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A new AMS facility based on a Cockcroft–Walton type 1 MV tandetron at IFIN-HH Magurele, Romania
Nuclear Instruments and Methods in Physics Research B 319 (2014) 117–122
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
A new AMS facility based on a Cockcroft–Walton type 1 MV tandetron at IFIN-HH Magurele, Romania C. Stan-Sion a,⇑, M. Enachescu a, D.G. Ghita a, C.I. Calinescu a, A. Petre a, D.V. Mosu a, M. Klein b a b
Horia Hulubei National Institute of Physics and Nuclear Engineering (IFIN-HH), Bucharest, Romania High Voltage Engineering Europa B.V., Amsterdamsweg 63, 3812 RR Amersfoort P.O. Box 99, 3800 AB Amersfoort, The Netherlands
a r t i c l e
i n f o
Article history: Received 22 May 2013 Received in revised form 30 July 2013 Available online 4 December 2013 Keywords: Accelerator Mass Spectrometry 14 C 10 Be 26 Al 129 I
a b s t r a c t A 1 MV AMS machine was recently installed in the National Institute for Physics and Nuclear Engineering IFIN-HH, Bucharest Romania. It is the second AMS facility at IFIN-HH having the goal not only to continue but mainly to enlarge the research area of this highly sensitive analyzing method. The multi-element AMS was developed by HVEE to measure 14C, 10Be, and 26Al, and 129I. The results of an acceptance test are presented and demonstrate that this machine is capable of routine 14C age dating and of measurements of other radioisotopes in terms of accuracy and precision as well as a low background level. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The highly sensitive analyzing method named today Accelerator Mass Spectrometry (AMS) was discovered 1939 in California, USA, by L.W. Alvarez and Robert Cornog. They used, for the first time, an accelerator (cyclotron) as a mass spectrometer to demonstrate that 3 He is stable and correctly concluded that the other mass-3 isotope tritium is radioactive [1]. 38 years later, stimulated by the high interest of sensitive carbon dating, laboratories at Rochester [2], at Lawrence Livermore National Laboratory [3] and at McMaster [4] supplanted the older ‘‘decay counting’’ method for 14C with the accelerator technique that was about a factor of 1000 more sensitive. They all recognized that modern accelerators could accelerate radioactive particles to energies where magnetic and electric forces are sufficiently high to eliminate the background interferences. During the next years, due to its exceptional analyzing sensitivity of counting individual atoms and performing applications never imaginable before, the AMS method was implemented in many laboratories around the world. Since the central part of such a machine is an energy precision accelerator, besides the stand alone dedicated systems also multipurpose tandem accelerators were adapted for AMS. Studies were undertaken to increase performances of charged particle selection, detection systems, isobar ⁄ Corresponding author. Tel.: +40 2124042346; fax: +40 212321906. E-mail address: [email protected] (C. Stan-Sion). 0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.07.073
separation contributing to the rapid progress of the AMS method [5]. In the recent years, the low-energy accelerator mass spectroscopy with stand alone machines has gained the leading supremacy. Several AMS machines with energies from 6 MV down to 0.5 MV are now in operation around the world [6–9]. Specially, the low energy machines of 0.5–1 MV terminal voltage may be called a balanced system because it is a compact system, with much lower running costs and manpower requirements, as compared to higher energy AMS systems. Moreover, single stage systems with 250 kV acceleration voltage and 200 kV vacuum insulated system (MICADAS) have recently gained remarkable importance [10]. Such systems demonstrate very good performances not only for 14C dating measurements but also for the measurements of other isotopes such as 10Be, 27Al, 41Ca, 129I, 239 Pu, and 240Pu. At the NIPNE in Bucharest a new Cockcroft–Walton type 1 MV HVEE tandetron system, was commissioned in April 2012 and is dedicated for ultra-sensitivity AMS analyses using C, Be, Al, I and Pu elements. This is the second AMS machine in our institute, after the one constructed based on the old 9 MV FN tandem accelerator [11]. The results of the acceptance test of this new machine are presented in this current paper. They will demonstrate its high efficiency in terms of accuracy, precision and low background level, routine 14C age dating and of measurements of other radioisotopes (10Be, 26Al, and 129I).
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C. Stan-Sion et al. / Nuclear Instruments and Methods in Physics Research B 319 (2014) 117–122
2. Description of the 1 MV AMS facility in Bucharest 2.1. The AMS machine This is the sixth 1 MV HVEE (High Voltage Engineering Europe B.V., Amersfoort, the Netherlands) AMS machine delivered by the company and several published papers have already described its main constructive features [12–18]. Therefore, we will give here only a short description. The device from Bucharest is based on a 1 MV tandetron, with a Cockcroft–Walton type high power supply and two separated ion beam injectors at 120° (see Fig. 1). A so called Q-Snout device [19] situated at accelerator entrance is performing two types of tasks: on one hand, it is performing the matching of the ion source emittance with the accelerator acceptance (by the ion beam supplementary acceleration) and on the other hand it is focusing the ion beam on the gas stripper channel. Together with a powerful electric Q-pole triplet lens at the accelerator exit, the two devices optimize the transmission of the overall beam transport for every ion and charge state used in an AMS experiment. The ion stripping in the tandem terminal is produced in an Ar gas stripper channel. The ion sources are both Sources of Negative Ions by Cesium Sputtering (SNICS) type with a 50-target carousel (SO110/50). The sequential injection system of ions consists in an injector 90° analyzing magnet, having its vacuum chamber electrically isolated. To this chamber a step-variable voltage is applied. It changes sequentially to different values lasting for different durations in such a way that the different ion species can be injected to the tandem accelerator according to their current value (9.75 ms for the rare isotopes and 100 ls for the stable isotopes with a delay between species of 0.25 ls). The cyclic change of this voltage for the injection of a sequence of isotopes is referred as bouncing [20]. However, during switching between the isotopes, the magnet chamber voltage requires some time to settle to a constant value otherwise it will result in an unstable beam position. In this
respect, the developed bouncer system by HVEE in 2003 was designed to overcome adverse effects of the switching process by use of a beam-blanking unit, which stops the beam unless the measurement conditions are stable. In our machine, the bouncer is working at a frequency of 100 Hz and a maximum voltage of 3 kV, resulting in very short measurement times for the stable isotopes and in achieving a high time efficiency for the counting of the rare isotope. By bringing through the tandem accelerator the stable and the rare isotope beam, it allows to be both measured. The first in a Faraday cup placed on the high energy side and the later in the detector system. In this way, the AMS analyses are continuously monitored, improving substantially their precision. An electrical static analyzer (ESA), polarized up to ±60 kV and 120° bending, is also used. By its independent action, a degerancy W/q (ions with same ratio of energy W over charge state q will pass the analyzer) remains. That means that ESA will separate ions having different energies from that one of the rare isotope of interest. However, the joint action, with the HE magnetic analyzer, resumes in a final degerancy of M/q (mass M over charge state) implying more restrictive discrimination of undesired ions. After these filter only ions having both the same energy and the same ratio mass over charge (M/q) will pass to the detection system. Finally, the rare isotope ion detector system is a large gas-filled ionization chamber with two anodes for DE Eres measurement and having a detection efficiency of nearly 100%. A 75-nm-thick Si3N4 entrance window separates the detector gas from the beam line vacuum. The separation of ions at low energies is based on the difference between their stopping powers. The DE and Eres electrodes are 10 and 20 cm long. The optimum separation is performed when the crossing of the stopping power curves happens exactly at the splitting boundary between the electrodes. A bi-parametric plot using these two electric signals will enhance the separation between different ions as will be shown in figures of the next chapter. The specifications of the machine are summarized in Table 1.
Fig. 1. The general layout of 1 MV HVEE AMS facility in Bucharest, Romania. (1) Two ion beam injectors equipped with SNICS ion sources of type SO110 – 50 sample carousel HVEE and focusing ion lenses; (2) electrostatic switching system; (3) injection Magnet with multi beam switcher (bouncer); (4) faraday cup and Q-Snout device; (5) 1 MV tandetron accelerator, with gas stripper channel; (6) analyzing Magnet; (7) three offset Faraday cups; (8) electrostatic analyzer; (9) particle detector; (10) Cockcroft–Walton type HV power supply.
C. Stan-Sion et al. / Nuclear Instruments and Methods in Physics Research B 319 (2014) 117–122
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accelerator is not equal for all the three isotopes. In the case of carbon isotopes their fragmentation may play an important role. However, it does not affect the age dating, because measured values will be corrected using reference samples (AMS is usually a relative measurement). When 2+ ions are used for the 1 MV 14C analyses, interference by the 7Li2 molecular ions is severe in some cases. To avoid any Li contribution to the 14C counts, ESA voltage was set close its maximum value (±60 kV) and a relative high stripper gas pressure (2.3 10 2 Torr) was used. Fig. 2 shows a spectrum free of this molecular interference. Due to the higher pressure of the stripper gas the beam transmission was slightly decreased.
Table 1 Specifications of the 1 MV AMS system of Bucharest. Dimension 4.2 6.2 m Ion source 100 lA max. – 2 injectors separated 120° Pre-acceleration voltage 35 kV Bouncer 3 kV, 100 Hz Terminal voltage and up charge current 1 MV, 2 mA Analyzing magnet 90°, 63 MeV amu, max current 300 A Electrical spherical analyzer (ESA) 120°, ±60 kV
3. Results and discussion of the acceptance test 3.1. Carbon
3.2. Aluminum The acceptance test for 14C at our machine was done by using four reference samples provided by HVEE for this purpose together with two background samples. Their 14C/12C nominal values were 1.6 10 12 for reference and for background samples a concentration lower than 3 10 15. In our experiments all samples were measured 4 times, being divided in the process in 30 blocks of 30 s (or 40,000 counts) each. A typical spectrum is shown in Fig. 1. The mean value of the 14C/12C was measured 1.4 10 12, with a statistical error of 0.25% and the relative standard deviation between the four samples of 0.37%. The 13C/12C ratio was measured from the 4 samples with a mean value of 1.02 10 2, with a relative standard deviation of 0.2%. The background value was measured immediately after a concentrated reference sample (10 12) and the obtained value was 1.8 10 15. These values were obtained based on the sequentially injecting ion beam system (bouncer) that permits to transport also the high ion currents of stable isotope beams through the accelerator in a sequential modus and for short durations. The corresponding currents are measured on the HE side, in Faraday cups placed of axis and after the analyzing magnet (see Fig. 1). The transmission between pre- and post accelerating Faraday cups was 58% for 12C2+ at a stripper gas pressure of 2.3 10 2 Torr. In our measurements, between this location and the Faraday cup placed in front of the detector, the beam transmission for 12C and 13C ions were each time 100%. It is expected the same transmission value for 14C on this path. Therefore, with the number of acquired counts of 14C in the detector and using the integrated beam current values of 13C and 12C, measured after the HE magnetic analyzer in the two Faraday cups located of axis, one can determine the 14C/12C and 13C/12C ratios. (This is obtained in an absolute way and without using reference samples.) Deviations of these values from the nominal values of the HVEE reference samples indicate that the transmission through the
Fig. 2. Spectrum of a carbon sample with a
14
C content of 1.6 10
Since aluminum ions in 1+ charge state have the best stripping yield in Ar-gas at 0.7 MV, the acceptance test was carried at this energy. For aluminum analyses, it was used reference material with a 26 Al/27Al ratio of 9.3 10 11. These samples, together with several background samples, were provided by Cerege – Aster AMS facility, France [21]. The AMS measurement was performed according to same procedure as used for carbon and described before. A spectrum is given below in Fig. 3. From the ion source we easily extracted a 300 nA current of 27 Al ions. This beam was fairly sufficient for the machine tuning and afterwards. By use of the bouncing system, it was integrated in the movable Faraday cup, to provide a permanent normalization of the analyze procedure. The measured mean value of 26Al/27Al of four reference samples was 7.5 10 11, with a statistical error of 0.8% and the relative standard deviation between the 4 samples was 1.3%. For the background samples the ratio 26Al/27Al was measured to be 1.2 10 15. The consensus value of the background samples was lower than 5 10 14. The beam transmission of the 27A11+ ion at a stripper pressure of 1.8 10 2 Torr was found to be 36% between pre- and postaccelerator Faraday cups. 3.3. Beryllium The acceptance test for beryllium was performed using four reference samples and several background samples, all supplied by CNRS-Cerege AMS laboratory of Universite Aix-Marseille, France. The difficulty of the 10Be measurement is associated with the interface of the isobar 10B. For the isobar suppression the passive absorber foil method was applied using a Si3N4 foil with a thickness of 150 nm placed in front of the ESA. The tandem terminal voltage
12
when using charge state 2+ at a terminal voltage of 1 MV.
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Fig. 3. Spectrum of a Al sample with a
26
Al content of 9.3 10
11
when using the charge state 1+ at a terminal voltage of 700 kV.
Fig. 4. The two-parametric spectrum is showing the registered counts as function of the initial energy loss (DE) and the residual energy (Eres) from the
was set to 1.0 MV and the 1+ state was selected by the high energy spectrometers before and after a Si3N4 absorber foil. As pilot beam, it was used the 9Be and the transmission through the accelerator, at a 1.8 10 2 Torr pressure in the stripper channel was 51%. However, the transmission efficiency for 10Be was extremely low because while passing through the absorber foil the charge states changes for many 10Be ions and the beam becomes spread out as a result of the straggling effect. Fig. 4 shows the bi-parametric spectrum DE Eres measured from the reference samples that had a consensus value of 2.78 10 11. The 10Be counts were always well separated from the interference with 10B. Each reference sample was measured 4 times and every measurement was divided into 30 blocks of 30 s (or 1500 counts). The mean ratio was measured to be 1.5 10 12, with a statistical error of 1.2% and relative standard deviation of 0.7%. The background sample 10Be/9Be ratio was measured 2.7 10 14. The application of passive absorber foil method gave good results and 10Be could be very well separated from the 10B isobar. The reduced transmission is however a subject of improvement, together with the sensitivity level of the AMS analyzes given by the background level. 3.4. Iodine The acceptance test for iodine was as well performed by using four reference samples. Their consensus value for the 129I/127I ratio was 1.30 10 11 and they were supplied ready to be used by the
10
Be AMS analyses.
Toronto AMS laboratory [22]. The background samples were supplied by Cerege [21] and expected values were below 3 10 13. The terminal voltage for iodine measurement was 1 MV and charge state 3+ was selected by the high energy spectrometer. After the first magnetic analyzer, the current selected for 127I was about 1.4 lA. However, usually a much lower current value was used (obtained by closing slits on the low energy side of the accelerator) in order to obtain the most suitable beam transport optics for the rare isotope. Since iodine is a very volatile material, it produces a deposition layer on the sample surfaces that can alter the constancy of the delivered current. To avoid such effects, new samples have to be conditioned at the beginning of an AMS analyzes by a 5 min Cs pre-sputtering procedure in the ion source. Because the iodine current is high, the batch mode registration for the reference samples was organized to run over 30 blocks of 30 s(or 500 counts). For each sample, more than 2500 counts were registered for the 129I selected area and an averaged statistical error of 1.95% with a relative standard deviation of 1.5% were obtained in measuring an averaged value of 1.2 10 11 for the 129 127 I/ I ratio. Fig. 5 shows a typical spectrum of iodine measurement performed at our AMS machine. In spite of use of high currents and of not very restricted ion beam transport by closed slits and apertures, the background was measured to be 1.7 10 13. However, this value is subject of improvement in the future. Finally, in Table 2 we present the resume of all results of the acceptance test.
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Fig. 5. Two parametric plot of
129
I.
Table 2 Resume of the acceptance test results. Element
Sample
No. of samples analyzed
Terminal voltage (MV)
Charge state after stripper
Averaged negative ion current (nA)
Nominal value of isotope ratio
Averaged isotope ratio measured
Carbon
HVEEa Back-ground sampleb
4 2
1 1
2+ 2+
1500 1500
1.6 10–12 <3 10 15
1.4 10 1.8 10
12
CNRS-Cerege Back-ground Sampleb
4 2
0.7 0.7
1+ 1+
300 300
9.35 10 11 <5 10 14
7.5 10 1.2 10
11
CNRS-Cerege Back-ground Sampleb
4 2
1 1
1+ 1+
900c 900c
2.79 10 11 <3 10 14
1.5 10 2.7 10
12
Toronto Back-ground Sampled
4 2
1 1
3+ 3+
1400 1400
1.30 10 11 <3 10 13
1.2 10 1.7 10
11
Aluminum Beryllium Iodine a b c d
15
15
15
13
Averaged counting statistic per sample
SD (%)
170,165 444
0.37
17,477 4
1.3
6644 18
0.7
2751 105
1.5
Samples delivered by HVEE but produced by ORAU [23] Counts in 1 h. Current for BeO . Counts in 510 s.
4. Conclusions The experimental tests and AMS measurements performed during the installation and during the acceptance measurements of the HVEE 1 MV multi-elemental AMS machine at the National Institute for Physics and Nuclear Engineering have widely proved the capacity for high precision and low-background measurements of 14C, 10Be, 26Al, and 129I. The existence of a bouncing system and of a fully automated control system of the ion beam transport and diagnose, makes possible a precise tuning of the entire analyzing system. It also permits a permanent monitoring of an AMS analyze. The AMS machine construction concept proved to be very efficient for removal of ion interferences in the analyzing experiments. Two powerful 90° magnetic analyzers, an ESA of high voltage and large bending angle, a gas field detector of large volume with two anodes (DE, Eres), correlated with operation of the AMS machine at low energies, all contributed to a very good discrimination of ion and also isobar interferences. The sample carousel accepts small size samples (U = 1 mm aperture of the target holder, 0.5 mg target material) in the context of large sputtering currents (required by a good statistic for dating). However, the errors are low in the evaluation of the AMS concentrations measured this machine. The precision of carbon quantitative analysis was better than 4‰ and the background was found to be lower than 2 10 15. The sample preparation and the tuning of machine beam optics contributed to counting of 14C events free of interferences form Li2 molecules. For our multiple applications important and beneficial is also the highly flexibility of the 1 MV AMS machine for switching between
different charge states and also between different isotopes. Finally, since it is able to analyze concentration of heavy ions, Pu and 41Ca are future goals for our machine upgrading. Acknowledgements This work was supported by the Romanian National Council for Science and Research under the project No. 7 PM /1 / 2008 at IFIN-HH. The authors would like to thank Dr. C. Simion for the chemistry laboratory set up. We also thank to Prof H-A. Synal of ETH for helping us with devices devoted for carbon sample preparation and many advices. Special thanks also for Drs. A. Olariu and S. Olariu for contributions in the early construction and assembly stages of this project. References [1] L.M. Alvarez, R. Conrog, Helium and hydrogen of mass 3, Phys. Rev. 56 (1939) 613. [2] C.L. Bennett, R.P. Beukens, M.R. Clover, H.E. Gove, R.B. Liebert, A.E. Litherland, K.H. Purser, W.E. Sondheim, Science 198 (1977) 508–510. [3] R.A. Muller, E. Stephenson, T. Mast, Science 201 (1978) 347. [4] D.E. Nelson, R.G. Korteling, W.R. Stott, Science 198 (1977) 507–508. [5] J. Chen, Z. Guo, K. Liu, Rev. Accel. Sci. Technol. 4 (2011) 117–145. [6] M. Suter, St. Jacob, H.-A. Synal, Nucl. Instr. Meth. B 123 (1–4) (1997) 148–152. [7] H.-A. Synal, S. Jacob, M. Suter, Nucl. Instr. Meth. B 172 (1–4) (2000) 1–7. [8] A.M. Müller, M. Christl, M. Dobeli, P.W. Kubik, M. Suter, H.-A. Synal, Nucl. Instr. Meth. B 266 (10) (2008) 2207–2212. [9] B.J. Hughey, P.L. Skipper, R.E. Klinkowstein, R.E. Shefer, J.S. Wishnok, S.R. Tannenbaum, Nucl. Instr. Meth. B 172 (1–4) (2000) 40–46.
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[10] H.-A. Synal, M. Stocker, M. Suter, Nucl. Instr. Meth. B 259 (1) (2007) 7–13. [11] C. Stan-Sion, M. Enachescu, O. Constantinescu, M. Dogaru, Nucl. Instr. Meth. B 268 (7–8) (2010) 863–866. [12] M. Klein, D.J.W. Mous, A. Gottdang, Nucl. Instr. Meth. B 249 (2006) 764–767. [13] M. Klein, H.J. van Staveren, D.J.W. Mous, A. Gottdang, Nucl. Instr. Meth. B 259 (2007) 184–187. [14] A. Olariu, C. Ivan, N.V. Zamfir, Rom. Rep. Phys. 63 (4) (2011) 923–928. [15] E. Chamizo, J.M. López-Gutiérrez, A. Ruiz-Gómez, F.J. Santos, M. García-León, C. Maden, V. Alfimov, Nucl. Instr. Meth. B 266 (2008) 2217–2220. [16] M. Klein, D.J.W. Mous, A. Gottang, Radiocarbon 55 (2–3) (2013) 224–230. [17] M. Klein, W.H.J. Vaes, B. Fabriek, H. Sandman, D.J.W. Mous, A. Gottdang, Nucl. Instr. Meth. B 294 (2013) 14–17.
[18] W. Hong, J.H. Park, K.S. Sung, H.J. Woo, J.K. Kim, H.W. Choi, G.D. Kim, Radiocarbon 52 (2–3) (2010) 243–251. [19] K.H. Purser, HVEE, United States Patent no. 5274263 (1993). [20] M. Klein, D.J.W. Mous, A. Gottang, Radiocarbon 46 (2004) 77–82. [21] AMS laboratory, Cerege, CNRS UMR 6635 Universite Aix-Marseille, France, (2012). [22] IsoTrace Laboratory, University of Toronto (Ont), 60 St George, Toronto, ON M5S 1A7, Canada, (2012). [23] Oak Ridge National Laboratory, Environmental Sciences Division, Oak Ridge, Tennessee 37830–6290, USA, (2012).
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
A new AMS facility based on a Cockcroft–Walton type 1 MV tandetron at IFIN-HH Magurele, Romania C. Stan-Sion a,⇑, M. Enachescu a, D.G. Ghita a, C.I. Calinescu a, A. Petre a, D.V. Mosu a, M. Klein b a b
Horia Hulubei National Institute of Physics and Nuclear Engineering (IFIN-HH), Bucharest, Romania High Voltage Engineering Europa B.V., Amsterdamsweg 63, 3812 RR Amersfoort P.O. Box 99, 3800 AB Amersfoort, The Netherlands
a r t i c l e
i n f o
Article history: Received 22 May 2013 Received in revised form 30 July 2013 Available online 4 December 2013 Keywords: Accelerator Mass Spectrometry 14 C 10 Be 26 Al 129 I
a b s t r a c t A 1 MV AMS machine was recently installed in the National Institute for Physics and Nuclear Engineering IFIN-HH, Bucharest Romania. It is the second AMS facility at IFIN-HH having the goal not only to continue but mainly to enlarge the research area of this highly sensitive analyzing method. The multi-element AMS was developed by HVEE to measure 14C, 10Be, and 26Al, and 129I. The results of an acceptance test are presented and demonstrate that this machine is capable of routine 14C age dating and of measurements of other radioisotopes in terms of accuracy and precision as well as a low background level. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The highly sensitive analyzing method named today Accelerator Mass Spectrometry (AMS) was discovered 1939 in California, USA, by L.W. Alvarez and Robert Cornog. They used, for the first time, an accelerator (cyclotron) as a mass spectrometer to demonstrate that 3 He is stable and correctly concluded that the other mass-3 isotope tritium is radioactive [1]. 38 years later, stimulated by the high interest of sensitive carbon dating, laboratories at Rochester [2], at Lawrence Livermore National Laboratory [3] and at McMaster [4] supplanted the older ‘‘decay counting’’ method for 14C with the accelerator technique that was about a factor of 1000 more sensitive. They all recognized that modern accelerators could accelerate radioactive particles to energies where magnetic and electric forces are sufficiently high to eliminate the background interferences. During the next years, due to its exceptional analyzing sensitivity of counting individual atoms and performing applications never imaginable before, the AMS method was implemented in many laboratories around the world. Since the central part of such a machine is an energy precision accelerator, besides the stand alone dedicated systems also multipurpose tandem accelerators were adapted for AMS. Studies were undertaken to increase performances of charged particle selection, detection systems, isobar ⁄ Corresponding author. Tel.: +40 2124042346; fax: +40 212321906. E-mail address: [email protected] (C. Stan-Sion). 0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.07.073
separation contributing to the rapid progress of the AMS method [5]. In the recent years, the low-energy accelerator mass spectroscopy with stand alone machines has gained the leading supremacy. Several AMS machines with energies from 6 MV down to 0.5 MV are now in operation around the world [6–9]. Specially, the low energy machines of 0.5–1 MV terminal voltage may be called a balanced system because it is a compact system, with much lower running costs and manpower requirements, as compared to higher energy AMS systems. Moreover, single stage systems with 250 kV acceleration voltage and 200 kV vacuum insulated system (MICADAS) have recently gained remarkable importance [10]. Such systems demonstrate very good performances not only for 14C dating measurements but also for the measurements of other isotopes such as 10Be, 27Al, 41Ca, 129I, 239 Pu, and 240Pu. At the NIPNE in Bucharest a new Cockcroft–Walton type 1 MV HVEE tandetron system, was commissioned in April 2012 and is dedicated for ultra-sensitivity AMS analyses using C, Be, Al, I and Pu elements. This is the second AMS machine in our institute, after the one constructed based on the old 9 MV FN tandem accelerator [11]. The results of the acceptance test of this new machine are presented in this current paper. They will demonstrate its high efficiency in terms of accuracy, precision and low background level, routine 14C age dating and of measurements of other radioisotopes (10Be, 26Al, and 129I).
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C. Stan-Sion et al. / Nuclear Instruments and Methods in Physics Research B 319 (2014) 117–122
2. Description of the 1 MV AMS facility in Bucharest 2.1. The AMS machine This is the sixth 1 MV HVEE (High Voltage Engineering Europe B.V., Amersfoort, the Netherlands) AMS machine delivered by the company and several published papers have already described its main constructive features [12–18]. Therefore, we will give here only a short description. The device from Bucharest is based on a 1 MV tandetron, with a Cockcroft–Walton type high power supply and two separated ion beam injectors at 120° (see Fig. 1). A so called Q-Snout device [19] situated at accelerator entrance is performing two types of tasks: on one hand, it is performing the matching of the ion source emittance with the accelerator acceptance (by the ion beam supplementary acceleration) and on the other hand it is focusing the ion beam on the gas stripper channel. Together with a powerful electric Q-pole triplet lens at the accelerator exit, the two devices optimize the transmission of the overall beam transport for every ion and charge state used in an AMS experiment. The ion stripping in the tandem terminal is produced in an Ar gas stripper channel. The ion sources are both Sources of Negative Ions by Cesium Sputtering (SNICS) type with a 50-target carousel (SO110/50). The sequential injection system of ions consists in an injector 90° analyzing magnet, having its vacuum chamber electrically isolated. To this chamber a step-variable voltage is applied. It changes sequentially to different values lasting for different durations in such a way that the different ion species can be injected to the tandem accelerator according to their current value (9.75 ms for the rare isotopes and 100 ls for the stable isotopes with a delay between species of 0.25 ls). The cyclic change of this voltage for the injection of a sequence of isotopes is referred as bouncing [20]. However, during switching between the isotopes, the magnet chamber voltage requires some time to settle to a constant value otherwise it will result in an unstable beam position. In this
respect, the developed bouncer system by HVEE in 2003 was designed to overcome adverse effects of the switching process by use of a beam-blanking unit, which stops the beam unless the measurement conditions are stable. In our machine, the bouncer is working at a frequency of 100 Hz and a maximum voltage of 3 kV, resulting in very short measurement times for the stable isotopes and in achieving a high time efficiency for the counting of the rare isotope. By bringing through the tandem accelerator the stable and the rare isotope beam, it allows to be both measured. The first in a Faraday cup placed on the high energy side and the later in the detector system. In this way, the AMS analyses are continuously monitored, improving substantially their precision. An electrical static analyzer (ESA), polarized up to ±60 kV and 120° bending, is also used. By its independent action, a degerancy W/q (ions with same ratio of energy W over charge state q will pass the analyzer) remains. That means that ESA will separate ions having different energies from that one of the rare isotope of interest. However, the joint action, with the HE magnetic analyzer, resumes in a final degerancy of M/q (mass M over charge state) implying more restrictive discrimination of undesired ions. After these filter only ions having both the same energy and the same ratio mass over charge (M/q) will pass to the detection system. Finally, the rare isotope ion detector system is a large gas-filled ionization chamber with two anodes for DE Eres measurement and having a detection efficiency of nearly 100%. A 75-nm-thick Si3N4 entrance window separates the detector gas from the beam line vacuum. The separation of ions at low energies is based on the difference between their stopping powers. The DE and Eres electrodes are 10 and 20 cm long. The optimum separation is performed when the crossing of the stopping power curves happens exactly at the splitting boundary between the electrodes. A bi-parametric plot using these two electric signals will enhance the separation between different ions as will be shown in figures of the next chapter. The specifications of the machine are summarized in Table 1.
Fig. 1. The general layout of 1 MV HVEE AMS facility in Bucharest, Romania. (1) Two ion beam injectors equipped with SNICS ion sources of type SO110 – 50 sample carousel HVEE and focusing ion lenses; (2) electrostatic switching system; (3) injection Magnet with multi beam switcher (bouncer); (4) faraday cup and Q-Snout device; (5) 1 MV tandetron accelerator, with gas stripper channel; (6) analyzing Magnet; (7) three offset Faraday cups; (8) electrostatic analyzer; (9) particle detector; (10) Cockcroft–Walton type HV power supply.
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accelerator is not equal for all the three isotopes. In the case of carbon isotopes their fragmentation may play an important role. However, it does not affect the age dating, because measured values will be corrected using reference samples (AMS is usually a relative measurement). When 2+ ions are used for the 1 MV 14C analyses, interference by the 7Li2 molecular ions is severe in some cases. To avoid any Li contribution to the 14C counts, ESA voltage was set close its maximum value (±60 kV) and a relative high stripper gas pressure (2.3 10 2 Torr) was used. Fig. 2 shows a spectrum free of this molecular interference. Due to the higher pressure of the stripper gas the beam transmission was slightly decreased.
Table 1 Specifications of the 1 MV AMS system of Bucharest. Dimension 4.2 6.2 m Ion source 100 lA max. – 2 injectors separated 120° Pre-acceleration voltage 35 kV Bouncer 3 kV, 100 Hz Terminal voltage and up charge current 1 MV, 2 mA Analyzing magnet 90°, 63 MeV amu, max current 300 A Electrical spherical analyzer (ESA) 120°, ±60 kV
3. Results and discussion of the acceptance test 3.1. Carbon
3.2. Aluminum The acceptance test for 14C at our machine was done by using four reference samples provided by HVEE for this purpose together with two background samples. Their 14C/12C nominal values were 1.6 10 12 for reference and for background samples a concentration lower than 3 10 15. In our experiments all samples were measured 4 times, being divided in the process in 30 blocks of 30 s (or 40,000 counts) each. A typical spectrum is shown in Fig. 1. The mean value of the 14C/12C was measured 1.4 10 12, with a statistical error of 0.25% and the relative standard deviation between the four samples of 0.37%. The 13C/12C ratio was measured from the 4 samples with a mean value of 1.02 10 2, with a relative standard deviation of 0.2%. The background value was measured immediately after a concentrated reference sample (10 12) and the obtained value was 1.8 10 15. These values were obtained based on the sequentially injecting ion beam system (bouncer) that permits to transport also the high ion currents of stable isotope beams through the accelerator in a sequential modus and for short durations. The corresponding currents are measured on the HE side, in Faraday cups placed of axis and after the analyzing magnet (see Fig. 1). The transmission between pre- and post accelerating Faraday cups was 58% for 12C2+ at a stripper gas pressure of 2.3 10 2 Torr. In our measurements, between this location and the Faraday cup placed in front of the detector, the beam transmission for 12C and 13C ions were each time 100%. It is expected the same transmission value for 14C on this path. Therefore, with the number of acquired counts of 14C in the detector and using the integrated beam current values of 13C and 12C, measured after the HE magnetic analyzer in the two Faraday cups located of axis, one can determine the 14C/12C and 13C/12C ratios. (This is obtained in an absolute way and without using reference samples.) Deviations of these values from the nominal values of the HVEE reference samples indicate that the transmission through the
Fig. 2. Spectrum of a carbon sample with a
14
C content of 1.6 10
Since aluminum ions in 1+ charge state have the best stripping yield in Ar-gas at 0.7 MV, the acceptance test was carried at this energy. For aluminum analyses, it was used reference material with a 26 Al/27Al ratio of 9.3 10 11. These samples, together with several background samples, were provided by Cerege – Aster AMS facility, France [21]. The AMS measurement was performed according to same procedure as used for carbon and described before. A spectrum is given below in Fig. 3. From the ion source we easily extracted a 300 nA current of 27 Al ions. This beam was fairly sufficient for the machine tuning and afterwards. By use of the bouncing system, it was integrated in the movable Faraday cup, to provide a permanent normalization of the analyze procedure. The measured mean value of 26Al/27Al of four reference samples was 7.5 10 11, with a statistical error of 0.8% and the relative standard deviation between the 4 samples was 1.3%. For the background samples the ratio 26Al/27Al was measured to be 1.2 10 15. The consensus value of the background samples was lower than 5 10 14. The beam transmission of the 27A11+ ion at a stripper pressure of 1.8 10 2 Torr was found to be 36% between pre- and postaccelerator Faraday cups. 3.3. Beryllium The acceptance test for beryllium was performed using four reference samples and several background samples, all supplied by CNRS-Cerege AMS laboratory of Universite Aix-Marseille, France. The difficulty of the 10Be measurement is associated with the interface of the isobar 10B. For the isobar suppression the passive absorber foil method was applied using a Si3N4 foil with a thickness of 150 nm placed in front of the ESA. The tandem terminal voltage
12
when using charge state 2+ at a terminal voltage of 1 MV.
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Fig. 3. Spectrum of a Al sample with a
26
Al content of 9.3 10
11
when using the charge state 1+ at a terminal voltage of 700 kV.
Fig. 4. The two-parametric spectrum is showing the registered counts as function of the initial energy loss (DE) and the residual energy (Eres) from the
was set to 1.0 MV and the 1+ state was selected by the high energy spectrometers before and after a Si3N4 absorber foil. As pilot beam, it was used the 9Be and the transmission through the accelerator, at a 1.8 10 2 Torr pressure in the stripper channel was 51%. However, the transmission efficiency for 10Be was extremely low because while passing through the absorber foil the charge states changes for many 10Be ions and the beam becomes spread out as a result of the straggling effect. Fig. 4 shows the bi-parametric spectrum DE Eres measured from the reference samples that had a consensus value of 2.78 10 11. The 10Be counts were always well separated from the interference with 10B. Each reference sample was measured 4 times and every measurement was divided into 30 blocks of 30 s (or 1500 counts). The mean ratio was measured to be 1.5 10 12, with a statistical error of 1.2% and relative standard deviation of 0.7%. The background sample 10Be/9Be ratio was measured 2.7 10 14. The application of passive absorber foil method gave good results and 10Be could be very well separated from the 10B isobar. The reduced transmission is however a subject of improvement, together with the sensitivity level of the AMS analyzes given by the background level. 3.4. Iodine The acceptance test for iodine was as well performed by using four reference samples. Their consensus value for the 129I/127I ratio was 1.30 10 11 and they were supplied ready to be used by the
10
Be AMS analyses.
Toronto AMS laboratory [22]. The background samples were supplied by Cerege [21] and expected values were below 3 10 13. The terminal voltage for iodine measurement was 1 MV and charge state 3+ was selected by the high energy spectrometer. After the first magnetic analyzer, the current selected for 127I was about 1.4 lA. However, usually a much lower current value was used (obtained by closing slits on the low energy side of the accelerator) in order to obtain the most suitable beam transport optics for the rare isotope. Since iodine is a very volatile material, it produces a deposition layer on the sample surfaces that can alter the constancy of the delivered current. To avoid such effects, new samples have to be conditioned at the beginning of an AMS analyzes by a 5 min Cs pre-sputtering procedure in the ion source. Because the iodine current is high, the batch mode registration for the reference samples was organized to run over 30 blocks of 30 s(or 500 counts). For each sample, more than 2500 counts were registered for the 129I selected area and an averaged statistical error of 1.95% with a relative standard deviation of 1.5% were obtained in measuring an averaged value of 1.2 10 11 for the 129 127 I/ I ratio. Fig. 5 shows a typical spectrum of iodine measurement performed at our AMS machine. In spite of use of high currents and of not very restricted ion beam transport by closed slits and apertures, the background was measured to be 1.7 10 13. However, this value is subject of improvement in the future. Finally, in Table 2 we present the resume of all results of the acceptance test.
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Fig. 5. Two parametric plot of
129
I.
Table 2 Resume of the acceptance test results. Element
Sample
No. of samples analyzed
Terminal voltage (MV)
Charge state after stripper
Averaged negative ion current (nA)
Nominal value of isotope ratio
Averaged isotope ratio measured
Carbon
HVEEa Back-ground sampleb
4 2
1 1
2+ 2+
1500 1500
1.6 10–12 <3 10 15
1.4 10 1.8 10
12
CNRS-Cerege Back-ground Sampleb
4 2
0.7 0.7
1+ 1+
300 300
9.35 10 11 <5 10 14
7.5 10 1.2 10
11
CNRS-Cerege Back-ground Sampleb
4 2
1 1
1+ 1+
900c 900c
2.79 10 11 <3 10 14
1.5 10 2.7 10
12
Toronto Back-ground Sampled
4 2
1 1
3+ 3+
1400 1400
1.30 10 11 <3 10 13
1.2 10 1.7 10
11
Aluminum Beryllium Iodine a b c d
15
15
15
13
Averaged counting statistic per sample
SD (%)
170,165 444
0.37
17,477 4
1.3
6644 18
0.7
2751 105
1.5
Samples delivered by HVEE but produced by ORAU [23] Counts in 1 h. Current for BeO . Counts in 510 s.
4. Conclusions The experimental tests and AMS measurements performed during the installation and during the acceptance measurements of the HVEE 1 MV multi-elemental AMS machine at the National Institute for Physics and Nuclear Engineering have widely proved the capacity for high precision and low-background measurements of 14C, 10Be, 26Al, and 129I. The existence of a bouncing system and of a fully automated control system of the ion beam transport and diagnose, makes possible a precise tuning of the entire analyzing system. It also permits a permanent monitoring of an AMS analyze. The AMS machine construction concept proved to be very efficient for removal of ion interferences in the analyzing experiments. Two powerful 90° magnetic analyzers, an ESA of high voltage and large bending angle, a gas field detector of large volume with two anodes (DE, Eres), correlated with operation of the AMS machine at low energies, all contributed to a very good discrimination of ion and also isobar interferences. The sample carousel accepts small size samples (U = 1 mm aperture of the target holder, 0.5 mg target material) in the context of large sputtering currents (required by a good statistic for dating). However, the errors are low in the evaluation of the AMS concentrations measured this machine. The precision of carbon quantitative analysis was better than 4‰ and the background was found to be lower than 2 10 15. The sample preparation and the tuning of machine beam optics contributed to counting of 14C events free of interferences form Li2 molecules. For our multiple applications important and beneficial is also the highly flexibility of the 1 MV AMS machine for switching between
different charge states and also between different isotopes. Finally, since it is able to analyze concentration of heavy ions, Pu and 41Ca are future goals for our machine upgrading. Acknowledgements This work was supported by the Romanian National Council for Science and Research under the project No. 7 PM /1 / 2008 at IFIN-HH. The authors would like to thank Dr. C. Simion for the chemistry laboratory set up. We also thank to Prof H-A. Synal of ETH for helping us with devices devoted for carbon sample preparation and many advices. Special thanks also for Drs. A. Olariu and S. Olariu for contributions in the early construction and assembly stages of this project. References [1] L.M. Alvarez, R. Conrog, Helium and hydrogen of mass 3, Phys. Rev. 56 (1939) 613. [2] C.L. Bennett, R.P. Beukens, M.R. Clover, H.E. Gove, R.B. Liebert, A.E. Litherland, K.H. Purser, W.E. Sondheim, Science 198 (1977) 508–510. [3] R.A. Muller, E. Stephenson, T. Mast, Science 201 (1978) 347. [4] D.E. Nelson, R.G. Korteling, W.R. Stott, Science 198 (1977) 507–508. [5] J. Chen, Z. Guo, K. Liu, Rev. Accel. Sci. Technol. 4 (2011) 117–145. [6] M. Suter, St. Jacob, H.-A. Synal, Nucl. Instr. Meth. B 123 (1–4) (1997) 148–152. [7] H.-A. Synal, S. Jacob, M. Suter, Nucl. Instr. Meth. B 172 (1–4) (2000) 1–7. [8] A.M. Müller, M. Christl, M. Dobeli, P.W. Kubik, M. Suter, H.-A. Synal, Nucl. Instr. Meth. B 266 (10) (2008) 2207–2212. [9] B.J. Hughey, P.L. Skipper, R.E. Klinkowstein, R.E. Shefer, J.S. Wishnok, S.R. Tannenbaum, Nucl. Instr. Meth. B 172 (1–4) (2000) 40–46.
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