60Fe at the Cologne FN tandem accelerator

Nuclear Instruments and Methods in Physics Research B 361 (2015) 95–99

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Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb

A dedicated AMS setup for accelerator

53

Mn/60Fe at the Cologne FN tandem

M. Schiffer ⇑, A. Dewald, C. Feuerstein, R. Altenkirch, A. Stolz, S. Heinze University of Cologne, Institute for Nuclear Physics, Germany

a r t i c l e

i n f o

Article history: Received 5 December 2014 Received in revised form 9 February 2015 Accepted 10 February 2015 Available online 7 March 2015 Keywords: AMS Medium mass isotopes 53 Mn 60 Fe Isotopic ratios of (53Mn/55Mn)

a b s t r a c t Following demands for AMS measurements of medium mass isotopes, especially for 53Mn and 60Fe, we started to build a dedicated AMS setup at the Cologne FN tandem accelerator. This accelerator with a maximum terminal voltage of 10 MV can be reliably operated at a terminal voltage of 9.5 MV which corresponds to energies of 93–102 MeV for 60Fe or 53Mn beams using the 9þ or 10þ charge state. These charge states can be obtained by foil stripping with efficiencies of 30% and 20%, respectively. Energies around 100 MeV are sufficient to effectively suppress the stable isobars 60Ni and 53Cr by (dE/dx) techniques using combinations of energy degrader foils and dispersive elements like electrostatic analyzers and time of flight (TOF) systems as well as (dE/dx)E ion detectors. In this contribution we report on the actual status of the AMS setup and discuss details and expected features. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction CologneAMS, the center for accelerator mass spectrometry of the University of Cologne, uses a 6 MV AMS system from High Voltage Engineering Europe which is designed mainly for cosmogenic isotopes like 10Be, 14C, 26Al, 36Cl and 129I. In order to enlarge our spectrum of isotopic ratios towards medium mass nuclei we started to build an AMS setup at the Cologne 10 MV FN tandem accelerator. The very aim of this AMS setup is to measure isotopic ratios of (53Mn/55Mn). Even at energies around 100 MeV, the successful suppression of the isobar 53Cr poses an ambitious challenge. The (53Mn/55Mn) ratio in the order of 1012 are required by Geologists for their research of surface morphology and geochronology [1]. 2. The AMS-setup at the FN accelerator Fig. 1 shows a schematic overview of the planned Cologne facility for accelerator mass spectrometry with the FN tandem accelerator, including also beamlines used for nuclear physics experiments (see also Table 1). The injection system consists of a NEC MC-SNICS multicathode-sputter ion source followed by an achromatic mass spectrometer. It is planned to extract the manganese molecule MnF to suppress the isobaric chromium background in the ion ⇑ Corresponding author. http://dx.doi.org/10.1016/j.nimb.2015.02.034 0168-583X/Ó 2015 Elsevier B.V. All rights reserved.

source [2]. For iron we will extract the molecule FeO. The ion source has a spherical ionizer and a vacuum lock which allows a fast exchange of the sample wheel. The gate valve is equipped with a turbomolecular pump, located in close vicinity to the ionizer housing. The ion source is mounted on a sled to enable quick and easy source maintenance. The sled and all power supplies are placed on an extra insulated ion-source-platform mounted on top of an insulated injector-platform. The ion source is equipped with an einzellens, by which the beam is focused to the first pair of x-slits, positioned at the object point of the spherical 90° electrostatic analyzer which has a bending radius of 435 mm. A y-steerer is located in front of the electrostatic analyzer, which allows to correct for small up-down misalignments of the ion source. The electrostatic analyzer is followed by a double focusing second order corrected 90° magnet with a bending radius of 435 mm. Slit pairs as well as faraday cups are installed at positions which coincide with the waist points of the system. Assuming a 1 mm beam spot, the mass resolution of the spectrometer is m/Dm = 435. The experimentally measured mass resolution for a 107,108Ag beam yielded m/Dm = 320 with an open slit setting. The beam width measured with this setting was 1.13 mm. This width is determined by the emittance of the ion beam which was produced by the ion source operated with a conical ionizer. We expect an improved emittance with the upgrade to the spherical ionizer. Two xy-steerer pairs are located downstream of the magnet. The injector is placed on the injector-platform which can be applied to a potential of 60 kV. The ion beam is accelerated by this

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Fig. 1. Floor plan for the first basement of the accelerator building in the Institute of Nuclear Physics Cologne with the new 10 MV AMS system and the beamlines for nuclear physics experiments.

Table 1 Technical details of the set-up. Injector Ion source Electrostatical analyzer

Pre-acceleration Cooling Remote control

NEC MC-SNICS, 40 samples, spherical ionizer Spherical, bending angle 90°, radius 435 mm, plate separation 43.5 mm, plate height 160 mm Double focusing, second order corrrected, bending angle 90°, radius 435 mm, gap 60 mm Potential difference 60 kV, tube length 385 mm Syltherme XLT for ion source, water for magnet and pumps SIMATIC S7-200, off-grid Ethernet network with fiber optics

Tandem accelerator Terminal voltage Stripper

Max. 10 MV, pelletron charging, corona stabilized with slit error feedback or GVM control Carbon foil, 180 foils at terminal

Bouncing magnet

HE set-up Analyzing magnet Electrostatic analyzer Time of flight system Ionization detector

Double focusing, bending angle 90°, radius 1100 mm, gap 25 mm, mass-energy product 124 amu MeV Single focusing, bending angle 30°, radius 3500 mm, plate separation 30 mm, plate height 175 mm Reconfigurable structure, active area 165 mm2 and 1385 mm2, timing resolution 0.45 ns, 4 m flight path with quadrupol doublet lens Four anode gas ionization detector

voltage in addition to the extraction voltage of 20 kV and sputter voltage of 10 kV. The injector is followed by the beam transport system of the FN tandem accelerator. It consists of an electrostatic quadrupol triplet lens, two einzellenses and a gridded lens at the entrance of the accelerator. The Cologne FN tandem accelerator is equipped with a pelletron charging system and can be operated at a maximum terminal voltage of 10 MV. The terminal voltage is corona stabilized via slit current feedback or GVM controlled. For stripping carbon foils are used. 180 foils can be mounted in a carousel positioned in the

terminal stripper box and are mechanically controlled via plexiglass rods. The FN tandem accelerator can routinely be operated at a terminal voltage of 9.5 MV. For 53Mn and 60Fe by using the 9þ or 10þ charge state, energies of 93–102 MeV can be obtained, regarding the injected molecules MnF and FeO. Expected charge states yields are in the order of 30% and 20%, respectively. The FN tandem accelerator is followed by a magnetic quadrupol doublet lens which focuses the ion beam to the object point of the 90° AMS analyzer magnet II. The beam has to pass a first 90° analyzer magnet I, which can send the beam via a switching

M. Schiffer et al. / Nuclear Instruments and Methods in Physics Research B 361 (2015) 95–99

magnet to beamlines used for nuclear physics experiments, see Fig. 1. The AMS analyzer magnet II is double focusing with a radius of 1100 mm. It has a mass-energy product of 124 amu MeV and we will measure the magnetic field with an NMR gaussmeter. At the image plane of the magnet II two movable precision off-set faraday cups with magnetic secondary electron suppression are installed. They can be positioned to angles up to 4°. These faraday cups are equipped with split apertures to generate an error signal for the corona feedback system from the stable beams. On the central beam axis a retractable faraday cup with combined electrostatic and magnetic secondary electron suppression is placed. The system is followed by a quadrupol triplet lens to focus the beam to the object point of the electrostatic analyzer. At this position, the first Si3N4 foil can be inserted with a linear actuator into the beam at the position of the waist point. The single focusing 30° electrostatic analyzer has a bending radius of 3500 mm which results in an energy dispersion of 0.46 mm/% . With this system a first isobar suppression can be realized. Such systems are used since 1980 [3] and are a well working method for the isobar suppression at smaller machines [4,5]. At the image point of the electrostatic analyzer a second Si3N4 foil can be inserted into the beam. It is directly followed by the first detector of the time of flight system (TOF). This detector unit is similar to version (B) from [6], with a perpendicular carbon foil and an electrostatic mirror mounted under 45° with respect to the beam axis. The mirror deflects the secondary electrons generated at the first carbon foil by 90° towards the horizontally mounted microchannel plate (MCP) followed by an anode, where the timing signal is produced. This geometry results in a good time resolution and a large active area, which is defined by the F465513 MCP of Hamamatsu with a radius of 7.25 mm. The stop signal is provided by a second detector unit which is placed 4 m downstream. After 1.2 m flight path of the TOF system a quadrupol doublet lens with an aperture of 60 mm is located to refocus the beam to the stop detector [7]. As stop detector of the TOF system we plan to use either a detector identical to the start detector or one with a large MCP in order to increase the efficiency. For this purpose a F9892-14 MCP with a radius of 21 mm of Hamamatsu is available. First tests using the small F4655-13 MCP at the existing TOF system of CologneAMS showed an improved time resolution of 0.45 ns which is considerably better than 0.72 ns which was

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measured with the old MCP, see Fig. 2. Similar setups with degrader foil and TOF are used at VERA and Munich for the isobar suppression [8,9]. Downstream the TOF system we will use a gas ionization detector which enables a third isobar suppression. A dedicated gas ionization detector with optimized anode lengths is under construction. For the data-acquisition we will use first an existing MPA-3 system. It is planned to replace it by a digital data-acquisition with a high data throughput. 3. The isobar suppression For the design of the AMS facility it was important to estimate the expected isobar suppression e.g. for 53Mn, using the (dE/dx) method with thin Si3N4 foils. In order to get realistic results we measured first the energy loss and energyloss straggling with the TOF system at the Cologne 6 MV AMS system [6]. We measured also the stoichiometry and density of the Si3N4 foils, which were used as energy degrader foils, by Rutherford backscattering. The layer measurements were performed at the University of Bochum. The stoichiometry is Si3N3.75 and the density is 2.7 g/cm3. We compared the measured energy loss with calculations performed with the computer codes SRIM [10] and LISE++ [11]. LISE++ uses the formalism from Hubert et al. [12] which describes the energy loss well enough. The comparison of the measured energyloss straggling with the Yang-straggling [13] shows reasonable agreements. Finally the measured values were used to calculate the expected isobar suppression with stacked degrader foils in combination with energy sensitive elements. As an example the results for 53Mn and 60 Fe are presented in Table 2. For 53Mn and 60Fe we assumed the thickness of the first Si3N4 foil to be 3 lm and 2 lm, respectively. In both cases we assumed the thickness of the second Si3N4 foil to be 3 lm. For both isotopes a total suppression of 108 is required to reach a sensitivity of 1014 assuming a chemical suppression of 106 during the sample preparation. Comparable sensitivity for (53Mn/55Mn) ratio measurements of 7  1015 was achieved at Munich [2] and of 1014 at CIAE-AMS [14]. 4. The ion optics calculation for the high energy mass spectrometer The ion optics calculation for the new AMS system at the Cologne FN accelerator was performed with LIMIOPTIC II. The result is shown in Fig. 3. The code uses the matrix formalism and allows to modify input parameters conveniently via slide bars. The results of the input variations are displayed simultaneously which simplifies the investigation of specific ion optical features of complex systems significantly [7]. The actual calculation starts with an ion beam emittance of 4 mm pffiffiffiffiffiffiffiffiffiffiffi mrad MeV. We calculated the beam profile as well as the individual ion paths. We have also calculated the transmission through the first degrader foil, the electrostatic analyzer, the second degrader foil and the TOF system, taking into account the energy loss, energyloss straggling and angular straggling. One example for 53 Mn and 53Cr is shown in Fig. 4. The calculated transmissions for 53 Mn and 60Fe are presented in Table 2, values given in parentheses. 5. Beam sequencing system

Fig. 2. Comparison of the timing resolution for the TOF system at the Cologne 6 MV AMS system with different microchannel plates, separated by a delay for better presentation. Measured with a 14C beam.

The beam sequencing system applies a pulsed voltage to the vacuum chamber of the 90° injector magnet to measure the different isotopes sequentially. It is based on preset voltages of power supplies, switched by solid state switches operating under

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M. Schiffer et al. / Nuclear Instruments and Methods in Physics Research B 361 (2015) 95–99

Table 2 Isobar suppression of the isobaric interference of 53Mn and 60Fe. Corresponding transmissions are given in the parentheses. The first suppression is calculated for 53Mn/53Cr with a 3 lm Si3N4 foil and for 60Fe/60Ni with a 2 lm Si3N4 foil. The second suppression is calculated for both isotopes with a 3 lm Si3N4 foil. Isotope

1. suppr.:

2. suppr.:

charge state

(dE/dx)+ESA

(dE/dx)

53

Mn/53Cr

(30%)

104 (23%)

103 (42%)

60

Fe/60Ni

(29%)

2

3

10  10 (32%)

2

3

10  10 (96%)

3. suppr.: TOF

Gas det.

(60%)

10  102

(60%)

4

10

Total

108  109 (1.7%) 108  1010 (5.3%)

Fig. 3. Ion optics calculation of the new 10 MV AMS system for (a) the injector and the FN accelerator and (b) the high energy mass spectrometer. Shaded areas show spatial limitations.

microprocessor control [15]. The block diagram is shown in Fig. 5. This system is now controlled by an ARDUINO UNO microprocessor. The high voltage switching box placed on the insulated injector-platform is fiber optic controlled. Three high voltage power supplies are separately controlled. Each has a maximum voltage of 5 kV and the output polarity can be changed. Pulse rise and fall times are both 3 ls and are limited by the solid state switches. The timing control signal is generated by the microprocessor. The pulse length can be selected from 20 ls up to 1 for tuning purposes. To measure the pulsed stable beam component, we are testing a new pulsed current integrator with a low entrance resistance and a high gain, provided by the operational amplifier OPA 129. The current integrator is equipped with a driver stage, a balance stage and a buffer stage. A current integrator will be available for each input range. We expect a noise and ripple smaller than 10 pA and drift less than 5 pA. First tests were performed with satisfying results.

6. Computer control For the precision of an AMS measurement the tuning of the ion source and the beam transport system is crucial. The control of many parameters has to be done simultaneously. We decided to use a programmable logic controller which is controlled by a LabVIEW program. The system was found to work reliably. The system can be easily extended. Since the complete injector system is distributed over 3 different voltage levels, ground potential, injector platform (60 kV) and ion source platform (80 kV), see Fig. 5. It is necessary to have controllers on each potential level, centrally controlled from the control room via fiber optic links. For the communication we use an off-grid Ethernet network with fiber optic media converters. The programmable logic controller is a SIMATIC S7-200 with analog input and output modules, digital input and output modules,

M. Schiffer et al. / Nuclear Instruments and Methods in Physics Research B 361 (2015) 95–99

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Fig. 4. Ion optics calculation for the suppression of the isobaric interference of 53Mn with two 3 lm Si3N4 foils. Shown is the transmission through the electrostatic analyzer, the quadrupol doublet lens to the x-slits, where the second Si3N4 foil can be inserted p into the the ffiffiffiffiffiffiffiffiffiffi ffi beam axis and through pffiffiffiffiffiffiffiffiffiffi ffi quadrupol doublet lens to the x-slits in front of the gas ionization detector. The beam parameters are: E = 100 MeV, Ex= 3.2 p mm mrad MeV, Ey = 5.0 p mm mrad MeV and it is gaussian distributed.

The electrostatic analyzer and the fast TOF system will be available in the first half of 2015. New fast MCP’s for the TOF system are in a test phase. First AMS measurements with the 10 MV FN accelerator system are expected in 2016. Acknowledgement Supported partially by the Emerging Groups program of the University of Cologne. References

Fig. 5. Block diagram of the computer control and beam sequencing system.

temperature readout modules and Ethernet modules. The programmable logic controller and the LabVIEW program are connected over an OPC server. This system has been operated for nearly two years without problems. The current readout of the faraday cups on the insulated platform is done by a KEITHLEY picoamperemeter via the Ethernet network. The faraday cup selection is done by several relays, connecting the faraday cup to the picoamperemeter. Faraday cups not in use are grounded and the suppression voltage is switched off, to avoid interactions with the ion beam. 7. Summary A dedicated AMS facility for medium mass isotopes spectrometry, like 32Si, 41Ca, 53Mn and 60Fe, has been designed and is under construction. The injector as well as a computer control system are in operation since 2013. The high energy 90° analyzing magnet II and a multi offset faraday cup chamber are in position.

[1] L. Gladkis, L. Fifield, C. Morton, T. Barrows, S. Tims, Nucl. Instr. Meth. B 259 (2007) 236–240. [2] M. Poutivtsev, I. Dillmann, T. Faestermann, K. Krnie, G. Korschinek, J. Lachner, A. Meier, G. Rugel, A. Wallner, Nucl. Instr. Meth. B 268 (2010) 756–758. [3] W. Kutschera, W. Henning, M. Paul, R.K. Smither, E.J. Stephenson, J.L. Yntema, D.E. Alburger, J.B. Cumming, G. Harbottle, Phys. Rev. Lett. 45 (1980) 592–596. [4] M.G. Klein, A. Gottdang, D.J.W. Mous, D.L. Bourlès, M. Arnold, B. Hamelin, G. Aumaıˆtre, R. Braucher, S. Merchel, F. Chauvet, Nucl. Instr. Meth. B 266 (2008) 1828–1832. [5] A.M. Müller, M. Christl, J. Lachner, M. Suter, H.-A. Synal, Nucl. Instr. Meth. B 268 (2010) 2801–2807. [6] G. Pascovici, A. Dewald, S. Heinze, L. Fink, C. Müller-Gatermann, M. Schiffer, C. Feuerstein, M. Pfeiffer, J. Jolie, S. Thiel, K.O. Zell, O. Arnopolina, F. von Blanckenburg, Nucl. Instr. Meth. B 294 (2013) 410–415. [7] A. Dewald, S. Heinze, C. Feuerstein, C. Müller-Gatermann, A. Stolz, M. Schiffer, G. Zitzer, T. Dunai, J. Rethemeyer, M. Melles, H. Wiesel, F. von Blanckenburg, EPJ Web Conf. 63 (2013) 03006. [8] C. Vockenhuber, A. Bergmaier, T. Faestermann, K. Knie, G. Korschinek, W. Kutschera, G. Rugel, P. Steiner, K. Vorderwinkler, A. Wallner, Nucl. Instr. Meth. B 240 (2005) 490–494. [9] C. Vockenhuber, A. Bergmaier, T. Faestermann, K. Knie, G. Korschinek, W. Kutschera, G. Rugel, P. Steiner, K. Vorderwinkler, A. Wallner, Nucl. Instr. Meth. B 259 (2007) 250–255. [10] J.F. Ziegler, M.D. Ziegler, J.P. Biersack, Nucl. Instr. Meth. B 268 (2010) 1818– 1823. [11] O.B. Tarasov, D. Bazin, Nucl. Instr. Meth. B 266 (2008) 4657–4664. [12] F. Hubert, R. Bimbot, H. Gauvin, At. Data Nucl. Data Tables 46 (1990) 1–213. [13] Q. Yang, D.J. O’Connor, Z. Wang, Nucl. Instr. Meth. B 61 (1991) 149–155. [14] D. Kejun, H. Hao, W. Xianggao, L. Chaoli, H. Ming, L. Zhenyu, W. Shaoyong, L. Jiancheng, Z. Guowen, L. Heng, C. Zhiganh, L. Guangshan, Y. Jian, J. Shan, Nucl. Instr. Meth. B 285 (2012) 57–60. [15] K. van der Borg, C. Alderliesten, A.F.M. de Jong, A. van den Brink, A.P. de Haas, H.J.H. Kersemaekers, J.E.M.J. Raaymakers, Nucl. Instr. Meth. B 123 (1997) 97– 101.