- Email: [email protected]
MAFFTRAP: ion trap system for MAFF
Nuclear Instruments and Methods in Physics Research B 204 (2003) 512–516 www.elsevier.com/locate/nimb
MAFFTRAP: ion trap system for MAFF J. Szerypo *, D. Habs, S. Heinz, J. Neumayr, P. Thirolf, A. Wilfart, F. Voit Sektion Physik, LMU M€unchen, Am Coulombwall 1, D-85748 Garching, Germany Maier-Leibnitz Laboratory, D-85748 Garching, Germany
Abstract At the future MAFF/FRM-II facility in Munich an ion trap system MAFFTRAP will be built. Its goal is to investigate the neutron-rich, in particular heavy, nuclei from thermal neutron fission and fusion of fission products with heavy target nuclei. Basic aims of these investigations are: radioactive beam cooling and purification for nuclear spectroscopy experiments, precise nuclear mass measurements and charged particle spectroscopy ‘‘in-trap’’. 2002 Elsevier Science B.V. All rights reserved. PACS: 39.10.þj; 21.10.Dr; 07.75.þh Keywords: Penning trap; Radioactive ion beam; Ion cooling; Mass measurements; High-resolution spectroscopy
1. Introduction The physics with accelerated radioactive ion beams is an important new field in nuclear physics. For the production of neutron-rich isotopes nuclear fission is a very suitable method due to the large fission cross-sections of thermal neutrons and the high thermal neutron flux in modern research reactors. At the new research reactor FRM-II in Munich the dedicated facility Munich Accelerator for Fission Fragments (MAFF) [1] is foreseen to produce, cool and accelerate high-intensity and high-quality neutron-rich radioactive beams (Fig. 1). In order to achieve highest quality rare beams an emittance
* Corresponding author. Tel.: +49-89-289-14089; fax: +4989-289-14072. E-mail address: [email protected] (J. Szerypo).
cooling and bunching of the mass-separated fission fragments is performed before they are accelerated and sent to the target station. These beams with energies up to about 6 MeV/U will allow for investigations in several exciting fields like the production and study of heavy and superheavy elements or nuclear structure and astrophysics research. The experimental activities at MAFF will focus around nuclear spectroscopy studies (including nuclear mass measurements), atomic physics and chemistry (Fig. 2).
2. MAFFTRAP – main goals One of main experimental devices at the MAFF facility will be the ion trap system MAFFTRAP. Principal technical tasks of this system are: • decelerate, cool, bunch and purify the radioactive beam;
0168-583X/02/$ - see front matter 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-583X(02)02123-7
J. Szerypo et al. / Nucl. Instr. and Meth. in Phys. Res. B 204 (2003) 512–516
513
Fig. 1. MAFF layout.
• perform precise nuclear mass, in-trap spectroscopy and laser spectroscopy measurements.
Fig. 2. MAFF diagram. Initially MAFFTRAP will be positioned at MAFF primary beam, phase 1. When all MAFF components will be ready, MAFFTRAP will be shifted to its final position, phase 2.
A very similar ion trap project SHIPTRAP [2] has been developed at GSI Darmstadt and is now in a testing phase. LMU Munich is one of main members of the SHIPTRAP collaboration, being responsible for a beam stopping (in a buffer gas cell) and extraction (RFQ ion guide). This experience will be directly applicable for a development and construction of MAFFTRAP. The MAFFTRAP technical possibilities mentioned above will allow one to reach the following scientific goals: • deliver high-quality radioactive beam for nuclear and laser spectroscopy studies;
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J. Szerypo et al. / Nucl. Instr. and Meth. in Phys. Res. B 204 (2003) 512–516
• high-precision (107 ) mass measurements; • in-trap spectroscopy: atomic and conversion electron, a- and recoil spectroscopy aiming at identification of heavy elements. High quality beam will be obtained in a process consisting of: beam deceleration, cooling, bunching and purification with the use of RF traps and a Penning trap, filled with a buffer gas. High-precision mass measurements will be performed in the second (precision) Penning trap, operated under high-vacuum conditions. This technique is well established and allows achieving high measurement accuracy, of the order of 107 [3]. The measurements can be performed on nuclides delivered by both the primary and secondary beam (e.g. fusion reaction products), in MAFF phases 1 and 2, respectively (see Section 3). Main goal is to measure masses of trans-uranium and superheavy elements, on the way to a predicted superheavy shell at Z ¼ 114 and N ¼ 184. Since an experimental binding energy is key information on the stability in that region, mass measurements are of particular importance. A novel concept of the nuclear spectroscopy in a Penning trap interior (in-trap) was already tested experimentally at the REXTRAP Penning trap at ISOLDE/CERN [4]. In this technique, detectors of a needed type are placed directly inside the trap and the radioactive sample is positioned in front of them. Such a scheme has advantages over conventional spectroscopy: • very thin, small size radioactive source (no backscattering, energy degradation, etc.); • virtual suppression, in a given detector, of background coming from other types of radiation; • passive shielding with the magnet cryostat. In the heavy element region accessible with MAFF, main de-excitation channel is a-decay. Outgoing a particle may knock out a K-shell electron, which, in turn, may initiate a series of Auger and Coster–Kronig electron transitions. The corresponding electron lines are a signature of a given element. Therefore, in-trap measurements of atomic electrons in coincidence with a-particle and recoil nucleus may identify Z-number of the
latter (A-number is determined earlier, in a purification Penning trap). In addition, conversion electron lines following a de-excitation the recoil nuclear levels can be studied.
3. MAFFTRAP – technical solution In the first phase of MAFF (phase 1, Fig. 2), when only a primary, low energy (30 keV) radioactive beam will be available, an electrostatic beam deceleration will be applied. This can be done by placing the trap system at the high-voltage (30 kV) platform, as in case of the similar ion trap project JYFLTRAP [5]. Subsequently, decelerated (to about 100 eV) beam will be injected into an RF funnel (also known as RF ion guide [6,7]), which is a novel approach for an efficient cooling of highintensity radioactive beams. This beam must undergo bunching, in order to have high efficiency of injection into Penning trap. For bunching, a recently developed technique (see e.g. [8,9]) of an RFQ cooler/buncher will be used. Finally, the ion bunches will be injected into a cylindrical Penning trap (placed in a superconducting magnet) with buffer gas [10]. There, due to a mass-selective buffer gas cooling technique [11], a very high mass resolving power (R ¼ M=DM 6 105 ) will be reached [10]. This will allow rejecting even isobaric contaminants and ejecting a clean, mono-isotopic, high quality beam for further studies. At the same time, mass number determination of the ejected ions is achieved. That high resolving power is necessary in order to reach most exotic species by eliminating huge background coming from nuclides other than those of interest, e.g. members of the isobaric chain produced in fission. In the final phase of MAFF (phase 2, Fig. 2), a secondary high energy (<6 MeV/u) radioactive beam coming out of the MORRIS recoil separator will be a subject of studies. In this phase, since electrostatic deceleration is impossible at these energies, for the beam deceleration a gas stopping chamber technique will be used. Corresponding MAFFTRAP schemes are shown in Fig. 3. Both purification and mass measurement Penning traps will be positioned in the same superconducting magnet, which facilitates the ion
J. Szerypo et al. / Nucl. Instr. and Meth. in Phys. Res. B 204 (2003) 512–516
515
The mass measurement trap can be interchanged with the in-trap spectroscopy trap where relevant particle detectors will be placed. In collaboration within the SHIPTRAP project [2], mentioned before, a prototype of the gas stopping chamber (ion injection transversal to the
Fig. 3. MAFFTRAP diagram (gray rectangles show trap system components).
transfer between the traps. Basic trap system parameters are as follows: • superconducting magnet B ¼ 7 T, d ¼ 155 cm bore, two homogeneous places of 106 and 107 homogeneity in 1 cm3 , field drift 108 /h; • additional coil for a compensation of perturbations from external magnetic fields [12]; • two cylindrical Penning traps [10] (d ¼ 32 mm diameter), separated by a vacuum barrier, placed in one superconducting magnet, with pressures of 103 –104 and 106 mbar, respectively (Fig. 4).
Fig. 5. MAFFTRAP gas stopping chamber (example of SHIPTRAP [2]). Beam comes via a thin (2–3 lg/cm2 ) entrance window (e.g. Ni) separating buffer gas (usually He) from vacuum and is stopped by collisions with gas atoms. Backside electrode makes confinement for stopped ions, which are extracted from the cell via a combination of RF-funnel and RFQ ion guides.
Fig. 4. MAFFTRAP Penning trap system (example of JYFLTRAP [5]). MCP ¼ microchannel plate detector. Vacuum barrier means a tight disc between the vacuum tube and the central trap electrode, separating two different pressure regions.
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J. Szerypo et al. / Nucl. Instr. and Meth. in Phys. Res. B 204 (2003) 512–516
ejection beamline axis, Fig. 5) has been developed at LMU. A second version of the gas stopping chamber (axial ion injection) is now being set-up at the LMU tandem accelerator, as a first part of MAFFTRAP. It is planned to make MAFFTRAP tests and experiments there, both with stable and radioactive beam, before positioning it finally at MAFF.
[3]
Acknowledgements [4]
This work, as well as this conferenceÕs travel costs of one of co-authors (J.S.), were supported by NIPNET project under contract no. HPRI-CT2001-50034.
[5]
[6]
References [1] D. Habs, T.V. Egidy, M. Gross, O. Kester, P. Kienle, U. Koester, J.-A. Pinston, P.G.Thirolf (Eds.), MAFF – Physics Case and Technical Description, October 1998, http:// www.ha.physik.uni-muenchen.de/maff/; D. Habs, M. Gross, W. Assman, F. Ames, H. Bongers, S. Emhofer, S. Heinz, S. Henry, O. Kester, J. Neumayr, F. Ospald, P. Reiter, T. Sieber, J. Szerypo, P.G. Thirolf, V. Varentsov, T. Wilfart, T. Faestermann, R. Kr€ ucken, Nucl. Instr. and Meth. B, these Proceedings. doi:10.1016/S0168583X(02)00496-8. [2] G. Marx, D. Ackermann, J. Dilling, F.P. Hessberger, S. Hoffman, H.-J. Kluge, R. Mann, G. M€ unzenberg, Z. Qamhieh, W. Quint, D. Rodriguez, M. Sch€adel, J. Sch€ onfelder, G. Sikler, C. Toader, C. Weber, O. Engels, D. Habs, P. Thirolf, H. Backe, A. Dretzke, W. Lauth, W. Ludolphs, M. Sewtz, Hyperfine Int. 132 (2001) 463;
[7] [8]
[9]
[10]
[11]
[12]
G. Sikler, D. Ackermann, F. Attallah, D. Beck, J. Dilling, A.F. Dodonov, S.A. Elisseev, H. Geissel, D. Habs, S. Heinz, F. Herfurth, F. Heßberger, S. Hofmann, H.-J. Kluge, Ch. Kozhuharov, G. Marx, M. Mukherjee, J. Neumayr, Yu.N. Novikov, W.R. Plaß, W. Quint, S. Rahaman, D. Rodriguez, C. Scheidenberger, M. Tarisien, P. Thirolf, V. Varentsov, Z. Wang, C. Weber, M. Weidenm€ uller, Z. Zhou, Nucl. Instr. and Meth. B, these Proceedings. doi:10.1016/S0168-583X(02)02116-X. G. Bollen, S. Becker, H.-J. Kluge, M. K€ onig, R.B. Moore, T. Otto, H. Raimbault-Hartmann, G. Savard, L. Schweikhard, H. Stolzenberg, Nucl. Instr. and Meth. A 368 (1996) 675. € yst€ L. Weissman, F. Ames, J. A o, O. Forstner, K. Reisinger, S. Rinta-Antila, Nucl. Instr. and Meth. A 492 (2002) 451. J. Szerypo, A. Jokinen, V.S. Kolhinen, A. Nieminen, € yst€ S. Rinta-Antila, J. A o, Acta Phys. Pol. B 32 (2001) 985. M. Wada, I. Katayama, S. Okada, Y. Ishida, T. Nakamura, Y. Nakai, T.M. Kojima, Y. Kanai, T. Kambara, Y. Yamazaki, K. Okada, K. Noda, H. Kawakami, S. Ohtani, RIKEN Rev. 31 (2000) 56. V.L. Varentsov, D. Habs, Nucl. Instr. and Meth. A 490 (2002) 16. € yst€ A. Nieminen, J. Huikari, A. Jokinen, J. A o, P. Campbell, E.C.A. Cochrane, Nucl. Instr. and Meth. 469 (2001) 244. F. Herfurth, A. Kellerbauer, J. Dilling, G. Bollen, S. Henry, H.-J. Kluge, E. Lamour, D. Lunney, R.B. Moore, C. Scheidenberger, S. Schwarz, G. Sikler, J. Szerypo, Nucl. Instr. and Meth. A 469 (2001) 254. H. Raimbault-Hartmann, D. Beck, G. Bollen, M. K€ onig, H.-J. Kluge, E. Schark, J. Stein, S. Schwarz, J. Szerypo, Nucl. Instr. and Meth. B 126 (1997) 378. G. Savard, S. Becker, G. Bollen, H.-J. Kluge, R.B. Moore, T. Otto, L. Schweikhard, H. Stolzenberg, U. Wiess, Phys. Lett. A 158 (1991) 247. G. Gabrielse, J. Tan, J. Appl. Phys. 63 (1988) 5143.
MAFFTRAP: ion trap system for MAFF J. Szerypo *, D. Habs, S. Heinz, J. Neumayr, P. Thirolf, A. Wilfart, F. Voit Sektion Physik, LMU M€unchen, Am Coulombwall 1, D-85748 Garching, Germany Maier-Leibnitz Laboratory, D-85748 Garching, Germany
Abstract At the future MAFF/FRM-II facility in Munich an ion trap system MAFFTRAP will be built. Its goal is to investigate the neutron-rich, in particular heavy, nuclei from thermal neutron fission and fusion of fission products with heavy target nuclei. Basic aims of these investigations are: radioactive beam cooling and purification for nuclear spectroscopy experiments, precise nuclear mass measurements and charged particle spectroscopy ‘‘in-trap’’. 2002 Elsevier Science B.V. All rights reserved. PACS: 39.10.þj; 21.10.Dr; 07.75.þh Keywords: Penning trap; Radioactive ion beam; Ion cooling; Mass measurements; High-resolution spectroscopy
1. Introduction The physics with accelerated radioactive ion beams is an important new field in nuclear physics. For the production of neutron-rich isotopes nuclear fission is a very suitable method due to the large fission cross-sections of thermal neutrons and the high thermal neutron flux in modern research reactors. At the new research reactor FRM-II in Munich the dedicated facility Munich Accelerator for Fission Fragments (MAFF) [1] is foreseen to produce, cool and accelerate high-intensity and high-quality neutron-rich radioactive beams (Fig. 1). In order to achieve highest quality rare beams an emittance
* Corresponding author. Tel.: +49-89-289-14089; fax: +4989-289-14072. E-mail address: [email protected] (J. Szerypo).
cooling and bunching of the mass-separated fission fragments is performed before they are accelerated and sent to the target station. These beams with energies up to about 6 MeV/U will allow for investigations in several exciting fields like the production and study of heavy and superheavy elements or nuclear structure and astrophysics research. The experimental activities at MAFF will focus around nuclear spectroscopy studies (including nuclear mass measurements), atomic physics and chemistry (Fig. 2).
2. MAFFTRAP – main goals One of main experimental devices at the MAFF facility will be the ion trap system MAFFTRAP. Principal technical tasks of this system are: • decelerate, cool, bunch and purify the radioactive beam;
0168-583X/02/$ - see front matter 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-583X(02)02123-7
J. Szerypo et al. / Nucl. Instr. and Meth. in Phys. Res. B 204 (2003) 512–516
513
Fig. 1. MAFF layout.
• perform precise nuclear mass, in-trap spectroscopy and laser spectroscopy measurements.
Fig. 2. MAFF diagram. Initially MAFFTRAP will be positioned at MAFF primary beam, phase 1. When all MAFF components will be ready, MAFFTRAP will be shifted to its final position, phase 2.
A very similar ion trap project SHIPTRAP [2] has been developed at GSI Darmstadt and is now in a testing phase. LMU Munich is one of main members of the SHIPTRAP collaboration, being responsible for a beam stopping (in a buffer gas cell) and extraction (RFQ ion guide). This experience will be directly applicable for a development and construction of MAFFTRAP. The MAFFTRAP technical possibilities mentioned above will allow one to reach the following scientific goals: • deliver high-quality radioactive beam for nuclear and laser spectroscopy studies;
514
J. Szerypo et al. / Nucl. Instr. and Meth. in Phys. Res. B 204 (2003) 512–516
• high-precision (107 ) mass measurements; • in-trap spectroscopy: atomic and conversion electron, a- and recoil spectroscopy aiming at identification of heavy elements. High quality beam will be obtained in a process consisting of: beam deceleration, cooling, bunching and purification with the use of RF traps and a Penning trap, filled with a buffer gas. High-precision mass measurements will be performed in the second (precision) Penning trap, operated under high-vacuum conditions. This technique is well established and allows achieving high measurement accuracy, of the order of 107 [3]. The measurements can be performed on nuclides delivered by both the primary and secondary beam (e.g. fusion reaction products), in MAFF phases 1 and 2, respectively (see Section 3). Main goal is to measure masses of trans-uranium and superheavy elements, on the way to a predicted superheavy shell at Z ¼ 114 and N ¼ 184. Since an experimental binding energy is key information on the stability in that region, mass measurements are of particular importance. A novel concept of the nuclear spectroscopy in a Penning trap interior (in-trap) was already tested experimentally at the REXTRAP Penning trap at ISOLDE/CERN [4]. In this technique, detectors of a needed type are placed directly inside the trap and the radioactive sample is positioned in front of them. Such a scheme has advantages over conventional spectroscopy: • very thin, small size radioactive source (no backscattering, energy degradation, etc.); • virtual suppression, in a given detector, of background coming from other types of radiation; • passive shielding with the magnet cryostat. In the heavy element region accessible with MAFF, main de-excitation channel is a-decay. Outgoing a particle may knock out a K-shell electron, which, in turn, may initiate a series of Auger and Coster–Kronig electron transitions. The corresponding electron lines are a signature of a given element. Therefore, in-trap measurements of atomic electrons in coincidence with a-particle and recoil nucleus may identify Z-number of the
latter (A-number is determined earlier, in a purification Penning trap). In addition, conversion electron lines following a de-excitation the recoil nuclear levels can be studied.
3. MAFFTRAP – technical solution In the first phase of MAFF (phase 1, Fig. 2), when only a primary, low energy (30 keV) radioactive beam will be available, an electrostatic beam deceleration will be applied. This can be done by placing the trap system at the high-voltage (30 kV) platform, as in case of the similar ion trap project JYFLTRAP [5]. Subsequently, decelerated (to about 100 eV) beam will be injected into an RF funnel (also known as RF ion guide [6,7]), which is a novel approach for an efficient cooling of highintensity radioactive beams. This beam must undergo bunching, in order to have high efficiency of injection into Penning trap. For bunching, a recently developed technique (see e.g. [8,9]) of an RFQ cooler/buncher will be used. Finally, the ion bunches will be injected into a cylindrical Penning trap (placed in a superconducting magnet) with buffer gas [10]. There, due to a mass-selective buffer gas cooling technique [11], a very high mass resolving power (R ¼ M=DM 6 105 ) will be reached [10]. This will allow rejecting even isobaric contaminants and ejecting a clean, mono-isotopic, high quality beam for further studies. At the same time, mass number determination of the ejected ions is achieved. That high resolving power is necessary in order to reach most exotic species by eliminating huge background coming from nuclides other than those of interest, e.g. members of the isobaric chain produced in fission. In the final phase of MAFF (phase 2, Fig. 2), a secondary high energy (<6 MeV/u) radioactive beam coming out of the MORRIS recoil separator will be a subject of studies. In this phase, since electrostatic deceleration is impossible at these energies, for the beam deceleration a gas stopping chamber technique will be used. Corresponding MAFFTRAP schemes are shown in Fig. 3. Both purification and mass measurement Penning traps will be positioned in the same superconducting magnet, which facilitates the ion
J. Szerypo et al. / Nucl. Instr. and Meth. in Phys. Res. B 204 (2003) 512–516
515
The mass measurement trap can be interchanged with the in-trap spectroscopy trap where relevant particle detectors will be placed. In collaboration within the SHIPTRAP project [2], mentioned before, a prototype of the gas stopping chamber (ion injection transversal to the
Fig. 3. MAFFTRAP diagram (gray rectangles show trap system components).
transfer between the traps. Basic trap system parameters are as follows: • superconducting magnet B ¼ 7 T, d ¼ 155 cm bore, two homogeneous places of 106 and 107 homogeneity in 1 cm3 , field drift 108 /h; • additional coil for a compensation of perturbations from external magnetic fields [12]; • two cylindrical Penning traps [10] (d ¼ 32 mm diameter), separated by a vacuum barrier, placed in one superconducting magnet, with pressures of 103 –104 and 106 mbar, respectively (Fig. 4).
Fig. 5. MAFFTRAP gas stopping chamber (example of SHIPTRAP [2]). Beam comes via a thin (2–3 lg/cm2 ) entrance window (e.g. Ni) separating buffer gas (usually He) from vacuum and is stopped by collisions with gas atoms. Backside electrode makes confinement for stopped ions, which are extracted from the cell via a combination of RF-funnel and RFQ ion guides.
Fig. 4. MAFFTRAP Penning trap system (example of JYFLTRAP [5]). MCP ¼ microchannel plate detector. Vacuum barrier means a tight disc between the vacuum tube and the central trap electrode, separating two different pressure regions.
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J. Szerypo et al. / Nucl. Instr. and Meth. in Phys. Res. B 204 (2003) 512–516
ejection beamline axis, Fig. 5) has been developed at LMU. A second version of the gas stopping chamber (axial ion injection) is now being set-up at the LMU tandem accelerator, as a first part of MAFFTRAP. It is planned to make MAFFTRAP tests and experiments there, both with stable and radioactive beam, before positioning it finally at MAFF.
[3]
Acknowledgements [4]
This work, as well as this conferenceÕs travel costs of one of co-authors (J.S.), were supported by NIPNET project under contract no. HPRI-CT2001-50034.
[5]
[6]
References [1] D. Habs, T.V. Egidy, M. Gross, O. Kester, P. Kienle, U. Koester, J.-A. Pinston, P.G.Thirolf (Eds.), MAFF – Physics Case and Technical Description, October 1998, http:// www.ha.physik.uni-muenchen.de/maff/; D. Habs, M. Gross, W. Assman, F. Ames, H. Bongers, S. Emhofer, S. Heinz, S. Henry, O. Kester, J. Neumayr, F. Ospald, P. Reiter, T. Sieber, J. Szerypo, P.G. Thirolf, V. Varentsov, T. Wilfart, T. Faestermann, R. Kr€ ucken, Nucl. Instr. and Meth. B, these Proceedings. doi:10.1016/S0168583X(02)00496-8. [2] G. Marx, D. Ackermann, J. Dilling, F.P. Hessberger, S. Hoffman, H.-J. Kluge, R. Mann, G. M€ unzenberg, Z. Qamhieh, W. Quint, D. Rodriguez, M. Sch€adel, J. Sch€ onfelder, G. Sikler, C. Toader, C. Weber, O. Engels, D. Habs, P. Thirolf, H. Backe, A. Dretzke, W. Lauth, W. Ludolphs, M. Sewtz, Hyperfine Int. 132 (2001) 463;
[7] [8]
[9]
[10]
[11]
[12]
G. Sikler, D. Ackermann, F. Attallah, D. Beck, J. Dilling, A.F. Dodonov, S.A. Elisseev, H. Geissel, D. Habs, S. Heinz, F. Herfurth, F. Heßberger, S. Hofmann, H.-J. Kluge, Ch. Kozhuharov, G. Marx, M. Mukherjee, J. Neumayr, Yu.N. Novikov, W.R. Plaß, W. Quint, S. Rahaman, D. Rodriguez, C. Scheidenberger, M. Tarisien, P. Thirolf, V. Varentsov, Z. Wang, C. Weber, M. Weidenm€ uller, Z. Zhou, Nucl. Instr. and Meth. B, these Proceedings. doi:10.1016/S0168-583X(02)02116-X. G. Bollen, S. Becker, H.-J. Kluge, M. K€ onig, R.B. Moore, T. Otto, H. Raimbault-Hartmann, G. Savard, L. Schweikhard, H. Stolzenberg, Nucl. Instr. and Meth. A 368 (1996) 675. € yst€ L. Weissman, F. Ames, J. A o, O. Forstner, K. Reisinger, S. Rinta-Antila, Nucl. Instr. and Meth. A 492 (2002) 451. J. Szerypo, A. Jokinen, V.S. Kolhinen, A. Nieminen, € yst€ S. Rinta-Antila, J. A o, Acta Phys. Pol. B 32 (2001) 985. M. Wada, I. Katayama, S. Okada, Y. Ishida, T. Nakamura, Y. Nakai, T.M. Kojima, Y. Kanai, T. Kambara, Y. Yamazaki, K. Okada, K. Noda, H. Kawakami, S. Ohtani, RIKEN Rev. 31 (2000) 56. V.L. Varentsov, D. Habs, Nucl. Instr. and Meth. A 490 (2002) 16. € yst€ A. Nieminen, J. Huikari, A. Jokinen, J. A o, P. Campbell, E.C.A. Cochrane, Nucl. Instr. and Meth. 469 (2001) 244. F. Herfurth, A. Kellerbauer, J. Dilling, G. Bollen, S. Henry, H.-J. Kluge, E. Lamour, D. Lunney, R.B. Moore, C. Scheidenberger, S. Schwarz, G. Sikler, J. Szerypo, Nucl. Instr. and Meth. A 469 (2001) 254. H. Raimbault-Hartmann, D. Beck, G. Bollen, M. K€ onig, H.-J. Kluge, E. Schark, J. Stein, S. Schwarz, J. Szerypo, Nucl. Instr. and Meth. B 126 (1997) 378. G. Savard, S. Becker, G. Bollen, H.-J. Kluge, R.B. Moore, T. Otto, L. Schweikhard, H. Stolzenberg, U. Wiess, Phys. Lett. A 158 (1991) 247. G. Gabrielse, J. Tan, J. Appl. Phys. 63 (1988) 5143.