- Email: [email protected]
14C AMS at the University of Washington: measurements in a shared facility at the 1% level on 0.4 mg samples
Nuclear
Instruments
and Methods
in Physics Research
B52 (1990) 351-356
351
North-Holland
14C AMS at the University of Washington: facility at the 1% level on 0.4 mg samples
measurements
in a shared
T.A. Brown 1*2),G.W. Farwell ‘I, P.M. Grootes 1.3), P.D. Quay 4, and F.H. Schmidt ‘I Department of Physics and Nuclear Physics Luhorarory GL-IO,
‘)
” Geoph_vsrcsProgram A K-50. ” Quarernay Isolope Laboratory AK-MI and Deparimenr 01 Geological Sciences AJ-20, ‘I School oj Oceanography WB-10, Unioersily oj Washington, SearlIe, WA 98195, USA
The FN tandem accelerator at the University of Washington has been used for AMS measurements for more than ten years. The current sample preparation methods. measurement procedures and system capacity are described. Preliminary results of an ongoing study of 14CH, in atmospheric methane and methane from wetlands environments are presented. The development of new preparation methods has reduced our sputter-source sample size requirements from 2-4 mg to 200-400 pg and allows essentially complete utilization of the sample during the sputtering process. At the *l’% level, the current precision and accuracy of our measurements are limited by counting statistics uncertainties. Ongoing improvements to the measurement system are also discussed.
1. Introduction
2. Sample preparation
In this paper we describe the current sample preparation methods, measurement procedures and capacity of the AMS system at the University of Washington Nuclear Physics Laboratory and present preliminary results of an ongoing scientific study. The system has been described in detail previously [1,2] and recent technological improvements are discussed elsewhere [3]. Briefly, C- ions are produced from our prepared carbon samples by a UNIS-type sputter source which uses a reflected Cs+ beam geometry [4] and has a remotely controlled 20-sample wheel. The source is elevated to 60 kV. The 85 keV C- ions produced are mass-analyzed by an NMR-regulated 45” inflection magnet which allows the injection of mass 13 or 14 ions into the tandem accelerator through a 3.9 mm diameter aperture at the image position of the inflection magnet. The accelerator is operated at 7.0 MV. An NMR-regulated 90” analyzing magnet, which alternates between 14C4+ and t3C4+ settings synchronously with the inflection magnet, allows sequential measurement of the “C4+ current on a movable Faraday cup at the image position of the 90” analyzing magnet and of the 14C4+ ions by a d E-E detector telescope following a 30” switching magnet and a velocity filter. A schematic diagram of our AMS system can be found in ref. [3].
Our current sample preparation method is based on the Ta encapsulation method presented previously [5]. As before, the initial steps in processing samples are the conversion of the pretreated carbonaceous materials to CO, and iron-catalyzed hydrogen reduction of the CO, to graphite [6]. Both of these steps give almost 100% yield. For the reasons discussed in ref. [5], we have continued to use a Fe:C atom ratio of - 1:20 in the reduction process ( - 1:4 Fe:C by weight) rather than the much higher Fe amounts used by other groups (7.81. The 14C standards we use are prepared by reduction of subsamples from CO1 working standards prepared from 1964 and 1968 year rings of a Washington Sitka Spruce [9] and from a Chinese sucrose 14C standard of which the t4C activity has been determined by P-decay counting [M. Stuiver, personal communication]. At the time of the initial description of the Ta encapsulation method, a lower limit on the diameter of the carbon sample was imposed by inconsistencies in the alignment of the sample in the ion source with respect to the sputtering Cs’ beam. Since that time we have developed procedures which allow us to ensure the alignment of samples with the sputtering beam to within kO.2 mm and have adjusted our tuning of the ion source to reduce our sensitivity to small variations in
0168-583X/90/$03.50
a 1990 - Elsevier Science Publishers
B.V. (North-Holland)
III. TECHNICAL
DEVELOPMENTS
352
T.A. Brown er al. / ‘CAMS
sample alignment. These improvements have allowed us to reduce the diameter of our carbon samples. The encapsulation apparatus has been completely redesigned and our procedures have been adapted so that the sample well which is pressed into the Ta disk is now only 0.7 mm in diameter. Weighing of the Ta disk and associated parts before and after filling the well with graphite indicates that the samples contain 200-400 pg carbon. The minimum amount of original sample carbon needed is somewhat larger due to difficulties in removing the reduction product from the reduction tube. As in the original procedure, the graphite in the sample well is encapsulated in Ta, compressed to - 14 kbar and heated to - 2500 o C in vacuum. The combination of pressure and temperature is sufficient to convert the filamentous graphite produced by iron-catalyzed hydrogen reduction to solid graphitic carbon. The Ta capsule is then pressed into an aluminum sample disk for use in the ion source and the top of the capsule is machined off to expose the sample surface. In fig. 1 we show a typical - 0.4 mg sample after it has been run in the ion source. This Chinese sucrose standard sample produced the equivalent of - 20 PA of ‘*C- beam for a total of 20 min during its use in the measurement of two unknown samples and was not run to exhaustion. The star-shaped pattern in the surface of the sample indicates the distribution of the sputtering Cs+ ions under our current tuning of the Cs+ reflection potential. Previously we tuned the reflection potential to maximize C- output and produced a much deeper - 0.4 mm diameter central sputtered spot with much shallower and shorter star-pattern arms. Although defocussing the Cs + ions to produce the star-shaped pattern results in a slight (- 10%) decrease in ion source output, it has two important benefits: (1) it improves the sample utilization efficiency by exposing more of the sample to the sputtering Cs+ ions, and (2) it reduces our sensitivity to small misalignments of the samples by spreading the Cs’ ions more evenly over the surface of the samples. Additionally, we have found that the defocussing does not reduce the C- transmission through the defining aperture at the image position of the 45” inflection magnet. During normal AMS operation we purposely run the ion source at moderate output (- 20 PA ‘*Cm) to ensure long-term stability and a reasonably long lifetime for the tandem terminal stripper foils (- 8 h/foil). We have run the ion source with the 0.7 mm samples at 50-60 PA ‘*C- output on some occasions but believe that injection into the tandem of the 13C- beams produced at such high output levels is detrimental to the overall performance of the AMS system. We have determined the ‘*C- production and transport efficiency of our ion source and low-energy beam line through the defining aperature at the image position of the inflection magnet to be in the range 2.4-2.9%,
al the University o/ Washington
i.e., our 400 pg samples produce - 25 PA h of usable ‘*C- beam. Correcting for beam loses during transmission through the tandem (10% on entrance grid and - 58% due to stripper foil charge-state distribution), this implies that approximately 1% of the 14C atoms in a sample placed in our ion source could be measured at the detector.
3. Current measurement
procedure
and system capacity
During routine measurement operation the various components of our system, including the ion source sample wheel. the image Faraday cup. and the NMRregulated inflection and analyzing magnets, are controlled by an industrial-type (5TI) Texas Instruments Company controller. Among its other functions, the controller allows us to drive the sample wheel back and forth between two adjacent samples in the sample wheel. Under our routine measurement scheme we determine the ratio of the 14Cqc count rates of two adjacent samples normalized to the 13C4+ currents produced by the samples. After loading the sample wheel with alternating standard and unknown samples, we currently use the following automated measurement sequence. (1) With the magnets set for “C, we measure the analyzed 13C4+ current on the image Faraday cup from the first of two adjacent samples for 30 s and then from the second sample for 30 s. (2) The magnets are then changed to 14C settings and we count 14C4’ ions at the dE-E detector telescope from the first sample for 30 s and then from the second sample for 30 s. (3) Steps 1 and 2 are repeated a total of ten times on the same known/unknown sample pair. (4) Tbe measurement c cling is ended with final 30 s measurements of the ‘1 C4+ currents from the two samples. The data from each cycle are collected by a VAX3200 which runs the XSYS data aquisition system; on-line analysis of the collected data provides cycle-by-cycle and summary calculations of various results. The primary result obtained from this cycling pattern is the ratio of the 14C4+ count rate of the unknown to that of an adjacent standard normalized to the ratio of the 13C4+ currents from these samples. Further off-line calculations are performed on a VAX 11/780. The entire measurement cycling, including switching of samples and magnet settings, takes approximately 30 min. As mentioned above, we purposely run the ion source at moderate output levels (- 20 PA ‘*C- equivalent) during routine measurment operations. Our typical contemporary carbon 14C count rate is 40-60 cps and we obtain (12-20) x lo3 count for each contemporary sample during routine measurement cycling. We normally measure each unknown sample against both of the adjacent standards, i.e., the standards preceding and
T.A. Brown et al. / ‘“CAMS at the University of Washington
353
Fig. 1. A typical - 0.4 mg sample after it has been run in the ion source and has produced the equivalent of - 7 pA h of “C- beam. Left: The entire ion source sample disk showing the eight holes through which the sputtering Cs+ enters before being reflected and focus& back onto the sample, the Ta capsule mounted in the middle of the disk, and the sputtered sample at the center of the disk. The diameter of the aluminum sample disk is 12.7 mm, Right: Magnified view of the carbon sample and star-shaped Cs+ sputtering pattern. The superimposed circle indicates the outer edge of the carbon sample; the carbon sample has a diameter of 0.7 mm.
following the unknown in the sample wheel. Thus, we can obtain sufficient counts from contemporary samples to determine an unknown to within a counting statistics uncertainty of < 1% in approximately one hour of measurement time. The FN tandem accelerator at the University of Was~ngton is operated p~ncipally as a nuclear physics research facility; we are able to obtain about 10% of the available beam-time for AMS measurements. In general, we obtain beam-time in blocks of 4-6 days and for each block of beam-time we must install the sputter ion source, install the LE beam-tube entrance grid lens, and set up the 14C detector beamline and electronics. Thus, the first day of our beam-time is used in setting up the AMS measurement system. Once we have set up and debugged the system, we are able to load and measure eight unknown samples of approximately contemporary carbon to a counting statistics uncertainty of - 1% in about 18 hours. Thus, under good conditions about 30 unknown samples can be measured in a block of beamtime.
4. Precision and accuracy of measurements We have analyzed data obtained from two recent AMS runs to determine the current precision and accuracy of our measurement system. To determine the precision of the measurements we have combined two
data sets. The first set is composed of measured ratios of the normalized i4C contents of two standard samples; by dividing the difference between each measured ratio and the known value of the ratio by the uncertainty in the measured ratio derived from counting statistics we converted the measured ratios into data which should have a Gaussian dis~bution. The second set is composed of pairs of measured ratios, namely the ratios of the normalized 14C content of each unknown to that of each of its two adjacent standard samples. By dividing the difference between the two measured ratios for each unknown by the uncertainty in the difference derived from the uncert~nties in the measured ratios based on counting statistics alone, we converted the pairs of measured ratios into data which should also have a Gaussian distribution. Fig. 2 shows the combined distribution obtained from these calculations along with an appropriately normalized Gaussian curve. Mi~mum x2 fitting of this distribution shows that it is indeed Gaussian, and that the counting-statistics-based estimates of the uncertainties in our measurements accurately represent the precision of the measurements. Almost all of the measurements used in obtaining the distribution had about 1% counting statistics uncertainties, therefore at this level the uncertainty in our measurements is almost entirely att~butable to counting statistics. To determine the accuracy of our measurement system we have expressed measured ratios of the normalIII. TECHNICAL DEVELOPMENTS
T.A. Brown et al. / ‘“CAMS at the University of Washington
354 40
r
our system are limited uncertainty level.
by counting
statistics
at the 1%
t
5. Recent scientific results
30
B g
20
z 10
0 -2
0
2
Sigma
Fig. 2. Histogram showing the distribution obtained from the two data sets described in the text superimposed on an appropriately normalized Gaussian curve.
ized 14C contents of two standard samples as percentage deviations from the known values of the ratios (fig. 3). These data show that the mean deviation of our measured ratios from the expected values is not significantly different from zero (- 0.17 f 0.18%) and that the scatter of the data is consistent with the 1% counting statistics uncertainties of the measurements. Thus, for measurements of approximately contemporary samples, the current precision and accuracy of
1
I
We have recently completed a series of measurements of the radiocarbon content of atmospheric CH, as a part of an ongoing NASA-funded project to use isotopic measurements as a means of improving our knowledge of the sources of the l%/yr increase in atmospheric CH,. In particular, the 14C content of CH, reflects the magnitude of fossil versus modem CH, sources. One conclusion that emerges is that nuclear power reactors contribute a major portion (30 f 9%) of the current 14CH, loading rate. The results presented here are preliminary; a complete discussion of our results will be presented in a separate publication [lo]. The t4C measurements were designed to measure temporal and spatial trends in the r4CH4. Air samples were collected at approximately biweekly intervals from a clean air site on the Washington coast (48 o N 126’ W) over a three-year period from 1987 to the present (fig. 4). This time series shows a significant (P < 0.1) increase of 1.4 f 0.5 pM/yr in the 14C content of atmospheric CH,, somewhat lower than the 2-3%/yr rate reported previously [ll] (PM = absolute percent modem, as defined in ref. [12]). This increase is primaily due to the increase in nuclear reactor release rates. The mean r4C content of CH, at this site over the three-year period was 123 + 2 pM (N = 30). Air samples were collected for 14CH, analysis on three NOAA sponsored cruises in the Pacific Ocean in 1987-1989; these cruises sampled air between 4O”N
.
% diff NW
.
% diff Jan
SAMPLE Fig. 3. The percentage deviation of our measured 14C content ratios of two standards from the known values of the ratios for two recent AMS runs (November 1989 and January 1990).
T.A. Brown et al. / ‘“CAMS
Mar 87
at the University of Washington
Jan 88
Nov 88
355
Sep 89
TIME Fig. 4. The 14C content in Percent modem (PM) of atmospheric methane at a clean air site on the Pacific coast of Washington (4g” N 126OW) from 1987 to the present (adapted from ref. [lo]).
and 60’s. Generally, the 14C content of atmospheric CH, is similar in both hemispheres at - 122 pM, although additional samples will be measured to verify these results. The preliminary data from the southern hemisphere agree well with the 120 pM value determined by Wahlen et al. [ll]. Our measured global average 14C content of 122 pM for atmospheric CH, is in good agreement with the measured value of 123 pM obtained by Wahlen et al. [ll]. This i4C content, when combined with the 1.4 pM/yr increase rate and current estimates of the nuclear power production rate of 14CH,, indicate that = 16 f 12% of the current CH, is derived from fossil sources. For a total CH, source strength of = 500 Tg CH,/yr, the fossil source strength is thus = 84 Tg/yr.
6. Ongoing developmental projects As mentioned above, our Cs sputter ion source is capable of producing 50-60 pA *‘C- from the 0.7 mm diameter carbon samples, but we run the source at much lower output levels during routine operation to avoid injecting the large (almost 1 PA mass 13-) beams which are produced when the source is run at the higher output levels. We have constructed and are currently testing a low energy (LE) beam chopper which operates at a frequency of 100 Hz and allows us to attenuate the mass-I3 beam by selectable amounts ranging from 50% to 99.9%. The LE chopper is mounted near the exit of the inflection magnet and electrostatically deflects the beam away from the 3.9 mm aperture at the image position of the infection magnet for selectable fractions of the 10 ms duty cycle. The LE chopper is controlled by the 5TI controller during automated cycling and does not operate unless the NMR-regulated inflection magnet is on the mass-13 setting. We hope that the LE beam chopper will allow us to run our ion source at output levels of 50-60 PA 12C- while ensuring a rea-
sonable lifetime for the tandem terminal stripper foils and avoiding potential problems, such as loading of the tandem and/or changes in ion beam transmission, caused by the injection of large mass-13 beams. We have also developed and are currently testing a measurement system in which the cycling sequence is controlled by a program running on the VAX3200 workstation rather than the 5TI controller. The 5TI controller measurement sequencing we now use for routine measurements, in which isotope settings and samples are both alternated, allows approximately 5 min of the 30 min routine measurement cycling to be spent counting 14C4+ ions from the unknown sample. The current version of the VAX3200 sequencing program cycles between measuring 13C4+ and i4C4+ ions from a single sample rather than alternating samples as well as isotope settings. While this new sequencing program is only in the earliest stages of testing, we hope that its implementation will allow us to significantly increase the fraction of measurement time spent counting 14C4+ ions from unknown samples. Finally, we are making small alterations to the Ta encapsulation apparatus to reduce the depth of the sample well pressed into the Ta disk to about one-fourth its current depth. With these changes we should be able to measure 100 Pg samples to < 1% counting statistics uncertainties with our current AMS system.
7. Summary The current status of the AMS system at the University of Washington Nuclear Physics Laboratory can be summarized as follows. (1) Carbon samples containing 200-400 Pg are now routinely produced by the Ta encapsulation method and provide beams of - 20 pA i2C- equivalent when our ion source is operated at moderate output levels. (2) The system efficiency in terms of producing C- ions from a sample and transIII. TECHNICAL DEVELOPMENTS
356
T.A. Brown et al. / t% AMS at the University of Washington
porting them through the tandem accelerator to the detectors is = 1%. (3) The accuracy and precision of the system have been shown to be limited by counting statistics uncertainties at the 1% level. (4) The system throughput is - 30 unknowns for each block of 4-6 days of beam time obtained on the nuclear physics dominated tandem accelerator. (5) System improvements which are currently underway should allow us to decrease our sample size to - 100 pg, increase our 14C4+ counting rate and the percentage of time spent counting 14C4+ ions, and, hence, allow us to increase the throughput of the system and/or the precision of our measurements. As a part of one of the scientific studies we are currently undert~ng, we have measured the radiocarbon content of methane from the atmosphere and wetlands environments. These measurements have improved our knowledge of the sources, sinks and cycling of atmospheric methane and our understanding of the source of the lW/yr increase in the atmospheric concentration of methane.
Acknowledgements
We thank Melinda Denton and Devra Jarvis for advice and assistance in preparing the photographs in fig. 1, the staff at the University of Washington Nuclear Physics Laboratory for their assistance, and Minze Stuiver (Quaternary Isotope Laboratory, VW) for advice and the use of facilities. Lastly, we thank William Weitkamp (Technical Director, NPL VW), John Cramer (former Director, NPL VW) and Derek Storm (current Director, NPL VW) for their en~uragement and support.
This work was supported by the US Department of Energy, the National Science Foundation (grant EAR8115994, En~ronment~ Geosciences Program) and the National Aeronautics and Space Administration (grant NAGW-844).
References [l] G.W. Farwell,
P.M. Grootes, D.D. Leach and F.H.
Schmidt, Nuci. Instr. and Meth. BS (1984) 144. [2] P.M. Grootes, M. Stuiver, G.W. Farwell, D.D. Leach and F.H. Schmidt, Radiocarbon 28 (2A) (1986) 237. [3] F.H. Schmidt, T.A. Brown, P.M. Grootes and G.W. Farwell, these Proceedings (AMS 5) Nucl. Instr. and Meth. B52 (1990) 229. [4] F.H. Schmidt and G.W. Farwell, Bull. Am. Phys. Sot. 24 (1979) 540. [S] D.R. Balsley, G.W. Farwell, P.M. Grootes and F.H. Schmidt, Nucl. Instr. and Meth. B29 (1987) 37. [6] J.S. Vogel, J.R. Southon, D.E. Nelson and T.A. Brown, Nucl. Instr. and Meth. BS (1984) 289. [7] J.S. Vogel, J.R. Southon, D.E. Nelson, Nucl. Insrr. and Meth. B29 (1987) 50. [S] M. Arnold, E. Bard, P. Maurice and J.C. Duplessy, Nucl. Instr. and Meth. B29 (1987) 120. [9] G.W. Farwell, P.M. Grootes, D.D. Leach, F.H. Schmidt and M. Stuiver, Radiocarbon 25 (2) (1983) 711. [lo] P.D. Quay, S.L. King, J. Stursman, D.O. Wilbur, L.P. Steele, I. Fung, R.H. Goon, P.M. Grooms, G.W. Farwell, F.H. Schmidt and T.A. Brown, Carbon isotopic composition of atomospheric CH,: fossil and biomass during source strengths, Global Biogeochemical Cycles
(1990) in press. [Ill M. Wahlen, N. Tanaka,
R. Henry, B. Deck. J. Zeglen, J.S. Vogel, J. Southon, A. Shemesh, R. Fairbanks and W. Broecker, Science 245 (1989) 286. [12] M. Stuiver and H.A. Polach. Radiocarbon 19 (1977) 355.
Instruments
and Methods
in Physics Research
B52 (1990) 351-356
351
North-Holland
14C AMS at the University of Washington: facility at the 1% level on 0.4 mg samples
measurements
in a shared
T.A. Brown 1*2),G.W. Farwell ‘I, P.M. Grootes 1.3), P.D. Quay 4, and F.H. Schmidt ‘I Department of Physics and Nuclear Physics Luhorarory GL-IO,
‘)
” Geoph_vsrcsProgram A K-50. ” Quarernay Isolope Laboratory AK-MI and Deparimenr 01 Geological Sciences AJ-20, ‘I School oj Oceanography WB-10, Unioersily oj Washington, SearlIe, WA 98195, USA
The FN tandem accelerator at the University of Washington has been used for AMS measurements for more than ten years. The current sample preparation methods. measurement procedures and system capacity are described. Preliminary results of an ongoing study of 14CH, in atmospheric methane and methane from wetlands environments are presented. The development of new preparation methods has reduced our sputter-source sample size requirements from 2-4 mg to 200-400 pg and allows essentially complete utilization of the sample during the sputtering process. At the *l’% level, the current precision and accuracy of our measurements are limited by counting statistics uncertainties. Ongoing improvements to the measurement system are also discussed.
1. Introduction
2. Sample preparation
In this paper we describe the current sample preparation methods, measurement procedures and capacity of the AMS system at the University of Washington Nuclear Physics Laboratory and present preliminary results of an ongoing scientific study. The system has been described in detail previously [1,2] and recent technological improvements are discussed elsewhere [3]. Briefly, C- ions are produced from our prepared carbon samples by a UNIS-type sputter source which uses a reflected Cs+ beam geometry [4] and has a remotely controlled 20-sample wheel. The source is elevated to 60 kV. The 85 keV C- ions produced are mass-analyzed by an NMR-regulated 45” inflection magnet which allows the injection of mass 13 or 14 ions into the tandem accelerator through a 3.9 mm diameter aperture at the image position of the inflection magnet. The accelerator is operated at 7.0 MV. An NMR-regulated 90” analyzing magnet, which alternates between 14C4+ and t3C4+ settings synchronously with the inflection magnet, allows sequential measurement of the “C4+ current on a movable Faraday cup at the image position of the 90” analyzing magnet and of the 14C4+ ions by a d E-E detector telescope following a 30” switching magnet and a velocity filter. A schematic diagram of our AMS system can be found in ref. [3].
Our current sample preparation method is based on the Ta encapsulation method presented previously [5]. As before, the initial steps in processing samples are the conversion of the pretreated carbonaceous materials to CO, and iron-catalyzed hydrogen reduction of the CO, to graphite [6]. Both of these steps give almost 100% yield. For the reasons discussed in ref. [5], we have continued to use a Fe:C atom ratio of - 1:20 in the reduction process ( - 1:4 Fe:C by weight) rather than the much higher Fe amounts used by other groups (7.81. The 14C standards we use are prepared by reduction of subsamples from CO1 working standards prepared from 1964 and 1968 year rings of a Washington Sitka Spruce [9] and from a Chinese sucrose 14C standard of which the t4C activity has been determined by P-decay counting [M. Stuiver, personal communication]. At the time of the initial description of the Ta encapsulation method, a lower limit on the diameter of the carbon sample was imposed by inconsistencies in the alignment of the sample in the ion source with respect to the sputtering Cs’ beam. Since that time we have developed procedures which allow us to ensure the alignment of samples with the sputtering beam to within kO.2 mm and have adjusted our tuning of the ion source to reduce our sensitivity to small variations in
0168-583X/90/$03.50
a 1990 - Elsevier Science Publishers
B.V. (North-Holland)
III. TECHNICAL
DEVELOPMENTS
352
T.A. Brown er al. / ‘CAMS
sample alignment. These improvements have allowed us to reduce the diameter of our carbon samples. The encapsulation apparatus has been completely redesigned and our procedures have been adapted so that the sample well which is pressed into the Ta disk is now only 0.7 mm in diameter. Weighing of the Ta disk and associated parts before and after filling the well with graphite indicates that the samples contain 200-400 pg carbon. The minimum amount of original sample carbon needed is somewhat larger due to difficulties in removing the reduction product from the reduction tube. As in the original procedure, the graphite in the sample well is encapsulated in Ta, compressed to - 14 kbar and heated to - 2500 o C in vacuum. The combination of pressure and temperature is sufficient to convert the filamentous graphite produced by iron-catalyzed hydrogen reduction to solid graphitic carbon. The Ta capsule is then pressed into an aluminum sample disk for use in the ion source and the top of the capsule is machined off to expose the sample surface. In fig. 1 we show a typical - 0.4 mg sample after it has been run in the ion source. This Chinese sucrose standard sample produced the equivalent of - 20 PA of ‘*C- beam for a total of 20 min during its use in the measurement of two unknown samples and was not run to exhaustion. The star-shaped pattern in the surface of the sample indicates the distribution of the sputtering Cs+ ions under our current tuning of the Cs+ reflection potential. Previously we tuned the reflection potential to maximize C- output and produced a much deeper - 0.4 mm diameter central sputtered spot with much shallower and shorter star-pattern arms. Although defocussing the Cs + ions to produce the star-shaped pattern results in a slight (- 10%) decrease in ion source output, it has two important benefits: (1) it improves the sample utilization efficiency by exposing more of the sample to the sputtering Cs+ ions, and (2) it reduces our sensitivity to small misalignments of the samples by spreading the Cs’ ions more evenly over the surface of the samples. Additionally, we have found that the defocussing does not reduce the C- transmission through the defining aperture at the image position of the 45” inflection magnet. During normal AMS operation we purposely run the ion source at moderate output (- 20 PA ‘*Cm) to ensure long-term stability and a reasonably long lifetime for the tandem terminal stripper foils (- 8 h/foil). We have run the ion source with the 0.7 mm samples at 50-60 PA ‘*C- output on some occasions but believe that injection into the tandem of the 13C- beams produced at such high output levels is detrimental to the overall performance of the AMS system. We have determined the ‘*C- production and transport efficiency of our ion source and low-energy beam line through the defining aperature at the image position of the inflection magnet to be in the range 2.4-2.9%,
al the University o/ Washington
i.e., our 400 pg samples produce - 25 PA h of usable ‘*C- beam. Correcting for beam loses during transmission through the tandem (10% on entrance grid and - 58% due to stripper foil charge-state distribution), this implies that approximately 1% of the 14C atoms in a sample placed in our ion source could be measured at the detector.
3. Current measurement
procedure
and system capacity
During routine measurement operation the various components of our system, including the ion source sample wheel. the image Faraday cup. and the NMRregulated inflection and analyzing magnets, are controlled by an industrial-type (5TI) Texas Instruments Company controller. Among its other functions, the controller allows us to drive the sample wheel back and forth between two adjacent samples in the sample wheel. Under our routine measurement scheme we determine the ratio of the 14Cqc count rates of two adjacent samples normalized to the 13C4+ currents produced by the samples. After loading the sample wheel with alternating standard and unknown samples, we currently use the following automated measurement sequence. (1) With the magnets set for “C, we measure the analyzed 13C4+ current on the image Faraday cup from the first of two adjacent samples for 30 s and then from the second sample for 30 s. (2) The magnets are then changed to 14C settings and we count 14C4’ ions at the dE-E detector telescope from the first sample for 30 s and then from the second sample for 30 s. (3) Steps 1 and 2 are repeated a total of ten times on the same known/unknown sample pair. (4) Tbe measurement c cling is ended with final 30 s measurements of the ‘1 C4+ currents from the two samples. The data from each cycle are collected by a VAX3200 which runs the XSYS data aquisition system; on-line analysis of the collected data provides cycle-by-cycle and summary calculations of various results. The primary result obtained from this cycling pattern is the ratio of the 14C4+ count rate of the unknown to that of an adjacent standard normalized to the ratio of the 13C4+ currents from these samples. Further off-line calculations are performed on a VAX 11/780. The entire measurement cycling, including switching of samples and magnet settings, takes approximately 30 min. As mentioned above, we purposely run the ion source at moderate output levels (- 20 PA ‘*C- equivalent) during routine measurment operations. Our typical contemporary carbon 14C count rate is 40-60 cps and we obtain (12-20) x lo3 count for each contemporary sample during routine measurement cycling. We normally measure each unknown sample against both of the adjacent standards, i.e., the standards preceding and
T.A. Brown et al. / ‘“CAMS at the University of Washington
353
Fig. 1. A typical - 0.4 mg sample after it has been run in the ion source and has produced the equivalent of - 7 pA h of “C- beam. Left: The entire ion source sample disk showing the eight holes through which the sputtering Cs+ enters before being reflected and focus& back onto the sample, the Ta capsule mounted in the middle of the disk, and the sputtered sample at the center of the disk. The diameter of the aluminum sample disk is 12.7 mm, Right: Magnified view of the carbon sample and star-shaped Cs+ sputtering pattern. The superimposed circle indicates the outer edge of the carbon sample; the carbon sample has a diameter of 0.7 mm.
following the unknown in the sample wheel. Thus, we can obtain sufficient counts from contemporary samples to determine an unknown to within a counting statistics uncertainty of < 1% in approximately one hour of measurement time. The FN tandem accelerator at the University of Was~ngton is operated p~ncipally as a nuclear physics research facility; we are able to obtain about 10% of the available beam-time for AMS measurements. In general, we obtain beam-time in blocks of 4-6 days and for each block of beam-time we must install the sputter ion source, install the LE beam-tube entrance grid lens, and set up the 14C detector beamline and electronics. Thus, the first day of our beam-time is used in setting up the AMS measurement system. Once we have set up and debugged the system, we are able to load and measure eight unknown samples of approximately contemporary carbon to a counting statistics uncertainty of - 1% in about 18 hours. Thus, under good conditions about 30 unknown samples can be measured in a block of beamtime.
4. Precision and accuracy of measurements We have analyzed data obtained from two recent AMS runs to determine the current precision and accuracy of our measurement system. To determine the precision of the measurements we have combined two
data sets. The first set is composed of measured ratios of the normalized i4C contents of two standard samples; by dividing the difference between each measured ratio and the known value of the ratio by the uncertainty in the measured ratio derived from counting statistics we converted the measured ratios into data which should have a Gaussian dis~bution. The second set is composed of pairs of measured ratios, namely the ratios of the normalized 14C content of each unknown to that of each of its two adjacent standard samples. By dividing the difference between the two measured ratios for each unknown by the uncertainty in the difference derived from the uncert~nties in the measured ratios based on counting statistics alone, we converted the pairs of measured ratios into data which should also have a Gaussian distribution. Fig. 2 shows the combined distribution obtained from these calculations along with an appropriately normalized Gaussian curve. Mi~mum x2 fitting of this distribution shows that it is indeed Gaussian, and that the counting-statistics-based estimates of the uncertainties in our measurements accurately represent the precision of the measurements. Almost all of the measurements used in obtaining the distribution had about 1% counting statistics uncertainties, therefore at this level the uncertainty in our measurements is almost entirely att~butable to counting statistics. To determine the accuracy of our measurement system we have expressed measured ratios of the normalIII. TECHNICAL DEVELOPMENTS
T.A. Brown et al. / ‘“CAMS at the University of Washington
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our system are limited uncertainty level.
by counting
statistics
at the 1%
t
5. Recent scientific results
30
B g
20
z 10
0 -2
0
2
Sigma
Fig. 2. Histogram showing the distribution obtained from the two data sets described in the text superimposed on an appropriately normalized Gaussian curve.
ized 14C contents of two standard samples as percentage deviations from the known values of the ratios (fig. 3). These data show that the mean deviation of our measured ratios from the expected values is not significantly different from zero (- 0.17 f 0.18%) and that the scatter of the data is consistent with the 1% counting statistics uncertainties of the measurements. Thus, for measurements of approximately contemporary samples, the current precision and accuracy of
1
I
We have recently completed a series of measurements of the radiocarbon content of atmospheric CH, as a part of an ongoing NASA-funded project to use isotopic measurements as a means of improving our knowledge of the sources of the l%/yr increase in atmospheric CH,. In particular, the 14C content of CH, reflects the magnitude of fossil versus modem CH, sources. One conclusion that emerges is that nuclear power reactors contribute a major portion (30 f 9%) of the current 14CH, loading rate. The results presented here are preliminary; a complete discussion of our results will be presented in a separate publication [lo]. The t4C measurements were designed to measure temporal and spatial trends in the r4CH4. Air samples were collected at approximately biweekly intervals from a clean air site on the Washington coast (48 o N 126’ W) over a three-year period from 1987 to the present (fig. 4). This time series shows a significant (P < 0.1) increase of 1.4 f 0.5 pM/yr in the 14C content of atmospheric CH,, somewhat lower than the 2-3%/yr rate reported previously [ll] (PM = absolute percent modem, as defined in ref. [12]). This increase is primaily due to the increase in nuclear reactor release rates. The mean r4C content of CH, at this site over the three-year period was 123 + 2 pM (N = 30). Air samples were collected for 14CH, analysis on three NOAA sponsored cruises in the Pacific Ocean in 1987-1989; these cruises sampled air between 4O”N
.
% diff NW
.
% diff Jan
SAMPLE Fig. 3. The percentage deviation of our measured 14C content ratios of two standards from the known values of the ratios for two recent AMS runs (November 1989 and January 1990).
T.A. Brown et al. / ‘“CAMS
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at the University of Washington
Jan 88
Nov 88
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TIME Fig. 4. The 14C content in Percent modem (PM) of atmospheric methane at a clean air site on the Pacific coast of Washington (4g” N 126OW) from 1987 to the present (adapted from ref. [lo]).
and 60’s. Generally, the 14C content of atmospheric CH, is similar in both hemispheres at - 122 pM, although additional samples will be measured to verify these results. The preliminary data from the southern hemisphere agree well with the 120 pM value determined by Wahlen et al. [ll]. Our measured global average 14C content of 122 pM for atmospheric CH, is in good agreement with the measured value of 123 pM obtained by Wahlen et al. [ll]. This i4C content, when combined with the 1.4 pM/yr increase rate and current estimates of the nuclear power production rate of 14CH,, indicate that = 16 f 12% of the current CH, is derived from fossil sources. For a total CH, source strength of = 500 Tg CH,/yr, the fossil source strength is thus = 84 Tg/yr.
6. Ongoing developmental projects As mentioned above, our Cs sputter ion source is capable of producing 50-60 pA *‘C- from the 0.7 mm diameter carbon samples, but we run the source at much lower output levels during routine operation to avoid injecting the large (almost 1 PA mass 13-) beams which are produced when the source is run at the higher output levels. We have constructed and are currently testing a low energy (LE) beam chopper which operates at a frequency of 100 Hz and allows us to attenuate the mass-I3 beam by selectable amounts ranging from 50% to 99.9%. The LE chopper is mounted near the exit of the inflection magnet and electrostatically deflects the beam away from the 3.9 mm aperture at the image position of the infection magnet for selectable fractions of the 10 ms duty cycle. The LE chopper is controlled by the 5TI controller during automated cycling and does not operate unless the NMR-regulated inflection magnet is on the mass-13 setting. We hope that the LE beam chopper will allow us to run our ion source at output levels of 50-60 PA 12C- while ensuring a rea-
sonable lifetime for the tandem terminal stripper foils and avoiding potential problems, such as loading of the tandem and/or changes in ion beam transmission, caused by the injection of large mass-13 beams. We have also developed and are currently testing a measurement system in which the cycling sequence is controlled by a program running on the VAX3200 workstation rather than the 5TI controller. The 5TI controller measurement sequencing we now use for routine measurements, in which isotope settings and samples are both alternated, allows approximately 5 min of the 30 min routine measurement cycling to be spent counting 14C4+ ions from the unknown sample. The current version of the VAX3200 sequencing program cycles between measuring 13C4+ and i4C4+ ions from a single sample rather than alternating samples as well as isotope settings. While this new sequencing program is only in the earliest stages of testing, we hope that its implementation will allow us to significantly increase the fraction of measurement time spent counting 14C4+ ions from unknown samples. Finally, we are making small alterations to the Ta encapsulation apparatus to reduce the depth of the sample well pressed into the Ta disk to about one-fourth its current depth. With these changes we should be able to measure 100 Pg samples to < 1% counting statistics uncertainties with our current AMS system.
7. Summary The current status of the AMS system at the University of Washington Nuclear Physics Laboratory can be summarized as follows. (1) Carbon samples containing 200-400 Pg are now routinely produced by the Ta encapsulation method and provide beams of - 20 pA i2C- equivalent when our ion source is operated at moderate output levels. (2) The system efficiency in terms of producing C- ions from a sample and transIII. TECHNICAL DEVELOPMENTS
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porting them through the tandem accelerator to the detectors is = 1%. (3) The accuracy and precision of the system have been shown to be limited by counting statistics uncertainties at the 1% level. (4) The system throughput is - 30 unknowns for each block of 4-6 days of beam time obtained on the nuclear physics dominated tandem accelerator. (5) System improvements which are currently underway should allow us to decrease our sample size to - 100 pg, increase our 14C4+ counting rate and the percentage of time spent counting 14C4+ ions, and, hence, allow us to increase the throughput of the system and/or the precision of our measurements. As a part of one of the scientific studies we are currently undert~ng, we have measured the radiocarbon content of methane from the atmosphere and wetlands environments. These measurements have improved our knowledge of the sources, sinks and cycling of atmospheric methane and our understanding of the source of the lW/yr increase in the atmospheric concentration of methane.
Acknowledgements
We thank Melinda Denton and Devra Jarvis for advice and assistance in preparing the photographs in fig. 1, the staff at the University of Washington Nuclear Physics Laboratory for their assistance, and Minze Stuiver (Quaternary Isotope Laboratory, VW) for advice and the use of facilities. Lastly, we thank William Weitkamp (Technical Director, NPL VW), John Cramer (former Director, NPL VW) and Derek Storm (current Director, NPL VW) for their en~uragement and support.
This work was supported by the US Department of Energy, the National Science Foundation (grant EAR8115994, En~ronment~ Geosciences Program) and the National Aeronautics and Space Administration (grant NAGW-844).
References [l] G.W. Farwell,
P.M. Grootes, D.D. Leach and F.H.
Schmidt, Nuci. Instr. and Meth. BS (1984) 144. [2] P.M. Grootes, M. Stuiver, G.W. Farwell, D.D. Leach and F.H. Schmidt, Radiocarbon 28 (2A) (1986) 237. [3] F.H. Schmidt, T.A. Brown, P.M. Grootes and G.W. Farwell, these Proceedings (AMS 5) Nucl. Instr. and Meth. B52 (1990) 229. [4] F.H. Schmidt and G.W. Farwell, Bull. Am. Phys. Sot. 24 (1979) 540. [S] D.R. Balsley, G.W. Farwell, P.M. Grootes and F.H. Schmidt, Nucl. Instr. and Meth. B29 (1987) 37. [6] J.S. Vogel, J.R. Southon, D.E. Nelson and T.A. Brown, Nucl. Instr. and Meth. BS (1984) 289. [7] J.S. Vogel, J.R. Southon, D.E. Nelson, Nucl. Insrr. and Meth. B29 (1987) 50. [S] M. Arnold, E. Bard, P. Maurice and J.C. Duplessy, Nucl. Instr. and Meth. B29 (1987) 120. [9] G.W. Farwell, P.M. Grootes, D.D. Leach, F.H. Schmidt and M. Stuiver, Radiocarbon 25 (2) (1983) 711. [lo] P.D. Quay, S.L. King, J. Stursman, D.O. Wilbur, L.P. Steele, I. Fung, R.H. Goon, P.M. Grooms, G.W. Farwell, F.H. Schmidt and T.A. Brown, Carbon isotopic composition of atomospheric CH,: fossil and biomass during source strengths, Global Biogeochemical Cycles
(1990) in press. [Ill M. Wahlen, N. Tanaka,
R. Henry, B. Deck. J. Zeglen, J.S. Vogel, J. Southon, A. Shemesh, R. Fairbanks and W. Broecker, Science 245 (1989) 286. [12] M. Stuiver and H.A. Polach. Radiocarbon 19 (1977) 355.