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The new 6 MV AMS-facility DREAMS at Dresden
Nuclear Instruments and Methods in Physics Research B 294 (2013) 5–10
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The new 6 MV AMS-facility DREAMS at Dresden Shavkat Akhmadaliev, René Heller, Daniel Hanf, Georg Rugel, Silke Merchel ⇑ Helmholtz-Zentrum Dresden-Rossendorf, D-01314 Dresden, Germany
a r t i c l e
i n f o
Article history: Received 1 July 2011 Received in revised form 18 January 2012 26 January 2012 Available online 5 March 2012 Keywords: Accelerator mass spectrometry Electrostatic accelerator
a b s t r a c t A new 6 MV electrostatic tandem accelerator has been put into operation at Helmholtz-Zentrum Dresden-Rossendorf (HZDR). The system is equipped for accelerator mass spectrometry and opens a new research field at HZDR and the Helmholtz Association. It will be also used for ion beam analysis as well as for material modification via high-energy ion implantation. The research activity at the DREsden Accelerator Mass Spectrometry facility (DREAMS) based on a 6 MV Tandetron is primarily dedicated to the long-lived radioisotopes of 10Be, 26Al, 36Cl, 41Ca, and 129I. DREAMS background levels have been found to be at 4.5 10 16 for 10Be/9Be, 8 10 16 for 26Al/27Al, 3 10 15 for 36Cl/35Cl and 8 10 15 for 41Ca/40Ca, respectively. The observed background of 2 10 13 for 129I/127I originates from intrinsic 129I from AgI produced from commercial KI. The introduction of quality assurance approaches for AMS, such as the use of traceable calibration materials and taking part in interlaboratory comparisons, guarantees high accuracy data for future DREAMS users. During first experiments an energy calibration of the accelerator has been carried out using the nuclear reaction 1H(15N,ca)12C yielding an energy correction factor of 1.019. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction The installation of a new 6 MV electrostatic tandem accelerator opens a new research discipline at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR): accelerator mass spectrometry (AMS). Beginning as a method of radiocarbon dating AMS has been developed during the last decades to a powerful technique for the detection of many rare radionuclides with a sensitivity in the range of 10 15–10 16 [1–5]. The benefits from using AMS for research in the field of radiation protection, nuclear safety, nuclear waste, radioecology, phytology, nutrition, toxicology, and pharmacology are obvious and manifold: Smaller sample sizes, easier and faster sample preparation, higher sample throughput and the redundancy for radiochemistry laboratories will largely reduce costs. The AMS research program at HZDR is primarily concentrated on investigations of cosmogenic long-lived radionuclides like 10 Be, 26Al and 36Cl as well as 41Ca and 129I, which have growing interest especially for environmental and geosciences, astrophysics, medicine and climatology. Using these nuclides dating of suddenly occurring prehistoric mass movements, e.g. volcanic eruptions, rock avalanches, tsunamis, meteor impacts, earth quakes and glacier movements, is possible. Additionally, glacier movements and data from ice cores give hints for the reconstruction of historic climate changes and providing information for the validation of climate model predicting future changes. Re⇑ Corresponding author. Tel.: +49 351 260 2802; fax: +49 351 260 12802. E-mail address: [email protected] (S. Merchel). 0168-583X/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2012.01.053
search topics are addressed in close national and international cooperation with other research institutes as e.g. from the Helmholtz and Max-Planck Association, universities and industrial partners. Two dedicated chemistry laboratories, one for 10Be, 26Al, 41Ca, 53 Mn, 59Ni and 55/60Fe, the second for the volatile radionuclides 36Cl and 129I, are available on-site for sample preparation. The implementation of quality assurance schemes such as the fabrication and use of traceable calibration materials and low-level carriers, and taking part in interlaboratory comparisons, allows for high accuracy data for all DREAMS users. Following decades of experience at HZDR, the accelerator is also used for ion beam analysis such as Particle-Induced X-ray and Gamma-Emission (PIXE/PIGE), nuclear reaction analysis (NRA) and elastic recoil detection (ERD) as well as for material modification via high-energy ion implantation. 2. System set-up The new system is based on a 6 MV Tandetron (HVEE) with two separate ion injectors (Fig. 1). The multipurpose ion injector, not in use for AMS and containing two ion sources, is applied for ion beam analysis and high-energy ion implantation. Negative He-ions are produced by a duoplasmatron Model 358 in combination with a Li-charge exchange canal. All other ion species are emitted by a Cs-sputter ion source (Model 860-C). Ten cathodes can be installed simultaneously into this ion source. The low-energy side of the AMS set-up consists of two identical Cs-sputter ion sources (SO-110) with wheels for up to 200 samples
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Fig. 1. Layout of the new 6 MV Tandetron system with high-energy implantation, AMS and interconnecting beamlines.
each. The energy of the negative ions are analysed by a 54° electrostatic analyser (ESA). The fast bouncing system with voltages of 3 to +3 kV, installed at the 90° magnet chamber, injects sequentially the isotopes of interest into the accelerator. The ion accelerating is based on the tandem principle with injection of low-energy negative ions and using argon gas as an electron stripping medium. The terminal voltage is generated by a parallel driven Dynamitron-type solid state high-voltage cascade [6] and can be varied between 300 kV and 6.0 MV. The drift is less than 10 4 per hour; the residual ripples from the high voltage generating cascade are in the range of 10 5. A maximum terminal voltage of 6.6 MV can be reached for a short time during conditioning. All high-voltage elements are insulated by SF6 gas inside the highpressure tank. The electrostatic quadrupole triplet lens at the output of the accelerator focuses the ion beam. The first high-energy 90° analysing magnet with a bending radius of 1.5 m and ME/Q2 = 185 MeV amu deflects the ions into the AMS section. The high-energy part of the AMS system is most similar to the one at the 5 MV Tandetron at CEREGE (ASTER, Aixen-Provence, France) [7–9]. Both consist of a 35° high-energy electrostatic analyser (ESA) with E/Q up to 6.85 MV, a 30° vertical magnet for ambiguity suppression in the horizontal plane and a gas ionisation detector (Fig. 1). A 1 lm thick silicon nitride absorber foil placed after the 90° analysing magnet is applied for isobar suppression of 10B and 36S. Additionally, there are two electrostatic quadrupole lenses before and after the ESA for refocusing the ion beam being subject of straggling processes in the absorber foil. The final counting of the particles is performed by an isobutanefilled four-anode gas ionisation detector with a 75 nm thick silicon nitride window. The signals from the four anodes are analysed by the FAST ComTec MPANT software [10]. As a special feature the DREAMS-facility has two moveable Faraday cups after the high-energy analysing magnet. They allow the simultaneous measurement of any two stable isotopes, which is important for controlling possible fractionation effects [11], and, moreover, indispensable for the application of isotope dilution AMS [12]. Hence, the detection of 35Cl and 37Cl, when using isotope-enriched carrier for chemistry, yield simultaneously the natural chlorine abundance in the investigated sample. Each of the slit systems in the Faraday cups can be alternatively used for energy stabilisation.
Besides the AMS-system, a second 90° analysing magnet (Fig. 1), is employed for high-energy ion implantation using an automatic wafer handler system operating wafers up to a size of 200 mm. The interconnecting beamline connects the new accelerator with the existing experimental ion beam analysis equipment of the old 5 MV tandem accelerator, which has been shutdown in December 2010. 3. Energy calibration Accurately knowing the energy and energy spread of the ions within the beam allows for correct performance of ion beam analysis like hydrogen depth-profiling and for calculation of the exact energy loss in material e.g. the detector gas. Thus, right after the installation, an energy calibration of the new accelerator has been performed. The ion energy depends on the terminal voltage and can be stabilised by either using a generating voltmeter or by a feedback slit error signal coming from one of the Faraday cups after the 90° analysing magnet [13]. The energy calibration of the accelerator was carried out using the resonant nuclear reaction 1H(15N,ca) 12C with a narrow width of 1.87 keV [14]. The 29 keV 12C15N negative ion beam was produced by a Cs-sputter ion source using a Cu-cathode filled with a mixture of nitrogen-15 enriched sodium azide and graphite. After stripping to positive 15N-ions with different charge states, their energy was scanned near the resonance of 6.385 MeV. An ultracleaned silicon wafer chip containing several monolayers of absorbed water molecules on the surface was taken as a target for the measurements. The produced 4.439 MeV gamma-rays were detected by a BGO scintillator with a photomultiplier. The fine tuning of the ion energy near the resonance was performed by changing the bias voltage of the Si-target in the range of ±20 kV. The resulting dependence of the scintillator counts on the bias voltage gives information about the true energy of the beam (Fig. 2). The full width of the half-maximum of this curve (15 keV) is determined by Doppler energy broadening, the thickness of the hydrogen-containing layer, the energy straggling in this layer and the energy spread in the beam. Assuming the Doppler broadening of about 11 keV [15] and neglecting the thickness of the water layer absorbed on the silicon surface, the energy spread of ions in the beam
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Fig. 2. (a) Resonance curve of the reaction 1H(15N, ca) 12C using 15N2+ at a terminal voltage of 2538 kV. (b) Measured energy of 15N-ions vs. calculated one ECalc. = UTetmQ + (UTerm + UInj) m/M + UBiasQ for charge states Q = +1,+2 and +3, negative ions mass M = 27, positive ion mass m = 15,the injector voltage UInj = 29 kV and target bias UBias was varied using ±20 kV power supply.
can be calculated to not exceeding 10 keV. The calibration curve was measured with 15N+, 15N2+ and 15N3+ corresponding to a terminal voltage of 4094, 2492 and 1791 kV, respectively. During the measurements the energy of the ions was stabilised using the slits. The terminal voltage deviation was determined assuming a linear function to about 1.9%, corresponding to an energy correction factor of 1.019. This deviation can be reduced by an improved adjusting of the feedback loop of the high voltage power supply. We have performed similar experiments at the 5 MV AMS facility ASTER, which have shown a smaller deviation of 0.35% between the calculated ion energy and the measured one. 4. AMS measurements Measurement conditions for the individual radionuclides as used for acceptance (14C only) and later machine and chemistry performance tests (all, but 14C) are summarised in Table 1. Only for those nuclides taking advantage of the (nearly) maximum terminal voltage, i.e. 36Cl and 41Ca some modifications in comparison to the well-established 5 MV AMS facility ASTER have been made. 4.1. Tuning for
14
C
Despite the fact that it is not foreseen to perform 14C measurements in the future at DREAMS, we took the best chance to become acquainted with our new system by using it for 14C, the radionuclide needing the highest measurement accuracy. The accelerator is run at a terminal voltage of 3 MV for 14C. The injection energy
Table 1 Performance of DREAMS for
Be,
14
C,
26
Al,
36
Cl,
41
Ca, and
129
4.2. Tests with
10
Be
The machine performance for 10Be samples is of course equally good as the one from the 5 MV AMS system ASTER at CEREGE [7,8], as this AMS set-up is most similar to the DREAMS one. The measurements are carried out at 4.5 MV terminal voltage using BeO extracted from a BeO–Nb cathode, resulting in energy of 10.742 MeV for 10Be2+. The 9BeO -ion beam current is typically 2–7 lA (for standards, blanks and well-prepared samples), a similar Be2+-ion beam current is reached in the Faraday cup at the highenergy site. The 10B isobar suppression is provided by 1 lm thick silicon nitride absorber foil followed by a 35° electrostatic analyser.
I using Ar stripper gas.
Rare nuclide
Injected ion
Injected current (lA)
Terminal voltage (MV)
Charge state after stripping
Charge state after absorber
Total transmission (%)a
Background
Precisionat isotope ratio
10
BeO C Al AlO Cl
2–7 30–45 0.2–0.5 3–6 20–30
4.5 3.0 2.7 6.0 6.0
2+ 3+ 3+ 4+ 5+
4+ – – 8+ 11+
21 40 35 2 3
0.3% at 10 0.4% at 10 1.5% at 10 2% at 10 2.0% at 10
CaF3 I
0.2–0.35 3–5
5.8 5.0
4+ 5+
– –
16 9
4.5 10 16 1.2 10 15 8 10 16 1 10 14 3 10 15 (36Cl/35Cl) 8 10 15 2 10 13
Be C 26 Al 14
36
Cl
41
Ca I
129 a
10
of negative carbon ions is 29 keV. The charge state 3+ is chosen to avoid interferences from 7Li+. As a pilot beam for adjustment of ion optical components of the high-energy part of the AMS-system 13C3+ is used. Typical negative ion currents from the ion source are in the range of 30–45 lA. After the 90° bouncer magnet the fraction of 13C (together with molecules) is about 300 nA without bouncing voltage. Finally, the 12 MeV 13C3+ beam current measured by the Faraday cup after the 90° analysing magnet is nearly 370 nA with a corresponding 12C3+ current of about 35 lA and an overall transmission of about 40%. For the acceptance test four standard samples have been measured resulting in a relative standard deviation of 4‰ at a 14C/12C ratio of 1.6 10 12, thus, passing the guaranteed value of 5‰. A 14C/12C background of 1.2 10 15 and a natural 13C/12C ratio of (1.080 ± 0.024) 10 2 have been determined.
Defined as the ratio between the number of ions detected in the ionisation chamber and the number of ions injected into the accelerator.
2.5% at 10 1.2% at 10
12 12 11 11 12
11 11
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Meltzow / ETH Klas / ETH Knauer / ETH Neupert / ETH Altmaier / ETH Merchel / ETH Schnabel / PRIMElab Mokos / PRIMElab DREAMS
22
20
10
Be [dpm/kg]
24
18
0
10 20 30 40 50 Chronological sample order (1-40 only)
Fig. 3. Replica measurements of 10Be isolated from material originating from the LL chondrite Dhurmsala (Fragment 2/3A,70 g without 1 g of metal, homogenised to size fraction of <125 lm [20]. AMS targets have been prepared at four chemistry labs (U of Cologne,U Hanover,Rutgers U,PRIMELab) by eight scientists (M. Altmaier,W. Klas,M. Knauer,B. Meltzow,S. Merchel,J. Mokos,U. Neupert,C. Schnabel) [21 and references therein]. AMS measurements have been performed from 1994–1998 at ETH Zurich (values adapted to new half-life and standard values [22]) and PRIMElab. The new DREAMS 10Be value is from BeO prepared in 2011 from 150 mg of the same Dhurmsala fraction.
About 36% of 10Be-ions, changing their charge state from 2+ to 4+, reach the detector, thus, the total transmission from the injector to the detector is about 21%. An excellent machine background (far below the guaranteed value of 3 10 15) of 4.5 10 16 for 10 Be/9Be as a mean of seven machine blank samples has been achieved using Be-containing minerals originating from a deep mine [16]. Due to the loss of 10Be ions in the absorber foil and the 35° ESA, the correction factor (nominal standard ratio divided by the measured one) is usually about 2.2. A new in-house secondary standard material ‘‘SMD-Be-12’’ has been cross-calibrated vs. the primary standard NIST 4325 at ASTER, VERA, and DREAMS. This material with a weighted mean value of (1.704 ± 0.030) 10 12 will be used for all 10Be measurements at DREAMS in the future. Its total uncertainty is mainly dominated by the original uncertainty of 1% of the primary standard. Thus, all DREAMS results are traceable to NIST 4325 with the most widely accepted 10Be/9Be value of (2.79 ± 0.03) 10 11 [17] and a half-life of (1.387 ± 0.012) Ma [18]. Taking part in 10Be round-robin exercises [19], and intercomparison measurements of high-activity samples such as meteorites (Fig. 3) have already shown that DREAMS produces internationally comparable 10Be data. Additionally, the chances of a German– French project further improved quality assurance. Samples containing in situ produced and atmospheric 10Be have been chemically processed both at DREAMS and ASTER and measured at DREAMS and/or ASTER [9]. The results clearly demonstrate that despite different chemistry laboratories and procedures, reproducibility and precision is guaranteed for high activity samples, but also for 10Be samples as low as 104 atoms g 1 (either water or quartz).
4.3. Tests with
26
Al
The 26Al measurements are carried out at a terminal voltage of 2.7 MV. The injection of Al into the accelerator provides excellent suppression of the isobar 26Mg directly in the source, thus, an absorber foil for further isobar suppression in not needed. However, the current is, compared to other nuclides, relatively low, in the order of 0.3–0.5 lA. About 35% of this is transformed into 27Al3+ current (300–500 nA) after acceleration. The machine background has been demonstrated to be better than 8 10 16 for 26Al/27Al. The
transmission at the high-energy side is nearly 90–95%, corresponding to a correction factor of about 1.05–1.1. First tests at DREAMS has been carried out using standard-type material at different ratio levels from a 26Al round-robin exercise [23] proofing the accuracy of DREAMS 26Al/27Al results at high levels. However, production of a new secondary in-house standard SMD-Al-11 (1 10 11) and subsequent cross-calibration vs. primary standards following the approach of Arnold et al. [8], hence, guaranteeing highest traceability for future users, are in preparation. As demonstrated by high-energy AMS facilities [24–26], the use of AlO can increase currents by one order of magnitude. High terminal voltages of 10–20 MV and foil stripper allow to accelerate the ions up to energies of about 100 MeV and to apply a gas-filled magnet for isobar suppression [27] or realise fully stripped ions (Al13+ and Mg12+) at energies of more than 150 MeV [25]. To test the performance of DREAMS’ absorber foil technique for 26Mg suppression, AlO has been extracted up to currents of 3–6 lA. Preliminary results have shown that the maximum terminal voltage of 6 MV using a charge state of 3+ (21.7 MeV) is not sufficient for a clear separation of 26Al and 26Mg in the detector. If choosing the charge state of 4+ (with a yield of about 15%), higher energies (27.7 MeV) thus, better separation is gained. The first tests were carried out using a 1 lm thick silicon nitride absorber foil allowing about 25% of the ions with charge state of 8+ to reach the detector. A satisfactory background of about 1 10 14 26Al/27Al can only be reached by defining very small regions of interests (ROIs) for 26Al in the detector signal spectra and by reducing the slit size after the 35° ESA. However, this way, a significant amount of the 26Al signal is lost and the total transmission decreases to about 2%. Compared to the overall transmission of 35% when using Al , the advantage of more than ten times higher AlO current is nearly nullified. Knowing chances are reasonable, further tests optimising the absorber foil thickness and charge state variations are in preparation. 4.4. Tests with
36
Cl
For applying isotope dilution AMS, the DREAMS system is equipped with two moveable Faraday cups that can be positioned for collecting simultaneously both stable isotopes of chlorine. Typically negative ion currents of the 35Cl -isotope from AgCl cathodes are 20–30 lA after the bouncer magnet during measurements. The
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Fig. 4. (a) 2D spectrum combined with signals from first (DE1) and last (DErest) anodes of the detector. (b) DErest spectrum for 29.0 MeV 36Cl and 29.3 MeV 36S. The sample ‘‘SM-Cl-12’’ has a 36Cl/35+37Cl ratio of 1.082 10 12 [28].
system is operated at a terminal voltage of 6.0 MV. About 27% of injected ions are accelerated up to 36 MeV as Cl5+. Ions of 35Cl5+ and 37Cl5+ are measured separately, each in its own Faraday cup. The absorber foil plays an important role for the suppression of the disturbing 36S isobar. Due to the difference in energy loss of about 0.3 MeV for 36Cl and 36S in the 1 lm silicon nitride foil, a suppression factor for sulphur by the ESA of more than 200 is obtained, resulting in a reasonable low background of better than 5 10 15, if the source has been very recently cleaned. As seen for other sources all over the world, under ‘‘every-day‘‘ measurement conditions, the 36Cl background (3 10 15) is solely limited by sample-to-sample cross-contamination and long-term source memory. A new source development [8] improves the performance [11,28], but not to an extend [9] allowing high sample-throughput as planned for AMS operation at DREAMS. The optimum for transmission of chlorine through the foil is about 12% for ions with charge state 11+ (in contrast to 10+ at ASTER [8]). The total transmission to the detector is about 3% and the measured ratios must be corrected by a factor of 9 to yield true ratios. Typical spectra from the detector for a standard-type sample ‘‘SM-Cl-12’’ at the 10 11 level [28] are shown in Fig. 4. As the high volatility and facile ionisation of chlorine promotes cross-contamination and source memory, 36Cl testing materials at different 36Cl/35+37Cl levels [28] have been used for the first 36 Cl test at DREAMS. The preparation of large quantities of inhouse secondary standards for DREAMS at the 10 11, 10 12, and 10 13 level and subsequent cross-calibration vs. primary-type materials at the same isotopic levels from a round-robin exercise [28] are foreseen for the very near future, therefore, establishing a traceability chain for 36Cl AMS. 4.5. Tests with
41
Ca
In the case of 41Ca, 41K is suppressed by extracting CaF3 from the ion source with currents of 200–350 nA. The ions of charge state 4+ are accelerated using a terminal voltage of 5.8 MV. The potassium suppression of the system has been measured to be at least 104 by comparing the count rates of 41K4+ in the detector while injecting either CaF or CaF3 into the accelerator. The resulting 41Ca/40Ca ratio for blank samples is lower than 1 10 14 without the use of an absorber foil. The remaining signal is true machine background, as the blank material, originating from a shielded calcite sample, has been measured at an energywise superior AMS facility at Munich down to 4 10 16 [29]. The current of two stable isotopes, such as 40Ca and 42Ca, can be quasi-simultaneously obtained from the two Faraday cups.
Following the approach of Arnold et al. [8], a set of different inhouse standards has been prepared as CaF2 and used for first 41Ca tests at DREAMS. The owing cross-calibration of these secondary standards vs. the corresponding primary ones from ERMÒ-AE701 [30] is already scheduled and will guarantee all DREAMS users traceable results, which is especially important for topics in biomedical research. However, earlier experiences have already shown that the calculated 41Ca/40Ca ratios of the secondary standards agree within permille-deviation with the cross-calibrated ones [8]. Following the approach of 10Be, 41Ca isolated as CaF2 from the already described special Dhurmsala meteorite sample, yield a 41 Ca-activity of about 31 dpm/kg, which is in astonishing good agreement with the value of (31 ± 5) dpm/kg, originally published by Merchel [21] as (37 ± 6) dpm/kg, but here treated by a correction factor after finding that the at that time used 41Ca calibration material ‘‘Juel’’ was about 15% lower than presumed [31]. 4.6. Tests with
129
I
Iodine measurements are carried out at the same terminal voltage using the same charge state, i.e. 5.0 MV and 5+, as demonstrated at ASTER [8]. As already seen for chlorine severe memory and cross-contamination could be identified. However, the remaining background of 2 10 13 for 129I/127I at DREAMS originates from intrinsic 129I, as we have used AgI prepared from commercial KI (MERCK) instead of preferable Woodward iodine. Shared inhouse standards (ASTER and DREAMS), ‘‘SM-I-09/10/11’’, have been made traceable to AgI precipitated from NIST 3231 (Level II, 1 10 8) via cross-calibration at ASTER [9]. 5. Summary The 6 MV Tandetron has been installed and put into operation at HZDR in 2010. DREAMS has been tuned for measurements of samples containing 14C, 10Be, 26Al, 36Cl, 41Ca and 129I. The demonstrated background levels are in the same order as those from the 5 MV ASTER (for same materials) with the exception of 36Cl being higher at DREAMS due to increased source memory. Acknowledgements The authors are grateful to M. Klein A. Gottdang and A. Terpstra (HVEE) for the excellent cooperation while the installation and tuning of the AMS system. We would like to thank W. Kutschera (VERA, Wien) for helpful discussions and suggestions, A. Steinhof
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(MPI for Biogeochemistry, Jena) and E. M. Wild (VERA, Wien) for 14 C test samples, M. Bichler (Atominstitut Vienna) and A. Wallner (VERA, Wien) for producing a 10Be sample via thermal neutron activation, and U. Herpers and B. Kuczewski (U Cologne) for the Dhurmsala sample. The unpublished 10Be data from the nineteen-nineties has been kindly provided by M. Altmaier, W. Klas, M. Knauer, B. Meltzow, J. Mokos, U. Neupert and C. Schnabel. Very valuable discussions and on-going help by the ASTER team, especially, M. Arnold, G. Aumaître, D. L. Bourlés, R. Braucher, R. C. Finkel, and K. Keddadouche, is highly appreciated. References [1] D.E. Nelson, R.G. Korteling, W.R. Stott, Carbon-14: Direct detection at natural concentrations, Science 198 (1977) 507. [2] D. Elmore et al., Determination of 129I using tandem accelerator mass spectrometry, Nature 286 (1980) 138. [3] J. Klein, R. Middleton, H. Tang, Modifications of an FN tandem for quantitative 10 Be measurement, Nucl. Instr. Meth. 193 (1982) 601. [4] R. Middleton, J. Klein, G.M. Raisbeck, F. Yiou, Accelerator mass spectrometry with 26Al, Nucl. Instr. Meth. 218 (1983) 430. [5] L.K. Fifield, Accelerator mass spectrometry and its applications, Rep. Prog. Phys. 92 (1999) 1223. [6] K.H. Purser, R.B. Liebert, C.J. Russo, MACS: An accelerator-based radioisotope measuring system, Radiocarbon 22 (1980) 794. [7] M.G. Klein, A. Gottdang, D.J.W. Mous, D.L. Bourlès, M. Arnold, B. Hamelin, G. Aumaître, R. Braucher, S. Merchel, F. Chauvet, Performance of the HVE 5 MV AMS system at CEREGE using an absorber foil for isobar suppression, Nucl. Instr. Meth. Res. B 266 (2008) 1828. [8] M. Arnold, S. Merchel, D.L. Bourlès, R. Braucher, L. Benedetti, R.C. Finkel, G. Aumaître, A. Gottdang, M. Klein, The French accelerator mass spectrometry facility ASTER: Improved performance and developments, Nucl. Instr. Meth. Res. B 268 (2010) 1954. [9] M. Arnold, G. Aumaître, D. L Bourlès, K. Keddadouche, R. Braucher, R. C. Finkel, E. Nottoli, L. Benedetti, S. Merchel, The French accelerator mass spectrometry facility ASTER after 4 years: Status and recent developments on 36Cl and 129I, these proceedings. http://dx.doi.org/10.1016/j.nimb.2012.01.049. [10] www.fastcomtec.com/products/product-lines/operating-software. [11] R. Finkel, M. Arnold, G. Aumaître, L. Benedetti, D. Bourlés, K. Keddadouche, S. Merchel, Improved 36Cl Performance at the ASTER HVE 5 MV Accelerator Mass Spectrometer National Facility, these proceedings. [12] D. Elmore, X. Ma, T. Miller, K. Mueller, M. Perry, F. Rickey, P. Sharma, P. Simms, M. Lipschutz, S. Vogt, Status and plans for the PRIME Lab AMS facility, Nucl. Instr. Meth. 123 (1997) 69. [13] D.J.W. Mous, A. Gottdang, J. van der Plicht, Status of the first HVEE 14C AMS in Groningen, Nucl. Instr. Meth. Res. B 92 (1994) 12. [14] A. Schardt, W.A. Fowler, C.C. Lauritsen, The disintegration of N15 by protons, Phys. Rev. 86 (1952) 527.
[15] W.A. Lanford, Analysis for hydrogen by nuclear reaction and energy recoil detection, Nucl. Instr. Meth. Res. B 66 (1992) 65. [16] S. Merchel, M. Arnold, G. Aumaître, L. Benedetti, D.L. Bourlès, R. Braucher, V. Alfimov, S.P.H.T. Freeman, P. Steier, A. Wallner, Towards more precise 10Be and 36 Cl data from measurements at the 10 14 level: Influence of sample preparation, Nucl. Instr. Meth. Res. B 266 (2008) 4921. [17] K. Nishiizumi, M. Imamura, M.W. Caffee, J.R. Southon, R.C. Finkel, J. McAninch, Absolute calibration of 10Be AMS standards, Nucl. Instr. Meth. Res. B 258 (2007) 403. [18] G. Korschinek et al., A new value for the half-life of 10Be by Heavy-Ion Elastic Recoil Detection and liquid scintillation counting, Nucl. Instr. Meth. Res. B 268 (2010) 187. [19] S. Merchel, et al., Quality assurance in accelerator mass spectroscopy: Results from an international round-robin exercise for 10Be, Nucl. Instr. Meth. Res. B, submitted for publication. [20] U. Neupert, Untersuchungen Kosmogener Radionuklide in extraterrestrischer Materie mit Hilfe der Beschleunigermassenspektrometrie (AMS), diploma thesis, University Hanover, Hanover,Germany, 1993. [21] S. Merchel, Über die Wechselwirkung der kosmischen Strahlung mit extraterrestrischer Materie: Radiochemische Bestimmung der Produktionsraten von kosmogenen langlebigen Radionukliden in Meteoriten, Ph.D. thesis, University of Cologne, Cologne, Germany, 1998. [22] P.W. Kubik, M. Christl, 10Be and 26Al measurements at the Zurich 6 MV tandem AMS facility, Nucl. Instr. Meth. 268 (2010) 880. [23] S. Merchel, W. Bremser, First international 26Al interlaboratory comparison Part I, Nucl. Instr. Meth. Res. B 223–224 (2004) 393. [24] A. Arazi, T. Faestermann, J.O. Fernández Niello, D. Frischke, K. Knie, G. Korschinek, H.J. Maier, E. Richter, G. Rugel, A. Wallner, Magnesium suppression for 26Al measurements using AlO ions, Nucl. Instr. Meth. Res. B 223–224 (2004) 259. [25] J. Barker, J.P. Day, N.D. Priest, D. Newton, P.V. Drumm, J.S. Lilley, G.W.A. Newton, Development of 26Al accelerator mass spectrometry for aluminium absorption experiments in humans, Nucl. Instr. Meth. Res. B 68 (1992) 319. [26] L.K. Fifield, S.G. Tims, L.G. Gladkis, C.R. Morton, 26Al measurements with 10Be counting statistics, Nucl. Instr. Meth. Res. B 259 (2007) 178. [27] K. Knie, T. Faestermann, G. Korschinek, G. Rugel, W. Rühm, C. Wallner, Highsensitivity AMS for heavy nuclides at the Munich tandem accelerator, Nucl. Instr. Meth. Res. B 172 (2000) 717. [28] S. Merchel et al., Ultra-trace analysis of 36Cl by accelerator mass spectrometry: An interlaboratory study, Anal. Bioanal. Chem. 400 (2011) 3125. [29] S. Merchel et al., A multi-radionuclide approach for in situ produced terrestrial cosmogenic nuclides: 10Be, 26Al, 36Cl and 41Ca from carbonate rocks, Nucl. Instr. Meth. Res. B 268 (2010) 1179. [30] C. Hennessy, M. Berglund, M. Ostermann, T. Walczyk, H.-A. Synal, C. Geppert, K. Wendt, P.D.P. Taylor, Certification of a 41Ca dose material for use in human studies (IRMM-3703) and a corresponding set of isotope reference materials for 41Ca measurements (IRMM-3701), Nucl. Instr. Meth. Res. B 229 (2005) 281. [31] Ch. Geppert, P. Müller, K. Wendt, Ch. Schnabel, H.-A. Synal, U. Herpers, S. Merchel, Intercomparison measurements between accelerator and laser-based mass spectrometry for ultra-trace determination of 41Ca in the 10 11–10 10 isotopic range, Nucl. Instr. Meth. Res. B 229 (2005) 519.
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
The new 6 MV AMS-facility DREAMS at Dresden Shavkat Akhmadaliev, René Heller, Daniel Hanf, Georg Rugel, Silke Merchel ⇑ Helmholtz-Zentrum Dresden-Rossendorf, D-01314 Dresden, Germany
a r t i c l e
i n f o
Article history: Received 1 July 2011 Received in revised form 18 January 2012 26 January 2012 Available online 5 March 2012 Keywords: Accelerator mass spectrometry Electrostatic accelerator
a b s t r a c t A new 6 MV electrostatic tandem accelerator has been put into operation at Helmholtz-Zentrum Dresden-Rossendorf (HZDR). The system is equipped for accelerator mass spectrometry and opens a new research field at HZDR and the Helmholtz Association. It will be also used for ion beam analysis as well as for material modification via high-energy ion implantation. The research activity at the DREsden Accelerator Mass Spectrometry facility (DREAMS) based on a 6 MV Tandetron is primarily dedicated to the long-lived radioisotopes of 10Be, 26Al, 36Cl, 41Ca, and 129I. DREAMS background levels have been found to be at 4.5 10 16 for 10Be/9Be, 8 10 16 for 26Al/27Al, 3 10 15 for 36Cl/35Cl and 8 10 15 for 41Ca/40Ca, respectively. The observed background of 2 10 13 for 129I/127I originates from intrinsic 129I from AgI produced from commercial KI. The introduction of quality assurance approaches for AMS, such as the use of traceable calibration materials and taking part in interlaboratory comparisons, guarantees high accuracy data for future DREAMS users. During first experiments an energy calibration of the accelerator has been carried out using the nuclear reaction 1H(15N,ca)12C yielding an energy correction factor of 1.019. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction The installation of a new 6 MV electrostatic tandem accelerator opens a new research discipline at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR): accelerator mass spectrometry (AMS). Beginning as a method of radiocarbon dating AMS has been developed during the last decades to a powerful technique for the detection of many rare radionuclides with a sensitivity in the range of 10 15–10 16 [1–5]. The benefits from using AMS for research in the field of radiation protection, nuclear safety, nuclear waste, radioecology, phytology, nutrition, toxicology, and pharmacology are obvious and manifold: Smaller sample sizes, easier and faster sample preparation, higher sample throughput and the redundancy for radiochemistry laboratories will largely reduce costs. The AMS research program at HZDR is primarily concentrated on investigations of cosmogenic long-lived radionuclides like 10 Be, 26Al and 36Cl as well as 41Ca and 129I, which have growing interest especially for environmental and geosciences, astrophysics, medicine and climatology. Using these nuclides dating of suddenly occurring prehistoric mass movements, e.g. volcanic eruptions, rock avalanches, tsunamis, meteor impacts, earth quakes and glacier movements, is possible. Additionally, glacier movements and data from ice cores give hints for the reconstruction of historic climate changes and providing information for the validation of climate model predicting future changes. Re⇑ Corresponding author. Tel.: +49 351 260 2802; fax: +49 351 260 12802. E-mail address: [email protected] (S. Merchel). 0168-583X/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2012.01.053
search topics are addressed in close national and international cooperation with other research institutes as e.g. from the Helmholtz and Max-Planck Association, universities and industrial partners. Two dedicated chemistry laboratories, one for 10Be, 26Al, 41Ca, 53 Mn, 59Ni and 55/60Fe, the second for the volatile radionuclides 36Cl and 129I, are available on-site for sample preparation. The implementation of quality assurance schemes such as the fabrication and use of traceable calibration materials and low-level carriers, and taking part in interlaboratory comparisons, allows for high accuracy data for all DREAMS users. Following decades of experience at HZDR, the accelerator is also used for ion beam analysis such as Particle-Induced X-ray and Gamma-Emission (PIXE/PIGE), nuclear reaction analysis (NRA) and elastic recoil detection (ERD) as well as for material modification via high-energy ion implantation. 2. System set-up The new system is based on a 6 MV Tandetron (HVEE) with two separate ion injectors (Fig. 1). The multipurpose ion injector, not in use for AMS and containing two ion sources, is applied for ion beam analysis and high-energy ion implantation. Negative He-ions are produced by a duoplasmatron Model 358 in combination with a Li-charge exchange canal. All other ion species are emitted by a Cs-sputter ion source (Model 860-C). Ten cathodes can be installed simultaneously into this ion source. The low-energy side of the AMS set-up consists of two identical Cs-sputter ion sources (SO-110) with wheels for up to 200 samples
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Fig. 1. Layout of the new 6 MV Tandetron system with high-energy implantation, AMS and interconnecting beamlines.
each. The energy of the negative ions are analysed by a 54° electrostatic analyser (ESA). The fast bouncing system with voltages of 3 to +3 kV, installed at the 90° magnet chamber, injects sequentially the isotopes of interest into the accelerator. The ion accelerating is based on the tandem principle with injection of low-energy negative ions and using argon gas as an electron stripping medium. The terminal voltage is generated by a parallel driven Dynamitron-type solid state high-voltage cascade [6] and can be varied between 300 kV and 6.0 MV. The drift is less than 10 4 per hour; the residual ripples from the high voltage generating cascade are in the range of 10 5. A maximum terminal voltage of 6.6 MV can be reached for a short time during conditioning. All high-voltage elements are insulated by SF6 gas inside the highpressure tank. The electrostatic quadrupole triplet lens at the output of the accelerator focuses the ion beam. The first high-energy 90° analysing magnet with a bending radius of 1.5 m and ME/Q2 = 185 MeV amu deflects the ions into the AMS section. The high-energy part of the AMS system is most similar to the one at the 5 MV Tandetron at CEREGE (ASTER, Aixen-Provence, France) [7–9]. Both consist of a 35° high-energy electrostatic analyser (ESA) with E/Q up to 6.85 MV, a 30° vertical magnet for ambiguity suppression in the horizontal plane and a gas ionisation detector (Fig. 1). A 1 lm thick silicon nitride absorber foil placed after the 90° analysing magnet is applied for isobar suppression of 10B and 36S. Additionally, there are two electrostatic quadrupole lenses before and after the ESA for refocusing the ion beam being subject of straggling processes in the absorber foil. The final counting of the particles is performed by an isobutanefilled four-anode gas ionisation detector with a 75 nm thick silicon nitride window. The signals from the four anodes are analysed by the FAST ComTec MPANT software [10]. As a special feature the DREAMS-facility has two moveable Faraday cups after the high-energy analysing magnet. They allow the simultaneous measurement of any two stable isotopes, which is important for controlling possible fractionation effects [11], and, moreover, indispensable for the application of isotope dilution AMS [12]. Hence, the detection of 35Cl and 37Cl, when using isotope-enriched carrier for chemistry, yield simultaneously the natural chlorine abundance in the investigated sample. Each of the slit systems in the Faraday cups can be alternatively used for energy stabilisation.
Besides the AMS-system, a second 90° analysing magnet (Fig. 1), is employed for high-energy ion implantation using an automatic wafer handler system operating wafers up to a size of 200 mm. The interconnecting beamline connects the new accelerator with the existing experimental ion beam analysis equipment of the old 5 MV tandem accelerator, which has been shutdown in December 2010. 3. Energy calibration Accurately knowing the energy and energy spread of the ions within the beam allows for correct performance of ion beam analysis like hydrogen depth-profiling and for calculation of the exact energy loss in material e.g. the detector gas. Thus, right after the installation, an energy calibration of the new accelerator has been performed. The ion energy depends on the terminal voltage and can be stabilised by either using a generating voltmeter or by a feedback slit error signal coming from one of the Faraday cups after the 90° analysing magnet [13]. The energy calibration of the accelerator was carried out using the resonant nuclear reaction 1H(15N,ca) 12C with a narrow width of 1.87 keV [14]. The 29 keV 12C15N negative ion beam was produced by a Cs-sputter ion source using a Cu-cathode filled with a mixture of nitrogen-15 enriched sodium azide and graphite. After stripping to positive 15N-ions with different charge states, their energy was scanned near the resonance of 6.385 MeV. An ultracleaned silicon wafer chip containing several monolayers of absorbed water molecules on the surface was taken as a target for the measurements. The produced 4.439 MeV gamma-rays were detected by a BGO scintillator with a photomultiplier. The fine tuning of the ion energy near the resonance was performed by changing the bias voltage of the Si-target in the range of ±20 kV. The resulting dependence of the scintillator counts on the bias voltage gives information about the true energy of the beam (Fig. 2). The full width of the half-maximum of this curve (15 keV) is determined by Doppler energy broadening, the thickness of the hydrogen-containing layer, the energy straggling in this layer and the energy spread in the beam. Assuming the Doppler broadening of about 11 keV [15] and neglecting the thickness of the water layer absorbed on the silicon surface, the energy spread of ions in the beam
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Fig. 2. (a) Resonance curve of the reaction 1H(15N, ca) 12C using 15N2+ at a terminal voltage of 2538 kV. (b) Measured energy of 15N-ions vs. calculated one ECalc. = UTetmQ + (UTerm + UInj) m/M + UBiasQ for charge states Q = +1,+2 and +3, negative ions mass M = 27, positive ion mass m = 15,the injector voltage UInj = 29 kV and target bias UBias was varied using ±20 kV power supply.
can be calculated to not exceeding 10 keV. The calibration curve was measured with 15N+, 15N2+ and 15N3+ corresponding to a terminal voltage of 4094, 2492 and 1791 kV, respectively. During the measurements the energy of the ions was stabilised using the slits. The terminal voltage deviation was determined assuming a linear function to about 1.9%, corresponding to an energy correction factor of 1.019. This deviation can be reduced by an improved adjusting of the feedback loop of the high voltage power supply. We have performed similar experiments at the 5 MV AMS facility ASTER, which have shown a smaller deviation of 0.35% between the calculated ion energy and the measured one. 4. AMS measurements Measurement conditions for the individual radionuclides as used for acceptance (14C only) and later machine and chemistry performance tests (all, but 14C) are summarised in Table 1. Only for those nuclides taking advantage of the (nearly) maximum terminal voltage, i.e. 36Cl and 41Ca some modifications in comparison to the well-established 5 MV AMS facility ASTER have been made. 4.1. Tuning for
14
C
Despite the fact that it is not foreseen to perform 14C measurements in the future at DREAMS, we took the best chance to become acquainted with our new system by using it for 14C, the radionuclide needing the highest measurement accuracy. The accelerator is run at a terminal voltage of 3 MV for 14C. The injection energy
Table 1 Performance of DREAMS for
Be,
14
C,
26
Al,
36
Cl,
41
Ca, and
129
4.2. Tests with
10
Be
The machine performance for 10Be samples is of course equally good as the one from the 5 MV AMS system ASTER at CEREGE [7,8], as this AMS set-up is most similar to the DREAMS one. The measurements are carried out at 4.5 MV terminal voltage using BeO extracted from a BeO–Nb cathode, resulting in energy of 10.742 MeV for 10Be2+. The 9BeO -ion beam current is typically 2–7 lA (for standards, blanks and well-prepared samples), a similar Be2+-ion beam current is reached in the Faraday cup at the highenergy site. The 10B isobar suppression is provided by 1 lm thick silicon nitride absorber foil followed by a 35° electrostatic analyser.
I using Ar stripper gas.
Rare nuclide
Injected ion
Injected current (lA)
Terminal voltage (MV)
Charge state after stripping
Charge state after absorber
Total transmission (%)a
Background
Precisionat isotope ratio
10
BeO C Al AlO Cl
2–7 30–45 0.2–0.5 3–6 20–30
4.5 3.0 2.7 6.0 6.0
2+ 3+ 3+ 4+ 5+
4+ – – 8+ 11+
21 40 35 2 3
0.3% at 10 0.4% at 10 1.5% at 10 2% at 10 2.0% at 10
CaF3 I
0.2–0.35 3–5
5.8 5.0
4+ 5+
– –
16 9
4.5 10 16 1.2 10 15 8 10 16 1 10 14 3 10 15 (36Cl/35Cl) 8 10 15 2 10 13
Be C 26 Al 14
36
Cl
41
Ca I
129 a
10
of negative carbon ions is 29 keV. The charge state 3+ is chosen to avoid interferences from 7Li+. As a pilot beam for adjustment of ion optical components of the high-energy part of the AMS-system 13C3+ is used. Typical negative ion currents from the ion source are in the range of 30–45 lA. After the 90° bouncer magnet the fraction of 13C (together with molecules) is about 300 nA without bouncing voltage. Finally, the 12 MeV 13C3+ beam current measured by the Faraday cup after the 90° analysing magnet is nearly 370 nA with a corresponding 12C3+ current of about 35 lA and an overall transmission of about 40%. For the acceptance test four standard samples have been measured resulting in a relative standard deviation of 4‰ at a 14C/12C ratio of 1.6 10 12, thus, passing the guaranteed value of 5‰. A 14C/12C background of 1.2 10 15 and a natural 13C/12C ratio of (1.080 ± 0.024) 10 2 have been determined.
Defined as the ratio between the number of ions detected in the ionisation chamber and the number of ions injected into the accelerator.
2.5% at 10 1.2% at 10
12 12 11 11 12
11 11
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Meltzow / ETH Klas / ETH Knauer / ETH Neupert / ETH Altmaier / ETH Merchel / ETH Schnabel / PRIMElab Mokos / PRIMElab DREAMS
22
20
10
Be [dpm/kg]
24
18
0
10 20 30 40 50 Chronological sample order (1-40 only)
Fig. 3. Replica measurements of 10Be isolated from material originating from the LL chondrite Dhurmsala (Fragment 2/3A,70 g without 1 g of metal, homogenised to size fraction of <125 lm [20]. AMS targets have been prepared at four chemistry labs (U of Cologne,U Hanover,Rutgers U,PRIMELab) by eight scientists (M. Altmaier,W. Klas,M. Knauer,B. Meltzow,S. Merchel,J. Mokos,U. Neupert,C. Schnabel) [21 and references therein]. AMS measurements have been performed from 1994–1998 at ETH Zurich (values adapted to new half-life and standard values [22]) and PRIMElab. The new DREAMS 10Be value is from BeO prepared in 2011 from 150 mg of the same Dhurmsala fraction.
About 36% of 10Be-ions, changing their charge state from 2+ to 4+, reach the detector, thus, the total transmission from the injector to the detector is about 21%. An excellent machine background (far below the guaranteed value of 3 10 15) of 4.5 10 16 for 10 Be/9Be as a mean of seven machine blank samples has been achieved using Be-containing minerals originating from a deep mine [16]. Due to the loss of 10Be ions in the absorber foil and the 35° ESA, the correction factor (nominal standard ratio divided by the measured one) is usually about 2.2. A new in-house secondary standard material ‘‘SMD-Be-12’’ has been cross-calibrated vs. the primary standard NIST 4325 at ASTER, VERA, and DREAMS. This material with a weighted mean value of (1.704 ± 0.030) 10 12 will be used for all 10Be measurements at DREAMS in the future. Its total uncertainty is mainly dominated by the original uncertainty of 1% of the primary standard. Thus, all DREAMS results are traceable to NIST 4325 with the most widely accepted 10Be/9Be value of (2.79 ± 0.03) 10 11 [17] and a half-life of (1.387 ± 0.012) Ma [18]. Taking part in 10Be round-robin exercises [19], and intercomparison measurements of high-activity samples such as meteorites (Fig. 3) have already shown that DREAMS produces internationally comparable 10Be data. Additionally, the chances of a German– French project further improved quality assurance. Samples containing in situ produced and atmospheric 10Be have been chemically processed both at DREAMS and ASTER and measured at DREAMS and/or ASTER [9]. The results clearly demonstrate that despite different chemistry laboratories and procedures, reproducibility and precision is guaranteed for high activity samples, but also for 10Be samples as low as 104 atoms g 1 (either water or quartz).
4.3. Tests with
26
Al
The 26Al measurements are carried out at a terminal voltage of 2.7 MV. The injection of Al into the accelerator provides excellent suppression of the isobar 26Mg directly in the source, thus, an absorber foil for further isobar suppression in not needed. However, the current is, compared to other nuclides, relatively low, in the order of 0.3–0.5 lA. About 35% of this is transformed into 27Al3+ current (300–500 nA) after acceleration. The machine background has been demonstrated to be better than 8 10 16 for 26Al/27Al. The
transmission at the high-energy side is nearly 90–95%, corresponding to a correction factor of about 1.05–1.1. First tests at DREAMS has been carried out using standard-type material at different ratio levels from a 26Al round-robin exercise [23] proofing the accuracy of DREAMS 26Al/27Al results at high levels. However, production of a new secondary in-house standard SMD-Al-11 (1 10 11) and subsequent cross-calibration vs. primary standards following the approach of Arnold et al. [8], hence, guaranteeing highest traceability for future users, are in preparation. As demonstrated by high-energy AMS facilities [24–26], the use of AlO can increase currents by one order of magnitude. High terminal voltages of 10–20 MV and foil stripper allow to accelerate the ions up to energies of about 100 MeV and to apply a gas-filled magnet for isobar suppression [27] or realise fully stripped ions (Al13+ and Mg12+) at energies of more than 150 MeV [25]. To test the performance of DREAMS’ absorber foil technique for 26Mg suppression, AlO has been extracted up to currents of 3–6 lA. Preliminary results have shown that the maximum terminal voltage of 6 MV using a charge state of 3+ (21.7 MeV) is not sufficient for a clear separation of 26Al and 26Mg in the detector. If choosing the charge state of 4+ (with a yield of about 15%), higher energies (27.7 MeV) thus, better separation is gained. The first tests were carried out using a 1 lm thick silicon nitride absorber foil allowing about 25% of the ions with charge state of 8+ to reach the detector. A satisfactory background of about 1 10 14 26Al/27Al can only be reached by defining very small regions of interests (ROIs) for 26Al in the detector signal spectra and by reducing the slit size after the 35° ESA. However, this way, a significant amount of the 26Al signal is lost and the total transmission decreases to about 2%. Compared to the overall transmission of 35% when using Al , the advantage of more than ten times higher AlO current is nearly nullified. Knowing chances are reasonable, further tests optimising the absorber foil thickness and charge state variations are in preparation. 4.4. Tests with
36
Cl
For applying isotope dilution AMS, the DREAMS system is equipped with two moveable Faraday cups that can be positioned for collecting simultaneously both stable isotopes of chlorine. Typically negative ion currents of the 35Cl -isotope from AgCl cathodes are 20–30 lA after the bouncer magnet during measurements. The
S. Akhmadaliev et al. / Nuclear Instruments and Methods in Physics Research B 294 (2013) 5–10
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Fig. 4. (a) 2D spectrum combined with signals from first (DE1) and last (DErest) anodes of the detector. (b) DErest spectrum for 29.0 MeV 36Cl and 29.3 MeV 36S. The sample ‘‘SM-Cl-12’’ has a 36Cl/35+37Cl ratio of 1.082 10 12 [28].
system is operated at a terminal voltage of 6.0 MV. About 27% of injected ions are accelerated up to 36 MeV as Cl5+. Ions of 35Cl5+ and 37Cl5+ are measured separately, each in its own Faraday cup. The absorber foil plays an important role for the suppression of the disturbing 36S isobar. Due to the difference in energy loss of about 0.3 MeV for 36Cl and 36S in the 1 lm silicon nitride foil, a suppression factor for sulphur by the ESA of more than 200 is obtained, resulting in a reasonable low background of better than 5 10 15, if the source has been very recently cleaned. As seen for other sources all over the world, under ‘‘every-day‘‘ measurement conditions, the 36Cl background (3 10 15) is solely limited by sample-to-sample cross-contamination and long-term source memory. A new source development [8] improves the performance [11,28], but not to an extend [9] allowing high sample-throughput as planned for AMS operation at DREAMS. The optimum for transmission of chlorine through the foil is about 12% for ions with charge state 11+ (in contrast to 10+ at ASTER [8]). The total transmission to the detector is about 3% and the measured ratios must be corrected by a factor of 9 to yield true ratios. Typical spectra from the detector for a standard-type sample ‘‘SM-Cl-12’’ at the 10 11 level [28] are shown in Fig. 4. As the high volatility and facile ionisation of chlorine promotes cross-contamination and source memory, 36Cl testing materials at different 36Cl/35+37Cl levels [28] have been used for the first 36 Cl test at DREAMS. The preparation of large quantities of inhouse secondary standards for DREAMS at the 10 11, 10 12, and 10 13 level and subsequent cross-calibration vs. primary-type materials at the same isotopic levels from a round-robin exercise [28] are foreseen for the very near future, therefore, establishing a traceability chain for 36Cl AMS. 4.5. Tests with
41
Ca
In the case of 41Ca, 41K is suppressed by extracting CaF3 from the ion source with currents of 200–350 nA. The ions of charge state 4+ are accelerated using a terminal voltage of 5.8 MV. The potassium suppression of the system has been measured to be at least 104 by comparing the count rates of 41K4+ in the detector while injecting either CaF or CaF3 into the accelerator. The resulting 41Ca/40Ca ratio for blank samples is lower than 1 10 14 without the use of an absorber foil. The remaining signal is true machine background, as the blank material, originating from a shielded calcite sample, has been measured at an energywise superior AMS facility at Munich down to 4 10 16 [29]. The current of two stable isotopes, such as 40Ca and 42Ca, can be quasi-simultaneously obtained from the two Faraday cups.
Following the approach of Arnold et al. [8], a set of different inhouse standards has been prepared as CaF2 and used for first 41Ca tests at DREAMS. The owing cross-calibration of these secondary standards vs. the corresponding primary ones from ERMÒ-AE701 [30] is already scheduled and will guarantee all DREAMS users traceable results, which is especially important for topics in biomedical research. However, earlier experiences have already shown that the calculated 41Ca/40Ca ratios of the secondary standards agree within permille-deviation with the cross-calibrated ones [8]. Following the approach of 10Be, 41Ca isolated as CaF2 from the already described special Dhurmsala meteorite sample, yield a 41 Ca-activity of about 31 dpm/kg, which is in astonishing good agreement with the value of (31 ± 5) dpm/kg, originally published by Merchel [21] as (37 ± 6) dpm/kg, but here treated by a correction factor after finding that the at that time used 41Ca calibration material ‘‘Juel’’ was about 15% lower than presumed [31]. 4.6. Tests with
129
I
Iodine measurements are carried out at the same terminal voltage using the same charge state, i.e. 5.0 MV and 5+, as demonstrated at ASTER [8]. As already seen for chlorine severe memory and cross-contamination could be identified. However, the remaining background of 2 10 13 for 129I/127I at DREAMS originates from intrinsic 129I, as we have used AgI prepared from commercial KI (MERCK) instead of preferable Woodward iodine. Shared inhouse standards (ASTER and DREAMS), ‘‘SM-I-09/10/11’’, have been made traceable to AgI precipitated from NIST 3231 (Level II, 1 10 8) via cross-calibration at ASTER [9]. 5. Summary The 6 MV Tandetron has been installed and put into operation at HZDR in 2010. DREAMS has been tuned for measurements of samples containing 14C, 10Be, 26Al, 36Cl, 41Ca and 129I. The demonstrated background levels are in the same order as those from the 5 MV ASTER (for same materials) with the exception of 36Cl being higher at DREAMS due to increased source memory. Acknowledgements The authors are grateful to M. Klein A. Gottdang and A. Terpstra (HVEE) for the excellent cooperation while the installation and tuning of the AMS system. We would like to thank W. Kutschera (VERA, Wien) for helpful discussions and suggestions, A. Steinhof
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(MPI for Biogeochemistry, Jena) and E. M. Wild (VERA, Wien) for 14 C test samples, M. Bichler (Atominstitut Vienna) and A. Wallner (VERA, Wien) for producing a 10Be sample via thermal neutron activation, and U. Herpers and B. Kuczewski (U Cologne) for the Dhurmsala sample. The unpublished 10Be data from the nineteen-nineties has been kindly provided by M. Altmaier, W. Klas, M. Knauer, B. Meltzow, J. Mokos, U. Neupert and C. Schnabel. Very valuable discussions and on-going help by the ASTER team, especially, M. Arnold, G. Aumaître, D. L. Bourlés, R. Braucher, R. C. Finkel, and K. Keddadouche, is highly appreciated. References [1] D.E. Nelson, R.G. Korteling, W.R. Stott, Carbon-14: Direct detection at natural concentrations, Science 198 (1977) 507. [2] D. Elmore et al., Determination of 129I using tandem accelerator mass spectrometry, Nature 286 (1980) 138. [3] J. Klein, R. Middleton, H. Tang, Modifications of an FN tandem for quantitative 10 Be measurement, Nucl. Instr. Meth. 193 (1982) 601. [4] R. Middleton, J. Klein, G.M. Raisbeck, F. Yiou, Accelerator mass spectrometry with 26Al, Nucl. Instr. Meth. 218 (1983) 430. [5] L.K. Fifield, Accelerator mass spectrometry and its applications, Rep. Prog. Phys. 92 (1999) 1223. [6] K.H. Purser, R.B. Liebert, C.J. Russo, MACS: An accelerator-based radioisotope measuring system, Radiocarbon 22 (1980) 794. [7] M.G. Klein, A. Gottdang, D.J.W. Mous, D.L. Bourlès, M. Arnold, B. Hamelin, G. Aumaître, R. Braucher, S. Merchel, F. Chauvet, Performance of the HVE 5 MV AMS system at CEREGE using an absorber foil for isobar suppression, Nucl. Instr. Meth. Res. B 266 (2008) 1828. [8] M. Arnold, S. Merchel, D.L. Bourlès, R. Braucher, L. Benedetti, R.C. Finkel, G. Aumaître, A. Gottdang, M. Klein, The French accelerator mass spectrometry facility ASTER: Improved performance and developments, Nucl. Instr. Meth. Res. B 268 (2010) 1954. [9] M. Arnold, G. Aumaître, D. L Bourlès, K. Keddadouche, R. Braucher, R. C. Finkel, E. Nottoli, L. Benedetti, S. Merchel, The French accelerator mass spectrometry facility ASTER after 4 years: Status and recent developments on 36Cl and 129I, these proceedings. http://dx.doi.org/10.1016/j.nimb.2012.01.049. [10] www.fastcomtec.com/products/product-lines/operating-software. [11] R. Finkel, M. Arnold, G. Aumaître, L. Benedetti, D. Bourlés, K. Keddadouche, S. Merchel, Improved 36Cl Performance at the ASTER HVE 5 MV Accelerator Mass Spectrometer National Facility, these proceedings. [12] D. Elmore, X. Ma, T. Miller, K. Mueller, M. Perry, F. Rickey, P. Sharma, P. Simms, M. Lipschutz, S. Vogt, Status and plans for the PRIME Lab AMS facility, Nucl. Instr. Meth. 123 (1997) 69. [13] D.J.W. Mous, A. Gottdang, J. van der Plicht, Status of the first HVEE 14C AMS in Groningen, Nucl. Instr. Meth. Res. B 92 (1994) 12. [14] A. Schardt, W.A. Fowler, C.C. Lauritsen, The disintegration of N15 by protons, Phys. Rev. 86 (1952) 527.
[15] W.A. Lanford, Analysis for hydrogen by nuclear reaction and energy recoil detection, Nucl. Instr. Meth. Res. B 66 (1992) 65. [16] S. Merchel, M. Arnold, G. Aumaître, L. Benedetti, D.L. Bourlès, R. Braucher, V. Alfimov, S.P.H.T. Freeman, P. Steier, A. Wallner, Towards more precise 10Be and 36 Cl data from measurements at the 10 14 level: Influence of sample preparation, Nucl. Instr. Meth. Res. B 266 (2008) 4921. [17] K. Nishiizumi, M. Imamura, M.W. Caffee, J.R. Southon, R.C. Finkel, J. McAninch, Absolute calibration of 10Be AMS standards, Nucl. Instr. Meth. Res. B 258 (2007) 403. [18] G. Korschinek et al., A new value for the half-life of 10Be by Heavy-Ion Elastic Recoil Detection and liquid scintillation counting, Nucl. Instr. Meth. Res. B 268 (2010) 187. [19] S. Merchel, et al., Quality assurance in accelerator mass spectroscopy: Results from an international round-robin exercise for 10Be, Nucl. Instr. Meth. Res. B, submitted for publication. [20] U. Neupert, Untersuchungen Kosmogener Radionuklide in extraterrestrischer Materie mit Hilfe der Beschleunigermassenspektrometrie (AMS), diploma thesis, University Hanover, Hanover,Germany, 1993. [21] S. Merchel, Über die Wechselwirkung der kosmischen Strahlung mit extraterrestrischer Materie: Radiochemische Bestimmung der Produktionsraten von kosmogenen langlebigen Radionukliden in Meteoriten, Ph.D. thesis, University of Cologne, Cologne, Germany, 1998. [22] P.W. Kubik, M. Christl, 10Be and 26Al measurements at the Zurich 6 MV tandem AMS facility, Nucl. Instr. Meth. 268 (2010) 880. [23] S. Merchel, W. Bremser, First international 26Al interlaboratory comparison Part I, Nucl. Instr. Meth. Res. B 223–224 (2004) 393. [24] A. Arazi, T. Faestermann, J.O. Fernández Niello, D. Frischke, K. Knie, G. Korschinek, H.J. Maier, E. Richter, G. Rugel, A. Wallner, Magnesium suppression for 26Al measurements using AlO ions, Nucl. Instr. Meth. Res. B 223–224 (2004) 259. [25] J. Barker, J.P. Day, N.D. Priest, D. Newton, P.V. Drumm, J.S. Lilley, G.W.A. Newton, Development of 26Al accelerator mass spectrometry for aluminium absorption experiments in humans, Nucl. Instr. Meth. Res. B 68 (1992) 319. [26] L.K. Fifield, S.G. Tims, L.G. Gladkis, C.R. Morton, 26Al measurements with 10Be counting statistics, Nucl. Instr. Meth. Res. B 259 (2007) 178. [27] K. Knie, T. Faestermann, G. Korschinek, G. Rugel, W. Rühm, C. Wallner, Highsensitivity AMS for heavy nuclides at the Munich tandem accelerator, Nucl. Instr. Meth. Res. B 172 (2000) 717. [28] S. Merchel et al., Ultra-trace analysis of 36Cl by accelerator mass spectrometry: An interlaboratory study, Anal. Bioanal. Chem. 400 (2011) 3125. [29] S. Merchel et al., A multi-radionuclide approach for in situ produced terrestrial cosmogenic nuclides: 10Be, 26Al, 36Cl and 41Ca from carbonate rocks, Nucl. Instr. Meth. Res. B 268 (2010) 1179. [30] C. Hennessy, M. Berglund, M. Ostermann, T. Walczyk, H.-A. Synal, C. Geppert, K. Wendt, P.D.P. Taylor, Certification of a 41Ca dose material for use in human studies (IRMM-3703) and a corresponding set of isotope reference materials for 41Ca measurements (IRMM-3701), Nucl. Instr. Meth. Res. B 229 (2005) 281. [31] Ch. Geppert, P. Müller, K. Wendt, Ch. Schnabel, H.-A. Synal, U. Herpers, S. Merchel, Intercomparison measurements between accelerator and laser-based mass spectrometry for ultra-trace determination of 41Ca in the 10 11–10 10 isotopic range, Nucl. Instr. Meth. Res. B 229 (2005) 519.