500 samples per year.
1. Introduction In the spring of 1987 the United States National Science Foundation (NSF) issued a Request For Proposals (RFP) for establishing a national accelerator facility dedicated largely to the mission of the ocean sciences community. This RFP was precipitated as a result of the rapidly increasing demand for accelerator radiocarbon analyses within the ocean sciences community, increasing delays in obtaining results from existing facilities due to these new demands and the requirement for an estimated 3-4000 additional radiocarbon analyses per year through the 1990s for the two international projects WOCE (World Ocean Circulation Experiment) and GOFS (Global Ocean Flux Study). In July 1988 it was announced that the Woods Hole Oceanographic Institution (WHOI) was the successful requestor for establishing such a facility. Funding for the $5 million, three-year project began in January 1989. The facility as we conceived it in 1987 would have to be able to meet the following criteria to be successful: (1) process up to 4500 samples per year, (2) routinely achieve precisions of 3-5 per mil for seawater samples of near modern age, and (3) be able to distribute the results of these analyses rapidly to the scientific community. Successful implementation of these goals requires a new accelerator design; a commitment to automation at virtually all stages of sample preparation, analysis and data reduction; and access to facilities designed specifically for efficient AMS research. The Woods Hole Oceanographic Institution committed approximately 85% of the funds required to build an 864 m2 building designed specifically for, and dedicated exclusively to accelerator mass spectrometry. The remaining 15% of the building funds were supplied as part of the above-mentioned NSF funding. Construction of this building was begun in July 1989 and com0168-583X/90/$03.50
pleted in May 1990. The construction of a new-generation 3 MV tandem accelerator was contracted with US-AMS Corporation in February 1989 and delivery was initiated in September 1990. The details of the new capabilities of this high precision/high throughput machine are covered elsewhere within this volume [1,2]. We anticipate that we will be in an initial operational mode in early 1991. This will be in time to process water samples from the first Pacific leg of the WOCE project, scheduled to begin in March 1991. Once operational we anticipate the breakdown of analyses to be approximately 70% for global programs such as WOCE and GOFS; 20% for marine geology, paleoceanography and related subjects; and 10% for a wide range of miscellaneous radiocarbon applications. The facility will operate with a staff of ten (fig. 1). We started with a staff of two in January 1989 and presently (September 1990) consist of eight. Scientific and operational input is received from an intemal-toWHO1 advisory board consisting of seven members serving staggered three-year terms. In addition there is a joint WHOI-NSF appointed external national/intemational advisory board consisting of six members also with staggered three-year terms.
2. Facility The new AMS facility occupies an 864 m2 addition to the Institution’s existing McLean Laboratory. The floor plan of this addition is shown in fig. 2. In addition to the accelerator room, sample preparation, stable isotope mass spectrometer and engineering laboratories are included. In designing these laboratories, we attempted to utilize knowledge gained from experience with existing facilities. We have installed separate circuits for the various parts of the AMS system, with grounds retum-
0 1990 - Elsevier Science Publishers B.V. (North-Holland)
G.A. Jones et al. / The National
Ocean Sciences AMS
Facility
279
.-l,.r~+giq&q Fig. 1. Personnel wire diagram and relationships with WHO1 and National Advisory Boards.
ing to a common point at the transformer. This will avoid many of the common electrical ground-loop problems. The 0.64 m thick floor for the AMS system was poured separately from the rest of the building, and planed to within 8 ~ll~~lof being flat and level. It is 11 m wide by 17 m long. A gas handling system, including compressor, dryer, filters and two 1000 1 storage tanks are installed for the SF, insulating gas. All vacuum pump exhausts are ducted outside of the building. Special efforts were made to keep the sample preparation laboratory clean, with sealed windows, restricted entry with particle retention floor mats, filtered air and lowparticle generating ceiling tiles and furniture. It is located adjacent to the AMS laboratory, so that samples are not carried down a hall, or into another building. Sedimentology and beta-decay radiocarbon laboratories are located in the adjacent building.
2. Sample preparation The 92 mz sample preparation laboratory is designed to prepare graphite targets from the carbon in seawater, carbonate shells and organic matter. Large numbers of seawater samples, collected in the sampler shown in fig. 3, are expected beginning in the first quarter of 1991 and continuing throughout the 1990s. We expect to receive samples in 500 ml ground-glass stoppered bot-
tles. The water samples will be poisoned when collected to inhibit the microbial oxidation of organic matter. A portion of the water (300-400 ml) will be transferred to a stripping vessel and acidified to facilitate stripping the inorganic carbon as CO,. Using the line shown in fig. 4, CO, is entrained in a helium stream bubbled through the water, then condensed out and separated from the water vapor by a series of cold traps. Although the amount of CO, contained in seawater can vary with its origin, generally at least a millimole of carbon is contained in 500 ml of seawater. The CO, stripped from the water is measured and divided into aliquots which are transferred to the graphite preparation line. Several procedures have been developed for the conversion of CO, to graphite [3-61. Many of these have specific advantages/disadvantages regarding their ease of modification to the automation requirements of the National Ocean Sciences AMS Facility. We are presently evaluating the modifications required to each method to preserve sample quality and integrity while at the same time allowing the preparation laboratory to produce up to 60 targets per day. An example of one such extraction line, modified from ref. [3], that we have built and are testing is shown in fig. 5. Samples of 80-120 ymol carbon are reduced in reactors of approximately 6 ml volume and samples of 120-280 umol carbon are reduced in 11 ml volume II. NEW & FUTURE FACILITIES
G.A. Jones et al. / The National Ocean Sciences AMS Facility
(G)
(E)
(F)
(A)
(A)
Accelerator
(B)
SampldTerget
(Cl
Stable
(0)
Engineering
Spsctrometer
Preparation Spectrometer
(E)
Office Storage
Room
(G)
Utility
Roam
l
HP 9000 Work St&ion
l
Pwsonal computer
l
HP 3852 Oata Aquisith&ntml
l
HP StarLAN 10 N&n+ HP
Room
Laboratory Room
Laboratory
(F)
a
McLeen
Mess
Mess
hit
ti
Rwitable Optical Oisk Orive
Leboretory
Fig. 2. Floor plan of the National Ocean Sciences AMS Facility. The longest dimension is 37 m.
reactors. Briefly, the reaction occurs at high temperatures (600-650 o C) over a Co catalyst in the presence of a stoichiometric excess of H,. Water vapor produced during the reaction is trapped in a cold finger because it can slow down or inhibit the formation of graphite. The reaction takes 4-5 hours to go to completion. Although
the relative pressures are the same, the reaction is faster in the larger volume reactors. Currently, targets with a C/Co molar ratio of 0.8-1.5 are prepared. At the end of this process, the graphite-cobalt mixture is pressed mechanically into a solid pellet, which becomes the sputter target for the AMS ion source.
G.A. Jones ei al. / The National Ocean Sciences AMS Facility
3. Automation We have set up an automated process controller (Hewlett Packard 3852A) to operate valves and ovens, and record temperatures and pressures on a prototype
(6
281
sample preparation line (see above). We are using software originally developed for accelerator lab controls (Continuous Electron Beam Accelerator Facility, Newport News, Virginia) which allows graphics setup of the process on a screen prior to actual interfacing. It
210 CM FT.-IO IN.)
Fig. 3. Seawater sampler designed by Battelle Inc. for the WOCE project. This instrument is designed to collect up to 36 samples of 7 1 each per each lowering. This device will be used on all WOCE voyages throughout the 1990s. II. NEW & FUTURE
FACILITIES
282
G.A. Jones et al. / The National Ocean Sciences AMS Facility
Fig. 4. Extraction of CO, from seawater samples. The stripping vessel is used to bubble helium through the water, which carries CO2 with it. The gas is quantified before reduction.
runs on a UNIX workstation (HP 360). At present we have 40 pneumatically actuated glass valves (Vacutap) installed. Once we have finished evaluating the different sample preparation procedures, we will adapt one for our automation processes. Several parts of this process will utilize robotics, and we are investigating currently several laboratory and industrial style robots. The loading of sample bottles and the transfer of seawater to the stripper lines at the beginning, and the handling of
vscuw h/y i-
catalysts and graphites are the most cost effective applications. We are setting up a barcode system to track samples from the water bottle to the ion source.
4. AMS and MS systems The AMS instrument has been described in detail elsewhere [1,2,7] and is shown in fig. 6. It was conceived as an improvement over the original generation of AMS
Pin? ----_
II-11
Stainless Steel Line
/ L
Kwifold Atkhmt
Port %nifold Attachsent Pert
Fig. 5. Schematic of one of the graphitization processes being tested for automation. A measured amount of CC& is introduced into a quartz tube containing a quantity of catalyst. The reactors operate at 650°C with hydrogen to remove the oxygen, as the graphite deposits on the catalyst. After Vogel et al. [3].
283
Fig. 6. System built by US-AMS for Woods Hole Oceanographic Institution. This system contains dual 60.sample cartrlges with high current hemispherical ion sources, recombinator for simultaneous injection of the three carbon isotopes [12,13,14], and an enlarged accelerator tube and stripper canal.
instruments (at Oxford, Arizona, Toronto, Nagoya and
Gif-stir-Yvette) built by General Ionex Corporation in the early 1980s. Simultaneous acceleration and detection of all three carbon isotopes, rather than sequential meas~ements, should reduce machine fractionation. The accelerator column uses 32 cm diameter accelerator tubes, rather than 22 cm as in the earlier instruments. The column is under longitudinal compression and carefully voltage-graded. A larger, 1 cm diameter X 100 cm long argon gas stripping canal with pressure readout is provided. The c~ent-car~g capacity is 100 PA of carbon ions, as compared to 20 JJA for the presently operating systems. This is especially important when using the recombinator-type injector. The first ion source has been operational since March 1990 and we have analyzed “C beams from a few different target materials produced in our prototype grap~~tion lines. Spectroscopic graphite yields currents in excess of 100 PA, and the graphite-cobalt mixture yields currents about half as large. The cesium sputtering beam from the hemispherical ionizer focuses to less than 0.5 mm in diameter. We are planning to use 2 mm diameter targets. They are pressed into the ends of cartridges which are inserted into the 60-sample wheel of the ion source. If the targets yield 60 pA current for 30 min each, then 0.3% counting statistics can be obtained for samples of modem 14C activity. In this mode each full target wheel
would take 30 hours to run. The entire system has been designed to meet a minimum specification of 0.5% accuracy for carbon 14/12 ratios after any fractionation corrections are determined from the 13/12 ratios. A VG Prism stable isotope ratio mass spectrometer (VG ISOTECH) has been installed in the new facility. It is capable of measuring the carbon 13/12 and oxygen 18/16 ratios to better than 0.01 parts per mil. We intend to use this instrument for baseline measurements to check performance of the 13/12 measuring capability of the AMS inst~ment, and for stand-alone i3C and I80 measurements of scientific interest. Data from both of these instruments will be archived on an optical disk. All labs and offices are interconnected via a Hewlett Packard Starlan-10 network, which incorporates our HP360 workstations, PCs and the HP650A optical disk. We will have an interco~~tion to the WHOINET institution-DDE network, hence to the outside world. Authorized users will be able to obtain sample status and analysis results from an electronic bulletin board.
The authors would like to acknowledge the great amount of help from our research assistants Alan R. II. NEW & FUTURE FACILITIES
G.A. Jones et al. / The National Ocean Sciences AMS Facility
284
Gagnon and Gregory J. Cohen, as well as secretary C. Annie
[3] J.S. Vogel, J.R. Southon
Kauffman. [4] [5] [6]
References [l] K.H. Purser and T.H. Smick, these Proceedings Nucl. Instr. and Meth. B52 (1990) 263. (21 A.E. Litherland and K.H. Purser, ibid., p. 424.
(AMS
5)
[7]
and D.E. Nelson, Nucl. Instr. and Meth. B29 (1987) 50. P.J. Slota, Jr., A.J.T. Jull, T.W. Linick and L.J. Toolin, Radiocarbon 29 (1987) 303. D.M. Gurfinkel, Radiocarbon 29 (1987) 335. D.C. Lowe and W.J. Judd, Nucl. Instr. and Meth. B28 (1987) 113. K.H. Purser, T. Snuck, A.E. Litherland, R.P. Beukens, W.E. Kieser and L.R. Kilius, Nucl. Instr. and Meth. B35 (1988) 284.