- Email: [email protected]
Comparison of detector systems for the separation of 36Cl and 36S with a 3-MV tandem
Nuclear Instruments and Methods in Physics Research B 268 (2010) 847–850
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Comparison of detector systems for the separation of with a 3-MV tandem
36
Cl and
36
S
Tobias Orlowski a,*, Oliver Forstner a, Robin Golser a, Walter Kutschera a, Silke Merchel b,c, Martin Martschini a, Alfred Priller a, Peter Steier a, Christof Vockenhuber d, Anton Wallner a a
VERA Laboratory, Fakultät für Physik, Universität Wien, Währinger Str. 17, Wien 1090, Austria CEREGE, CNRS-IRD-Université Aix-Marseille, Europôle Méditerranéen de L’Arbois, BP 80, Aix-en-Provence 13545, France Forschungszentrum Dresden-Rossendorf, D-01314 Dresden, Germany d HPK H 31, Institut für Teilchenphysik, ETH Zürich, Schafmattstr. 20 CH-8093 Zürich, Switzerland b c
a r t i c l e
i n f o
Article history: Available online 9 October 2009 Keywords: 36 Cl AMS Isobar separation Ionization chamber TOF
a b s t r a c t The possibility of detecting 36Cl for geological exposure dating has been explored for several years at VERA (the Vienna Environmental Research Accelerator). First results on real samples were obtained with an ionization chamber (developed at the ETH/PSI, Zürich, Switzerland) with two anodes. To improve the suppression of 36S, we equipped the ionization chamber with an exit window and added a Time-of-Flight (TOF) system with a double-sided silicon strip detector (50 50 mm2) as stop detector. We optimized the TOF setup by using silicon nitride foils to reduce scattering tails in the energy spectra. At 3 MV terminal voltage, corresponding to a particle energy of 24 MeV of 36Cl7+, we achieved a 36S7+suppression of 21,500 (50% 36Cl-detector-efficiency). Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Continuing technical progress has allowed medium size accelerators with a terminal voltage of 3 MV to measure all the AMS isotopes for which no stable isobars which form negative ions exist [1]. Suppression of a stable isobar based on the different energy loss in matter is only established for 10Be (stable isobar 10 B). This isobar suppression has been limited to 10Be because of the large relative energy straggling below the maximum of the Bragg curve (1 MeV/amu). At VERA (the Vienna Environmental Research Accelerator with nominally a 3.0 MV terminal voltage) 36 Cl can be measured with a 36Cl vs. 36S-suppression of 30,000 (at 50% 36Cl-detector-efficiency) [2]. This measurement was made with an ionization chamber with two anodes [3] at 28 MeV (terminal voltage 3.5 MV, 7+ charge state). However, with the same detection efficiency a suppression of only 9300 at 26.4 MeV (3.3 MV, 7+) and 1600 at 24 MeV (3 MV, 7+) was reached. We have to condition the accelerator for at least two days to reach 3.5 MV. Therefore it would be advantageous to measure 36Cl at 3.0 MV. In the present work we present an improved detectionsystem which reaches a comparably good 36S-suppression at this lower voltage.
* Corresponding author. Address: VERA Laboratory, Fakultät für Physik – Isotopenforschung, Universität Wien, Währinger Straße 17, A-1090 Wien, Austria. Tel.: +43 1 4277 51729; fax: +43 1 4277 9517. E-mail address: [email protected] (T. Orlowski). 0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.10.046
The best suppression with the ionization chamber alone is achieved if the chamber pressure is chosen such that the crossover of the energy loss curves lies under the center of the second anode. At the end of the chamber the ions still have one third of their energy. To exploit the Z-information also contained in the last third of the energy, we equipped the ionization chamber with an exit window and placed a double-sided silicon strip detector (50 50 mm2, 256 pixels) behind the ionization chamber. Furthermore we added a Time-of-Flight (TOF) system. The idea is to combine the ionization chamber with our previous 36Cl-detectionsystem called DTOF in [4], as well as with a possibility for an independent measurement of the residual energy. We think that this ‘‘complicated” detection setup will provide the highest separation, but also allows to study the contributions of the individual detection systems separately. A similar detection system was used with a stop detector inside the ionization chamber [5,6]. The disadvantage of this setup is that the differing energy loss close to the stop detector does not fully translate into a TOF difference. Therefore useful Z-information for the separation is lost. We simulated the separation of 36S by an energy loss in a foil followed by a momentum analysis (first used by [7] for 10Be) with TRIM [8] and Simion (Scientific Instrument Services). With the optimum setup (2500 nm SiN foil, magnetic quadrupole doublet for focusing and a 28° bending magnet with 56 mm gap), a maximum 36Cl-transmission of only 15% is expected, but 99.88% of 36S could be separated. However, in comparison to the detector setup
848
T. Orlowski et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 847–850
presented in this work, no improvement of the 36S suppression at the same 36Cl efficiency can be expected (as a result of the high loss of 36Cl in the simulation). Therefore we decided not to investigate the energy loss setup experimentally. TOF (95 cm)
Start
Stop
ΔE1, ΔE2
E Rp , ER n Pp , Pn
Beam
Strip Detector
Ionization Chamber
Fig. 1. Schematic view of the detector setup. There are two energy loss signals from the ionization chamber (DE1 and DE2). Both sides of the strip detector provide energy signals (ERp or ERn) and position signals (Pp or Pn, respectively), which together specify the hit pixel, as well as the stop signal for the TOF.
10000
SiN
1000 nm SiN + 15 µg/cm² C 1000
SiN + Al coating
100
counts
1050 nm SiN + Al coating
10
500
450
400
350
300
250
1 200
TOF(channels) Fig. 2. TOF spectra for different foils used in the start MCP detector.
2. Detector setup Fig. 1 shows a schematic view of the detector setup. We equipped the ionization chamber (two anodes, 40 mbar isobutane) [3] with a 5 5 mm2 exit window (50 nm thick silicon nitride membrane with the probable composition Si3.0N3.1H0.06 [9], in the following abbreviated as SiN). At a distance of 63 cm behind the ionization chamber, a doublesided silicon strip detector (Micron Semiconductors Design W1) with an active area of 50 50 mm2 was placed. Both sides of the detector consist of 16 stripes (each 3 50 mm2). The n-side is rotated by 90° compared with the p-side, making up 256 pixels of 3 3 mm2 each. Whereas the energy resolution of a single strip for alpha-particles is 40–100 keV FWHM, the total energy resolution for alphaparticles is 60–150 keV FWHM. Therefore we calibrated every pixel with a monoenergetic beam to improve the energy resolution of the total detector. The strip detector also provides the stop signal for the TOF system (95 cm flight path). The start detector is equipped with a 50 nm SiN foil coated with Al (approximately 25 nm) introducing only a small energy-loss with a minimum of tailing. The ions which do not exactly hit the center of the strip detector follow a longer path. So we calibrated the TOF-measurement using the position signals of the strip detector to improve the energy resolution. Due to the energetic width of the 36Cl and the 36S measured in this work, the time resolution of the start detector (0.5 ns) and the stop detector (0.5–1 ns (private communication with Micron Semiconductors)) turned out to be negligible.
3. SiN timing foils In [4], we observed significant low-energy tails caused by the DLC (diamond like carbon) start foil (0.6 lg/cm2) [10]. Therefore, we experimented with SiN foils (from Silson Ltd., UK) as start foils, which are known to be of superior homogeneity (Fig. 2). First we equipped the detector with a 1000 nm thick SiN foil. The result is a TOF peak with a perfect Gaussian shape. We attri-
Fig. 3. 36Cl detection efficiency vs. 36S-suppression of our detection systems (7+ charge state, 24 MeV) obtained by varying the ellipsoidal integration bin (position, axis length, orientation) in the n-dimensional spectra (n = number of detector signals used). For comparison, the results with the ionization chamber alone at higher energies are also shown.
T. Orlowski et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 847–850
bute this to the smoothness of the surfaces and to the amorphous structure, which avoids grain boundaries and channeling. However, the detection efficiency for transmitted ions seemed to be poor. About 20–90% of the ions passed through the membrane without producing detectable secondary electrons, although several 100 electrons per ion are expected [11]. Actually, SiN has insulator properties. In the case of a perfect insulator the average number of electrons emitted has to balance the number of electrons lost in the stripping process in the membrane. So we assume that the SiN locally charges up to several kV what prevents low-energy electrons to escape. Because of this only very few electrons per ion can be expected. Unlike SiN foils, carbon foils provide 100% detection-efficiency for transmitted ions. But although the tested carbon foil was much thinner (15 lg/cm2) than the SiN foil, it lead to a large low-energy tail. For reaching 100% detection-efficiency with a minimum of tails, we coated the thinnest SiN foil available (50 nm) with conducting material (Al, approximately 25 nm). The aluminum still introduced some low-energy tailing, but gave acceptable results. 4.
36
S-suppression
From the same data set of acquired ions, the suppression was first determined for utilizing the information from the ionization chamber alone, then for the ionization chamber in combination
849
with the strip detector as well as for the ionization chamber in combination with the TOF and finally for the whole detector setup (Fig. 3). In the present setup, 35% of all ions are lost on the small exit window of the ionization chamber. Since this could probably be avoided by a larger window, we have renormalized all data for the strip and TOF detectors for this factor. In previous measurements, using only the ionization chamber, we achieved a 36S-suppression of 7750 (50% 36Cl-detector-efficiency) on average with a terminal voltage of 3.3 MV. At 3 MV and with the ionization chamber alone a similar suppression can only be achieved if the integration bin of 36Cl7+ is chosen to be very narrow to avoid the interfering 36S7+. The 36Cl-efficiency, however, is reduced to 12%. By adding the information of the residual energy measured by the strip detector or the TOF detector, an even better 36 S-suppression (12,000) is possible with the same 36Cl-efficiency (50%, including scattering losses in the TOF etc.). A better suppression is reached with the TOF detector compared to the strip detector, because its energy resolution is better. The information provided by the residual energy improves the suppression of 36S considerably (Fig. 4). Using all the information of our detector system the 36S-suppression can still be improved, e.g. 21,500 (50% efficiency). Although this number is still slightly worse than the suppression achieved with the ionization chamber alone at 3.5 MV, a slight reduction of the integration bin leading to a 36Cl-efficiency of about 46% results in a suppression of 35,000. The fact that a small change
Fig. 4. Improvement of the 36S-suppression by adding the information from the TOF detector. A standard and a blank sample were measured under the same conditions but the blank was measured for a significantly longer time. Our region-of-interest (ellipse) in the DE/E spectrum of the ionization chamber (upper two plots) is contaminated by various 36S tails. The events in this region-of-interest are plotted in the lower two plots. With the TOF-detector most of these ions can clearly be identified as 36S.
850
T. Orlowski et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 847–850
of the integration bin has such a strong influence on the suppression, suggests that this numbers bear a significant uncertainty. For a better suppression we also tried to take advantage of the fact that anomalously scattered 36S-ions making up the low-energy-tails would probably have a bigger scattering angle. Therefore we also investigated the suppression as a function of the biggest detectable scattering angle using the position information of the strip detector. However, no further improvement was visible. This might partly result from the fact that the small exit window of the ionization chamber already limits the scattering angle.
5. Outlook and summary At 3 MV terminal voltage (24 MeV), our detection setup reaches a 36S-suppression of 21,500, while still accepting 50% of the 36Cl7+. Together with the 16% stripping yield in the tandem terminal (foil stripping) for the 7+ charge state (estimated by a measured yield of 19% at 3.2 MV) this results in an overall detection efficiency of 8% of all 36Cl-ions, which compares favorably to larger facilities. We are looking forward to employ our detection setup for measurements with true environmental samples. We will equip the ionization chamber with a larger exit window to improve the detection efficiency and to investigate the suppres-
sion depending on angular scattering. In addition, an ionization chamber with three anodes would be worth investigating.
References [1] P. Steier, R. Golser, V. Liechtenstein, W. Kutschera, A. Priller, C. Vockenhuber, A. Wallner, Nucl. Instrum. Meth. Phys. Res. B 240 (2005) 445. [2] P. Steier, O. Forstner, R. Golser, W. Kutschera, S. Merchel, T. Orlowski, A. Priller, C. Vockenhuber, A. Wallner, Cl-36 exposure dating with a 3-MV tandem, Contribution to these Proceedings. [3] M. Stocker, M. Döbeli, M. Grajcar, M. Suter, H.-A. Synal, L. Wacker, Nucl. Instrum. Meth. Phys. Res. B 240 (2005) 483. [4] C. Vockenhuber, R. Golser, W. Kutschera, A. Priller, P. Steier, K. Vorderwinkler, A. Wallner, Nucl. Instrum. Meth. Phys. Res. B 240 (2005) 490. [5] M. Paul, D. Berkovits, L.D. Cecil, H. Feldstein, A. Hershkowitz, Y. Kashiv, S. Vogt, Nucl. Instrum. Meth. Phys. Res. B 123 (1997) 394. [6] X. Ruan, Y. Guan, M. He, K. Dong, Y. Zheng, S. Wu, S. Jiang, W. Wu, K. Liu, B. Han, Nucl. Instrum. Meth. Phys. Res. B 223–224 (2004) 172. [7] G.M. Raisbeck, F. Yiou, D. Bourles, J. Lestringuez, D. Deboffle, Nucl. Instrum. Meth. Phys. Res. B 5 (1984) 175. [8] J.F. Ziegler, James Ziegler – SRIM & TRIM,
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Comparison of detector systems for the separation of with a 3-MV tandem
36
Cl and
36
S
Tobias Orlowski a,*, Oliver Forstner a, Robin Golser a, Walter Kutschera a, Silke Merchel b,c, Martin Martschini a, Alfred Priller a, Peter Steier a, Christof Vockenhuber d, Anton Wallner a a
VERA Laboratory, Fakultät für Physik, Universität Wien, Währinger Str. 17, Wien 1090, Austria CEREGE, CNRS-IRD-Université Aix-Marseille, Europôle Méditerranéen de L’Arbois, BP 80, Aix-en-Provence 13545, France Forschungszentrum Dresden-Rossendorf, D-01314 Dresden, Germany d HPK H 31, Institut für Teilchenphysik, ETH Zürich, Schafmattstr. 20 CH-8093 Zürich, Switzerland b c
a r t i c l e
i n f o
Article history: Available online 9 October 2009 Keywords: 36 Cl AMS Isobar separation Ionization chamber TOF
a b s t r a c t The possibility of detecting 36Cl for geological exposure dating has been explored for several years at VERA (the Vienna Environmental Research Accelerator). First results on real samples were obtained with an ionization chamber (developed at the ETH/PSI, Zürich, Switzerland) with two anodes. To improve the suppression of 36S, we equipped the ionization chamber with an exit window and added a Time-of-Flight (TOF) system with a double-sided silicon strip detector (50 50 mm2) as stop detector. We optimized the TOF setup by using silicon nitride foils to reduce scattering tails in the energy spectra. At 3 MV terminal voltage, corresponding to a particle energy of 24 MeV of 36Cl7+, we achieved a 36S7+suppression of 21,500 (50% 36Cl-detector-efficiency). Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Continuing technical progress has allowed medium size accelerators with a terminal voltage of 3 MV to measure all the AMS isotopes for which no stable isobars which form negative ions exist [1]. Suppression of a stable isobar based on the different energy loss in matter is only established for 10Be (stable isobar 10 B). This isobar suppression has been limited to 10Be because of the large relative energy straggling below the maximum of the Bragg curve (1 MeV/amu). At VERA (the Vienna Environmental Research Accelerator with nominally a 3.0 MV terminal voltage) 36 Cl can be measured with a 36Cl vs. 36S-suppression of 30,000 (at 50% 36Cl-detector-efficiency) [2]. This measurement was made with an ionization chamber with two anodes [3] at 28 MeV (terminal voltage 3.5 MV, 7+ charge state). However, with the same detection efficiency a suppression of only 9300 at 26.4 MeV (3.3 MV, 7+) and 1600 at 24 MeV (3 MV, 7+) was reached. We have to condition the accelerator for at least two days to reach 3.5 MV. Therefore it would be advantageous to measure 36Cl at 3.0 MV. In the present work we present an improved detectionsystem which reaches a comparably good 36S-suppression at this lower voltage.
* Corresponding author. Address: VERA Laboratory, Fakultät für Physik – Isotopenforschung, Universität Wien, Währinger Straße 17, A-1090 Wien, Austria. Tel.: +43 1 4277 51729; fax: +43 1 4277 9517. E-mail address: [email protected] (T. Orlowski). 0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.10.046
The best suppression with the ionization chamber alone is achieved if the chamber pressure is chosen such that the crossover of the energy loss curves lies under the center of the second anode. At the end of the chamber the ions still have one third of their energy. To exploit the Z-information also contained in the last third of the energy, we equipped the ionization chamber with an exit window and placed a double-sided silicon strip detector (50 50 mm2, 256 pixels) behind the ionization chamber. Furthermore we added a Time-of-Flight (TOF) system. The idea is to combine the ionization chamber with our previous 36Cl-detectionsystem called DTOF in [4], as well as with a possibility for an independent measurement of the residual energy. We think that this ‘‘complicated” detection setup will provide the highest separation, but also allows to study the contributions of the individual detection systems separately. A similar detection system was used with a stop detector inside the ionization chamber [5,6]. The disadvantage of this setup is that the differing energy loss close to the stop detector does not fully translate into a TOF difference. Therefore useful Z-information for the separation is lost. We simulated the separation of 36S by an energy loss in a foil followed by a momentum analysis (first used by [7] for 10Be) with TRIM [8] and Simion (Scientific Instrument Services). With the optimum setup (2500 nm SiN foil, magnetic quadrupole doublet for focusing and a 28° bending magnet with 56 mm gap), a maximum 36Cl-transmission of only 15% is expected, but 99.88% of 36S could be separated. However, in comparison to the detector setup
848
T. Orlowski et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 847–850
presented in this work, no improvement of the 36S suppression at the same 36Cl efficiency can be expected (as a result of the high loss of 36Cl in the simulation). Therefore we decided not to investigate the energy loss setup experimentally. TOF (95 cm)
Start
Stop
ΔE1, ΔE2
E Rp , ER n Pp , Pn
Beam
Strip Detector
Ionization Chamber
Fig. 1. Schematic view of the detector setup. There are two energy loss signals from the ionization chamber (DE1 and DE2). Both sides of the strip detector provide energy signals (ERp or ERn) and position signals (Pp or Pn, respectively), which together specify the hit pixel, as well as the stop signal for the TOF.
10000
SiN
1000 nm SiN + 15 µg/cm² C 1000
SiN + Al coating
100
counts
1050 nm SiN + Al coating
10
500
450
400
350
300
250
1 200
TOF(channels) Fig. 2. TOF spectra for different foils used in the start MCP detector.
2. Detector setup Fig. 1 shows a schematic view of the detector setup. We equipped the ionization chamber (two anodes, 40 mbar isobutane) [3] with a 5 5 mm2 exit window (50 nm thick silicon nitride membrane with the probable composition Si3.0N3.1H0.06 [9], in the following abbreviated as SiN). At a distance of 63 cm behind the ionization chamber, a doublesided silicon strip detector (Micron Semiconductors Design W1) with an active area of 50 50 mm2 was placed. Both sides of the detector consist of 16 stripes (each 3 50 mm2). The n-side is rotated by 90° compared with the p-side, making up 256 pixels of 3 3 mm2 each. Whereas the energy resolution of a single strip for alpha-particles is 40–100 keV FWHM, the total energy resolution for alphaparticles is 60–150 keV FWHM. Therefore we calibrated every pixel with a monoenergetic beam to improve the energy resolution of the total detector. The strip detector also provides the stop signal for the TOF system (95 cm flight path). The start detector is equipped with a 50 nm SiN foil coated with Al (approximately 25 nm) introducing only a small energy-loss with a minimum of tailing. The ions which do not exactly hit the center of the strip detector follow a longer path. So we calibrated the TOF-measurement using the position signals of the strip detector to improve the energy resolution. Due to the energetic width of the 36Cl and the 36S measured in this work, the time resolution of the start detector (0.5 ns) and the stop detector (0.5–1 ns (private communication with Micron Semiconductors)) turned out to be negligible.
3. SiN timing foils In [4], we observed significant low-energy tails caused by the DLC (diamond like carbon) start foil (0.6 lg/cm2) [10]. Therefore, we experimented with SiN foils (from Silson Ltd., UK) as start foils, which are known to be of superior homogeneity (Fig. 2). First we equipped the detector with a 1000 nm thick SiN foil. The result is a TOF peak with a perfect Gaussian shape. We attri-
Fig. 3. 36Cl detection efficiency vs. 36S-suppression of our detection systems (7+ charge state, 24 MeV) obtained by varying the ellipsoidal integration bin (position, axis length, orientation) in the n-dimensional spectra (n = number of detector signals used). For comparison, the results with the ionization chamber alone at higher energies are also shown.
T. Orlowski et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 847–850
bute this to the smoothness of the surfaces and to the amorphous structure, which avoids grain boundaries and channeling. However, the detection efficiency for transmitted ions seemed to be poor. About 20–90% of the ions passed through the membrane without producing detectable secondary electrons, although several 100 electrons per ion are expected [11]. Actually, SiN has insulator properties. In the case of a perfect insulator the average number of electrons emitted has to balance the number of electrons lost in the stripping process in the membrane. So we assume that the SiN locally charges up to several kV what prevents low-energy electrons to escape. Because of this only very few electrons per ion can be expected. Unlike SiN foils, carbon foils provide 100% detection-efficiency for transmitted ions. But although the tested carbon foil was much thinner (15 lg/cm2) than the SiN foil, it lead to a large low-energy tail. For reaching 100% detection-efficiency with a minimum of tails, we coated the thinnest SiN foil available (50 nm) with conducting material (Al, approximately 25 nm). The aluminum still introduced some low-energy tailing, but gave acceptable results. 4.
36
S-suppression
From the same data set of acquired ions, the suppression was first determined for utilizing the information from the ionization chamber alone, then for the ionization chamber in combination
849
with the strip detector as well as for the ionization chamber in combination with the TOF and finally for the whole detector setup (Fig. 3). In the present setup, 35% of all ions are lost on the small exit window of the ionization chamber. Since this could probably be avoided by a larger window, we have renormalized all data for the strip and TOF detectors for this factor. In previous measurements, using only the ionization chamber, we achieved a 36S-suppression of 7750 (50% 36Cl-detector-efficiency) on average with a terminal voltage of 3.3 MV. At 3 MV and with the ionization chamber alone a similar suppression can only be achieved if the integration bin of 36Cl7+ is chosen to be very narrow to avoid the interfering 36S7+. The 36Cl-efficiency, however, is reduced to 12%. By adding the information of the residual energy measured by the strip detector or the TOF detector, an even better 36 S-suppression (12,000) is possible with the same 36Cl-efficiency (50%, including scattering losses in the TOF etc.). A better suppression is reached with the TOF detector compared to the strip detector, because its energy resolution is better. The information provided by the residual energy improves the suppression of 36S considerably (Fig. 4). Using all the information of our detector system the 36S-suppression can still be improved, e.g. 21,500 (50% efficiency). Although this number is still slightly worse than the suppression achieved with the ionization chamber alone at 3.5 MV, a slight reduction of the integration bin leading to a 36Cl-efficiency of about 46% results in a suppression of 35,000. The fact that a small change
Fig. 4. Improvement of the 36S-suppression by adding the information from the TOF detector. A standard and a blank sample were measured under the same conditions but the blank was measured for a significantly longer time. Our region-of-interest (ellipse) in the DE/E spectrum of the ionization chamber (upper two plots) is contaminated by various 36S tails. The events in this region-of-interest are plotted in the lower two plots. With the TOF-detector most of these ions can clearly be identified as 36S.
850
T. Orlowski et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 847–850
of the integration bin has such a strong influence on the suppression, suggests that this numbers bear a significant uncertainty. For a better suppression we also tried to take advantage of the fact that anomalously scattered 36S-ions making up the low-energy-tails would probably have a bigger scattering angle. Therefore we also investigated the suppression as a function of the biggest detectable scattering angle using the position information of the strip detector. However, no further improvement was visible. This might partly result from the fact that the small exit window of the ionization chamber already limits the scattering angle.
5. Outlook and summary At 3 MV terminal voltage (24 MeV), our detection setup reaches a 36S-suppression of 21,500, while still accepting 50% of the 36Cl7+. Together with the 16% stripping yield in the tandem terminal (foil stripping) for the 7+ charge state (estimated by a measured yield of 19% at 3.2 MV) this results in an overall detection efficiency of 8% of all 36Cl-ions, which compares favorably to larger facilities. We are looking forward to employ our detection setup for measurements with true environmental samples. We will equip the ionization chamber with a larger exit window to improve the detection efficiency and to investigate the suppres-
sion depending on angular scattering. In addition, an ionization chamber with three anodes would be worth investigating.
References [1] P. Steier, R. Golser, V. Liechtenstein, W. Kutschera, A. Priller, C. Vockenhuber, A. Wallner, Nucl. Instrum. Meth. Phys. Res. B 240 (2005) 445. [2] P. Steier, O. Forstner, R. Golser, W. Kutschera, S. Merchel, T. Orlowski, A. Priller, C. Vockenhuber, A. Wallner, Cl-36 exposure dating with a 3-MV tandem, Contribution to these Proceedings. [3] M. Stocker, M. Döbeli, M. Grajcar, M. Suter, H.-A. Synal, L. Wacker, Nucl. Instrum. Meth. Phys. Res. B 240 (2005) 483. [4] C. Vockenhuber, R. Golser, W. Kutschera, A. Priller, P. Steier, K. Vorderwinkler, A. Wallner, Nucl. Instrum. Meth. Phys. Res. B 240 (2005) 490. [5] M. Paul, D. Berkovits, L.D. Cecil, H. Feldstein, A. Hershkowitz, Y. Kashiv, S. Vogt, Nucl. Instrum. Meth. Phys. Res. B 123 (1997) 394. [6] X. Ruan, Y. Guan, M. He, K. Dong, Y. Zheng, S. Wu, S. Jiang, W. Wu, K. Liu, B. Han, Nucl. Instrum. Meth. Phys. Res. B 223–224 (2004) 172. [7] G.M. Raisbeck, F. Yiou, D. Bourles, J. Lestringuez, D. Deboffle, Nucl. Instrum. Meth. Phys. Res. B 5 (1984) 175. [8] J.F. Ziegler, James Ziegler – SRIM & TRIM,