- Email: [email protected]
Performance characteristics of the 3 MV Tandetron AMS system at the National Ocean Sciences AMS facility
Nuclear Instruments North-Holland
and Methods
in Physics
Research
B 92 (1994) 7-11 Beam Interactions with Materials 8 Atoms
Performance characteristics of the 3 MV Tandetron at the National Ocean Sciences AMS facility
AMS system
Karl F. von Reden *, Robert J. Schneider, Gregory J. Cohen and Glenn A. Jones National Ocean Sciences Accelerator Mass Spectrometry Facility, Woods Hole Oceanographic Institution, Woods Hole, MA 02540, USA
performance parameters are discussed for the National Ocean Sciences AMS system. The system now 50 and 100 carbon samples per week in largely unattended mode using one of two functional high-current ion sources. System development and procedures are described that enable us to reach and maintain the high precision level required for the measurement of deep sea water dissolved inorganic carbon samples. Operational
and machine
routinely measures between
1. Introduction
The National Ocean Sciences AMS (NOSAMS) facility is in its third year of operation. Since January 1991, steady progress has been made to achieve a state of the art level of performance for AMS systems. The main goals for the facility in full operation are to process in excess of 4000 carbon samples per year at precision levels of 0.5% or better. In this paper we will describe the machine specific steps that were taken to reach these goals. Aspects of the NOSAMS facility operation, relating to sample preparation, specific data sets, and data analysis are discussed in other papers at this conference [ 11.
2. System configuration The Woods Hole AMS system has been described in several references [2-51. We will give here a more detailed account of our experience with the system and its present performance. The tandem accelerator, built by US-AMS Co., a successor to General Ionex Corp. (GIG), is a further development of the GIC Tandetron, currently used in several labs worldwide. Fig. 1 shows the AMS setup at the NOSAMS facility in its present configuration. The two virtually identical injectors can be operated in alternating mode to allow quasi-uninterrupted data acquisition. The injectors consist of high-current hemispherical ionizer cesium sputter ion sources and pre-acceleration mass selectors for A = 12,13,14 (“recombinator”) [3,6,7]. Faraday cups and flaps in the symmetry plane of the recombinator allow
analysis of the individual components of the injected beam. In addition, the “C beam is chopped by a factor of about 95 with a rotating slotted disk to reduce beam loading in the high energy spectrometer. The third generation Tandetron has enlarged acceleration tubes for higher current capability (0.32 m outer diameter), magnetic and electrostatic electron suppression, and the positive ion acceleration tube has an inclined-field design [4] to reduce the backgrounds due to ME/q2 ambiguities. The parallel-fed Cockcroft-Walton type solid state power supply operates at 35 kHz and has been quite reliable and stable, when the system is allowed to reach thermal equilibrium, and the accelerator tank is pressurized to 5 atm of SF, gas, dried to a dew point of below -40°C. The high-energy mass spectrometer, consisting of a 110” magnet, a 33” electrostatic deflector, and a 90” magnet, is designed to suppress backgrounds in the mass 14 channel by lo-l6 at a vacuum of better than lo-’ Torr in this section. The seven major sections of the AMS system are electrically isolated from each other, powered independently, and grounded at a common ground at one point in the building. Any crosstalk between sections in this star configuration is greatly reduced and has not been a problem in our operation. In order to maintain the electrical isolation between the sections all digital and analog device control and monitor lines are fiber optic links [8] with better than 10V3 resolution in the voltage-to-frequency conversion. Faraday cups and 14C gas detector are isolated from the beam line and grounded at the central control station.
3. System controls * Corresponding author. Tel.+ 1 508 457 2000 (ext. 3384), fax+ 1 508 457 2183, e-mail [email protected]. 0168-583X/94/$07.00 0 1994 - Eisevier SSDI 0168-583X(94)00008-J
One of the central features of the Woods Hole AMS system is its three-stage control design: hard-
Science B.V. All rights reserved
I. FACILITY
REPORTS
K.F. uon Reden et al. / Nucl. Instr. and Mrth. in Phys. Res. B 92 (1994) 7-11
8
ware-level, low-level processor, and high-level control processor. The two processor levels and the critical ion source controls are backed by uninterruptible power supplies. This allows a safe shutdown of the system in the event of a sudden failure, while retaining important system parameters, especially the ion source parameters. The entire laboratory (60 kW average total load, 30 kW of which is for the AMS) is switched to a backup generator in case of power failure, with a switch-over time of less than 20 s. This is sufficient to maintain the high vacuum in the beam line. In normal operation no direct hardware level intervention by the operator is necessary or possible, all devices are controlled through the low-level processor, an HP3852A Data Acquisition and Control unit (DATAC) [8]. This processor is programmed in DATAC Basic and serves to monitor all system parameters with a multiplexed
--
T
high-speed voltmeter, maintain the digital logic table, run the system interlock task, and apply the control voltages to the power supplies. DATAC and the highlevel processor stage are linked via an IEEE-488 bus, controlled by the main system computer, a PC-386. A Keithley 619 Electrometer [9] for “C and 13C current measurements and an eight-channel Kepco precision power supply controller [lo] are also connected to this bus. Commands issued by the operator or operating system on the main computer are filtered by the interlock task in the DATAC to ensure safe system conditions. For instance, gate valves will not open unless the pressure on both sides of the valve is near equilibrium (instead a warning message is displayed on the system monitor). The two ion source sample changers are independently controlled by two PC 286, running a monitoring/control program in continuous mode. They,
CH
SL Y2 DE
Fig. 1. Three-megavolt Tandetron AMS system at the NOSAMS facility. ISlJS2, cesium sputter ion sources; EL, einzel lens; X(n),Y(n), beam steerers; M45, 4.5” magnet; SL, slot lens; CH, “C beam chopper; QS, Q-snout lens; LE, low-energy acceleration tube; HE, high-energy acceleration tube; QPD, electrostatic quadrupole lens doublet; MllO, 110” spectrometer magnet; FC, Faraday cup; D33, 33” electrostatic deflector; M90, 90” spectrometer magnet; DET, isobutane gas ionization detector; T(n), turbomolecular pump; C(n), cryogenic pump. Not shown in this schematic plot is the center section of the Tandetron with the Cockcroft-Walton-type solid state power supply and the terminal, containing the 1 m long, 1.25 cm diameter argon stripper canal with turbomolecular recirculation pump (see ref. [4] for a description).
KF. uon Reden et al. / Nucl. Instr. and Meth. in Phys. Res. B 92 (1994) 7-11
too, are interfaced via fiber optics to maintain electric isolation. In data acquisition mode, the on-line source PC is switched into slave status and controlled over serial link by the main system PC. Deviating from the initial US-AMS Corp. design we have replaced the Kepco control for the EMI bending magnet supplies [ll] with precision Zeranin shunt regulated controls by Congruent Design [12], operated through the parallel PC port with opto-coupling (instead of fiber-optics link). This measure increased the magnet stability from 5 x lop4 to better than 10-4. We also replaced the unipolar EM1 magnet power supply for the central injector magnet (switching magnet) with a Lakeshore [13] bipolar supply that allows us to switch remotely between the two injectors within less than 5 min. The isobutane gas ionization chamber is operated at 20 mbar static pressure, in AE-E mode. Using standard NIM electronics a gate is generated from the AE-E coincidence signal to allow only particles with proper kinematics to be counted in the pulse height spectrum.
9
4. Operation
Fig. 2. Comparison of the performance of the two injectors at NOSAMS. For the same sample wheel one measurement cycle was run on ion source #l and a second cycle on ion source #2. Most of the repeat measurements fall within one standard deviation of the initial data. A few samples showed signs of burn-out in the second run and have larger deviations.
As might be expected, the ion source parameters have been the most critical system parameters. Good vacuum ( < 5 X 10m6 Torr) and thermal equilibrium of all components are essential. Excessive space charge effects and Cs saturation effects have to be avoided. For extracted 12C currents of up to 100 uA, calculations with the ion source modeling code “SORCERY” [14] show that space charge related changes in the source emittance (i.e. fractionation) are small, if deep sputter cratering is avoided. We therefore “scan” the cylindrical carbon sample (0.75 mm radius, up to 2 mm deep) laterally in an octagonal pattern to achieve a shallow sputter well of 0.5 mm radius, up to 0.25 mm depth. The total current between target and ionizer has to be kept below 2 mA for stable operation. A cleaning of the ion source is necessary roughly every 20 Coulomb of 12C extracted (approximately two wheels of 56 samples at 50 uA ‘*Cl. Usually the end of the lifetime cycle is indicated by increased ionizer-target currents and eventual breakdown of the ionizer-target voltage. As the main cause for the breakdown, a buildup of filaments between target cone and ionizer shroud has been identified ( Egap = 20 kV/cm). In most cases a fast regeneration procedure can be performed, preserving the Cs reservoir under Ar atmosphere. A dry clean-up of the ionizer shroud, target cone, and target holder and replacement of the target cone aperture insert take about 30 min, bringing the entire procedure to less than an hour between venting and reevacuation. The presence of two ion sources in our system has the advantage that any work on a source can be done
off-line, while the other source is on-line. This mode of operation will soon be a significant factor when up to three wheels are run per week (currently the rate is 1.5 wheels per week). A point of concern has been the beam-optical compatibility of the two injectors with respect to the remainder of the spectrometer. A slight mass dependence of the focal length of the recombinator design, expected from beam-optics calculations using measured fringe field parameters for the 45” magnets, does not seem to affect the transmission of the three carbon isotopes in the remainder of the system. Except for small steering differences the tuning parameters for the two injectors are very similar. No retuning of the remainder of the spectrometer is necessary when the operation is switched from one injector to the other. Fig. 2 shows a comparison of the performance of the two injectors. A sample wheel was first run on ion source 1 and then on source 2 for a repeat measurement. The reproducibility is generally within one standard deviation of the measurements. It is planned to do cross calibrations of this kind on a regular basis to maintain the confidence level in our system. The accelerator terminal is routinely conditioned between 2-3 day data acquisition cycles. An automatic routine, initiated from the main console, cycles the terminal voltage in a sawtooth pattern of 100 kV height, up to base levels of 2.9 MV, thus reaching the design value of 3 MV, without excessive terminal instability. The set point for data acquisition is presently 2.5 MV, somewhat below the value for optimal stripping effiI. FACILITY REPORTS
10
K.F. von Reden et al. /Nucl.
Instr. and Meth. in Phys. Res. B 92 (1994) 7-11
ciency for the combined carbon isotopes in argon [15]. It is planned to raise the set point to the optimal value of about 2.7 MV in the near future. At this time the terminal voltage is regulated using the feedback from a generating voltmeter, mounted in the tank wall near the terminal. In steady environmental conditions the terminal stability is adequate: a 35 kHz ripple of less than 40 V p/p, and a low frequency (360 Hz) ripple of 140 V p/p were measured using a capacitive pick-up plate of 10 cm diameter in the accelerator tank wall near the terminal. In thermal equilibrium, dc drifts are less than 200 V per day. Even though these numbers are good enough for high-quality data acquisition, a terminal stabilization scheme using the feed back from a beam position monitor for one of the analyzed beams is presently being designed. This measure would make the system more independent of environmental fluctuations while running in unattended mode. All system parameters are continually kept at a basic tune level. Thermal drifting of parameters has been observed for up to 24 h after a cold start. In this mode, after the initial warm-up of one of the ion sources, 14C signals from a reasonably modern sample can instantaneously be observed in the gas ionization chamber. From this starting point a fine-tune procedure is performed to optimize first the 12,13C currents after the 110” magnet and then the 14C counting rate in the detector. Using a known modern standard sample (NIST Oxalic Acid I or II) the sum of the ‘*C and 13C currents and their ratio are monitored simultaneously, while in an iterative procedure all beam optics devices are cycled around their established set points to ensure optimal transmission in the center of a flat-topped region for the device parameter. The i2C/13C ratio is a sensitive parameter that helps to avoid lopsided tunes, overemphasizing one or the other of the two currents. Depending on the hydrogen;onat+ents in a;amp+le relaand C tively intense beams of C from stripper-dissociated CH- ions can be present in the analyzed spectrum of the 110” magnet. Their separation from the CC+ C3+) main beams is 25 mm (A = 12) and 22.5 mm (A = 13) because of their slightly lower energy. We therefore reduced the width of the aperture in front of the 13C Faraday cup from 12 to 9 mm to reduce the possibility of halo from the hydride beam entering the cup. The 14C rate is optimized by inserting a 6.3 mm wide slit at the entrance of the 33” deflector. Iteratively, the terminal voltage, the deflector voltage, and the 90” magnet field are cycled around their set points, in order to optimize the 14C rate in the detector. The small changes (&SO0 V) of the terminal voltage necessary for 14C optimization do not affect the 12C,13C parameters established before. After the optimal transmission for 14C has been established, the deflector entrance slit is removed, and the system is ready for data acquisition.
5. Data handling A sample wheel contains 58 samples: 45 unknown samples, 10 known modern standards, two known background standards, and one aluminum machine blank. One of the modern standards is used for tuning and the aluminum blank for “parking” the cesium sputter beam between run cycles and after completion of data acquisition. Before the start of the data acquisition, a wheel inventory file, containing origin, preparation, and wheel position information for the samples, is transferred from the central database via network link to the main control PC. That file is then downloaded to the on-line source PC; a wheel position directory is created and displayed on the PC monitor as part of the source control screen. From the same file the data acquisition program extracts and forms file handles for AMS data storage. Note that at no time is manual data entry required, thus removing human errors in the data handling. The only actions performed by the operator are file transfers, file sorting, and rearrangement of sample records to establish a run order for the samples. The data acquisition is performed in unattended run cycles of 40-50 h periods, during which each of the unknown 45 and 11 standard samples on a wheel is repeatedly measured up to 45 min per sample. Monitors at the console and outside the laboratory in an exhibit area display the 14C pulse height spectrum together with on-line estimates of the sample radiocarbon “age”. A second monitor at the console tabulates the real-time acquisition data. Examples of measurements and data handling on our system can be found in the accompanying papers [l]. For a 50% modern sample in normal machine operation our system will yield better than 0.3% statistical precision in about 45 min of data acquisition. This is the minimal condition to reach the required deep water sample accuracy of better than 0.4%. In the described run mode the overall system dead time is less than 20%. The individual measurement on a specific sample occurs in blocks of 20 s, during which the gate of the PCA II pulse height analyzer [16] is opened and 10 readings each of the 12C and 13C Faraday cup currents are taken with the Keithley 619 dual channel electrometer. The two currents are averaged and block ratios of the three isotopes are calculated. All raw block data are stored in binary form for a later off-line replay of the data, if necessary. At the same time a summary text file, containing the block data, is updated after completion of each block. Sporadic source instabilities, poor target performance or other intermittent events in the system may lead to sudden fluctuations in the data, leading to a flawed data block. The typical stability of the 13C/12C ratio is kO.l% per block. For the on-line processing some acceptance criteria are set for the data. Two windows are set for the 13C/12Cci,,,p ratio, a coarse
ELF. uon Reden et al. / Nucl. Instr. and Meth. in Phys. Res. B 92 (1994) 7-11
static window of [0.85,1.15], and a 1% dynamic window between consecutive readings (when two consecutive ratios deviate by more than l%, the data block is interrupted, marked “rejected”, and a new data block is started). Each data block corresponds to one of eight sputter positions of the octagonal “scanning” pattern described above. The PCA II pulse height spectrum contains a single mass 14 peak at 10 MeV with 5% resolution (independent of the presence of the AE-E gate), indicating the high efficiency of the magnet-deflector-magnet filter arrangement (see refs. [4,5] for examples of typical pulse height spectra). A software window of 2 MeV around the centroid is set to determine the peak integral and the peak-to-background ratio, in case events are observed outside that window. While data acquisition is running, all machine parameters are continually monitored by the DATAC interlock task and in the event of a primary failure the system is shut down to a safe state. Intermittent faults cause specific routines to attempt recovery on-line or while data acquisition is suspended. For instance, sparking in the ion sources may trip the local turbomolecular pump controller. Automatic restart of the pump is initiated by the interlock task for a preset number of times in a given time period. If the restart fails, gate valves will be closed and the system will be shut down. After completion of a run cycle all data files are transferred to permanent storage on a network disk. The contents of the status file are also imported into the central data base for final processing and storage.
6. Conclusions The Woods Hole AMS system has reached full operation 2.5 years after installation. With two functional high-current ion sources the system is now capable of measuring in excess of 100 carbon samples per week at accuracy levels of better than 0.5% for samples with a fraction modern carbon content of more than 50%. Future developments will include beam position feed back terminal stabilization and the possible extension of the range of isotopes to other light isotopes (“Be, 26Al>in th e near term, and heavy isotopes in the long term (iz91). Development of a specialized ion source for organic compounds is also being studied.
11
Acknowledgment
We would like to acknowledge support from the U.S. National Science Foundation, Cooperative Agreement OCE-8801015.
References [ll R.J. Schneider et al., these Proceedings (6th Int. Conf.
on Accelerator Mass Spectrometry CAMS-6),CanberraSydney, Australia, 19931 Nucl. Instr. and Meth. B 92 (1994) 172; A.P. McNichoI et al., these Proceedings (6th Int. Conf. on Accelerator Mass Spectrometry CAMS-61, CanberraSydney, Australia, 1993) Nucl. Instr. and Meth. B 92 (1994) 162; G.J. Cohen et al., these Proceedings (6th Int. Conf. on Accelerator Mass Spectrometry CAMS-61,Canberra-Sydney, Australia, 19931 Nucl. Instr. and Meth. B 92 (1994) 133; G.A. Jones et al., these Proceedings (6th Int. Conf. on Accelerator Mass Spectrometry CAMS-61, Canberra-Sydney, Australia, 1993) Nucl. Instr. and Meth. B 92 (1994) 426; F.H. Stguin et al., these Proceedings (6th Int. Conf. on Accelerator Mass Spectrometry CAMS-61,Canberra-Sydney, Australia, 1993) Nucl. Instr. and Meth. B 92 (1994) 176. 121 K.H. Purser et al., Nucl. Instr. and Meth. B 52 (1990) 263. [31 R.J. Schneider, K.F. von Reden and K.H. Purser, IEEE Part. Accel. Conf. 2 (1991) 878. I41 K.H. Purser, Radiocarbon 34 (1992) 458. 151 K.F. von Reden et al., Radiocarbon 34 (1992) 478. [61 D.E. Lobb, J.R. Southon, D.E. Nelson, W. Wiesenhan and R.G. Korteling, Nucl. Instr. and Meth. 179 (1981) 171. 171A.E. Litherland and L.R. Kilius, Nucl. Instr. and Meth. B 52 (1990) 375. [81 Hewlett-Packard Co., Palo Alto, CA. 191 Keithley Corp., Cleveland, Ohio, Model 619. [lo] Kepco Corp., Flushing, NY, Model SNR-488-8. 1111Electronics Measurements Inc., Neptune, NJ, Model TCR 40T125. 1121 Congruent Design Inc., Guilford, CT. [13] Lakeshore Cryotronics, Westerville, OH, Model 637. [14] Computer code SORCERY, N.R. White, Wenham, MA. [151 G. Bonani et al., Nucl. Instr. and Meth. B 52 (1990) 338. 1161The Nucleus Inc., Oak Ridge, TN, Personal Computer Analyzer II.
I. FACILITY REPORTS
and Methods
in Physics
Research
B 92 (1994) 7-11 Beam Interactions with Materials 8 Atoms
Performance characteristics of the 3 MV Tandetron at the National Ocean Sciences AMS facility
AMS system
Karl F. von Reden *, Robert J. Schneider, Gregory J. Cohen and Glenn A. Jones National Ocean Sciences Accelerator Mass Spectrometry Facility, Woods Hole Oceanographic Institution, Woods Hole, MA 02540, USA
performance parameters are discussed for the National Ocean Sciences AMS system. The system now 50 and 100 carbon samples per week in largely unattended mode using one of two functional high-current ion sources. System development and procedures are described that enable us to reach and maintain the high precision level required for the measurement of deep sea water dissolved inorganic carbon samples. Operational
and machine
routinely measures between
1. Introduction
The National Ocean Sciences AMS (NOSAMS) facility is in its third year of operation. Since January 1991, steady progress has been made to achieve a state of the art level of performance for AMS systems. The main goals for the facility in full operation are to process in excess of 4000 carbon samples per year at precision levels of 0.5% or better. In this paper we will describe the machine specific steps that were taken to reach these goals. Aspects of the NOSAMS facility operation, relating to sample preparation, specific data sets, and data analysis are discussed in other papers at this conference [ 11.
2. System configuration The Woods Hole AMS system has been described in several references [2-51. We will give here a more detailed account of our experience with the system and its present performance. The tandem accelerator, built by US-AMS Co., a successor to General Ionex Corp. (GIG), is a further development of the GIC Tandetron, currently used in several labs worldwide. Fig. 1 shows the AMS setup at the NOSAMS facility in its present configuration. The two virtually identical injectors can be operated in alternating mode to allow quasi-uninterrupted data acquisition. The injectors consist of high-current hemispherical ionizer cesium sputter ion sources and pre-acceleration mass selectors for A = 12,13,14 (“recombinator”) [3,6,7]. Faraday cups and flaps in the symmetry plane of the recombinator allow
analysis of the individual components of the injected beam. In addition, the “C beam is chopped by a factor of about 95 with a rotating slotted disk to reduce beam loading in the high energy spectrometer. The third generation Tandetron has enlarged acceleration tubes for higher current capability (0.32 m outer diameter), magnetic and electrostatic electron suppression, and the positive ion acceleration tube has an inclined-field design [4] to reduce the backgrounds due to ME/q2 ambiguities. The parallel-fed Cockcroft-Walton type solid state power supply operates at 35 kHz and has been quite reliable and stable, when the system is allowed to reach thermal equilibrium, and the accelerator tank is pressurized to 5 atm of SF, gas, dried to a dew point of below -40°C. The high-energy mass spectrometer, consisting of a 110” magnet, a 33” electrostatic deflector, and a 90” magnet, is designed to suppress backgrounds in the mass 14 channel by lo-l6 at a vacuum of better than lo-’ Torr in this section. The seven major sections of the AMS system are electrically isolated from each other, powered independently, and grounded at a common ground at one point in the building. Any crosstalk between sections in this star configuration is greatly reduced and has not been a problem in our operation. In order to maintain the electrical isolation between the sections all digital and analog device control and monitor lines are fiber optic links [8] with better than 10V3 resolution in the voltage-to-frequency conversion. Faraday cups and 14C gas detector are isolated from the beam line and grounded at the central control station.
3. System controls * Corresponding author. Tel.+ 1 508 457 2000 (ext. 3384), fax+ 1 508 457 2183, e-mail [email protected]. 0168-583X/94/$07.00 0 1994 - Eisevier SSDI 0168-583X(94)00008-J
One of the central features of the Woods Hole AMS system is its three-stage control design: hard-
Science B.V. All rights reserved
I. FACILITY
REPORTS
K.F. uon Reden et al. / Nucl. Instr. and Mrth. in Phys. Res. B 92 (1994) 7-11
8
ware-level, low-level processor, and high-level control processor. The two processor levels and the critical ion source controls are backed by uninterruptible power supplies. This allows a safe shutdown of the system in the event of a sudden failure, while retaining important system parameters, especially the ion source parameters. The entire laboratory (60 kW average total load, 30 kW of which is for the AMS) is switched to a backup generator in case of power failure, with a switch-over time of less than 20 s. This is sufficient to maintain the high vacuum in the beam line. In normal operation no direct hardware level intervention by the operator is necessary or possible, all devices are controlled through the low-level processor, an HP3852A Data Acquisition and Control unit (DATAC) [8]. This processor is programmed in DATAC Basic and serves to monitor all system parameters with a multiplexed
--
T
high-speed voltmeter, maintain the digital logic table, run the system interlock task, and apply the control voltages to the power supplies. DATAC and the highlevel processor stage are linked via an IEEE-488 bus, controlled by the main system computer, a PC-386. A Keithley 619 Electrometer [9] for “C and 13C current measurements and an eight-channel Kepco precision power supply controller [lo] are also connected to this bus. Commands issued by the operator or operating system on the main computer are filtered by the interlock task in the DATAC to ensure safe system conditions. For instance, gate valves will not open unless the pressure on both sides of the valve is near equilibrium (instead a warning message is displayed on the system monitor). The two ion source sample changers are independently controlled by two PC 286, running a monitoring/control program in continuous mode. They,
CH
SL Y2 DE
Fig. 1. Three-megavolt Tandetron AMS system at the NOSAMS facility. ISlJS2, cesium sputter ion sources; EL, einzel lens; X(n),Y(n), beam steerers; M45, 4.5” magnet; SL, slot lens; CH, “C beam chopper; QS, Q-snout lens; LE, low-energy acceleration tube; HE, high-energy acceleration tube; QPD, electrostatic quadrupole lens doublet; MllO, 110” spectrometer magnet; FC, Faraday cup; D33, 33” electrostatic deflector; M90, 90” spectrometer magnet; DET, isobutane gas ionization detector; T(n), turbomolecular pump; C(n), cryogenic pump. Not shown in this schematic plot is the center section of the Tandetron with the Cockcroft-Walton-type solid state power supply and the terminal, containing the 1 m long, 1.25 cm diameter argon stripper canal with turbomolecular recirculation pump (see ref. [4] for a description).
KF. uon Reden et al. / Nucl. Instr. and Meth. in Phys. Res. B 92 (1994) 7-11
too, are interfaced via fiber optics to maintain electric isolation. In data acquisition mode, the on-line source PC is switched into slave status and controlled over serial link by the main system PC. Deviating from the initial US-AMS Corp. design we have replaced the Kepco control for the EMI bending magnet supplies [ll] with precision Zeranin shunt regulated controls by Congruent Design [12], operated through the parallel PC port with opto-coupling (instead of fiber-optics link). This measure increased the magnet stability from 5 x lop4 to better than 10-4. We also replaced the unipolar EM1 magnet power supply for the central injector magnet (switching magnet) with a Lakeshore [13] bipolar supply that allows us to switch remotely between the two injectors within less than 5 min. The isobutane gas ionization chamber is operated at 20 mbar static pressure, in AE-E mode. Using standard NIM electronics a gate is generated from the AE-E coincidence signal to allow only particles with proper kinematics to be counted in the pulse height spectrum.
9
4. Operation
Fig. 2. Comparison of the performance of the two injectors at NOSAMS. For the same sample wheel one measurement cycle was run on ion source #l and a second cycle on ion source #2. Most of the repeat measurements fall within one standard deviation of the initial data. A few samples showed signs of burn-out in the second run and have larger deviations.
As might be expected, the ion source parameters have been the most critical system parameters. Good vacuum ( < 5 X 10m6 Torr) and thermal equilibrium of all components are essential. Excessive space charge effects and Cs saturation effects have to be avoided. For extracted 12C currents of up to 100 uA, calculations with the ion source modeling code “SORCERY” [14] show that space charge related changes in the source emittance (i.e. fractionation) are small, if deep sputter cratering is avoided. We therefore “scan” the cylindrical carbon sample (0.75 mm radius, up to 2 mm deep) laterally in an octagonal pattern to achieve a shallow sputter well of 0.5 mm radius, up to 0.25 mm depth. The total current between target and ionizer has to be kept below 2 mA for stable operation. A cleaning of the ion source is necessary roughly every 20 Coulomb of 12C extracted (approximately two wheels of 56 samples at 50 uA ‘*Cl. Usually the end of the lifetime cycle is indicated by increased ionizer-target currents and eventual breakdown of the ionizer-target voltage. As the main cause for the breakdown, a buildup of filaments between target cone and ionizer shroud has been identified ( Egap = 20 kV/cm). In most cases a fast regeneration procedure can be performed, preserving the Cs reservoir under Ar atmosphere. A dry clean-up of the ionizer shroud, target cone, and target holder and replacement of the target cone aperture insert take about 30 min, bringing the entire procedure to less than an hour between venting and reevacuation. The presence of two ion sources in our system has the advantage that any work on a source can be done
off-line, while the other source is on-line. This mode of operation will soon be a significant factor when up to three wheels are run per week (currently the rate is 1.5 wheels per week). A point of concern has been the beam-optical compatibility of the two injectors with respect to the remainder of the spectrometer. A slight mass dependence of the focal length of the recombinator design, expected from beam-optics calculations using measured fringe field parameters for the 45” magnets, does not seem to affect the transmission of the three carbon isotopes in the remainder of the system. Except for small steering differences the tuning parameters for the two injectors are very similar. No retuning of the remainder of the spectrometer is necessary when the operation is switched from one injector to the other. Fig. 2 shows a comparison of the performance of the two injectors. A sample wheel was first run on ion source 1 and then on source 2 for a repeat measurement. The reproducibility is generally within one standard deviation of the measurements. It is planned to do cross calibrations of this kind on a regular basis to maintain the confidence level in our system. The accelerator terminal is routinely conditioned between 2-3 day data acquisition cycles. An automatic routine, initiated from the main console, cycles the terminal voltage in a sawtooth pattern of 100 kV height, up to base levels of 2.9 MV, thus reaching the design value of 3 MV, without excessive terminal instability. The set point for data acquisition is presently 2.5 MV, somewhat below the value for optimal stripping effiI. FACILITY REPORTS
10
K.F. von Reden et al. /Nucl.
Instr. and Meth. in Phys. Res. B 92 (1994) 7-11
ciency for the combined carbon isotopes in argon [15]. It is planned to raise the set point to the optimal value of about 2.7 MV in the near future. At this time the terminal voltage is regulated using the feedback from a generating voltmeter, mounted in the tank wall near the terminal. In steady environmental conditions the terminal stability is adequate: a 35 kHz ripple of less than 40 V p/p, and a low frequency (360 Hz) ripple of 140 V p/p were measured using a capacitive pick-up plate of 10 cm diameter in the accelerator tank wall near the terminal. In thermal equilibrium, dc drifts are less than 200 V per day. Even though these numbers are good enough for high-quality data acquisition, a terminal stabilization scheme using the feed back from a beam position monitor for one of the analyzed beams is presently being designed. This measure would make the system more independent of environmental fluctuations while running in unattended mode. All system parameters are continually kept at a basic tune level. Thermal drifting of parameters has been observed for up to 24 h after a cold start. In this mode, after the initial warm-up of one of the ion sources, 14C signals from a reasonably modern sample can instantaneously be observed in the gas ionization chamber. From this starting point a fine-tune procedure is performed to optimize first the 12,13C currents after the 110” magnet and then the 14C counting rate in the detector. Using a known modern standard sample (NIST Oxalic Acid I or II) the sum of the ‘*C and 13C currents and their ratio are monitored simultaneously, while in an iterative procedure all beam optics devices are cycled around their established set points to ensure optimal transmission in the center of a flat-topped region for the device parameter. The i2C/13C ratio is a sensitive parameter that helps to avoid lopsided tunes, overemphasizing one or the other of the two currents. Depending on the hydrogen;onat+ents in a;amp+le relaand C tively intense beams of C from stripper-dissociated CH- ions can be present in the analyzed spectrum of the 110” magnet. Their separation from the CC+ C3+) main beams is 25 mm (A = 12) and 22.5 mm (A = 13) because of their slightly lower energy. We therefore reduced the width of the aperture in front of the 13C Faraday cup from 12 to 9 mm to reduce the possibility of halo from the hydride beam entering the cup. The 14C rate is optimized by inserting a 6.3 mm wide slit at the entrance of the 33” deflector. Iteratively, the terminal voltage, the deflector voltage, and the 90” magnet field are cycled around their set points, in order to optimize the 14C rate in the detector. The small changes (&SO0 V) of the terminal voltage necessary for 14C optimization do not affect the 12C,13C parameters established before. After the optimal transmission for 14C has been established, the deflector entrance slit is removed, and the system is ready for data acquisition.
5. Data handling A sample wheel contains 58 samples: 45 unknown samples, 10 known modern standards, two known background standards, and one aluminum machine blank. One of the modern standards is used for tuning and the aluminum blank for “parking” the cesium sputter beam between run cycles and after completion of data acquisition. Before the start of the data acquisition, a wheel inventory file, containing origin, preparation, and wheel position information for the samples, is transferred from the central database via network link to the main control PC. That file is then downloaded to the on-line source PC; a wheel position directory is created and displayed on the PC monitor as part of the source control screen. From the same file the data acquisition program extracts and forms file handles for AMS data storage. Note that at no time is manual data entry required, thus removing human errors in the data handling. The only actions performed by the operator are file transfers, file sorting, and rearrangement of sample records to establish a run order for the samples. The data acquisition is performed in unattended run cycles of 40-50 h periods, during which each of the unknown 45 and 11 standard samples on a wheel is repeatedly measured up to 45 min per sample. Monitors at the console and outside the laboratory in an exhibit area display the 14C pulse height spectrum together with on-line estimates of the sample radiocarbon “age”. A second monitor at the console tabulates the real-time acquisition data. Examples of measurements and data handling on our system can be found in the accompanying papers [l]. For a 50% modern sample in normal machine operation our system will yield better than 0.3% statistical precision in about 45 min of data acquisition. This is the minimal condition to reach the required deep water sample accuracy of better than 0.4%. In the described run mode the overall system dead time is less than 20%. The individual measurement on a specific sample occurs in blocks of 20 s, during which the gate of the PCA II pulse height analyzer [16] is opened and 10 readings each of the 12C and 13C Faraday cup currents are taken with the Keithley 619 dual channel electrometer. The two currents are averaged and block ratios of the three isotopes are calculated. All raw block data are stored in binary form for a later off-line replay of the data, if necessary. At the same time a summary text file, containing the block data, is updated after completion of each block. Sporadic source instabilities, poor target performance or other intermittent events in the system may lead to sudden fluctuations in the data, leading to a flawed data block. The typical stability of the 13C/12C ratio is kO.l% per block. For the on-line processing some acceptance criteria are set for the data. Two windows are set for the 13C/12Cci,,,p ratio, a coarse
ELF. uon Reden et al. / Nucl. Instr. and Meth. in Phys. Res. B 92 (1994) 7-11
static window of [0.85,1.15], and a 1% dynamic window between consecutive readings (when two consecutive ratios deviate by more than l%, the data block is interrupted, marked “rejected”, and a new data block is started). Each data block corresponds to one of eight sputter positions of the octagonal “scanning” pattern described above. The PCA II pulse height spectrum contains a single mass 14 peak at 10 MeV with 5% resolution (independent of the presence of the AE-E gate), indicating the high efficiency of the magnet-deflector-magnet filter arrangement (see refs. [4,5] for examples of typical pulse height spectra). A software window of 2 MeV around the centroid is set to determine the peak integral and the peak-to-background ratio, in case events are observed outside that window. While data acquisition is running, all machine parameters are continually monitored by the DATAC interlock task and in the event of a primary failure the system is shut down to a safe state. Intermittent faults cause specific routines to attempt recovery on-line or while data acquisition is suspended. For instance, sparking in the ion sources may trip the local turbomolecular pump controller. Automatic restart of the pump is initiated by the interlock task for a preset number of times in a given time period. If the restart fails, gate valves will be closed and the system will be shut down. After completion of a run cycle all data files are transferred to permanent storage on a network disk. The contents of the status file are also imported into the central data base for final processing and storage.
6. Conclusions The Woods Hole AMS system has reached full operation 2.5 years after installation. With two functional high-current ion sources the system is now capable of measuring in excess of 100 carbon samples per week at accuracy levels of better than 0.5% for samples with a fraction modern carbon content of more than 50%. Future developments will include beam position feed back terminal stabilization and the possible extension of the range of isotopes to other light isotopes (“Be, 26Al>in th e near term, and heavy isotopes in the long term (iz91). Development of a specialized ion source for organic compounds is also being studied.
11
Acknowledgment
We would like to acknowledge support from the U.S. National Science Foundation, Cooperative Agreement OCE-8801015.
References [ll R.J. Schneider et al., these Proceedings (6th Int. Conf.
on Accelerator Mass Spectrometry CAMS-6),CanberraSydney, Australia, 19931 Nucl. Instr. and Meth. B 92 (1994) 172; A.P. McNichoI et al., these Proceedings (6th Int. Conf. on Accelerator Mass Spectrometry CAMS-61, CanberraSydney, Australia, 1993) Nucl. Instr. and Meth. B 92 (1994) 162; G.J. Cohen et al., these Proceedings (6th Int. Conf. on Accelerator Mass Spectrometry CAMS-61,Canberra-Sydney, Australia, 19931 Nucl. Instr. and Meth. B 92 (1994) 133; G.A. Jones et al., these Proceedings (6th Int. Conf. on Accelerator Mass Spectrometry CAMS-61, Canberra-Sydney, Australia, 1993) Nucl. Instr. and Meth. B 92 (1994) 426; F.H. Stguin et al., these Proceedings (6th Int. Conf. on Accelerator Mass Spectrometry CAMS-61,Canberra-Sydney, Australia, 1993) Nucl. Instr. and Meth. B 92 (1994) 176. 121 K.H. Purser et al., Nucl. Instr. and Meth. B 52 (1990) 263. [31 R.J. Schneider, K.F. von Reden and K.H. Purser, IEEE Part. Accel. Conf. 2 (1991) 878. I41 K.H. Purser, Radiocarbon 34 (1992) 458. 151 K.F. von Reden et al., Radiocarbon 34 (1992) 478. [61 D.E. Lobb, J.R. Southon, D.E. Nelson, W. Wiesenhan and R.G. Korteling, Nucl. Instr. and Meth. 179 (1981) 171. 171A.E. Litherland and L.R. Kilius, Nucl. Instr. and Meth. B 52 (1990) 375. [81 Hewlett-Packard Co., Palo Alto, CA. 191 Keithley Corp., Cleveland, Ohio, Model 619. [lo] Kepco Corp., Flushing, NY, Model SNR-488-8. 1111Electronics Measurements Inc., Neptune, NJ, Model TCR 40T125. 1121 Congruent Design Inc., Guilford, CT. [13] Lakeshore Cryotronics, Westerville, OH, Model 637. [14] Computer code SORCERY, N.R. White, Wenham, MA. [151 G. Bonani et al., Nucl. Instr. and Meth. B 52 (1990) 338. 1161The Nucleus Inc., Oak Ridge, TN, Personal Computer Analyzer II.
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