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Competitive 10Be measurements below 1 MeV with the upgraded ETH–TANDY AMS facility
Nuclear Instruments and Methods in Physics Research B 268 (2010) 2801–2807
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Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Competitive AMS facility
10
Be measurements below 1 MeV with the upgraded ETH–TANDY
A.M. Müller *, M. Christl, J. Lachner, M. Suter, H.-A. Synal Laboratory of Ion Beam Physics, ETH Zurich, Schafmattstrasse 20, CH-8093 Zurich, Switzerland
a r t i c l e
i n f o
Article history: Received 31 March 2010 Received in revised form 31 May 2010 Available online 18 June 2010 Keywords: 10 Be Low energy AMS TANDY Ion source Magnetic spectrometer 9 Be background Boron suppression Achromatic system
a b s t r a c t Competitive 10Be measurements at energies as low as 0.75 MeV are now possible with the compact ETH AMS system TANDY. In this paper we describe and discuss the modifications that led to the significantly improved performance for 10Be at the 0.6 MV accelerator. Results from the first routine measurement show that 10Be on the upgraded TANDY is now fully competitive with larger AMS systems with respect to background and measurement precision. The total efficiency for 10Be is comparable to our large 6 MV Tandem system and thus sufficient for the full range of applications in the Earth and Environmental Sciences. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction The compact ETH AMS facility – TANDY – was built in the late 90s with the intention to demonstrate that precise and accurate radiocarbon measurements are possible at energies of about 1 MeV [1,2]. But, the instrumental design of the TANDY was not restricted to radiocarbon and in principle allowed to transport ions over the whole mass rage (e.g. from 10Be to Pu isotopes). It soon turned out that the TANDY is suitable as a multi isotope AMS system. Many systematic studies in combination with further technical developments demonstrated that the TANDY (operating at a maximal terminal voltage of 600 kV) is competitive to conventional AMS in many cases (as summarized in [3]). At ETH, further miniaturization and technical improvement led to the development of the even more compact MIni Carbon DAting System (MICADAS) [4,5] leaving the field of all the other radionuclides for the TANDY. In the recent past routine measurements of 129I [6] and actinides [7–9] have been established while measurements of 10Be, 26Al, and 41Ca were possible [10,11] but suffered from a higher level of background compared to conventional AMS and/ or were limited by low currents from the ion source. To solve these limitations an additional magnet was installed on the high energy side and the ion source including the vacuum system was
* Corresponding author. Tel.: +41 44 633 20 26. E-mail address: [email protected] (A.M. Müller). 0168-583X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2010.05.104
upgraded, respectively. In this paper, we first describe these two major technical modifications and in the second part we discuss the performance of the upgraded TANDY for 10Be measurements at low (0.75 MeV) energies. 2. Experimental setup 2.1. Ion source modifications Several changes were made to the original NEC 40 MC-SNICS ion source. Modifications of the interior of the ion source largely followed the suggestions by Southon and Santos [12] (and references therein). The original conical NEC ionizer was replaced by a spherical SpectramatÒ ionizer [13] – as it is now used in most of the ETH ion sources. A new ionizer housing was built with a fixed Cs feeding line that is heated by thermal conduction from the ionizer. Accordingly, the Cs oven was rebuilt with a vacuum insulated Cs feeding line. The Cs lens was removed from the ion source and an immersion lens at cathode potential was installed instead. To compensate for the lack of a separately adjustable focusing element, both cathode and immersion lens were placed closer to the geometrical focus (f = 1.75 cm) of the ionizer. As a consequence the field gradient at the surface of the ionizer significantly increased, which is thought to be the limiting parameter for the efficient extraction of the primary Cs+ beam [14]. The ion optics of the original and the new geometry was modeled using SIMIONÒ (Fig. 1) [15]. Compared to the simulation of the
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Fig. 1. SIMION simulation of the rebuilt TANDY MC-SNICS ion source. The blue numbers indicate the electrostatic potentials used in this simulation. On the length scale below the position of the cathode (0 mm), immersion lens (2 mm), top of the ionizer housing (6 mm), bottom of the ionizer housing (31 mm), and extraction lens (44 mm) are indicated. The lower half is magnified by a factor of 5. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
original design the computed primary Cs beam (blue lines) is well constrained on the cathode and the modeled secondary ion beam (red lines) is of good quality with respect to small spatial extent and low divergence. However, since crucial effects like the influence of space charges and the complete sputtering process are not included in the simulations this result has to be considered qualitatively only.
Similar to modifications at the VERA AMS facility, an online target positioning system was implemented [16]. During the measurement the target can be tilted or moved along the beam axis by individually turning three lucite rods that are connected to the original positioning screws via chains. The movement of the target can be observed with an IP-camera mounted on the viewport of the low energy magnet. The possibility to visually control the position of the cathode and its behaviour during sputtering turned out to be very useful in the first routine measurements. Further modifications were made concerning the vacuum system of the ion source. To increase pumping capacity near the cathode wheel a new MDCÒ gate valve with an additional CF 100 port for a turbo pump was installed. The turbo pump is mounted overhead the valve in 90 degree position. Both, the turbo pump and the associated membrane pump are operated at source potential. After a few month of operation no malfunction due to sparking of the source has been observed. The increased pumping capacity decreased the idle time during wheel changes to about 2–3 h and reduced the pressure after the ion source down to the 10 8 mbar range during operation. During first test runs significant variations of the Cs-oven temperature were observed (up to 2 °C within 24 h). To reduce this variability, which in our opinion affects the stability of the system the manually adjustable Cs oven temperature control is currently being replaced by an active computer controlled regulation system. Therefore, the thermocouple signal is amplified, digitized and transferred (via an optical link) to the measurement computer. The feedback signal is (optically) transferred from the measurement software to a power amplifier operating on source potential that directly adjusts the power of the Cs oven. The regulation software will be installed within the next months. With the new active temperature regulation system a very good temperature stability of about one tenth of a degree is expected. With this new setup, first tests with BeO (mixed with Nb powder; see Section 3) at a sample potential of 10 kV were performed and negative ion currents of up to 8 lA could be extracted. However, the source still has to be thoroughly tested for reliability
Fig. 2. Perspective view of the upgraded TANDY system. Between the ESA and the new 130° Magnet a retractable gas ionization chamber is mounted, which can be used for setting up the system.
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under routine operation. The first routine run of 10Be was performed with more conservative ion source settings of 8.5 kV sample potential and lower Cs-temperature compared to the test run resulting in about 2–4 lA negative ion currents of 9BeO . The performance of the TANDY during the first routine 10Be run is further discussed in Section 3. 2.2. The additional 130° bending magnet 2.2.1. Motivation With the standard setup for 10Be [17,18] radionuclide measurements are performed directly after the ESA in a DE–Eres gas ionization chamber providing sufficient resolution to discriminate the isobar 10B [19]. In order to reduce the 10B intensity from BeO samples down to a level that can be handled by the detection system, a thin SiN degrader foil (67–75 nm) is placed in front of the ESA [10].
Table 1 Specifications of the additional magnetic spectrometer. Entrance/exit angle
48.5°
Bending angle Bending radius Mass dispersion Dx/(Dm/m) Field strength Ability of deflection m E/q2 Pole gap Pole width Total weight
130° 750 mm 1601 mm 1.25 Tesla 42,600 amu keV/e2 50 mm 200 mm 4410 kg
Using this setup for 10Be measurements a severe background at a Be/9Be level of 1 10 13 is observed. Background caused by (highly abundant) neighboring masses passing the high energy spectrometer is one of the major problems for radionuclide measurements at low energies. Scattering and electron capture cross sections increase by orders of magnitude in the sub MeV region [20], dramatically enhancing the probability for interaction with residual gas atoms. As a consequence, background caused by abundant neighboring masses becomes increasingly dominant when going to lower energies. When injecting mass 26 (amu) from BeO samples 9Be16O1H and 9 Be17O are present at the level of 10 3 and 10 4 compared to 10 Be16O, respectively [21]. Due to the processes described above, 9 Be from the breakup of these molecules may pass the magnetic filter after the accelerator and reach the degrader foil with the same momentum but at higher energy compared to 10Be. The fact that 9 Be ions in the sub MeV range loose more energy in the degrader foil because they have a higher stopping power than 10Be and the broadening of the energy distribution when passing through the degrader foil both increase the probability of 9Be ions reaching the detector after the ESA with the same energy as 10Be [10]. Initial tests with an additional magnet (taken from the ETH–MICADAS system) mounted on the high energy spectrometer have demonstrated that this 9Be background can be removed efficiently [10]. Furthermore, beam losses caused by energy and angular straggling in the SiN degrader foil are reduced due to the energy and angular focusing effect of the magnet. Consequently, it was decided to design and install an additional magnet to the high energy side of the TANDY, which is optimized for 10Be as well as for much heavier isotopes like the actinides. 10
TANDY HE SPEKTROMETER AFTER DEGRADER 743 mm
1178 mm
777 mm
712 mm
BEAM PLOT 1702 mm
761 mm
X-MAX 0.100 m
ELECTRIC SECTOR FIELD
MAGNETIC SECTOR FIELD
Y-MAX 0.100 m
SiN Degrader Foil
Focus Slits Retractable Detector
Detector
Fig. 3. GICOSY beam plot for the ESA 130° magnet combination [24]. Beam starting position corresponds to the position of the SiN degrader foil. The drift lengths in free and sector field regions are shown at the top. The course of beam is illustrated for three different initial energies (DE/E = ±0.5, 0) and five angles (a = ±15 mrad, ±7.5 mrad, 0).
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2.2.2. Ion optics and specifications The optimized design for the new magnet had to meet several prerequisites: The magnetic deflection ability should be equal to that of the existing high energy magnet (33,000 amu keV/e2) in order to use the new spectrometer for the entire mass range of radionuclides analyzed at the TANDY (from 10Be to Pu isotopes). Therefore, the bending radius of the new magnet was specified to be equal to the radius of the existing 90° high magnet. The focal distances in the dispersive and the orthogonal plane, respectively were set to be equal resulting in a stigmatic beam mapping of the new magnet. The design was further optimized so that the combination of the new magnet with the spherical electrostatic analyzer (ESA) forms an achromatic system with the SiN degrader in the object plane. In order to define an energy acceptance for the detector, the beam has to be focused after the ESA. Because of this intermediate focus the bending direction of the magnet has to be towards the inner part of the combined spectrometer in order to achieve an achromatic ESA – magnet configuration [22]. This in turn constrains the space available for the new spectrometer. The ion optical properties of the high energy spectrometer were calculated using ETHoptics [3] and a detailed simulation of the magnet was performed using the TRANSPORT code [23]. Meeting the space and weight constraints at a given bending radius of 750 mm an optimal design for the new magnet was found with a bending angle of 130° and shim angles of 48.5°. Smaller bending angles would lead to large focal distances that are in conflict with the space constraints. Larger bending angles would reduce the distance between ESA and magnet in a way that it would be impossible to place a retractable detector and other beam line equipment (bellows, valves etc.) at the exit of the ESA. Furthermore, this would increase the weight of the magnet and exceed the allowed floor load. The final design of the magnet and how it fits into the
existing AMS system is shown in Fig. 2, the detailed specifications of the magnet are listed in Table 1. The angular focussing and the achromatic beam transport from the object position (degrader foil) into the detector through the combined ESA – 130° magnet spectrometer is shown in Fig. 3. The x-, y-beam plot is calculated using the GICOSY ion optics program by Weick [24] with the x-plot corresponding to the dispersive plane. In this plot an initial energy dispersion of 0.5% and an initial angular divergence of ±15 mrad was assumed. It is shown how the beam is expanded in the dispersive plane according to the initial energy dispersion (after passing the ESA), and how this energy dispersion is refocused after the magnet (achromatic design). Furthermore it demonstrates that within the given energy dispersion the initial angular dispersion is refocused (in the x and y plane) at the detector position (double focusing and stigmatic design). The position and the width of the slits in the focal plane of the ESA can be adjusted in order to define an energy window that is accepted by the detector. 3. Performance
10
Be
In this section we describe the performance of the upgraded TANDY for 10Be measurements as obtained during initial tests with the new spectrometer and during the first routine measurement of 10 Be. 3.1. Instrumental setup and performance The basic procedure with respect to sample preparation and measurement technique has not changed significantly compared to previous measurements. Samples are prepared as BeO and (now) mixed with Nb powder [25]. It has been found that mixing
400
300
ΔE (Channels)
10
B 10
Be
200
100
0
100
300
200
400
Eresidual (Channels) Fig. 4. Spectrum of our in in-house standard sample S2007 (nominal 1.5–3 kHz.
10
Be/9Be = 30.87 10
12
). The measurement time was 145 s. The rate of 10B entering the detector was
A.M. Müller et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 2801–2807
BeO with Nb powder produces higher output of negative ions from the TANDY ion source compared to mixing with Cu or Ag. Such an effect has been reported by many other labs working with different ion sources [26], it is, however, not observed with the high current source (HCS) on our 6 MV Tandem system. BeO is extracted from the ion source and Be1+ is selected on the high energy side. 10B is suppressed by several orders of magnitude by applying the degrader foil technique [27]. As degrader foil we use Si3N3.1 membranes of different size and thickness that can be inserted in front of the ESA. The detector consists of the ETH DE–Eres gas ionization detector, which has sufficient resolution to suppresses the residual boron in the ion beam [28,10,19]. Within a certain range, the 10B counting rate in the detector can be adjusted to the contamination level of the samples by varying the position and the width of the slits in the focal image plane of the ESA (Fig. 2). The maximal possible aperture is 30 30 mm2 according to an energy window of DE/E 2%. Tests have shown that the ETH gas ionization chamber can handle boron rates up to 4–5 kHz without any effect on the measured 10Be/9Be ratio. A total energy resolution of 18 keV for 10B ions at 701 keV incident energy allows full separation of the isobar from 10Be (Fig. 4).
The transmission of 10Be from the low energy side to the detector is determined by the transmission through both the gas stripper and the accelerator and by losses after passing the SiN degrader. At a terminal voltage of 525 kV maximum 9Be stripper transmission of 58% could be achieved for the 1+ charge state with typical values ranging between 50% and 55%. Beam losses when passing the SiN degrader foil (for a given thickness) depend on both, the energy and the angular straggling of the incident ion beam and on the stripping yield for the 2+ charge state, which is selected after the degrader foil [10]. Tests with different SiN foils indicate a yield of less than 55% for 10Be2+ at 745 keV. This is significantly lower than the values obtained for 9Be projectiles in thin carbon foils [29]. Further studies to determine the stripping yield from SiN and carbon foils are currently undertaken to explain the observed discrepancy. Based on theory both, energy and angular straggling of a beam, which is within the angular acceptance of the ESA, are compensated by the achromatic and angular focussing characteristics of the combined ESA-magnet spectrometer. Fig. 5 shows a scan run of the 130° magnet at a beam energy of 745 keV (after passing the 67 nm SiN degrader foil). In this experiment, a detector entrance window of 12 8 mm2 and was used. According to Sun et al. [30] the energy straggling for 10Be projectile passing the 67 nm SiN degrader at 745 keV is 7.7 keV FWHM corresponding to a spatial distribution of about 17 mm in the image plane of the ESA. The flat top demonstrates how the energy spread is refocused to a beam of about 3–4 mm width (FWHM). By confining the aperture in the image plane asymmetrically to an opening of 6 mm towards the low energy side and 15 mm towards the high energy side (values are given relative to the optical beam axis) a 10Be transmission through the foil into the detector of 20–22% was achieved, which is in agreement with theoretical estimates made for losses due to angular and energy straggling (Fig. 6) and due to the stripping yield in the SiN degrader foil. Based on these results we conclude that a total 10Be transmission (transport efficiency) of 10–13% is achievable with this configuration.
3.2. First routine operation for Fig. 5. Scan of the 130° bending magnet over a SiN entrance window with the dimensions 12 8 mm2.
10
Be
The first routine run of 10Be on the TANDY was carried out in December 2009. All measurements were performed with a SiN
Optical axis (701 keV)
10
2805
B
10
2.7 keV
Be
6.8 keV
Fig. 6. Calculated energy distribution transfered to spatial coordinates represented in the focal plane of the ESA. It is assumed that a beam of 745 keV energy passed a 67 nm SiN membrane (mean 10Be energy after passing: 701 keV). The widths of the distributions are based on energy loss straggling [30]. The shaded areas indicate the regions covered by slits. The energy acceptance (white area under the 10Be energy distribution) of this configuration of slits is about 78%.
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degrader foil of 75 nm nominal thickness and an area of 10 6 mm2. The TANDY operated continuously and largely unattended for three days and about 100 10Be samples (75 unknown samples from the Northgrip ice core, and about 25 blanks and standards) were measured (with about 60 h up time over 72 h). The results and the interpretation of the ice core samples are subject to another paper. Here we focus on the general performance of the upgraded TANDY during first routine operation (as summarized in Table 2). Typical 9BeO currents ranged between 2 and 4 lA and a stripper transmission in the 1+ charge state was larger than 50%. Few samples showed lower transmission, which probably was caused by contamination with other molecules of mass 25. Transmission through the degrader was about 20% resulting in an efficiency (e) of 10–11% during the first routine run, which is fully competitive to the large (6 MV) TANDEM AMS system at ETH [31]. The measured 10Be ratio of the in-house standard S2007 (Fig. 7) varied by only 1.4% over one sample wheel (mean error of a single measurement) while the final uncertainty of the 10Be concentration in the ice core samples typically was 2–3% (in most cases dominated by counting statistics). Thus, a very good time stability and high precision of the data was achieved. To estimate a background value for the new setup a low-level Be carrier from University of Hannover (provided by F.v. Blanckenburg) was analyzed. 10Be/9Be ratios for this carrier material reported by other AMS laboratories (SUERC, VERA, ASTER) range between 0.4 and 1.2 10 15) (1r uncertainty) [32]. The mean 10 Be/9Be ratio of our measurements is (1.0 ± 0.4) 10 15 (Fig. 8) clearly showing that no background at the 10 15 level is observed with the upgraded TANDY system. In particular, the 9Be background has been eliminated completely.
Table 2 Performance of the TANDY during first routine operation for 10Be. Nuclide
10
I (lA) Transmission (%) Transport efficiency e (%) 10 Be/9Be background
2–4 >50 10–11 <1 10
Be
15
Fig. 8. Low-level Be carrier measurements. The 1r-errors of the measurements are dominated by counting statistics. The average value (straight line) represents the error weighted mean of all measurements. The dashed lines represent the mean 1rerror for a single measurement.
4. Conclusion Several technical modifications of the compact low energy AMS system TANDY led to a significantly improved performance for all radionuclides. In this study we focused on 10Be, which is (besides radiocarbon) the second most important radionuclide in the Earth and Environmental Sciences. Modifications of the ion source and the vacuum system led to a higher and more stable output of negative ions and decreased the idle time during wheel changes. The extension of the high energy spectrometer with a new magnet and its design as an achromatic and stigmatic system in combination with the ESA led to an improved beam transport and effectively removed the background caused by scattered 9Be. The combination of all the modifications now enables us to measure 10 Be at a competitive level of performance compared to much larger AMS systems. The results from the first routine operation of 10 Be at 0.75 MeV demonstrate that samples covering a broad range from 10 11 (standard) to 10 15 (carrier) can be precisely measured with an average measurement time of about 50 min/sample. We conclude that the ETH low energy AMS system TANDY is now ready for routine operation of 10Be at negligible background and therefore is capable to significantly extend our capacity for 10Be measurements covering the full range of applications in the Earth and Environmental Sciences. Acknowledgement The authors want to acknowledge the technical staff of the Laboratory of Ion Beam Physics (in particular Joël Bourquin, Andreas Herrmann and René Gruber) for their dedication and commitment making all the technical modifications possible. F.v. Blanckenburg (GFZ-Potsdam, Germany) for providing the low level carrier, H. Weick (University of Giessen, Germany) for the GICOSY program, J. Beer (EAWAG, Switzerland) and H. Fischer (University of Berne, Switzerland) for providing access to the ice core samples and last but not least Silvia Lueck-Bollhalder (EAWAG, Switzerland) for sample prepation.
Fig. 7. Standard S2007 over 17 h (raw data). The 1r-error of the data is given by whichever is larger the error related with counting statistics or the standard deviation of the repeated (3–6 times) measurement of the same sample. The current weighted mean value for the whole run is displayed by the straight line. The dashed lines represent the mean 1r-error of a single measurement.
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Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Competitive AMS facility
10
Be measurements below 1 MeV with the upgraded ETH–TANDY
A.M. Müller *, M. Christl, J. Lachner, M. Suter, H.-A. Synal Laboratory of Ion Beam Physics, ETH Zurich, Schafmattstrasse 20, CH-8093 Zurich, Switzerland
a r t i c l e
i n f o
Article history: Received 31 March 2010 Received in revised form 31 May 2010 Available online 18 June 2010 Keywords: 10 Be Low energy AMS TANDY Ion source Magnetic spectrometer 9 Be background Boron suppression Achromatic system
a b s t r a c t Competitive 10Be measurements at energies as low as 0.75 MeV are now possible with the compact ETH AMS system TANDY. In this paper we describe and discuss the modifications that led to the significantly improved performance for 10Be at the 0.6 MV accelerator. Results from the first routine measurement show that 10Be on the upgraded TANDY is now fully competitive with larger AMS systems with respect to background and measurement precision. The total efficiency for 10Be is comparable to our large 6 MV Tandem system and thus sufficient for the full range of applications in the Earth and Environmental Sciences. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction The compact ETH AMS facility – TANDY – was built in the late 90s with the intention to demonstrate that precise and accurate radiocarbon measurements are possible at energies of about 1 MeV [1,2]. But, the instrumental design of the TANDY was not restricted to radiocarbon and in principle allowed to transport ions over the whole mass rage (e.g. from 10Be to Pu isotopes). It soon turned out that the TANDY is suitable as a multi isotope AMS system. Many systematic studies in combination with further technical developments demonstrated that the TANDY (operating at a maximal terminal voltage of 600 kV) is competitive to conventional AMS in many cases (as summarized in [3]). At ETH, further miniaturization and technical improvement led to the development of the even more compact MIni Carbon DAting System (MICADAS) [4,5] leaving the field of all the other radionuclides for the TANDY. In the recent past routine measurements of 129I [6] and actinides [7–9] have been established while measurements of 10Be, 26Al, and 41Ca were possible [10,11] but suffered from a higher level of background compared to conventional AMS and/ or were limited by low currents from the ion source. To solve these limitations an additional magnet was installed on the high energy side and the ion source including the vacuum system was
* Corresponding author. Tel.: +41 44 633 20 26. E-mail address: [email protected] (A.M. Müller). 0168-583X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2010.05.104
upgraded, respectively. In this paper, we first describe these two major technical modifications and in the second part we discuss the performance of the upgraded TANDY for 10Be measurements at low (0.75 MeV) energies. 2. Experimental setup 2.1. Ion source modifications Several changes were made to the original NEC 40 MC-SNICS ion source. Modifications of the interior of the ion source largely followed the suggestions by Southon and Santos [12] (and references therein). The original conical NEC ionizer was replaced by a spherical SpectramatÒ ionizer [13] – as it is now used in most of the ETH ion sources. A new ionizer housing was built with a fixed Cs feeding line that is heated by thermal conduction from the ionizer. Accordingly, the Cs oven was rebuilt with a vacuum insulated Cs feeding line. The Cs lens was removed from the ion source and an immersion lens at cathode potential was installed instead. To compensate for the lack of a separately adjustable focusing element, both cathode and immersion lens were placed closer to the geometrical focus (f = 1.75 cm) of the ionizer. As a consequence the field gradient at the surface of the ionizer significantly increased, which is thought to be the limiting parameter for the efficient extraction of the primary Cs+ beam [14]. The ion optics of the original and the new geometry was modeled using SIMIONÒ (Fig. 1) [15]. Compared to the simulation of the
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Fig. 1. SIMION simulation of the rebuilt TANDY MC-SNICS ion source. The blue numbers indicate the electrostatic potentials used in this simulation. On the length scale below the position of the cathode (0 mm), immersion lens (2 mm), top of the ionizer housing (6 mm), bottom of the ionizer housing (31 mm), and extraction lens (44 mm) are indicated. The lower half is magnified by a factor of 5. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
original design the computed primary Cs beam (blue lines) is well constrained on the cathode and the modeled secondary ion beam (red lines) is of good quality with respect to small spatial extent and low divergence. However, since crucial effects like the influence of space charges and the complete sputtering process are not included in the simulations this result has to be considered qualitatively only.
Similar to modifications at the VERA AMS facility, an online target positioning system was implemented [16]. During the measurement the target can be tilted or moved along the beam axis by individually turning three lucite rods that are connected to the original positioning screws via chains. The movement of the target can be observed with an IP-camera mounted on the viewport of the low energy magnet. The possibility to visually control the position of the cathode and its behaviour during sputtering turned out to be very useful in the first routine measurements. Further modifications were made concerning the vacuum system of the ion source. To increase pumping capacity near the cathode wheel a new MDCÒ gate valve with an additional CF 100 port for a turbo pump was installed. The turbo pump is mounted overhead the valve in 90 degree position. Both, the turbo pump and the associated membrane pump are operated at source potential. After a few month of operation no malfunction due to sparking of the source has been observed. The increased pumping capacity decreased the idle time during wheel changes to about 2–3 h and reduced the pressure after the ion source down to the 10 8 mbar range during operation. During first test runs significant variations of the Cs-oven temperature were observed (up to 2 °C within 24 h). To reduce this variability, which in our opinion affects the stability of the system the manually adjustable Cs oven temperature control is currently being replaced by an active computer controlled regulation system. Therefore, the thermocouple signal is amplified, digitized and transferred (via an optical link) to the measurement computer. The feedback signal is (optically) transferred from the measurement software to a power amplifier operating on source potential that directly adjusts the power of the Cs oven. The regulation software will be installed within the next months. With the new active temperature regulation system a very good temperature stability of about one tenth of a degree is expected. With this new setup, first tests with BeO (mixed with Nb powder; see Section 3) at a sample potential of 10 kV were performed and negative ion currents of up to 8 lA could be extracted. However, the source still has to be thoroughly tested for reliability
Fig. 2. Perspective view of the upgraded TANDY system. Between the ESA and the new 130° Magnet a retractable gas ionization chamber is mounted, which can be used for setting up the system.
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under routine operation. The first routine run of 10Be was performed with more conservative ion source settings of 8.5 kV sample potential and lower Cs-temperature compared to the test run resulting in about 2–4 lA negative ion currents of 9BeO . The performance of the TANDY during the first routine 10Be run is further discussed in Section 3. 2.2. The additional 130° bending magnet 2.2.1. Motivation With the standard setup for 10Be [17,18] radionuclide measurements are performed directly after the ESA in a DE–Eres gas ionization chamber providing sufficient resolution to discriminate the isobar 10B [19]. In order to reduce the 10B intensity from BeO samples down to a level that can be handled by the detection system, a thin SiN degrader foil (67–75 nm) is placed in front of the ESA [10].
Table 1 Specifications of the additional magnetic spectrometer. Entrance/exit angle
48.5°
Bending angle Bending radius Mass dispersion Dx/(Dm/m) Field strength Ability of deflection m E/q2 Pole gap Pole width Total weight
130° 750 mm 1601 mm 1.25 Tesla 42,600 amu keV/e2 50 mm 200 mm 4410 kg
Using this setup for 10Be measurements a severe background at a Be/9Be level of 1 10 13 is observed. Background caused by (highly abundant) neighboring masses passing the high energy spectrometer is one of the major problems for radionuclide measurements at low energies. Scattering and electron capture cross sections increase by orders of magnitude in the sub MeV region [20], dramatically enhancing the probability for interaction with residual gas atoms. As a consequence, background caused by abundant neighboring masses becomes increasingly dominant when going to lower energies. When injecting mass 26 (amu) from BeO samples 9Be16O1H and 9 Be17O are present at the level of 10 3 and 10 4 compared to 10 Be16O, respectively [21]. Due to the processes described above, 9 Be from the breakup of these molecules may pass the magnetic filter after the accelerator and reach the degrader foil with the same momentum but at higher energy compared to 10Be. The fact that 9 Be ions in the sub MeV range loose more energy in the degrader foil because they have a higher stopping power than 10Be and the broadening of the energy distribution when passing through the degrader foil both increase the probability of 9Be ions reaching the detector after the ESA with the same energy as 10Be [10]. Initial tests with an additional magnet (taken from the ETH–MICADAS system) mounted on the high energy spectrometer have demonstrated that this 9Be background can be removed efficiently [10]. Furthermore, beam losses caused by energy and angular straggling in the SiN degrader foil are reduced due to the energy and angular focusing effect of the magnet. Consequently, it was decided to design and install an additional magnet to the high energy side of the TANDY, which is optimized for 10Be as well as for much heavier isotopes like the actinides. 10
TANDY HE SPEKTROMETER AFTER DEGRADER 743 mm
1178 mm
777 mm
712 mm
BEAM PLOT 1702 mm
761 mm
X-MAX 0.100 m
ELECTRIC SECTOR FIELD
MAGNETIC SECTOR FIELD
Y-MAX 0.100 m
SiN Degrader Foil
Focus Slits Retractable Detector
Detector
Fig. 3. GICOSY beam plot for the ESA 130° magnet combination [24]. Beam starting position corresponds to the position of the SiN degrader foil. The drift lengths in free and sector field regions are shown at the top. The course of beam is illustrated for three different initial energies (DE/E = ±0.5, 0) and five angles (a = ±15 mrad, ±7.5 mrad, 0).
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2.2.2. Ion optics and specifications The optimized design for the new magnet had to meet several prerequisites: The magnetic deflection ability should be equal to that of the existing high energy magnet (33,000 amu keV/e2) in order to use the new spectrometer for the entire mass range of radionuclides analyzed at the TANDY (from 10Be to Pu isotopes). Therefore, the bending radius of the new magnet was specified to be equal to the radius of the existing 90° high magnet. The focal distances in the dispersive and the orthogonal plane, respectively were set to be equal resulting in a stigmatic beam mapping of the new magnet. The design was further optimized so that the combination of the new magnet with the spherical electrostatic analyzer (ESA) forms an achromatic system with the SiN degrader in the object plane. In order to define an energy acceptance for the detector, the beam has to be focused after the ESA. Because of this intermediate focus the bending direction of the magnet has to be towards the inner part of the combined spectrometer in order to achieve an achromatic ESA – magnet configuration [22]. This in turn constrains the space available for the new spectrometer. The ion optical properties of the high energy spectrometer were calculated using ETHoptics [3] and a detailed simulation of the magnet was performed using the TRANSPORT code [23]. Meeting the space and weight constraints at a given bending radius of 750 mm an optimal design for the new magnet was found with a bending angle of 130° and shim angles of 48.5°. Smaller bending angles would lead to large focal distances that are in conflict with the space constraints. Larger bending angles would reduce the distance between ESA and magnet in a way that it would be impossible to place a retractable detector and other beam line equipment (bellows, valves etc.) at the exit of the ESA. Furthermore, this would increase the weight of the magnet and exceed the allowed floor load. The final design of the magnet and how it fits into the
existing AMS system is shown in Fig. 2, the detailed specifications of the magnet are listed in Table 1. The angular focussing and the achromatic beam transport from the object position (degrader foil) into the detector through the combined ESA – 130° magnet spectrometer is shown in Fig. 3. The x-, y-beam plot is calculated using the GICOSY ion optics program by Weick [24] with the x-plot corresponding to the dispersive plane. In this plot an initial energy dispersion of 0.5% and an initial angular divergence of ±15 mrad was assumed. It is shown how the beam is expanded in the dispersive plane according to the initial energy dispersion (after passing the ESA), and how this energy dispersion is refocused after the magnet (achromatic design). Furthermore it demonstrates that within the given energy dispersion the initial angular dispersion is refocused (in the x and y plane) at the detector position (double focusing and stigmatic design). The position and the width of the slits in the focal plane of the ESA can be adjusted in order to define an energy window that is accepted by the detector. 3. Performance
10
Be
In this section we describe the performance of the upgraded TANDY for 10Be measurements as obtained during initial tests with the new spectrometer and during the first routine measurement of 10 Be. 3.1. Instrumental setup and performance The basic procedure with respect to sample preparation and measurement technique has not changed significantly compared to previous measurements. Samples are prepared as BeO and (now) mixed with Nb powder [25]. It has been found that mixing
400
300
ΔE (Channels)
10
B 10
Be
200
100
0
100
300
200
400
Eresidual (Channels) Fig. 4. Spectrum of our in in-house standard sample S2007 (nominal 1.5–3 kHz.
10
Be/9Be = 30.87 10
12
). The measurement time was 145 s. The rate of 10B entering the detector was
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BeO with Nb powder produces higher output of negative ions from the TANDY ion source compared to mixing with Cu or Ag. Such an effect has been reported by many other labs working with different ion sources [26], it is, however, not observed with the high current source (HCS) on our 6 MV Tandem system. BeO is extracted from the ion source and Be1+ is selected on the high energy side. 10B is suppressed by several orders of magnitude by applying the degrader foil technique [27]. As degrader foil we use Si3N3.1 membranes of different size and thickness that can be inserted in front of the ESA. The detector consists of the ETH DE–Eres gas ionization detector, which has sufficient resolution to suppresses the residual boron in the ion beam [28,10,19]. Within a certain range, the 10B counting rate in the detector can be adjusted to the contamination level of the samples by varying the position and the width of the slits in the focal image plane of the ESA (Fig. 2). The maximal possible aperture is 30 30 mm2 according to an energy window of DE/E 2%. Tests have shown that the ETH gas ionization chamber can handle boron rates up to 4–5 kHz without any effect on the measured 10Be/9Be ratio. A total energy resolution of 18 keV for 10B ions at 701 keV incident energy allows full separation of the isobar from 10Be (Fig. 4).
The transmission of 10Be from the low energy side to the detector is determined by the transmission through both the gas stripper and the accelerator and by losses after passing the SiN degrader. At a terminal voltage of 525 kV maximum 9Be stripper transmission of 58% could be achieved for the 1+ charge state with typical values ranging between 50% and 55%. Beam losses when passing the SiN degrader foil (for a given thickness) depend on both, the energy and the angular straggling of the incident ion beam and on the stripping yield for the 2+ charge state, which is selected after the degrader foil [10]. Tests with different SiN foils indicate a yield of less than 55% for 10Be2+ at 745 keV. This is significantly lower than the values obtained for 9Be projectiles in thin carbon foils [29]. Further studies to determine the stripping yield from SiN and carbon foils are currently undertaken to explain the observed discrepancy. Based on theory both, energy and angular straggling of a beam, which is within the angular acceptance of the ESA, are compensated by the achromatic and angular focussing characteristics of the combined ESA-magnet spectrometer. Fig. 5 shows a scan run of the 130° magnet at a beam energy of 745 keV (after passing the 67 nm SiN degrader foil). In this experiment, a detector entrance window of 12 8 mm2 and was used. According to Sun et al. [30] the energy straggling for 10Be projectile passing the 67 nm SiN degrader at 745 keV is 7.7 keV FWHM corresponding to a spatial distribution of about 17 mm in the image plane of the ESA. The flat top demonstrates how the energy spread is refocused to a beam of about 3–4 mm width (FWHM). By confining the aperture in the image plane asymmetrically to an opening of 6 mm towards the low energy side and 15 mm towards the high energy side (values are given relative to the optical beam axis) a 10Be transmission through the foil into the detector of 20–22% was achieved, which is in agreement with theoretical estimates made for losses due to angular and energy straggling (Fig. 6) and due to the stripping yield in the SiN degrader foil. Based on these results we conclude that a total 10Be transmission (transport efficiency) of 10–13% is achievable with this configuration.
3.2. First routine operation for Fig. 5. Scan of the 130° bending magnet over a SiN entrance window with the dimensions 12 8 mm2.
10
Be
The first routine run of 10Be on the TANDY was carried out in December 2009. All measurements were performed with a SiN
Optical axis (701 keV)
10
2805
B
10
2.7 keV
Be
6.8 keV
Fig. 6. Calculated energy distribution transfered to spatial coordinates represented in the focal plane of the ESA. It is assumed that a beam of 745 keV energy passed a 67 nm SiN membrane (mean 10Be energy after passing: 701 keV). The widths of the distributions are based on energy loss straggling [30]. The shaded areas indicate the regions covered by slits. The energy acceptance (white area under the 10Be energy distribution) of this configuration of slits is about 78%.
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degrader foil of 75 nm nominal thickness and an area of 10 6 mm2. The TANDY operated continuously and largely unattended for three days and about 100 10Be samples (75 unknown samples from the Northgrip ice core, and about 25 blanks and standards) were measured (with about 60 h up time over 72 h). The results and the interpretation of the ice core samples are subject to another paper. Here we focus on the general performance of the upgraded TANDY during first routine operation (as summarized in Table 2). Typical 9BeO currents ranged between 2 and 4 lA and a stripper transmission in the 1+ charge state was larger than 50%. Few samples showed lower transmission, which probably was caused by contamination with other molecules of mass 25. Transmission through the degrader was about 20% resulting in an efficiency (e) of 10–11% during the first routine run, which is fully competitive to the large (6 MV) TANDEM AMS system at ETH [31]. The measured 10Be ratio of the in-house standard S2007 (Fig. 7) varied by only 1.4% over one sample wheel (mean error of a single measurement) while the final uncertainty of the 10Be concentration in the ice core samples typically was 2–3% (in most cases dominated by counting statistics). Thus, a very good time stability and high precision of the data was achieved. To estimate a background value for the new setup a low-level Be carrier from University of Hannover (provided by F.v. Blanckenburg) was analyzed. 10Be/9Be ratios for this carrier material reported by other AMS laboratories (SUERC, VERA, ASTER) range between 0.4 and 1.2 10 15) (1r uncertainty) [32]. The mean 10 Be/9Be ratio of our measurements is (1.0 ± 0.4) 10 15 (Fig. 8) clearly showing that no background at the 10 15 level is observed with the upgraded TANDY system. In particular, the 9Be background has been eliminated completely.
Table 2 Performance of the TANDY during first routine operation for 10Be. Nuclide
10
I (lA) Transmission (%) Transport efficiency e (%) 10 Be/9Be background
2–4 >50 10–11 <1 10
Be
15
Fig. 8. Low-level Be carrier measurements. The 1r-errors of the measurements are dominated by counting statistics. The average value (straight line) represents the error weighted mean of all measurements. The dashed lines represent the mean 1rerror for a single measurement.
4. Conclusion Several technical modifications of the compact low energy AMS system TANDY led to a significantly improved performance for all radionuclides. In this study we focused on 10Be, which is (besides radiocarbon) the second most important radionuclide in the Earth and Environmental Sciences. Modifications of the ion source and the vacuum system led to a higher and more stable output of negative ions and decreased the idle time during wheel changes. The extension of the high energy spectrometer with a new magnet and its design as an achromatic and stigmatic system in combination with the ESA led to an improved beam transport and effectively removed the background caused by scattered 9Be. The combination of all the modifications now enables us to measure 10 Be at a competitive level of performance compared to much larger AMS systems. The results from the first routine operation of 10 Be at 0.75 MeV demonstrate that samples covering a broad range from 10 11 (standard) to 10 15 (carrier) can be precisely measured with an average measurement time of about 50 min/sample. We conclude that the ETH low energy AMS system TANDY is now ready for routine operation of 10Be at negligible background and therefore is capable to significantly extend our capacity for 10Be measurements covering the full range of applications in the Earth and Environmental Sciences. Acknowledgement The authors want to acknowledge the technical staff of the Laboratory of Ion Beam Physics (in particular Joël Bourquin, Andreas Herrmann and René Gruber) for their dedication and commitment making all the technical modifications possible. F.v. Blanckenburg (GFZ-Potsdam, Germany) for providing the low level carrier, H. Weick (University of Giessen, Germany) for the GICOSY program, J. Beer (EAWAG, Switzerland) and H. Fischer (University of Berne, Switzerland) for providing access to the ice core samples and last but not least Silvia Lueck-Bollhalder (EAWAG, Switzerland) for sample prepation.
Fig. 7. Standard S2007 over 17 h (raw data). The 1r-error of the data is given by whichever is larger the error related with counting statistics or the standard deviation of the repeated (3–6 times) measurement of the same sample. The current weighted mean value for the whole run is displayed by the straight line. The dashed lines represent the mean 1r-error of a single measurement.
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