- Email: [email protected]
Research on 3He spin filter cells made of quartz glass
ARTICLE IN PRESS
Physica B 356 (2005) 91–95 www.elsevier.com/locate/physb
Research on 3He spin filter cells made of quartz glass V. Hutanu, A. Rupp Department SF2, Hahn-Meitner-Institut Berlin, Glienicker St. 100, 14109 Berlin, Germany
Abstract Cesiated cells made out of quartz glass are widely used as vessels for hyperpolarised 3He to be applied in neutron scattering experiments. In spite of the significant progress achieved in the enhancement of the relaxation times, the crucial mechanisms of spin–wall interaction are not really understood and spin filter cells still suffer from a worse reproducibility regarding their relaxation behaviour. We report the first experimental observation of a T 1 dependence on the magnetic history in uncoated as well as in cesiated cells made out of quartz glass. A huge decrease (about one order of magnitude) of the relaxation time constant T 1 occurred solely due to short exposure of the cells to an external magnetic field of 0.7 T. Partial or total restoring of the initial relaxation time constant T 1 could be achieved by a careful degaussing of the magnetised cells. The magnetisation turned out to be very stable in time, keeping T 1 unmodified during a few months or even years. The relaxation time T 1 was measured directly in the neutron beam from the decay of the polarised neutron flipping ratio. The cell preparation procedures are also presented. r 2004 Elsevier B.V. All rights reserved. PACS: 33.25.+k; 34.50.Dy; 75.90.+w Keywords: Quartz cell; 3He polarization; Polarised 3He relaxation; T 1 hysteresis
1. Introduction Studies to reveal the relaxation mechanism of nuclear polarised 3He gas due to its contact with glass surfaces have been started long ago [1,2]. A phenomenological approach which attributed the relaxation to paramagnetic centres on or close to Corresponding author. Tel.: +49 30 8062 3177;
fax: +49 30 8062 2999. E-mail address: [email protected] (A. Rupp).
the surface remains the dominant explanation of the process over decades. In spite of some excellent results with some glass cells, Rb-coated to be used for spin exchange optical pumping (SEOP) [3,4] as well as Cs-coated [5], a detailed understanding is still obscure, practically resulting in a relatively poor reproducibility regarding the relaxation behaviour. On the other hand, a high reliability of such cells to be applied as neutron spin filters is indispensable. Cells must be able to undergo both, many refilling cycles and exposures to various
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.10.055
ARTICLE IN PRESS V. Hutanu, A. Rupp / Physica B 356 (2005) 91–95
92
magnetic environments without changing their relaxation properties. Jacob et al. [6] demonstrated that the 3He polarisation decay in Pyrex cells can
be very sensitive to their magnetic history. This paper presents preliminary measurements performed on cells made out of quartz glass, Cscoated as well as uncoated, which show that the phenomenon seems to be characteristic not only to Pyrex, but has a more general origin. Hence, further studies on the influence of various external magnetic fields on the cell properties should be performed.
2. Cell preparation A number of 14 quartz glass spin filter cells were manufactured and tested at the HMI during the last years. Two different types of cylindrical cell geometry have been chosen: first, 100 mm long and 50 mm in diameter, and second, 35 mm long and 50 mm in diameter, in particular designed for small angle neutron scattering applications. Fig. 1 shows photos of both types. Most of the cells were coated with Cs, only few of them are uncoated. A list is given in Table 1. After manufacturing by a glassblower, most of the cells were shortly rinsed with hydrofluoric acid of various concentrations in order to remove impurities. Lower concentrated acid solutions of 8–10% and higher concentrated solutions of 38% were used, but no systematic influence on the later relaxation behaviour could be observed. The time between manufacturing and the preparation of the Cs coating was usually a few weeks, for some cells even longer (a few months). During that time, no
Fig. 1. (a) Cs-coated filter cell (No. 9) designed to be used at the three-axis spectrometer E1; (b) short filter cell for small angle neutron scattering spectrometer V4 before attaching the Cs ampoule.
Table 1 List of the investigated cells made of quartz glass Cell no.
Cell length (cm)
Windows thickness (mm)
Pressure test (bar)
HF concentr. (%)
Relaxation time T 1 (h)
1 2 4 8 9 10 11 12 13 14 16
10 10 10 10 10 3.5 3.5 3.5 3.5 10 10
5 5 2 3.5 3.5 5 5 5 5 5 5
5 5 2 2.4 2.2 4 5 3 5 5 —
38 38 38 8 8 8 8 8 10 — —
50 31 10.5 43 22; 25 10 1.4 4.5 19 11.5 20
ARTICLE IN PRESS V. Hutanu, A. Rupp / Physica B 356 (2005) 91–95
The aim of the experiment was to investigate for some of the above-mentioned cells whether their spin–wall-relaxation properties will be changed by external magnetic fields. Therefore, for each cell the relaxation times T 1 ; before as well as after a short (about 1 min) exposure to a magnetic field of 7 and 5 kG, respectively, had to be compared. The measurements to determine T 1 were performed directly in the neutron beam of the three-axis spectrometer E1, using the method and the experimental set-up similar to that presented in Ref. [7], whereas T 1 is derived from the decay of the 3He polarisation, which depends on the flipping ratio of the outcoming neutron beam, following Eq. (1): 1 15nþ n PHe ¼ ln ; (1) x 15n nþ
0.45 0.40 0.35 He-Polarisation
3. Experiment
where x is the effective absorption coefficient [8], often called opacity, while nþ and n2 are the detected counting rates for the cases of a flipped and an unflipped incoming neutron beam. The shown equation is already corrected for the finite flipping ratio of the incoming beam. A typical example of the exponential decay of the 3He polarisation in a cell is shown in Fig. 2. Each cell to be studied was filled with polarised 3 He gas from a bigger reservoir with an initial pressure of about 2 bar. Then it was situated in a mobile, double shielded solenoid magnetic cavity (called ‘‘transport unit’’), inside creating a homogeneous magnetic field of about 8 G, described elsewhere [9]. The cell remained inside it during both the transportation and the experiment on the neutron beam. The relaxation caused by magnetic field inhomogeneity in the cavity representing 235 h for 1 bar pressure can be considered as negligible. After the measurement, the cell was removed, usually evacuated until 2 106 mbar were reached and exposed to the magnetic field, created by a strong electromagnet. The orientation of the external field regarding the cell geometry is perpendicular to the field direction in the ‘‘transport unit’’. Afterwards, the cell was refilled again from the same reservoir and the measurement to determine T 1 was repeated with the same relative orientation of the cell in the cavity.
3
special storing conditions were provided, but the cells were just exposed to ambient air. The further preparation procedure was applied to all cells: first, each cell was subjected to a pressure test, usually up to 5 bar (N2) for some days, in the case of thin windows also less (see Table 1). After attaching a Cs ampoule (Alfa Aesar), the cell was rinsed a few times with ethanol and deionised water. The glass valve was lubricated with Apiezon H25 (BOC Edwards) vacuum grease and the cell body was heated at 400 1C for about 1 week under high vacuum conditions (2 107 mbar). There is a risk to overheat the valve and to fluidise the vacuum grease during baking because of its short distance from the cell body. To avoid this, a specially designed, water-cooled chiller made out of glass was attached to the valve. After closing the valve and removing the cell from the vacuum setup, the inside tip of the ampoule was broken, and finally some hundred milligrams of Cs were filled into the cell and distributed all over the inner surface by evaporation and recondensation, making use of a simple hot air gun. The rest of the Cs remained in the ampoule which was removed by sealing-off.
93
Experimental data; Exponential fit: P0= 43.4 %, T1= 21.9 h;
0.30 0.25 0.20 0.15 0.10 0.05 0.00 0
10
20
30
40
50
60
70
80
90
100
Elapsed time [h]
Fig. 2. 3He relaxation in cell No. 9, fitted to an exponential decay function P ¼ P0 expðt=T 1 Þ; where P0 —initial polarisation in the cell after the filling, t—elapsed time and T 1 — relaxation time constant.
ARTICLE IN PRESS V. Hutanu, A. Rupp / Physica B 356 (2005) 91–95
The results for three different cells are presented in Table 2. An uncoated cell and Cs-coated cells with different initial relaxation times, measured before exposing them to the magnetic field, are selected. A strong decrease of T 1 for a factor of more than 10 has been observed in the unprepared cell (No. 11) and in one of the coated cells (No. 12) while even a factor of more than 20 occurred in the other coated cell (No. 13). The partial and total restoring of the initial T 1 in the magnetised cells could be achieved by a degaussing procedure performed in the same electromagnet by means of a periodical reversing of the polarity, combined with a gradual decrease of the field strength. Exemplarily for all investigated cells, in Fig. 3 the chronological evolution of T 1 in cell no. 13 is presented. A relaxation time T 1 of 19 h was measured in the Cs-coated cell before exposure to external field. After exposing to a field of 0.7 T, the T 1 of the cell is 0.82 h. Quick degaussing by decreasing the field in big steps of 500 or 200 G until the remanent field of the magnet of about 50 G is achieved, lead only to a T 1 of 2.4 h. Much carefully degaussing by decreasing the field in smaller steps and driving the field in the magnet as close as possible to zero lead not only to the total restoring of the relaxation time T 1 ; but to a slight improvement of T 1 at the value of 23.4 h. Repeated exposure of the cell to the external field of 0.5 T brings the relaxation of the cell again to the T 1 of 0.85 h. This fact may suggest the conclusion that a magnetisation of the cell achieved its saturation even at lower fields. The phenomenon is reproducible over multiple expositions and is observable in different cells. Thus, we observed in the quartz cells an effect similar to that described by Jacob et al. [6] for Rbcoated Pyrex cells (there called ‘‘T 1 hysteresis’’). The magnitude of the observed changes of factor
10 or more is the same as that observed in Rbcoated Pyrex or GE 180 glass cells. The concentration of the iron or other magnetic impurities in the quartz glass is more than two orders of magnitude lower than in the case of borosilicate or aluminosilicate glass. We registered a strong magnetisation effect also in an uncoated cell, what was not the fact for Pyrex cells, where the phenomenon of ‘‘T 1 hysteresis’’ is attributed to the Rb presence. One can suggest that the iron oxide concentration in the cell material (at least under or over certain levels) is not the main critical parameter for the 3He relaxation properties of the cell. But the nature of this rather unexpected influence of external fields on the relaxation in quartz spin filter cells still remains to be investigated. Much detailed studies of external field influence on the relaxation behaviour of quartz glass as well as other glasses could lead to a better
25
20
T1 [ hours ]
94
15 magnet.1 - at 7kG magnet.2 - at 5kG
10
5
0 unmagn. magnet.1 degaus.1 degaus.2 magnet.2
chronological order
Fig. 3. Characteristic relaxation time T 1 is plotted vs. the chronological history of magnetisation in the single cell. For all measurements, polarised 3He was transferred in the cell from a reservoir cell. All exposures to an external field as well as degaussing were done after evacuating the gas from the cell.
Table 2 Results of the T 1 measurements in three different cells before and after exposure to an external magnetic field of 0.7 T Cell no.
Cell preparation
T 1 before exposure to high field (h)
T 1 after exposure to high field (h)
12 13 11
Cs-coated Cs-coated Uncoated
4.470.4 1970.5 1.3270.04
0.4370.06 0.8370.01 0.1470.02
ARTICLE IN PRESS V. Hutanu, A. Rupp / Physica B 356 (2005) 91–95
understanding of the fundamental mechanisms of 3 He relaxation and progress reproducible fabrication of glass vessels for polarised 3He.
Acknowledgements The work has been partially supported by the European Union through the Integrated Infrastructure Initiative NMI3 project JRA4 (NSF). We are grateful to Prof. Heil and his group at the University of Mainz who supplied us with polarised 3He.
95
References [1] W.A. Fitzsimmons, L.L. Tankersley, G.K. Walters, Phys. Rev. 179 (1969) 156. [2] R.S. Tismit, J.M. Daniels, A.D. May, Can. J. Phys. 49 (1971) 561. [3] D.R. Rich, T.R. Gentile, T.B. Smith, T.K. Thophson, Appl. Phys. Lett. 80 (2002) 2210. [4] T.B. Smith, et al., Nucl. Instr. and Meth. A 402 (1998) 247. [5] W. Heil, H. Humblot, E. Otten, M. Schafer, R. Surkau, M. Leduc, Phys. Lett. A 201 (1995) 337. [6] R.E. Jacob, S.W. Morgan, B. Saam, J.C. Leawoods, Phys. Rev. Lett. 87 (2001) 143004. [7] A. Gorzel, J. Klenke, A. Rupp, Appl. Phys. A 74 (Suppl.) (2002) 89. [8] R. Surkau, et al., Nucl. Instr. and Meth. A 384 (1997) 444. [9] A. Danzig, A. Rupp, Physica B 267–268 (1999) 344.
Physica B 356 (2005) 91–95 www.elsevier.com/locate/physb
Research on 3He spin filter cells made of quartz glass V. Hutanu, A. Rupp Department SF2, Hahn-Meitner-Institut Berlin, Glienicker St. 100, 14109 Berlin, Germany
Abstract Cesiated cells made out of quartz glass are widely used as vessels for hyperpolarised 3He to be applied in neutron scattering experiments. In spite of the significant progress achieved in the enhancement of the relaxation times, the crucial mechanisms of spin–wall interaction are not really understood and spin filter cells still suffer from a worse reproducibility regarding their relaxation behaviour. We report the first experimental observation of a T 1 dependence on the magnetic history in uncoated as well as in cesiated cells made out of quartz glass. A huge decrease (about one order of magnitude) of the relaxation time constant T 1 occurred solely due to short exposure of the cells to an external magnetic field of 0.7 T. Partial or total restoring of the initial relaxation time constant T 1 could be achieved by a careful degaussing of the magnetised cells. The magnetisation turned out to be very stable in time, keeping T 1 unmodified during a few months or even years. The relaxation time T 1 was measured directly in the neutron beam from the decay of the polarised neutron flipping ratio. The cell preparation procedures are also presented. r 2004 Elsevier B.V. All rights reserved. PACS: 33.25.+k; 34.50.Dy; 75.90.+w Keywords: Quartz cell; 3He polarization; Polarised 3He relaxation; T 1 hysteresis
1. Introduction Studies to reveal the relaxation mechanism of nuclear polarised 3He gas due to its contact with glass surfaces have been started long ago [1,2]. A phenomenological approach which attributed the relaxation to paramagnetic centres on or close to Corresponding author. Tel.: +49 30 8062 3177;
fax: +49 30 8062 2999. E-mail address: [email protected] (A. Rupp).
the surface remains the dominant explanation of the process over decades. In spite of some excellent results with some glass cells, Rb-coated to be used for spin exchange optical pumping (SEOP) [3,4] as well as Cs-coated [5], a detailed understanding is still obscure, practically resulting in a relatively poor reproducibility regarding the relaxation behaviour. On the other hand, a high reliability of such cells to be applied as neutron spin filters is indispensable. Cells must be able to undergo both, many refilling cycles and exposures to various
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.10.055
ARTICLE IN PRESS V. Hutanu, A. Rupp / Physica B 356 (2005) 91–95
92
magnetic environments without changing their relaxation properties. Jacob et al. [6] demonstrated that the 3He polarisation decay in Pyrex cells can
be very sensitive to their magnetic history. This paper presents preliminary measurements performed on cells made out of quartz glass, Cscoated as well as uncoated, which show that the phenomenon seems to be characteristic not only to Pyrex, but has a more general origin. Hence, further studies on the influence of various external magnetic fields on the cell properties should be performed.
2. Cell preparation A number of 14 quartz glass spin filter cells were manufactured and tested at the HMI during the last years. Two different types of cylindrical cell geometry have been chosen: first, 100 mm long and 50 mm in diameter, and second, 35 mm long and 50 mm in diameter, in particular designed for small angle neutron scattering applications. Fig. 1 shows photos of both types. Most of the cells were coated with Cs, only few of them are uncoated. A list is given in Table 1. After manufacturing by a glassblower, most of the cells were shortly rinsed with hydrofluoric acid of various concentrations in order to remove impurities. Lower concentrated acid solutions of 8–10% and higher concentrated solutions of 38% were used, but no systematic influence on the later relaxation behaviour could be observed. The time between manufacturing and the preparation of the Cs coating was usually a few weeks, for some cells even longer (a few months). During that time, no
Fig. 1. (a) Cs-coated filter cell (No. 9) designed to be used at the three-axis spectrometer E1; (b) short filter cell for small angle neutron scattering spectrometer V4 before attaching the Cs ampoule.
Table 1 List of the investigated cells made of quartz glass Cell no.
Cell length (cm)
Windows thickness (mm)
Pressure test (bar)
HF concentr. (%)
Relaxation time T 1 (h)
1 2 4 8 9 10 11 12 13 14 16
10 10 10 10 10 3.5 3.5 3.5 3.5 10 10
5 5 2 3.5 3.5 5 5 5 5 5 5
5 5 2 2.4 2.2 4 5 3 5 5 —
38 38 38 8 8 8 8 8 10 — —
50 31 10.5 43 22; 25 10 1.4 4.5 19 11.5 20
ARTICLE IN PRESS V. Hutanu, A. Rupp / Physica B 356 (2005) 91–95
The aim of the experiment was to investigate for some of the above-mentioned cells whether their spin–wall-relaxation properties will be changed by external magnetic fields. Therefore, for each cell the relaxation times T 1 ; before as well as after a short (about 1 min) exposure to a magnetic field of 7 and 5 kG, respectively, had to be compared. The measurements to determine T 1 were performed directly in the neutron beam of the three-axis spectrometer E1, using the method and the experimental set-up similar to that presented in Ref. [7], whereas T 1 is derived from the decay of the 3He polarisation, which depends on the flipping ratio of the outcoming neutron beam, following Eq. (1): 1 15nþ n PHe ¼ ln ; (1) x 15n nþ
0.45 0.40 0.35 He-Polarisation
3. Experiment
where x is the effective absorption coefficient [8], often called opacity, while nþ and n2 are the detected counting rates for the cases of a flipped and an unflipped incoming neutron beam. The shown equation is already corrected for the finite flipping ratio of the incoming beam. A typical example of the exponential decay of the 3He polarisation in a cell is shown in Fig. 2. Each cell to be studied was filled with polarised 3 He gas from a bigger reservoir with an initial pressure of about 2 bar. Then it was situated in a mobile, double shielded solenoid magnetic cavity (called ‘‘transport unit’’), inside creating a homogeneous magnetic field of about 8 G, described elsewhere [9]. The cell remained inside it during both the transportation and the experiment on the neutron beam. The relaxation caused by magnetic field inhomogeneity in the cavity representing 235 h for 1 bar pressure can be considered as negligible. After the measurement, the cell was removed, usually evacuated until 2 106 mbar were reached and exposed to the magnetic field, created by a strong electromagnet. The orientation of the external field regarding the cell geometry is perpendicular to the field direction in the ‘‘transport unit’’. Afterwards, the cell was refilled again from the same reservoir and the measurement to determine T 1 was repeated with the same relative orientation of the cell in the cavity.
3
special storing conditions were provided, but the cells were just exposed to ambient air. The further preparation procedure was applied to all cells: first, each cell was subjected to a pressure test, usually up to 5 bar (N2) for some days, in the case of thin windows also less (see Table 1). After attaching a Cs ampoule (Alfa Aesar), the cell was rinsed a few times with ethanol and deionised water. The glass valve was lubricated with Apiezon H25 (BOC Edwards) vacuum grease and the cell body was heated at 400 1C for about 1 week under high vacuum conditions (2 107 mbar). There is a risk to overheat the valve and to fluidise the vacuum grease during baking because of its short distance from the cell body. To avoid this, a specially designed, water-cooled chiller made out of glass was attached to the valve. After closing the valve and removing the cell from the vacuum setup, the inside tip of the ampoule was broken, and finally some hundred milligrams of Cs were filled into the cell and distributed all over the inner surface by evaporation and recondensation, making use of a simple hot air gun. The rest of the Cs remained in the ampoule which was removed by sealing-off.
93
Experimental data; Exponential fit: P0= 43.4 %, T1= 21.9 h;
0.30 0.25 0.20 0.15 0.10 0.05 0.00 0
10
20
30
40
50
60
70
80
90
100
Elapsed time [h]
Fig. 2. 3He relaxation in cell No. 9, fitted to an exponential decay function P ¼ P0 expðt=T 1 Þ; where P0 —initial polarisation in the cell after the filling, t—elapsed time and T 1 — relaxation time constant.
ARTICLE IN PRESS V. Hutanu, A. Rupp / Physica B 356 (2005) 91–95
The results for three different cells are presented in Table 2. An uncoated cell and Cs-coated cells with different initial relaxation times, measured before exposing them to the magnetic field, are selected. A strong decrease of T 1 for a factor of more than 10 has been observed in the unprepared cell (No. 11) and in one of the coated cells (No. 12) while even a factor of more than 20 occurred in the other coated cell (No. 13). The partial and total restoring of the initial T 1 in the magnetised cells could be achieved by a degaussing procedure performed in the same electromagnet by means of a periodical reversing of the polarity, combined with a gradual decrease of the field strength. Exemplarily for all investigated cells, in Fig. 3 the chronological evolution of T 1 in cell no. 13 is presented. A relaxation time T 1 of 19 h was measured in the Cs-coated cell before exposure to external field. After exposing to a field of 0.7 T, the T 1 of the cell is 0.82 h. Quick degaussing by decreasing the field in big steps of 500 or 200 G until the remanent field of the magnet of about 50 G is achieved, lead only to a T 1 of 2.4 h. Much carefully degaussing by decreasing the field in smaller steps and driving the field in the magnet as close as possible to zero lead not only to the total restoring of the relaxation time T 1 ; but to a slight improvement of T 1 at the value of 23.4 h. Repeated exposure of the cell to the external field of 0.5 T brings the relaxation of the cell again to the T 1 of 0.85 h. This fact may suggest the conclusion that a magnetisation of the cell achieved its saturation even at lower fields. The phenomenon is reproducible over multiple expositions and is observable in different cells. Thus, we observed in the quartz cells an effect similar to that described by Jacob et al. [6] for Rbcoated Pyrex cells (there called ‘‘T 1 hysteresis’’). The magnitude of the observed changes of factor
10 or more is the same as that observed in Rbcoated Pyrex or GE 180 glass cells. The concentration of the iron or other magnetic impurities in the quartz glass is more than two orders of magnitude lower than in the case of borosilicate or aluminosilicate glass. We registered a strong magnetisation effect also in an uncoated cell, what was not the fact for Pyrex cells, where the phenomenon of ‘‘T 1 hysteresis’’ is attributed to the Rb presence. One can suggest that the iron oxide concentration in the cell material (at least under or over certain levels) is not the main critical parameter for the 3He relaxation properties of the cell. But the nature of this rather unexpected influence of external fields on the relaxation in quartz spin filter cells still remains to be investigated. Much detailed studies of external field influence on the relaxation behaviour of quartz glass as well as other glasses could lead to a better
25
20
T1 [ hours ]
94
15 magnet.1 - at 7kG magnet.2 - at 5kG
10
5
0 unmagn. magnet.1 degaus.1 degaus.2 magnet.2
chronological order
Fig. 3. Characteristic relaxation time T 1 is plotted vs. the chronological history of magnetisation in the single cell. For all measurements, polarised 3He was transferred in the cell from a reservoir cell. All exposures to an external field as well as degaussing were done after evacuating the gas from the cell.
Table 2 Results of the T 1 measurements in three different cells before and after exposure to an external magnetic field of 0.7 T Cell no.
Cell preparation
T 1 before exposure to high field (h)
T 1 after exposure to high field (h)
12 13 11
Cs-coated Cs-coated Uncoated
4.470.4 1970.5 1.3270.04
0.4370.06 0.8370.01 0.1470.02
ARTICLE IN PRESS V. Hutanu, A. Rupp / Physica B 356 (2005) 91–95
understanding of the fundamental mechanisms of 3 He relaxation and progress reproducible fabrication of glass vessels for polarised 3He.
Acknowledgements The work has been partially supported by the European Union through the Integrated Infrastructure Initiative NMI3 project JRA4 (NSF). We are grateful to Prof. Heil and his group at the University of Mainz who supplied us with polarised 3He.
95
References [1] W.A. Fitzsimmons, L.L. Tankersley, G.K. Walters, Phys. Rev. 179 (1969) 156. [2] R.S. Tismit, J.M. Daniels, A.D. May, Can. J. Phys. 49 (1971) 561. [3] D.R. Rich, T.R. Gentile, T.B. Smith, T.K. Thophson, Appl. Phys. Lett. 80 (2002) 2210. [4] T.B. Smith, et al., Nucl. Instr. and Meth. A 402 (1998) 247. [5] W. Heil, H. Humblot, E. Otten, M. Schafer, R. Surkau, M. Leduc, Phys. Lett. A 201 (1995) 337. [6] R.E. Jacob, S.W. Morgan, B. Saam, J.C. Leawoods, Phys. Rev. Lett. 87 (2001) 143004. [7] A. Gorzel, J. Klenke, A. Rupp, Appl. Phys. A 74 (Suppl.) (2002) 89. [8] R. Surkau, et al., Nucl. Instr. and Meth. A 384 (1997) 444. [9] A. Danzig, A. Rupp, Physica B 267–268 (1999) 344.