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Development of AMS at Daresbury for fully-stripped heavy ions
Nuclear Instruments and Methods in Physics Research B52 (1990) 290-293 North-Holland
290
Development
of AMS at Daresbury for fully-stripped
G.W.A. Newton, T.W. Aitken, T.R. Charlesworth, J.S. Lilley and M.J. Smithson Nuclear Structure Facility, SERC Daresbury Loboratory,
R.A. Cunningham,
heavy ions
P.V. Drumm,
Warringron, Cheshire WA4 4AD, England
J. Barker, J.P. Day and F. Rehman Department
of Chemistry,
University of Manchester,
Manchester
MI3 9PL, England
The tandem Van de Graaff at Daresbury was designed for heavy-ion physics and operates with a maximum of 20 MV on the centre terminal. Development work has been carried out to use the Nuclear Structure Facility (NSF) for mass spectrometry and measure the isotopes 26Aland ‘% for biological and geological applications respectively. Extraction currents from a Middleton ion source have been optimised for AIO- and Cl-. The machine optics were aligned and stabilised using 26Mg and %. These ions, which were fully stripped by a carbon foil, were analysed in a QMG/2 magnetic spectrometer and measured in a detector at the focal plane. Linear calibrations over two orders of magnitude have been obtained for both masses and a concentration of one radioactive isotope in lOI of the stable isotope has been analysed. Geological ground water and biological samples have been measured.
1. lntraduction Several laboratories have developed methods for the measurement of % and 26A1by AMS [l], but none of these were based in the UK. However, in recent years there has been a growing demand for measurements of these isotopes, and this paper describes the initial development work which has been carried out to establish an AMS facility at Daresbury. Funding constraints demanded that maximum use be made of existing laboratory facilities. The method employs the full power of the NSF and takes advantage of the fact that at 200 MeV a good fraction of the chlorine and aluminium ions can be fully stripped and thus easily separated from isobars in a magnetic spectrometer and identified in a detector at the focal plane
*^.
Development work was required to ensure that the ratio of radioactive to stable isotope was measured precisely; this included sample preparation, monitoring the ion source output, accelerator stabilisation, and clean separation of the radioactive isotope from other masses.
2. Ion source development A modified Middleton source [3] was chosen because it has a large sample wheel comprising 23 pills but with low cross contamination (see results). In general, the source was operated at 20 kV and a vacuum of about 0168-583X/90/$03.50
6 x lo-’ Torr. Details of the development work for Al is described in a paper at this conference [4]. The main aims of this work was to establish the best pill composition, the optimum operating conditions for the ion source and the range of ion species produced. This work was carried out on a test rig available at the laboratory. Silver chloride was used for Cl- production and the output was proportional to the atomic percent Cl in the pill. Varying concentrations of chloride were obtained by diluting with bromide; the mass scan of the source output showed that apart from Cl- and Br- only a few hundred nanoamps of O- was observed up to mass 100; about 23 uA of 35C1 could be obtained from the source. Separation of S was unnecessary and indeed the presence of S was essential as described below. The anode bias, various lenses, Y steerer and switching magnet were all adjusted to give maximum output. All the variables had a broad peak, the important finding being that all elements had to be adjusted by a small amount to give the optimum output. The output current varied by as much as a factor of two before and after optimisation.
3. Machine development 3.1. Aperture
set
For early runs, the stable Cl- (AlO-) output was monitored by integrating the current recorded by the
0 1990 - Elsevier Science Publishers B.V. (North-Holland)
G. W.A. Newton et al. / Development ofAMS at Daresbury
291
enriched in 36S or 26Mg. For each sample pill the low-energy steerers (quadrupole triplet and XY steerers) were adjusted as required. ‘%l- (“S-) or 26A10(26Mg0-) beams were injected into the machine and accelerated with a terminal voltage of 17.3 MV (17.0 MV for Al). At the central terminal the Cl”+ (8 + for Al) ions were selected, after stripping, and accelerated to 187 MeV (147 MeV for Al). 3.3. Stabiksation
ax. tcMm.T
Fig. 1. A schematic view of the tandem and experimental layout.
The tandem was stabilised using the relatively intense fluxes of 36S or 26Mg transmitted by the system. Sulpbide was added to the solutions of samples and standards and pr~ipitated with the silver chloride. This ensured a high count rate of ‘“S through the machine; addition of Mg was not necessary. Part of this ‘% (26Mg) was intercepted by a pair of scintillator slits placed between the analysing magnet and the target chamber of the magnetic spectrometer. The slits were adjustable and were set symmetrically about the surveyed centre of the beamline. The output from the scintillators was fed through a servo system to control the terminal voltage so that the count rate on each slit was kept the same, thus ensuring centralisation of the beam. This system has worked very well.
4. Detection system ion-source beam scanner. However, it provides only a relative measurement of current and is subject to electronic noise, which limits its performance for the weaker beams of AIO- from small samples. Accordingly, an alternative system was developed which uses a suppressed aperture plate. This is shown schematically in fig. 2. During operation, the mass 36 (or mass 42) beam is focussed to be transmitted through the system, and the intense beams of 35 and 37 (or 43) are interrupted by the aperture plate and the current recorded. 3.2. Beam transport Fig. 1 is a diagram of the machine. All NSF settings were optimised for mass 36 or 42 using source pills
Before entering the spectrometer, the beam is passed through a carbon foil and a fraction of the ions are fully ( 26A1’3+) ions were separated stripped. The %l”+ from the more rigid 36S16+(26Mg12-t) ions in the magnetic spectrometer, and detected in the multiparameter focal plane detector. The focal plane detector has been fully described elsewhere [2]. A range of single- and two-dimensional spectra are obtained which enable 26Mg to be distinguished from 26A1and %S from 36C1. dE versus E and E versus position spectra are shown in fig. 3. It can be seen that these ions are well separated and only the ion of interest is counted.
5. Results The Al results are reported at this conference [4]. Four sets of results are reported here for Cl: tests, calibration, blanks and groundwater samples. 5.1. Tests Fig. 2. A side view of the device used to measure the current of neighbouring masses to the transmitted beam.
Cross contamination between samples in the ion source is about 0.1% and appears to be burned off in a few minutes. The evidence for this was obtained from II. NEW & FUTURE FACILITIES
G. W.A. Newton et al, / Development ojAMS at Daresbury
292
successive 5 minute runs on background samples: spectra from two such successive runs are shown in fig. 3a and b. Three 36C1 counts, shown ringed, were recorded in the first 5 minute run on the blank. No counts were recorded in the second run. Repeating the experiment on a second background sample gave two and zero counts respectively, confirming the earlier indication. A run on a neighbouring sample enriched to 10-l’ in 36C1 gave 2270 counts, fig. 3c, in the same time interval.
5.2. Calibration A standard solution of 36C1 was purchased from Amersham and a series of standards containing one radioactive isotope in 10” to 1013 were made using Saskatoon potash as the source of stable Cl. A small amount of sodium sulphide was added and precipitated with silver nitrate; 1 to 2 g of AgCl were prepared for each standard. Precautions such as using glove bags for each solution and new glassware for each preparation
S 16+
S 16t
I
(a )
POSITION
(W
$6+ fill’+
(4 Fig. 3. (a)-(c)
(a
ENERGY
Plots of energy versus position for ‘“C1’7+ and ‘0S16+. (d) Plot of energy versus energy loss for ‘%21”’
and 36S16+.
293
G. W.A. Newton et al. / Development of AMS at Daresbury
36
Cl calibranon
26Al calibration
approaching one in 10”; this is equivalent to a detection limit of 2.9 X 10-l’ g in our 30 mg sample.
0’
.d’
I ,
/’
ii I’
/
?’
1
lO0i
0.0
1.0
5.4. Groundwater samples
.a’
2.0
log.relanve “Cl cont.
log. relative 26A1cont.
Fig. 4. Plots of yield (arbitrary units related to counts in QMG spectrometer) versns relative concentration of isotope in the sample ( 36C1 and 26A1).
Several samples from Cornwall have been measured, giving “?I counts in 5 minutes ranging from 238 to 2812 when a lO_” standard gave 379 counts. These values are higher than expected and suggests that radioactivity in the granite matrix is a major source of 36C1 production in the local chloride.
6. Conclusion were used to avoid cross contamination. New gloves were worn at all handling stages. Gloves and separate presses were used for pill preparation. Initially 4 X 4 mm pills were made, but this was later reduced to 2 X 2 mm to match some low weight samples. Results are shown in fig. 4, indicating a good linear response over two orders of magnitude.
It has been possible to obtain good blanks, linear calibrations and some reproducible results for the two isotopes 26A1 and 36C1. It remains only to confirm the reproducibility of the procedures established.
References
5.3. Blanks
HI See, Several sources of stable Cl were used: analar NaCl, the mineral Stassfurt Camallite and potash from Saskatoon in Canada. A range of values have been obtained, the higher values being cause for concern. However, later runs have given good values and a Saskatoon blank gave zero counts in 40 minutes. Since one radioactive isotope in 1Ol3 of the stable isotope gave a count of 10 in 5 minutes, our limit of detection is
for example, Proc. 4th Int. Symp. on Accelerator Mass Spectrometry, Nucl. Instr. and Meth. B29 (1987). N.E. Sanderson, W.N.J. Snodgrass, PI R.A. Cunningham, D.W. Banes, S.D. Hoath and J.N. MO, Nucl. Instr. and Meth. A234 (1985) 67. [31 K. Brand, Nucl. Instr. and Meth. 154 (1977) 595. P.V. [41J. Barker, J.P. Day, T.W. Aitken, T.R. Charlesworth, Drnmm, J.S. Lilley, G.W.A. Newton and M.J. Smithson, these Proceedings (AMS 5) Nucl. Instr. and Meth. B52 (1990) 540.
II. NEW & FUTURE
FACILITIES
290
Development
of AMS at Daresbury for fully-stripped
G.W.A. Newton, T.W. Aitken, T.R. Charlesworth, J.S. Lilley and M.J. Smithson Nuclear Structure Facility, SERC Daresbury Loboratory,
R.A. Cunningham,
heavy ions
P.V. Drumm,
Warringron, Cheshire WA4 4AD, England
J. Barker, J.P. Day and F. Rehman Department
of Chemistry,
University of Manchester,
Manchester
MI3 9PL, England
The tandem Van de Graaff at Daresbury was designed for heavy-ion physics and operates with a maximum of 20 MV on the centre terminal. Development work has been carried out to use the Nuclear Structure Facility (NSF) for mass spectrometry and measure the isotopes 26Aland ‘% for biological and geological applications respectively. Extraction currents from a Middleton ion source have been optimised for AIO- and Cl-. The machine optics were aligned and stabilised using 26Mg and %. These ions, which were fully stripped by a carbon foil, were analysed in a QMG/2 magnetic spectrometer and measured in a detector at the focal plane. Linear calibrations over two orders of magnitude have been obtained for both masses and a concentration of one radioactive isotope in lOI of the stable isotope has been analysed. Geological ground water and biological samples have been measured.
1. lntraduction Several laboratories have developed methods for the measurement of % and 26A1by AMS [l], but none of these were based in the UK. However, in recent years there has been a growing demand for measurements of these isotopes, and this paper describes the initial development work which has been carried out to establish an AMS facility at Daresbury. Funding constraints demanded that maximum use be made of existing laboratory facilities. The method employs the full power of the NSF and takes advantage of the fact that at 200 MeV a good fraction of the chlorine and aluminium ions can be fully stripped and thus easily separated from isobars in a magnetic spectrometer and identified in a detector at the focal plane
*^.
Development work was required to ensure that the ratio of radioactive to stable isotope was measured precisely; this included sample preparation, monitoring the ion source output, accelerator stabilisation, and clean separation of the radioactive isotope from other masses.
2. Ion source development A modified Middleton source [3] was chosen because it has a large sample wheel comprising 23 pills but with low cross contamination (see results). In general, the source was operated at 20 kV and a vacuum of about 0168-583X/90/$03.50
6 x lo-’ Torr. Details of the development work for Al is described in a paper at this conference [4]. The main aims of this work was to establish the best pill composition, the optimum operating conditions for the ion source and the range of ion species produced. This work was carried out on a test rig available at the laboratory. Silver chloride was used for Cl- production and the output was proportional to the atomic percent Cl in the pill. Varying concentrations of chloride were obtained by diluting with bromide; the mass scan of the source output showed that apart from Cl- and Br- only a few hundred nanoamps of O- was observed up to mass 100; about 23 uA of 35C1 could be obtained from the source. Separation of S was unnecessary and indeed the presence of S was essential as described below. The anode bias, various lenses, Y steerer and switching magnet were all adjusted to give maximum output. All the variables had a broad peak, the important finding being that all elements had to be adjusted by a small amount to give the optimum output. The output current varied by as much as a factor of two before and after optimisation.
3. Machine development 3.1. Aperture
set
For early runs, the stable Cl- (AlO-) output was monitored by integrating the current recorded by the
0 1990 - Elsevier Science Publishers B.V. (North-Holland)
G. W.A. Newton et al. / Development ofAMS at Daresbury
291
enriched in 36S or 26Mg. For each sample pill the low-energy steerers (quadrupole triplet and XY steerers) were adjusted as required. ‘%l- (“S-) or 26A10(26Mg0-) beams were injected into the machine and accelerated with a terminal voltage of 17.3 MV (17.0 MV for Al). At the central terminal the Cl”+ (8 + for Al) ions were selected, after stripping, and accelerated to 187 MeV (147 MeV for Al). 3.3. Stabiksation
ax. tcMm.T
Fig. 1. A schematic view of the tandem and experimental layout.
The tandem was stabilised using the relatively intense fluxes of 36S or 26Mg transmitted by the system. Sulpbide was added to the solutions of samples and standards and pr~ipitated with the silver chloride. This ensured a high count rate of ‘“S through the machine; addition of Mg was not necessary. Part of this ‘% (26Mg) was intercepted by a pair of scintillator slits placed between the analysing magnet and the target chamber of the magnetic spectrometer. The slits were adjustable and were set symmetrically about the surveyed centre of the beamline. The output from the scintillators was fed through a servo system to control the terminal voltage so that the count rate on each slit was kept the same, thus ensuring centralisation of the beam. This system has worked very well.
4. Detection system ion-source beam scanner. However, it provides only a relative measurement of current and is subject to electronic noise, which limits its performance for the weaker beams of AIO- from small samples. Accordingly, an alternative system was developed which uses a suppressed aperture plate. This is shown schematically in fig. 2. During operation, the mass 36 (or mass 42) beam is focussed to be transmitted through the system, and the intense beams of 35 and 37 (or 43) are interrupted by the aperture plate and the current recorded. 3.2. Beam transport Fig. 1 is a diagram of the machine. All NSF settings were optimised for mass 36 or 42 using source pills
Before entering the spectrometer, the beam is passed through a carbon foil and a fraction of the ions are fully ( 26A1’3+) ions were separated stripped. The %l”+ from the more rigid 36S16+(26Mg12-t) ions in the magnetic spectrometer, and detected in the multiparameter focal plane detector. The focal plane detector has been fully described elsewhere [2]. A range of single- and two-dimensional spectra are obtained which enable 26Mg to be distinguished from 26A1and %S from 36C1. dE versus E and E versus position spectra are shown in fig. 3. It can be seen that these ions are well separated and only the ion of interest is counted.
5. Results The Al results are reported at this conference [4]. Four sets of results are reported here for Cl: tests, calibration, blanks and groundwater samples. 5.1. Tests Fig. 2. A side view of the device used to measure the current of neighbouring masses to the transmitted beam.
Cross contamination between samples in the ion source is about 0.1% and appears to be burned off in a few minutes. The evidence for this was obtained from II. NEW & FUTURE FACILITIES
G. W.A. Newton et al, / Development ojAMS at Daresbury
292
successive 5 minute runs on background samples: spectra from two such successive runs are shown in fig. 3a and b. Three 36C1 counts, shown ringed, were recorded in the first 5 minute run on the blank. No counts were recorded in the second run. Repeating the experiment on a second background sample gave two and zero counts respectively, confirming the earlier indication. A run on a neighbouring sample enriched to 10-l’ in 36C1 gave 2270 counts, fig. 3c, in the same time interval.
5.2. Calibration A standard solution of 36C1 was purchased from Amersham and a series of standards containing one radioactive isotope in 10” to 1013 were made using Saskatoon potash as the source of stable Cl. A small amount of sodium sulphide was added and precipitated with silver nitrate; 1 to 2 g of AgCl were prepared for each standard. Precautions such as using glove bags for each solution and new glassware for each preparation
S 16+
S 16t
I
(a )
POSITION
(W
$6+ fill’+
(4 Fig. 3. (a)-(c)
(a
ENERGY
Plots of energy versus position for ‘“C1’7+ and ‘0S16+. (d) Plot of energy versus energy loss for ‘%21”’
and 36S16+.
293
G. W.A. Newton et al. / Development of AMS at Daresbury
36
Cl calibranon
26Al calibration
approaching one in 10”; this is equivalent to a detection limit of 2.9 X 10-l’ g in our 30 mg sample.
0’
.d’
I ,
/’
ii I’
/
?’
1
lO0i
0.0
1.0
5.4. Groundwater samples
.a’
2.0
log.relanve “Cl cont.
log. relative 26A1cont.
Fig. 4. Plots of yield (arbitrary units related to counts in QMG spectrometer) versns relative concentration of isotope in the sample ( 36C1 and 26A1).
Several samples from Cornwall have been measured, giving “?I counts in 5 minutes ranging from 238 to 2812 when a lO_” standard gave 379 counts. These values are higher than expected and suggests that radioactivity in the granite matrix is a major source of 36C1 production in the local chloride.
6. Conclusion were used to avoid cross contamination. New gloves were worn at all handling stages. Gloves and separate presses were used for pill preparation. Initially 4 X 4 mm pills were made, but this was later reduced to 2 X 2 mm to match some low weight samples. Results are shown in fig. 4, indicating a good linear response over two orders of magnitude.
It has been possible to obtain good blanks, linear calibrations and some reproducible results for the two isotopes 26A1 and 36C1. It remains only to confirm the reproducibility of the procedures established.
References
5.3. Blanks
HI See, Several sources of stable Cl were used: analar NaCl, the mineral Stassfurt Camallite and potash from Saskatoon in Canada. A range of values have been obtained, the higher values being cause for concern. However, later runs have given good values and a Saskatoon blank gave zero counts in 40 minutes. Since one radioactive isotope in 1Ol3 of the stable isotope gave a count of 10 in 5 minutes, our limit of detection is
for example, Proc. 4th Int. Symp. on Accelerator Mass Spectrometry, Nucl. Instr. and Meth. B29 (1987). N.E. Sanderson, W.N.J. Snodgrass, PI R.A. Cunningham, D.W. Banes, S.D. Hoath and J.N. MO, Nucl. Instr. and Meth. A234 (1985) 67. [31 K. Brand, Nucl. Instr. and Meth. 154 (1977) 595. P.V. [41J. Barker, J.P. Day, T.W. Aitken, T.R. Charlesworth, Drnmm, J.S. Lilley, G.W.A. Newton and M.J. Smithson, these Proceedings (AMS 5) Nucl. Instr. and Meth. B52 (1990) 540.
II. NEW & FUTURE
FACILITIES