- Email: [email protected]
A universal and competitive compact AMS facility
Nuclear Instruments and Methods in Physics Research B 240 (2005) 483–489 www.elsevier.com/locate/nimb
A universal and competitive compact AMS facility Martin Stocker
b
a,*
, Max Do¨beli b, Michal Grajcar a, Martin Suter a, Hans-Arno Synal b, Lukas Wacker a
a Institute of Particle Physics, ETH Ho¨nggerberg, Zurich, Switzerland Paul Scherrer Institute c/o ETH Ho¨nggerberg, ETH Zurich, Zurich, Switzerland
Available online 3 October 2005
Abstract The 600 kV AMS facility at ETH/PSI has been upgraded to a universal platform for AMS at low energies allowing to perform studies of 10Be, 14C, 26Al, 41Ca, 129I, 236U and Pu. The most significant improvement in performance is due to a new gas ionisation chamber of much better energy resolution. Performance for 10Be, 14C, 26Al, 129I and Pu isotopes is now competitive with larger AMS facilities. The background in 41Ca measurements is sufficiently low to measure samples for biomedical applications. 2005 Elsevier B.V. All rights reserved. PACS: 07.75.+h; 41.85. p Keywords: Accelerator mass spectrometry; Gas ionisation chamber; Aluminium; Calcium; Iodine
1. Introduction The demand for radionuclide analysis by AMS has grown enormously during the last decade, although the cost per analysis is relatively high due to the size and complexity of the instruments. The ETH/PSI group has initiated a research program to develop smaller and simpler instruments for AMS. In a first step, a compact radiocarbon
*
Corresponding author. Tel.: +41 44 633 3885; fax: +41 44 633 1067. E-mail address: [email protected] (M. Stocker).
dating facility was developed in collaboration with National Electrostatic Corp. (NEC). If is based on a tandem type accelerator operating at voltages of 300–600 kV, but it requires only 6 · 4 m2 floor space [1–3]. This was made possible by applying a new concept for the elimination of molecules which are stable in charges states, 1+ and 2+: increase of the gas stripper density until the molecules are destroyed in successive collisions [4]. The prototype of this radiocarbon dating facility was designed flexible enough to carry out tests with heavier radionuclides. First experiments showed promising results [5– 8]. Stripping yields and ion optical transmission
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.06.224
484
M. Stocker et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 483–489
were competitive with those of larger facilities, but the background was in many cases much higher. Meanwhile, several instrumental improvements have been made, by which the background level was reduced. Our compact AMS system is now also competitive in this respect. In this paper, the instrumental developments and the present performance are described.
2. Experimental set-up The basic set-up has been described in several previous papers [2,3]. The accelerator is a NEC pelletron with a nominal voltage of 500 kV, but it can be operated under stable conditions at up to 600 kV. The ion source is a multi-cathode SNICS II source, also from NEC, which is mounted on a high-voltage platform (V 6 60 kV). The low-energy magnet (r = 50 cm, M · E/ q2 = 5.9 amu MeV/e2) deflects the beam by 90. The high-energy achromatic mass spectrometer consists of a 90 magnet (r = 75 cm, M · E/ q2 = 33 amu MeV/e2) and a 90 electrostatic analyser (r = 75 cm), allowing to transmit ions in charge state 1+ up to mass 27 at the maximum terminal voltage. For heavier elements higher charge states and/or lower terminal voltages have to be used. The following improvements and modifications for a more versatile instrument have been made:
(1) After the high-energy magnet a system of 5 Faraday cups have been installed. These can be positioned from the outside to measure most of the relevant stable isotopes. (2) The power supply for the fast isotope bouncing system at the low-energy side has been replaced by a bipolar module for the injection of all isotopes (including aluminium) of interest in fast pulsing mode. Stable isotopes can now also be measured in pulsed mode in a side cup on the low-energy side of the spectrometer in order to monitor quasi-continuously the transmission through the accelerator. (3) Improvement in the ion optics: in contrast to larger accelerators, the entrance to the highenergy tube of small accelerators forms a relatively strong lens. Therefore, the position of the focal point on the high-energy side depends strongly on the selected charge state. Additional apertures were put in the corresponding focal plane positions in order to optimise the mass separation of the spectrometer. The gas detector can now also be mounted either in the focal plane of the charge state 1+ or 3+ (see Fig. 1). (4) Construction of a new gas ionisation chamber: First studies performed with a gas detector from our 6 MV AMS facility showed that the energy resolution was not sufficient to properly separate background events. For
Fig. 1. Ion optical properties on the high-energy side of the 600 kV AMS system. To the left the real focus in the stripper is indicated. Due to the lens effect of the high-energy acceleration tube, a virtual focus is produced which is the object point for the high-energy spectrometer. The positions of Faraday cups and apertures installed in the analysing chamber are indicated. To the right, the positions, where the detector should be installed are displayed.
M. Stocker et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 483–489 10
Based on these findings, a new detector has been designed and built (see Fig. 2). The resolution could be improved by about a factor of 2. The detector has also a Si3N4 entrance window, which can be made much thinner and more homogeneous than a plastic foil. Measurements were performed with estimated foils thickness of 30, 50 and 100 nm. The detector is also small enough to be mounted in a 4 in. cross piece. Preamplifiers are attached directly to the anodes to reduce input capacity and to minimise cable length to avoid electronically coupled noise from high frequency fields. A reduction of electronic noise by a factor of 2 has been achieved. For light elements such
14
Pulser
C
FWHM: 40 keV
FWHM: 21 keV
Counts / (a.u.)
Be studies, the isobar 10B has to be suppressed by several orders of magnitudes. For 26Al analysis, molecules and their fragments are a strong background source and for heavier isotopes such as 129I and Pu (m/ q)-ambiguities were the limiting factors. The detector resolution was limited to 40– 50 keV by electronic noise for light ions and by the thickness of the entrance for the heavy elements [9].
485
200
400
600
800
1000
1200
Energy /keV Fig. 3. signal.
14
C energy spectrum (1 MeV) compared with a pulser
as Be, B and C the electronic noise now corresponds to about 20 keV (see Fig. 3).
3. Results 3.1. Beryllium The separation of 10Be–10B has been increased significantly with the new detector allowing the identification of 10Be ions in charge state 1+ (E = 0.8 MeV) instead of 2+ (E = 1.4 MeV). Consequently, the particle transmission through the accelerator increased from 13% to 50% due to higher stripping yield of charge state 1+ at terminal voltage of 0.6 MV. The 10B ions have been suppressed by more than 5 orders of magnitude at energy of 0.8 MeV. Even the identification and separation of residual 9BeH molecules is possible allowing the measurements with the stripper density for highest transmission (see Fig. 4). Combining the new detector technology with the BeF2 method [10] the overall boron suppression is sufficient to reduce background to the level of 10 14, which is competitive with larger facilities [7,11]. 3.2. Carbon
Fig. 2. Side view of detector set-up with detector housing (bottom) and an aperture to right of it. The detector is mounted on a 4 in. flange. The valves (left and right) are part of the gas handling system.
The new detector has also some potential for the identification of various background sources for carbon but does not provide sufficient resolution
486
M. Stocker et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 483–489
which the high-energy spectrometer was tuned to these background particles. The background level can be reduced by 30% by choosing an appropriate region of interest. With a large area entrance window of 8 · 8 mm2 (100 nm or 32 lg/cm2) the efficiency loss is minimised and the detector can be used for routine carbon measurements where both high precision and low detection limits are required.
200
10
B
∆ E / (a.u.)
9
BeH
150
10
Be
3.3. Aluminium
10 0
50
Eres / (a.u.) Fig. 4. Spectrum of BeF sample for charge state 1+ at beam energy of 0.8 MeV. 10B and 9BeH molecules are well separated from 10Be.
∆ E / (a.u.)
200
13
150
C[H]
13 CH
13
C[H]
14
N 14
C
50
100
Eres / (a.u.) Fig. 5. The positions of 13C molecules and scattered fragments of 13CH molecules with the same energy or momentum as 14C and 14N is indicated in a 14C blank spectrum. Potential background positions in the DE Eres spectrum were determined in separate experiments.
for complete separation of the background peaks. The background spectrum in Fig. 5 contains not only 14C but also molecular fragments of 13CH having the same energy or momentum. The position of the various background peaks in the spectrum was determined in different experiments, in
The samples were prepared as Al2O3, from which Al ions were extracted from the ion source (isobar 26Mg is not stable) with currents of 10– 100 nA at the low-energy side of the spectrometer. On the high-energy side the charge state 1+ or 3+ is selected because for charge state 2+ the ions from the break-up of the 13C2 molecules have counting rates too high to obtain a sufficient suppression in the detector. Due to the high stripping yield charge state 1+ is the first choice to measure aluminium at a compact AMS facility. The Mg-hydride molecules have to be destroyed completely by a sufficiently high stripper density of about 1.1 lg/cm2. A reasonable separation for this molecule in the detector is not possible, whereas other molecules such as 14N12C, 10B16O or 13C2 can be distinguished from 26Al. Fig. 6 (right) shows a DE Eres spectrum of an aluminium standard. The molecule peak is well separated from the aluminium peak. For comparison, a simulated spectrum is shown in Fig. 6 (left). The simulation, which is based on the SRIM program, agrees with the experimental data. A transmission of about 25% and a background level of 10 14 is obtained. 3.4. Calcium In the EU research project ‘‘OSTEODIET’’, a novel isotopic technique to monitor changes in bone metabolism will be developed and evaluated. The new technique will be employed as a tool for assessing the impact of diet on bone metabolism in postmenopausal women. The long-lived radionuclide 41Ca (half-life: 104000 a) will be used as a tracer to label Ca in bone. A method to determine reliably and precisely the 41Ca concentration
M. Stocker et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 483–489
487
1000
Molecules 800 13 26
∆E / (a.u.)
600
C2
Al
26
Al
400
200
Experiment 100
200
300
400
Simulation 500
100
200
300
400
500
Eres / (a.u.) Fig. 6. Right: simulated DE Eres spectrum of 26Al and 13C2 molecules at initial energy of 1 MeV. The simulation is based on the SRIM program. Left: DE Eres spectrum obtained from 26Al standard with molecules such as 12C14N, 10B16O or 13C2. The spectra shows, that the separation of 26Al from molecules is better in the experiment than expected from simulations.
(10 12 to 10 9) is required. A compact AMS facility using CaF2 is an ideal solution. 41 K cannot be separated from 41Ca at low energies. Extracting CaH3 ions from a CaH2 material leads to a 41K suppression which is sufficient for calcium AMS measurements at low energies. Calcium measurement with CaH2 is possible at low energy AMS. A background level of 10 12 and a transmission of about 15% is obtained in the charge state 2+. Sample preparation and handling would be much simpler if CaF2 instead of CaH2 could be used. In this case, 41 KF3 is suppressed by 8 orders of magnitude [12]. In the following, the calcium measurement with CaF2 at low-energy AMS facilities is discussed. In principle, calcium can be measured by selecting charge state 2+ or 3+. The background is primarily produced by scattered stable isotopes in the residual gas of the high-energy part of the spectrometer. In charge state 3+, the measurements can be made at low stripper pressure, because the interfering molecules are not stable in 3+. The background level is thus a factor of three lower than in charge state 2+ due to the better vacuum and thus less scattered particles. In charge state 3+, a background level of 3 · 10 12 and a trans-
mission of 6% is measured, while in charge state 2+ a background level of 10 11 and a transmission of 15% is obtained. 3.5. Iodine Iodine is measured in the charge state 4+ and a total energy of 2.5 MeV. Here, the m/q-interference of 97Mo is very well separated in the total energy spectrum. Molybdenum is injected as 97Mo16O2 (mass 129) into the pelletron and is destroyed in the stripper. Due to the almost equal m/q ratio (D(m/q) = 2.6%) 97Mo still reaches the detector. Instead of 8 · 8 mm2, a 4 · 4 mm2 aperture was used before the pelletron to improve the mass separation of the low-energy side of the spectrometer. This reduces the background caused by scattered stable isotopes in the residual gas of the high-energy part of the spectrometer. For 129I, a transmission of about 4%, a detector efficiency of 80% and a background level of 3 · 10 13 is obtained. 3.6. Plutonium and actinides Ultra-sensitive detection of the long-lived plutonium isotopes, 239Pu, 240Pu, 242Pu, 244Pu by
488
M. Stocker et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 483–489
accelerator mass spectrometry, has been demonstrated at an accelerating voltage of 300 kV [8]. Transmission efficiencies as high as 15% have been observed. Using the gas ionisation detector with the ultra-thin Si3N4 window, an energy resolution sufficient to separate Pu3+ ions from interfering Dy2+ ions with 2/3 of the energy was possible. Sensitivities about 106 atoms for the various Pu isotopes were achieved. We have successfully extended the test measurement to different nuclides of actinides such as 241 Am, 243Am and 236U, 238U. The AMS measurement efficiency is even better than that of larger and much more complex tandem accelerators. Detailed information about actinide analysis with a compact AMS system is given elsewhere [13]. 3.7. AMS performance For most of the isotopes discussed above, detailed performance studies have been made by using reference materials with known concentrations or by remeasuring series of samples which have been analysed before with our 6 MV AMS system. Good agreement and reproducible results were obtained with the compact AMS facility. Table 1 shows the transmission values and the background levels for the various radionuclides, which have been evaluated. As a comparison, the corresponding performance of the PSI/ETH EN-Tandem facility is shown. In general, the background determined with blank samples, which should not contain any of the radionuclides is slightly higher for the compact system. However,
the performance is sufficient for most applications. Only for 41Ca is the background almost 2 orders of magnitude higher. However, a level of 10 12 is sufficient for biomedical applications. Especially the use of CaF2 samples, which are easy to prepare, makes the system very appealing for application discussed above.
4. Conclusion The background sources and limitations of the compact AMS system were identified by systematic studies. The system was improved with simple solutions and small investments. It is now competitive with larger AMS systems for many radionuclides. With the new gas ionisation detector particle identification was improved significantly. The technology should also be useful for other ion beam techniques. The present limitations are mainly due to scattering and charge changing processes along the beam path. Further improvements seem feasible: Modifications of the vacuum system would reduce the rate of scattering and charge changing processes. Improvements in the ion optics could decrease the number of scattered particles reaching the final detector. The present developments should help to build future compact AMS instruments. These would be very attractive alternatives to the existing larger facilities and could be easily accommodated in earth science laboratories or biomedical research institutions.
Table 1 Transmission and background levels for the radionuclides evaluated at the 600 kV AMS facility Isotope
600 kV AMS Charge state
10
Be [BeO] Be [BeF] 14 C 26 Al 41 Ca [CaH] 41 Ca [CaF] 129 I Pu 10
+
2 1+ 1+ 1+ 2+ 3+ 4+ 3+
EN-Tandem Energy in MeV 1.4 0.8 1 1 1.4 1.9 2.5 1.2
Transmission in % Background 9
13 [ Be] 50 [9Be] 40–45 25 15 7 4 10–15
13
<10 10 14 5 · 10 15 10 14 10 12 3 · 10 12 3 · 10 13 106 atoms
Charge state
Transmission in %
Background
3
20
10
4+ 3+ 4+
40–45 15–20 10–12
5 · 10 10 15 5 · 10
15
5+
6
5 · 10
14
+
For comparison, the performance of the PSI/ETH EN-Tandem facility is given for respective charge states and energies.
14
to 10
14
15
M. Stocker et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 483–489
Acknowledgements The authors would like to thank Mark Stalder for his contribution to the investigation of calcium analysis. The Swiss National Science Foundation supports this work.
References [1] M. Suter, Nucl. Instr. and Meth. B 139 (1998) 150. [2] H.-A. Synal, S. Jacob, M. Suter, Nucl. Instr. and Meth. 161–163 (2000) 29. [3] H.-A. Synal, S. Jacob, M. Suter, Nucl. Instr. and Meth. 172 (2000) 1. [4] S. Jacob, M. Suter, H.-A. Synal, Nucl. Instr. and Meth. 172 (2000) 235. [5] M. Suter, S. Jacob, H.-A. Synal, Nucl. Instr. and Meth. 172 (2000) 144.
B B B B
489
[6] M. Stocker, R. Bertschinger, M. Do¨beli, M. Grajcar, S. Jakob, J. Scheer, M. Suter, H.-A. Synal, Nucl. Instr. and Meth. B 223–224 (2004) 104. [7] M. Grajcar, M. Do¨beli, P.W. Kubik, C. Maden, M. Suter, H.-A. Synal, Nucl. Instr. and Meth. B 223–224 (2004) 190. [8] L.K. Fifield, H.-A. Synal, M. Suter, Nucl. Instr. and Meth. B 223–224 (2004) 802. [9] M. Do¨beli, C. Kottler, M. Stocker, S. Weinmann, H.-A. Synal, M. Grajcar, M. Suter, Nucl. Instr. and Meth. B 219– 220 (2004) 415. [10] X.-L. Zhao, A.E. Litherland, J.P. Doupe´, W.E. Kieser, Nucl. Instr. and Meth. B 223–224 (2004) 199. [11] L. Wacker, M. Grajcar, S. Ivy-Ochs, P.W. Kubik, M. Suter, Radiocarbon 46 (1) (2004) 83. [12] D. Fink, R. Middleton, J. Klein, P. Sharma, Nucl. Instr. and Meth. B 47 (1990) 79. [13] L. Wacker, E. Chamizo, L.K. Fifield, M. Stocker, M. Suter, H.-A. Synal, Nucl. Instr. and Meth. B, this edition (2004) THU-PM-A4.
A universal and competitive compact AMS facility Martin Stocker
b
a,*
, Max Do¨beli b, Michal Grajcar a, Martin Suter a, Hans-Arno Synal b, Lukas Wacker a
a Institute of Particle Physics, ETH Ho¨nggerberg, Zurich, Switzerland Paul Scherrer Institute c/o ETH Ho¨nggerberg, ETH Zurich, Zurich, Switzerland
Available online 3 October 2005
Abstract The 600 kV AMS facility at ETH/PSI has been upgraded to a universal platform for AMS at low energies allowing to perform studies of 10Be, 14C, 26Al, 41Ca, 129I, 236U and Pu. The most significant improvement in performance is due to a new gas ionisation chamber of much better energy resolution. Performance for 10Be, 14C, 26Al, 129I and Pu isotopes is now competitive with larger AMS facilities. The background in 41Ca measurements is sufficiently low to measure samples for biomedical applications. 2005 Elsevier B.V. All rights reserved. PACS: 07.75.+h; 41.85. p Keywords: Accelerator mass spectrometry; Gas ionisation chamber; Aluminium; Calcium; Iodine
1. Introduction The demand for radionuclide analysis by AMS has grown enormously during the last decade, although the cost per analysis is relatively high due to the size and complexity of the instruments. The ETH/PSI group has initiated a research program to develop smaller and simpler instruments for AMS. In a first step, a compact radiocarbon
*
Corresponding author. Tel.: +41 44 633 3885; fax: +41 44 633 1067. E-mail address: [email protected] (M. Stocker).
dating facility was developed in collaboration with National Electrostatic Corp. (NEC). If is based on a tandem type accelerator operating at voltages of 300–600 kV, but it requires only 6 · 4 m2 floor space [1–3]. This was made possible by applying a new concept for the elimination of molecules which are stable in charges states, 1+ and 2+: increase of the gas stripper density until the molecules are destroyed in successive collisions [4]. The prototype of this radiocarbon dating facility was designed flexible enough to carry out tests with heavier radionuclides. First experiments showed promising results [5– 8]. Stripping yields and ion optical transmission
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.06.224
484
M. Stocker et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 483–489
were competitive with those of larger facilities, but the background was in many cases much higher. Meanwhile, several instrumental improvements have been made, by which the background level was reduced. Our compact AMS system is now also competitive in this respect. In this paper, the instrumental developments and the present performance are described.
2. Experimental set-up The basic set-up has been described in several previous papers [2,3]. The accelerator is a NEC pelletron with a nominal voltage of 500 kV, but it can be operated under stable conditions at up to 600 kV. The ion source is a multi-cathode SNICS II source, also from NEC, which is mounted on a high-voltage platform (V 6 60 kV). The low-energy magnet (r = 50 cm, M · E/ q2 = 5.9 amu MeV/e2) deflects the beam by 90. The high-energy achromatic mass spectrometer consists of a 90 magnet (r = 75 cm, M · E/ q2 = 33 amu MeV/e2) and a 90 electrostatic analyser (r = 75 cm), allowing to transmit ions in charge state 1+ up to mass 27 at the maximum terminal voltage. For heavier elements higher charge states and/or lower terminal voltages have to be used. The following improvements and modifications for a more versatile instrument have been made:
(1) After the high-energy magnet a system of 5 Faraday cups have been installed. These can be positioned from the outside to measure most of the relevant stable isotopes. (2) The power supply for the fast isotope bouncing system at the low-energy side has been replaced by a bipolar module for the injection of all isotopes (including aluminium) of interest in fast pulsing mode. Stable isotopes can now also be measured in pulsed mode in a side cup on the low-energy side of the spectrometer in order to monitor quasi-continuously the transmission through the accelerator. (3) Improvement in the ion optics: in contrast to larger accelerators, the entrance to the highenergy tube of small accelerators forms a relatively strong lens. Therefore, the position of the focal point on the high-energy side depends strongly on the selected charge state. Additional apertures were put in the corresponding focal plane positions in order to optimise the mass separation of the spectrometer. The gas detector can now also be mounted either in the focal plane of the charge state 1+ or 3+ (see Fig. 1). (4) Construction of a new gas ionisation chamber: First studies performed with a gas detector from our 6 MV AMS facility showed that the energy resolution was not sufficient to properly separate background events. For
Fig. 1. Ion optical properties on the high-energy side of the 600 kV AMS system. To the left the real focus in the stripper is indicated. Due to the lens effect of the high-energy acceleration tube, a virtual focus is produced which is the object point for the high-energy spectrometer. The positions of Faraday cups and apertures installed in the analysing chamber are indicated. To the right, the positions, where the detector should be installed are displayed.
M. Stocker et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 483–489 10
Based on these findings, a new detector has been designed and built (see Fig. 2). The resolution could be improved by about a factor of 2. The detector has also a Si3N4 entrance window, which can be made much thinner and more homogeneous than a plastic foil. Measurements were performed with estimated foils thickness of 30, 50 and 100 nm. The detector is also small enough to be mounted in a 4 in. cross piece. Preamplifiers are attached directly to the anodes to reduce input capacity and to minimise cable length to avoid electronically coupled noise from high frequency fields. A reduction of electronic noise by a factor of 2 has been achieved. For light elements such
14
Pulser
C
FWHM: 40 keV
FWHM: 21 keV
Counts / (a.u.)
Be studies, the isobar 10B has to be suppressed by several orders of magnitudes. For 26Al analysis, molecules and their fragments are a strong background source and for heavier isotopes such as 129I and Pu (m/ q)-ambiguities were the limiting factors. The detector resolution was limited to 40– 50 keV by electronic noise for light ions and by the thickness of the entrance for the heavy elements [9].
485
200
400
600
800
1000
1200
Energy /keV Fig. 3. signal.
14
C energy spectrum (1 MeV) compared with a pulser
as Be, B and C the electronic noise now corresponds to about 20 keV (see Fig. 3).
3. Results 3.1. Beryllium The separation of 10Be–10B has been increased significantly with the new detector allowing the identification of 10Be ions in charge state 1+ (E = 0.8 MeV) instead of 2+ (E = 1.4 MeV). Consequently, the particle transmission through the accelerator increased from 13% to 50% due to higher stripping yield of charge state 1+ at terminal voltage of 0.6 MV. The 10B ions have been suppressed by more than 5 orders of magnitude at energy of 0.8 MeV. Even the identification and separation of residual 9BeH molecules is possible allowing the measurements with the stripper density for highest transmission (see Fig. 4). Combining the new detector technology with the BeF2 method [10] the overall boron suppression is sufficient to reduce background to the level of 10 14, which is competitive with larger facilities [7,11]. 3.2. Carbon
Fig. 2. Side view of detector set-up with detector housing (bottom) and an aperture to right of it. The detector is mounted on a 4 in. flange. The valves (left and right) are part of the gas handling system.
The new detector has also some potential for the identification of various background sources for carbon but does not provide sufficient resolution
486
M. Stocker et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 483–489
which the high-energy spectrometer was tuned to these background particles. The background level can be reduced by 30% by choosing an appropriate region of interest. With a large area entrance window of 8 · 8 mm2 (100 nm or 32 lg/cm2) the efficiency loss is minimised and the detector can be used for routine carbon measurements where both high precision and low detection limits are required.
200
10
B
∆ E / (a.u.)
9
BeH
150
10
Be
3.3. Aluminium
10 0
50
Eres / (a.u.) Fig. 4. Spectrum of BeF sample for charge state 1+ at beam energy of 0.8 MeV. 10B and 9BeH molecules are well separated from 10Be.
∆ E / (a.u.)
200
13
150
C[H]
13 CH
13
C[H]
14
N 14
C
50
100
Eres / (a.u.) Fig. 5. The positions of 13C molecules and scattered fragments of 13CH molecules with the same energy or momentum as 14C and 14N is indicated in a 14C blank spectrum. Potential background positions in the DE Eres spectrum were determined in separate experiments.
for complete separation of the background peaks. The background spectrum in Fig. 5 contains not only 14C but also molecular fragments of 13CH having the same energy or momentum. The position of the various background peaks in the spectrum was determined in different experiments, in
The samples were prepared as Al2O3, from which Al ions were extracted from the ion source (isobar 26Mg is not stable) with currents of 10– 100 nA at the low-energy side of the spectrometer. On the high-energy side the charge state 1+ or 3+ is selected because for charge state 2+ the ions from the break-up of the 13C2 molecules have counting rates too high to obtain a sufficient suppression in the detector. Due to the high stripping yield charge state 1+ is the first choice to measure aluminium at a compact AMS facility. The Mg-hydride molecules have to be destroyed completely by a sufficiently high stripper density of about 1.1 lg/cm2. A reasonable separation for this molecule in the detector is not possible, whereas other molecules such as 14N12C, 10B16O or 13C2 can be distinguished from 26Al. Fig. 6 (right) shows a DE Eres spectrum of an aluminium standard. The molecule peak is well separated from the aluminium peak. For comparison, a simulated spectrum is shown in Fig. 6 (left). The simulation, which is based on the SRIM program, agrees with the experimental data. A transmission of about 25% and a background level of 10 14 is obtained. 3.4. Calcium In the EU research project ‘‘OSTEODIET’’, a novel isotopic technique to monitor changes in bone metabolism will be developed and evaluated. The new technique will be employed as a tool for assessing the impact of diet on bone metabolism in postmenopausal women. The long-lived radionuclide 41Ca (half-life: 104000 a) will be used as a tracer to label Ca in bone. A method to determine reliably and precisely the 41Ca concentration
M. Stocker et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 483–489
487
1000
Molecules 800 13 26
∆E / (a.u.)
600
C2
Al
26
Al
400
200
Experiment 100
200
300
400
Simulation 500
100
200
300
400
500
Eres / (a.u.) Fig. 6. Right: simulated DE Eres spectrum of 26Al and 13C2 molecules at initial energy of 1 MeV. The simulation is based on the SRIM program. Left: DE Eres spectrum obtained from 26Al standard with molecules such as 12C14N, 10B16O or 13C2. The spectra shows, that the separation of 26Al from molecules is better in the experiment than expected from simulations.
(10 12 to 10 9) is required. A compact AMS facility using CaF2 is an ideal solution. 41 K cannot be separated from 41Ca at low energies. Extracting CaH3 ions from a CaH2 material leads to a 41K suppression which is sufficient for calcium AMS measurements at low energies. Calcium measurement with CaH2 is possible at low energy AMS. A background level of 10 12 and a transmission of about 15% is obtained in the charge state 2+. Sample preparation and handling would be much simpler if CaF2 instead of CaH2 could be used. In this case, 41 KF3 is suppressed by 8 orders of magnitude [12]. In the following, the calcium measurement with CaF2 at low-energy AMS facilities is discussed. In principle, calcium can be measured by selecting charge state 2+ or 3+. The background is primarily produced by scattered stable isotopes in the residual gas of the high-energy part of the spectrometer. In charge state 3+, the measurements can be made at low stripper pressure, because the interfering molecules are not stable in 3+. The background level is thus a factor of three lower than in charge state 2+ due to the better vacuum and thus less scattered particles. In charge state 3+, a background level of 3 · 10 12 and a trans-
mission of 6% is measured, while in charge state 2+ a background level of 10 11 and a transmission of 15% is obtained. 3.5. Iodine Iodine is measured in the charge state 4+ and a total energy of 2.5 MeV. Here, the m/q-interference of 97Mo is very well separated in the total energy spectrum. Molybdenum is injected as 97Mo16O2 (mass 129) into the pelletron and is destroyed in the stripper. Due to the almost equal m/q ratio (D(m/q) = 2.6%) 97Mo still reaches the detector. Instead of 8 · 8 mm2, a 4 · 4 mm2 aperture was used before the pelletron to improve the mass separation of the low-energy side of the spectrometer. This reduces the background caused by scattered stable isotopes in the residual gas of the high-energy part of the spectrometer. For 129I, a transmission of about 4%, a detector efficiency of 80% and a background level of 3 · 10 13 is obtained. 3.6. Plutonium and actinides Ultra-sensitive detection of the long-lived plutonium isotopes, 239Pu, 240Pu, 242Pu, 244Pu by
488
M. Stocker et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 483–489
accelerator mass spectrometry, has been demonstrated at an accelerating voltage of 300 kV [8]. Transmission efficiencies as high as 15% have been observed. Using the gas ionisation detector with the ultra-thin Si3N4 window, an energy resolution sufficient to separate Pu3+ ions from interfering Dy2+ ions with 2/3 of the energy was possible. Sensitivities about 106 atoms for the various Pu isotopes were achieved. We have successfully extended the test measurement to different nuclides of actinides such as 241 Am, 243Am and 236U, 238U. The AMS measurement efficiency is even better than that of larger and much more complex tandem accelerators. Detailed information about actinide analysis with a compact AMS system is given elsewhere [13]. 3.7. AMS performance For most of the isotopes discussed above, detailed performance studies have been made by using reference materials with known concentrations or by remeasuring series of samples which have been analysed before with our 6 MV AMS system. Good agreement and reproducible results were obtained with the compact AMS facility. Table 1 shows the transmission values and the background levels for the various radionuclides, which have been evaluated. As a comparison, the corresponding performance of the PSI/ETH EN-Tandem facility is shown. In general, the background determined with blank samples, which should not contain any of the radionuclides is slightly higher for the compact system. However,
the performance is sufficient for most applications. Only for 41Ca is the background almost 2 orders of magnitude higher. However, a level of 10 12 is sufficient for biomedical applications. Especially the use of CaF2 samples, which are easy to prepare, makes the system very appealing for application discussed above.
4. Conclusion The background sources and limitations of the compact AMS system were identified by systematic studies. The system was improved with simple solutions and small investments. It is now competitive with larger AMS systems for many radionuclides. With the new gas ionisation detector particle identification was improved significantly. The technology should also be useful for other ion beam techniques. The present limitations are mainly due to scattering and charge changing processes along the beam path. Further improvements seem feasible: Modifications of the vacuum system would reduce the rate of scattering and charge changing processes. Improvements in the ion optics could decrease the number of scattered particles reaching the final detector. The present developments should help to build future compact AMS instruments. These would be very attractive alternatives to the existing larger facilities and could be easily accommodated in earth science laboratories or biomedical research institutions.
Table 1 Transmission and background levels for the radionuclides evaluated at the 600 kV AMS facility Isotope
600 kV AMS Charge state
10
Be [BeO] Be [BeF] 14 C 26 Al 41 Ca [CaH] 41 Ca [CaF] 129 I Pu 10
+
2 1+ 1+ 1+ 2+ 3+ 4+ 3+
EN-Tandem Energy in MeV 1.4 0.8 1 1 1.4 1.9 2.5 1.2
Transmission in % Background 9
13 [ Be] 50 [9Be] 40–45 25 15 7 4 10–15
13
<10 10 14 5 · 10 15 10 14 10 12 3 · 10 12 3 · 10 13 106 atoms
Charge state
Transmission in %
Background
3
20
10
4+ 3+ 4+
40–45 15–20 10–12
5 · 10 10 15 5 · 10
15
5+
6
5 · 10
14
+
For comparison, the performance of the PSI/ETH EN-Tandem facility is given for respective charge states and energies.
14
to 10
14
15
M. Stocker et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 483–489
Acknowledgements The authors would like to thank Mark Stalder for his contribution to the investigation of calcium analysis. The Swiss National Science Foundation supports this work.
References [1] M. Suter, Nucl. Instr. and Meth. B 139 (1998) 150. [2] H.-A. Synal, S. Jacob, M. Suter, Nucl. Instr. and Meth. 161–163 (2000) 29. [3] H.-A. Synal, S. Jacob, M. Suter, Nucl. Instr. and Meth. 172 (2000) 1. [4] S. Jacob, M. Suter, H.-A. Synal, Nucl. Instr. and Meth. 172 (2000) 235. [5] M. Suter, S. Jacob, H.-A. Synal, Nucl. Instr. and Meth. 172 (2000) 144.
B B B B
489
[6] M. Stocker, R. Bertschinger, M. Do¨beli, M. Grajcar, S. Jakob, J. Scheer, M. Suter, H.-A. Synal, Nucl. Instr. and Meth. B 223–224 (2004) 104. [7] M. Grajcar, M. Do¨beli, P.W. Kubik, C. Maden, M. Suter, H.-A. Synal, Nucl. Instr. and Meth. B 223–224 (2004) 190. [8] L.K. Fifield, H.-A. Synal, M. Suter, Nucl. Instr. and Meth. B 223–224 (2004) 802. [9] M. Do¨beli, C. Kottler, M. Stocker, S. Weinmann, H.-A. Synal, M. Grajcar, M. Suter, Nucl. Instr. and Meth. B 219– 220 (2004) 415. [10] X.-L. Zhao, A.E. Litherland, J.P. Doupe´, W.E. Kieser, Nucl. Instr. and Meth. B 223–224 (2004) 199. [11] L. Wacker, M. Grajcar, S. Ivy-Ochs, P.W. Kubik, M. Suter, Radiocarbon 46 (1) (2004) 83. [12] D. Fink, R. Middleton, J. Klein, P. Sharma, Nucl. Instr. and Meth. B 47 (1990) 79. [13] L. Wacker, E. Chamizo, L.K. Fifield, M. Stocker, M. Suter, H.-A. Synal, Nucl. Instr. and Meth. B, this edition (2004) THU-PM-A4.