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A novel high precision slit system
Nuclear Instruments and Methods in Physics Research B 158 (1999) 107±112
www.elsevier.nl/locate/nimb
A novel high precision slit system O. Schmelmer *, G. Dollinger, G. Datzmann, C. Goeden, H.-J. K orner Physik-Department E12, Technische Universit at M unchen, James-Franck-Strasse, D-85748 Garching, Germany
Abstract A new high precision slit system has been developed for the nano beam facility SNAKE (Supraleitendes Nanoskop f ur Angewandte Kernphysikalische Experimente) which is under construction at the Munich tandem accelerator. Cylindrically strained germanium wafers with a bending radius of 50 mm are used as optimized slit edges. High resolution and angle resolved energy distributions of the transmitted protons were measured by means of a Q3D magnetic spectrograph at incident energies of 20 MeV. The measurements revealed the expected strong correlation between small angle scattering and energy loss processes at the slits. Within SNAKE's angular acceptance the ratio of particles suering energy loss by interacting with the slit and particles not interacting with the slit is less than 0.6% even for aperture widths of 2 lm. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 07.78; 41.85; 61.85
1. Introduction For material analysis and modi®cation a new nanobeam facility SNAKE (Supraleitendes Nanoskop f ur Angewandte Kernphysikalische Experimente) is under construction at the Munich tandem accelerator [1]. In contrast to existing facilities this two stage nanoprobe system should achieve a sub micrometer beam spot with high energy ions. Using protons with energies up to 30 MeV a lateral resolution of 100 nm is strived for at a beam current of 100 pA which is sucient for elemental analysis. In addition, heavy ions with
* Corresponding author. Tel.: +49-89-289-14288; fax: +4989-289-1229; e-mail: [email protected]
energies up to 200 MeV q2 /A can be focused. Since the energy of the ions is almost one order of magnitude larger and the desired beam spot one order of magnitude smaller than in usual systems, various parts of the system have been completely new designed and are currently under construction. Besides a very bright Hÿ ion source [2] and a new superconducting multipole lens [1] new high precision slits are required. The new high precision slits shall especially be used as object slits de®ning the beam spot size at the target as well as at the intermediate focus of the two stage nanoprobe. A detailed description of the beam optics is given in [1]. In the following, the main requirements of the slits and their solutions are presented. In addition, a quality test is presented which was performed using 20 MeV protons.
0168-583X/99/$ - see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 3 5 7 - 2
108
O. Schmelmer et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 107±112
2. Technical details The object slits must be opened less than 20 lm if an image size of 100 nm is strived for, because the demagni®cation factor of SNAKE is 200 [1]. In ®rst approximation the width of the beam transmitted through the slits is equal to this distance between two slit edges. In reality, however, not only particles passing this aperture appear behind the slit but also particles penetrating at least a small part of the slit material. The region where the beam penetrates the slit edge is called transparency zone [3,4]. It depends on the geometrical shape of the slits and on the range of the particles in the slit material. Especially, this becomes important for energetic light ions. For example, the projected range of 30 MeV protons in germanium is 2.7 mm [5]. Particles interacting with the slits that are not completely stopped, suer from energy loss as well as from small angle scattering and consequently reduce the quality of the beam. As a result, a halo is produced around the focused beam. In general, the slit material should have a smooth surface and be made of a heavy element material [3], since small angle scattering is increased and the particles interacting with the slit are scattered towards large angles and directed o the accepted beam path. Investigations [3] have shown that the shape of the slits should be cylindrical since the transparency zone can be minimized in this geometry. Another advantage of a cylindrical shape is that particles being scattered at the slit edge are suppressed [6]. These aspects have been considered by the development of the new slits. The actual slit is made of a nearly atomically smooth germanium wafer (thickness 0.1 mm) which is cylindrically bend with a radius of 50 mm. In this geometry the depth of the transparency zone de®ned by the range of the ions is only 18 lm for 30 MeV protons. The germanium wafer is bend by a frame which is connected by a stick to a linear transporter outside the vacuum (see Fig. 1). An advantage of this mounting is that the germanium wafer can be replaced by another material, e.g. silicon, if necessary. Since the object slits will be opened less than 20 lm slit edges must be driven in micrometer
Fig. 1. Principal drawing of one half of a slit. Technical details are described in the text.
steps to get a convenient handling. The adjustment of the new slit is managed by a linear transporter (Berger Lineartisch 5101.1, linear travel 60 mm) outside the vacuum in 1 lm steps. By driving each slit edge separately the slits can be symmetrically adjusted to the beam axis. As vibrations of e.g. pumps mounted on the beam tube (up to 60 lm at some kHz) destroy the required resolution, the slit is mechanically isolated from the beam tube by a thin ¯exible coupling. The linear transporter and also the actual slit is mechanically ®xed to the ¯oor of the accelerator by a free standing frame. Another problem arises from the heating of the slits by the ion beam which in¯uences the width of the aperture. Regarding stainless steel, for example, an increase of the temperature of only 10 K due to a variation of the beam current leads to a linear expansion of 5 lm. In contrast to other facilities trying to cool their slits [7] this new slit system is heated. If the beam hits the slit the additional heat supply will be reduced to keep the temperature of the slit and therefore the total heating power constant. Inside the actual mounting of the slit a tungsten ®lament heats it (Fig. 1). Together with a thermocouple (type K) and a control unit (Eurotherm 2416) the temperature of the slit is kept constant. Thereby the variation of the slit opening is minimized. The temperature of the slit should be higher than the maximal
O. Schmelmer et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 107±112
temperature originating from the heating by the maximal beam current. The germanium wafer and the bending frame are thermally and electrically isolated from the rest of the mounting by a slice of ceramic (Fig. 1) in order to ensure a slit current measurement. Both halfs of the slit are tangentially shifted (30 mm). So the slits can be overlapped and the beam can be faded out without destroying the surface of the slit wafer (Fig. 2(b)).
109
3. Test of the slits 3.1. Experimental arrangement The test of one high precision slit was performed at the Munich tandem accelerator using 20 MeV protons. The slit was mounted vertically in front of the Q3D magnetic spectrograph [8] which was operated in transmission geometry. After the protons had passed the slit their energy was analysed by the magnetic ®eld of the Q3D. The particles were detected in the focal plane of the Q3D by a position sensitive ionisation chamber [9]. A principal drawing of the experimental arrangement is shown in Fig. 2. Using this setup high resolution energy distributions of the transmitted particles were measured. An energy resolution of the combined system of proton beam, magnetic spectrograph and focal plane detector of DE=E 4 10ÿ4 was achieved. As the ionisation chamber contains a vertical multi wire plane, angle resolved energy distributions within 2.5 mrad were measured simultaneously in the y±z plane (see Fig. 2). During the whole measurements the temperature of the slit was kept at 100:0 0:1°C by the automatic control mentioned above. 3.2. Beam preparation In order to measure the performance of the slits the beam was pre-cut by three beam reducers which consist of nets with a transmission of 1:1000 and 1:30. After the reduction the beam was de¯ected by 50° and 60° magnets in order to purify the beam. The diameter of the beam was about 0.4 mm at the position of the new slits. The beam current was reduced to measure the energy distribution of the beam without the precision slits (Fig. 4, dotted line). 3.3. Calibration of the slit aperture
Fig. 2. Experimental arrangement testing the new presision slits: (a) top view; (b) side view (principal drawing). The angle resolved energy distributions were measured in the y±z plane.
Before looking at the energy distributions of the transmitted protons, the position of the slits relative to the beam and the distance between the slit edges was determined. This was performed by the following procedure. To ensure that the slit is symmetrically adjusted to the beam, each slit edge
110
O. Schmelmer et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 107±112
is driven from the margin to the center of the beam axis. When the slit edge hits the margin of the symmetric beam pro®le the count rate of the detector decreases rapidly. If both slit edges reduce the beam intensity to the same extent the slit is adjusted symmetrically to the beam axis. To determine the distance of the slit edges, both halfs must be driven symmetrically to the beam axis. Then the count rate in the detector decreases monotonously (Fig. 3). Extrapolating the count rate to zero the closed position of the slit can be calculated. The accuracy of this measurement is 1 lm which is equivalent to one step of the stepping motor. The width of the slit can be determined from the calibration by the number of driven steps. 3.4. Integrated energy distributions After the slit opening has been calibrated energy distributions are considered. In order to compare the spectra with dierent apertures all energy distributions are scaled to the same integral contents over the displayed energy region (19.8± 20.1 MeV). Fig. 4 shows the energy distribution of the proton beam without the new slit (dotted line) and the distribution with the new slit with an aperture of 12 lm (solid line). The accepted vertical angle of these measurements is 2.5 mrad. Both
Fig. 4. Energy distribution of the 20 MeV proton beam without the new slit (dotted line) and the distribution using the new slit being 12 lm open (solid line). Both spectra are scaled to the same integral content.
distributions show a peak at 20 MeV which corresponds to the actual beam energy and a low energy tail below 19.98 MeV. Regarding the distribution of the measurement without the new slit the low energy tail is due to the beam preparation (conventional analysing slits, beam reduction, etc.). To characterize the in¯uence of the slit to the beam quality the ratio of the integral contents of the low energy tail and the peak around 20 MeV is introduced (tail/peak). For the distribution without the new slit it is 1.37%. By decreasing the distance of the slit edges the number of the transmitted ions decreases, while the number of particles which interact with the slit material remains constant. Therefore the ratio tail/peak should increase. However, considering the measurement with the new slit opened only 12 lm the ratio is 1.41%. Compared to the measurement without the new slit this ratio is almost the same. Since there is no increase of the ratio there is no signi®cant reduction of the quality of the beam for distances of the slit edges down to 12 lm. 3.5. Angle resolved energy distributions
Fig. 3. Determination of the distance of the slit edges by monitoring the counting rate in a detector at two dierent beam currents. The lower abscissa shows the distance of the slit edges in steps of the linear travels (left and right).
As the particles interacting with the slit material do not only suer from energy loss but also from small angle scattering it is usefull to look at angle resolved energy distributions. As mentioned above, the angle a is only measured in the y±z-plane (see Fig. 2). Fig. 5 depicts energy distributions of the
O. Schmelmer et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 107±112
beam using the new slit being 4 lm open for ®ve angle intervals. It shows a strong asymmetric behaviour. This is due to the tangentially shifted slit edges: The second slit edge shadows the projectiles being scattered o the beam axis by the ®rst slit edge (see Fig. 2). It can also be observed that the low energy tail is increased and the peak at 20 MeV is less pronounced for angles o the beam axis. This indicates that projectiles that suered from energy loss are scattered o the beam axis. In Fig. 6 the ratio tail/peak in dependence of the distance of the slit edges is shown for the ®ve angle intervals. For almost all measured angles the ratio increases when the opening decreases. It is even greater than one for angles between ÿ2.5 and ÿ1.5 mrad. Only for particles detected within 0.5
111
Fig. 6. Ratio of the integral contents of the low energy tail and the peak around 20 MeV versus distance of slit edges for different angle intervals.
mrad the ratio is almost constant. The additional number of particles suered from energy loss at the new slits larger than 20 keV and without scattering to angles larger than 0.5 mrad is less than 0.6% compared to the open slits. The behaviour of the ratio shows that ions suered from energy loss are mainly scattered o the beam axis. In general, these projectiles reduce the beam quality seriously. However, since the maximal accepted angle of SNAKE de®ned by divergence slits is smaller than 0.1 mrad in both transversal directions [1] particles suered energy loss and small angle scattering are faded out by a large extent and do not in¯uence beam quality. As a result a beam halo less than 0.6% of the beam intensity is expected even for a slit aperture of 4 lm. 4. Conclusion
Fig. 5. Energy distribution of the proton beam behind the new slit being only 4 lm open for ®ve angle intervals. The asymmetric behaviour is due to the tangentially shifted slit edges.
A new high precision slit system for the planned nanobeam SNAKE has been introduced which uses mechanically strained gemanium wafers forming a cylinder with a radius of 50 mm. Besides a mechanical isolation from the beam tube a new method to compensate linear expansion of the slit due to the heating up of the aperture by the beam is engaged. A test of one slit has shown a strong correlation of small angle scattering and energy loss processes. The additional number of particles suered from energy loss at the new slits larger
112
O. Schmelmer et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 107±112
than 20 keV and without scattering to angles larger than 0.5 mrad is less than 0.6% compared to the open slits. Therefore, the new developed slits ful®ll the high requirements of SNAKE. References [1] G. Hinderer, G. Dollinger, G. Datzmann, H.J. K orner, Nucl. Instr. and Meth. B 130 (1997) 51. [2] C. Goeden, G. Dollinger, to be published.
[3] J. Adamczewski, J. Meijer, A. Stephan, H.H. Bukow, C. Rolfs, Nucl. Instr. and Meth. B 114 (1996) 172. [4] E.J. Burger, D.A. Smith, Rev. Sci. Instr. 33 (1962) 1371. [5] J.F. Ziegler, TRIM 96, [email protected]. [6] M. Takai, Y. Horino, Y. Mokuno, A. Chayahara, M. Kiuchi, K. Fujii, M. Satou, Nucl. Instr. and Meth. B 77 (1993) 8. [7] B.E. Fischer, Nucl. Instr. and Meth. B 10/11 (1985) 693. [8] M. L oer, H.-J. Scheerer, H. Vonach, Nucl. Instr. and Meth. 111 (1973) 1. [9] E. Zanotti, M. Bisenberger, R. Hertenberger, H. Kader, Nucl. Instr. and Meth. B 310 (1991) 706.
www.elsevier.nl/locate/nimb
A novel high precision slit system O. Schmelmer *, G. Dollinger, G. Datzmann, C. Goeden, H.-J. K orner Physik-Department E12, Technische Universit at M unchen, James-Franck-Strasse, D-85748 Garching, Germany
Abstract A new high precision slit system has been developed for the nano beam facility SNAKE (Supraleitendes Nanoskop f ur Angewandte Kernphysikalische Experimente) which is under construction at the Munich tandem accelerator. Cylindrically strained germanium wafers with a bending radius of 50 mm are used as optimized slit edges. High resolution and angle resolved energy distributions of the transmitted protons were measured by means of a Q3D magnetic spectrograph at incident energies of 20 MeV. The measurements revealed the expected strong correlation between small angle scattering and energy loss processes at the slits. Within SNAKE's angular acceptance the ratio of particles suering energy loss by interacting with the slit and particles not interacting with the slit is less than 0.6% even for aperture widths of 2 lm. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 07.78; 41.85; 61.85
1. Introduction For material analysis and modi®cation a new nanobeam facility SNAKE (Supraleitendes Nanoskop f ur Angewandte Kernphysikalische Experimente) is under construction at the Munich tandem accelerator [1]. In contrast to existing facilities this two stage nanoprobe system should achieve a sub micrometer beam spot with high energy ions. Using protons with energies up to 30 MeV a lateral resolution of 100 nm is strived for at a beam current of 100 pA which is sucient for elemental analysis. In addition, heavy ions with
* Corresponding author. Tel.: +49-89-289-14288; fax: +4989-289-1229; e-mail: [email protected]
energies up to 200 MeV q2 /A can be focused. Since the energy of the ions is almost one order of magnitude larger and the desired beam spot one order of magnitude smaller than in usual systems, various parts of the system have been completely new designed and are currently under construction. Besides a very bright Hÿ ion source [2] and a new superconducting multipole lens [1] new high precision slits are required. The new high precision slits shall especially be used as object slits de®ning the beam spot size at the target as well as at the intermediate focus of the two stage nanoprobe. A detailed description of the beam optics is given in [1]. In the following, the main requirements of the slits and their solutions are presented. In addition, a quality test is presented which was performed using 20 MeV protons.
0168-583X/99/$ - see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 3 5 7 - 2
108
O. Schmelmer et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 107±112
2. Technical details The object slits must be opened less than 20 lm if an image size of 100 nm is strived for, because the demagni®cation factor of SNAKE is 200 [1]. In ®rst approximation the width of the beam transmitted through the slits is equal to this distance between two slit edges. In reality, however, not only particles passing this aperture appear behind the slit but also particles penetrating at least a small part of the slit material. The region where the beam penetrates the slit edge is called transparency zone [3,4]. It depends on the geometrical shape of the slits and on the range of the particles in the slit material. Especially, this becomes important for energetic light ions. For example, the projected range of 30 MeV protons in germanium is 2.7 mm [5]. Particles interacting with the slits that are not completely stopped, suer from energy loss as well as from small angle scattering and consequently reduce the quality of the beam. As a result, a halo is produced around the focused beam. In general, the slit material should have a smooth surface and be made of a heavy element material [3], since small angle scattering is increased and the particles interacting with the slit are scattered towards large angles and directed o the accepted beam path. Investigations [3] have shown that the shape of the slits should be cylindrical since the transparency zone can be minimized in this geometry. Another advantage of a cylindrical shape is that particles being scattered at the slit edge are suppressed [6]. These aspects have been considered by the development of the new slits. The actual slit is made of a nearly atomically smooth germanium wafer (thickness 0.1 mm) which is cylindrically bend with a radius of 50 mm. In this geometry the depth of the transparency zone de®ned by the range of the ions is only 18 lm for 30 MeV protons. The germanium wafer is bend by a frame which is connected by a stick to a linear transporter outside the vacuum (see Fig. 1). An advantage of this mounting is that the germanium wafer can be replaced by another material, e.g. silicon, if necessary. Since the object slits will be opened less than 20 lm slit edges must be driven in micrometer
Fig. 1. Principal drawing of one half of a slit. Technical details are described in the text.
steps to get a convenient handling. The adjustment of the new slit is managed by a linear transporter (Berger Lineartisch 5101.1, linear travel 60 mm) outside the vacuum in 1 lm steps. By driving each slit edge separately the slits can be symmetrically adjusted to the beam axis. As vibrations of e.g. pumps mounted on the beam tube (up to 60 lm at some kHz) destroy the required resolution, the slit is mechanically isolated from the beam tube by a thin ¯exible coupling. The linear transporter and also the actual slit is mechanically ®xed to the ¯oor of the accelerator by a free standing frame. Another problem arises from the heating of the slits by the ion beam which in¯uences the width of the aperture. Regarding stainless steel, for example, an increase of the temperature of only 10 K due to a variation of the beam current leads to a linear expansion of 5 lm. In contrast to other facilities trying to cool their slits [7] this new slit system is heated. If the beam hits the slit the additional heat supply will be reduced to keep the temperature of the slit and therefore the total heating power constant. Inside the actual mounting of the slit a tungsten ®lament heats it (Fig. 1). Together with a thermocouple (type K) and a control unit (Eurotherm 2416) the temperature of the slit is kept constant. Thereby the variation of the slit opening is minimized. The temperature of the slit should be higher than the maximal
O. Schmelmer et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 107±112
temperature originating from the heating by the maximal beam current. The germanium wafer and the bending frame are thermally and electrically isolated from the rest of the mounting by a slice of ceramic (Fig. 1) in order to ensure a slit current measurement. Both halfs of the slit are tangentially shifted (30 mm). So the slits can be overlapped and the beam can be faded out without destroying the surface of the slit wafer (Fig. 2(b)).
109
3. Test of the slits 3.1. Experimental arrangement The test of one high precision slit was performed at the Munich tandem accelerator using 20 MeV protons. The slit was mounted vertically in front of the Q3D magnetic spectrograph [8] which was operated in transmission geometry. After the protons had passed the slit their energy was analysed by the magnetic ®eld of the Q3D. The particles were detected in the focal plane of the Q3D by a position sensitive ionisation chamber [9]. A principal drawing of the experimental arrangement is shown in Fig. 2. Using this setup high resolution energy distributions of the transmitted particles were measured. An energy resolution of the combined system of proton beam, magnetic spectrograph and focal plane detector of DE=E 4 10ÿ4 was achieved. As the ionisation chamber contains a vertical multi wire plane, angle resolved energy distributions within 2.5 mrad were measured simultaneously in the y±z plane (see Fig. 2). During the whole measurements the temperature of the slit was kept at 100:0 0:1°C by the automatic control mentioned above. 3.2. Beam preparation In order to measure the performance of the slits the beam was pre-cut by three beam reducers which consist of nets with a transmission of 1:1000 and 1:30. After the reduction the beam was de¯ected by 50° and 60° magnets in order to purify the beam. The diameter of the beam was about 0.4 mm at the position of the new slits. The beam current was reduced to measure the energy distribution of the beam without the precision slits (Fig. 4, dotted line). 3.3. Calibration of the slit aperture
Fig. 2. Experimental arrangement testing the new presision slits: (a) top view; (b) side view (principal drawing). The angle resolved energy distributions were measured in the y±z plane.
Before looking at the energy distributions of the transmitted protons, the position of the slits relative to the beam and the distance between the slit edges was determined. This was performed by the following procedure. To ensure that the slit is symmetrically adjusted to the beam, each slit edge
110
O. Schmelmer et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 107±112
is driven from the margin to the center of the beam axis. When the slit edge hits the margin of the symmetric beam pro®le the count rate of the detector decreases rapidly. If both slit edges reduce the beam intensity to the same extent the slit is adjusted symmetrically to the beam axis. To determine the distance of the slit edges, both halfs must be driven symmetrically to the beam axis. Then the count rate in the detector decreases monotonously (Fig. 3). Extrapolating the count rate to zero the closed position of the slit can be calculated. The accuracy of this measurement is 1 lm which is equivalent to one step of the stepping motor. The width of the slit can be determined from the calibration by the number of driven steps. 3.4. Integrated energy distributions After the slit opening has been calibrated energy distributions are considered. In order to compare the spectra with dierent apertures all energy distributions are scaled to the same integral contents over the displayed energy region (19.8± 20.1 MeV). Fig. 4 shows the energy distribution of the proton beam without the new slit (dotted line) and the distribution with the new slit with an aperture of 12 lm (solid line). The accepted vertical angle of these measurements is 2.5 mrad. Both
Fig. 4. Energy distribution of the 20 MeV proton beam without the new slit (dotted line) and the distribution using the new slit being 12 lm open (solid line). Both spectra are scaled to the same integral content.
distributions show a peak at 20 MeV which corresponds to the actual beam energy and a low energy tail below 19.98 MeV. Regarding the distribution of the measurement without the new slit the low energy tail is due to the beam preparation (conventional analysing slits, beam reduction, etc.). To characterize the in¯uence of the slit to the beam quality the ratio of the integral contents of the low energy tail and the peak around 20 MeV is introduced (tail/peak). For the distribution without the new slit it is 1.37%. By decreasing the distance of the slit edges the number of the transmitted ions decreases, while the number of particles which interact with the slit material remains constant. Therefore the ratio tail/peak should increase. However, considering the measurement with the new slit opened only 12 lm the ratio is 1.41%. Compared to the measurement without the new slit this ratio is almost the same. Since there is no increase of the ratio there is no signi®cant reduction of the quality of the beam for distances of the slit edges down to 12 lm. 3.5. Angle resolved energy distributions
Fig. 3. Determination of the distance of the slit edges by monitoring the counting rate in a detector at two dierent beam currents. The lower abscissa shows the distance of the slit edges in steps of the linear travels (left and right).
As the particles interacting with the slit material do not only suer from energy loss but also from small angle scattering it is usefull to look at angle resolved energy distributions. As mentioned above, the angle a is only measured in the y±z-plane (see Fig. 2). Fig. 5 depicts energy distributions of the
O. Schmelmer et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 107±112
beam using the new slit being 4 lm open for ®ve angle intervals. It shows a strong asymmetric behaviour. This is due to the tangentially shifted slit edges: The second slit edge shadows the projectiles being scattered o the beam axis by the ®rst slit edge (see Fig. 2). It can also be observed that the low energy tail is increased and the peak at 20 MeV is less pronounced for angles o the beam axis. This indicates that projectiles that suered from energy loss are scattered o the beam axis. In Fig. 6 the ratio tail/peak in dependence of the distance of the slit edges is shown for the ®ve angle intervals. For almost all measured angles the ratio increases when the opening decreases. It is even greater than one for angles between ÿ2.5 and ÿ1.5 mrad. Only for particles detected within 0.5
111
Fig. 6. Ratio of the integral contents of the low energy tail and the peak around 20 MeV versus distance of slit edges for different angle intervals.
mrad the ratio is almost constant. The additional number of particles suered from energy loss at the new slits larger than 20 keV and without scattering to angles larger than 0.5 mrad is less than 0.6% compared to the open slits. The behaviour of the ratio shows that ions suered from energy loss are mainly scattered o the beam axis. In general, these projectiles reduce the beam quality seriously. However, since the maximal accepted angle of SNAKE de®ned by divergence slits is smaller than 0.1 mrad in both transversal directions [1] particles suered energy loss and small angle scattering are faded out by a large extent and do not in¯uence beam quality. As a result a beam halo less than 0.6% of the beam intensity is expected even for a slit aperture of 4 lm. 4. Conclusion
Fig. 5. Energy distribution of the proton beam behind the new slit being only 4 lm open for ®ve angle intervals. The asymmetric behaviour is due to the tangentially shifted slit edges.
A new high precision slit system for the planned nanobeam SNAKE has been introduced which uses mechanically strained gemanium wafers forming a cylinder with a radius of 50 mm. Besides a mechanical isolation from the beam tube a new method to compensate linear expansion of the slit due to the heating up of the aperture by the beam is engaged. A test of one slit has shown a strong correlation of small angle scattering and energy loss processes. The additional number of particles suered from energy loss at the new slits larger
112
O. Schmelmer et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 107±112
than 20 keV and without scattering to angles larger than 0.5 mrad is less than 0.6% compared to the open slits. Therefore, the new developed slits ful®ll the high requirements of SNAKE. References [1] G. Hinderer, G. Dollinger, G. Datzmann, H.J. K orner, Nucl. Instr. and Meth. B 130 (1997) 51. [2] C. Goeden, G. Dollinger, to be published.
[3] J. Adamczewski, J. Meijer, A. Stephan, H.H. Bukow, C. Rolfs, Nucl. Instr. and Meth. B 114 (1996) 172. [4] E.J. Burger, D.A. Smith, Rev. Sci. Instr. 33 (1962) 1371. [5] J.F. Ziegler, TRIM 96, [email protected]. [6] M. Takai, Y. Horino, Y. Mokuno, A. Chayahara, M. Kiuchi, K. Fujii, M. Satou, Nucl. Instr. and Meth. B 77 (1993) 8. [7] B.E. Fischer, Nucl. Instr. and Meth. B 10/11 (1985) 693. [8] M. L oer, H.-J. Scheerer, H. Vonach, Nucl. Instr. and Meth. 111 (1973) 1. [9] E. Zanotti, M. Bisenberger, R. Hertenberger, H. Kader, Nucl. Instr. and Meth. B 310 (1991) 706.