- Email: [email protected]
The Fudan nuclear microprobe set-up and performance
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 260 (2007) 109–113 www.elsevier.com/locate/nimb
The Fudan nuclear microprobe set-up and performance L. Zhong, W. Zhuang, H. Shen *, Y. Mi, Y. Wu, B. Liu, M. Yang, H. Cheng Institute of Modern Physics, Applied Ion Beam Physics Laboratory, Fudan University, Shanghai 200433, PR China Available online 14 February 2007
Abstract A new scanning nuclear microprobe has been constructed at the Institute of Modern Physics in Fudan University, to replace the old microbeam system which had been running for more than ten years. The key parts were purchased from Oxford Microbeams Ltd., including triplet quadrupole lens (model OM-150), collimator slits, scanning system, target chamber, and data acquisition system. Ion beams are provided from a NEC 9SDH-2 Tandem accelerator. Three CCD cameras and multiple monitors were installed to assist beam adjust. The design of beam line and beam monitors is described. Beam optics calculations were carried out based on the specific Fudan microprobe system geometry, and the results regarding beam line performance and limitations of the spacial resolution are presented and discussed here. A comparison with experimental results is given as well. About 1.5 lm beam spot size could be achieved with a 3 MeV proton beam at a current of around 10 pA. Recently, the new microprobe is applied to obtain information of fly ash particle, algae cell and otoliths. 2007 Elsevier B.V. All rights reserved. PACS: 07.79.v; 41.75.Ak; 41.85.p Keywords: Nuclear microprobe; Beam optics; Quadrupole lens
1. Introduction With the rapid development of the nuclear microprobe technique in 1970’s, research based on aperture-collimated nuclear microprobe was carried out in the early 1980’s at Fudan University. A scanning proton microprobe (SPM), essentially a doublet quadrupole lens, was established with the cooperation of SUNY/Albany and research work was carried out in the fields of materials science, biological science and geological science [1]. However, the performance of the system was not good enough for the increasing research requirements, and most of the instrumentation was outdated. There were only a Si (Li) and an annular Au (Si) surface barrier detector (SSB) installed because the target chamber had insufficient space for other detectors. The length of the beam line was limited by geometry of the lab hall, so there was no possibility to increase *
Corresponding author. Tel.: +86 021 65642176; fax: +86 021 65642787. E-mail address: [email protected] (H. Shen). 0168-583X/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.01.283
demagnification by extending the object distance. In 2000, supported by the 211 project, the Institute of Modern Physics in Fudan University imported key parts of a new microprobe system from Oxford Microbeams Ltd., and a new microbeam system was set-up. Detectors for PIXE, RBS and STIM techniques have been installed. From beam optical calculations it was found that the demagnification of the new microprobe with a triplet quadrupole lens was better than the old system with the same length of beamline [2].
2. Beam line In the new microprobe facility, ion beams are produced using an Alpha-Tross (for a beams) or a SINCS ion source. Then the ions are injected into a NEC 9SDH-2 Tandem Accelerator. The operating voltage of the accelerator ranges from 0.3 to 3 MV with a proton beam current of 10 lA at the exit of the ion source. Fig. 1 is a layout of the Tandem accelerator and the SPM beamline. Ions are mass/energy analyzed by the
110
L. Zhong et al. / Nucl. Instr. and Meth. in Phys. Res. B 260 (2007) 109–113
Fig. 1. Layout of tandem and SPM beamline.
high-energy analyzing magnet and injected into the microprobe facility installed at the 45 line. Fig. 2 is a schematic of the SPM at Fudan University. In order to minimize vibrations, the object slits (4) and the target chamber are supported by separate systems, a solid mount and a concrete platform. Bellows (2) are used for vibration isolation between the parts. There is an observation window (5) placed downstream from the object slits and before anti-scattering slits. The ions travel through the energy stabilization slits (3) placed before the deflection coil (8). Another set of slits (3) was installed just before the object slits (4) to limit the beam current impinging on the object slits. Two valves (6) divide the microprobe system between object slits and target chamber into three parts. There is a separate vacuum system for each part. In the first part, there are a pre-pump and a turbo molecular pump (450 l/s). The vacuum system of the middle part is constructed with a prepump, a turbo molecular pump (110 l/s) and a sputter ion pump (100 l/s). The vacuum of the target chamber is provided by a pre-pump, a turbo molecular pump (450 l/s) and a sputter ion pump (100 l/s), these pumps allow the quick resumption of measurements after sample changes. The vacuum in the target chamber is about 107 torr which can be achieved in less than 3 h. The turbo molecular pump of the target chamber is shut down during experiments to reduce vibration in the SPM target chamber. To avoid beam interference by magnetic fields, the distance between the beamline and the ion pump is enlarged, and the pipe of beamline is covered with magnetic shielding material.
3. Target chamber A schematic of the target chamber is shown in Fig. 3. The target chamber is based on the OM70 design (Oxford Microbeams Ltd.), with an x–y–z target manipulator (range x, y = ± 12.5 mm, z = 0–50 mm, step size = 5 lm). Three detectors are already installed. Particle induced X-ray emission (PIXE) for detection of light, medium
Fig. 3. Schematic of the target chamber.
Fig. 2. Schematic of the new microprobe facility.
L. Zhong et al. / Nucl. Instr. and Meth. in Phys. Res. B 260 (2007) 109–113
and heavy elements; Rutherford back scattering (RBS) in order to detect lighter elements (C, O, N, etc.) as well as depth and thickness distributions of heavier elements, and scanning transmission ion microscope (STIM). A Si (Li) detector (Sirius80, Gresham Ltd.) with an energy resolution of 150 eV at 5.9 keV for PIXE is set at 135 with respect to the direction of incident ion beam. The distance between target holder and detector is adjustable for various measurements. An annular Au (Si) surface barrier detector is situated at about 170 of the beam direction for RBS measurements. For STIM, another Si surface barrier detector is placed behind the target holder. A microscope is set at 135, connected to a CCD video camera. To illuminate the target holder, a LED was installed, which produced a low thermal load. With the help of CCD and LED, it is easy to observe the samples and the position of the beam spot.
111
where I is excitation of the coil, r0 is radius of the lens opening, N is the number of turns of the coil. The magnetic induction intensity B is then 8 < Bx ¼ l0 oWðx;yÞ ¼ l0 2NI y; ox r20 ð2Þ : By ¼ l0 oWðx;yÞ ¼ l0 2NI2 x: oy r 0
l0 is permeability in vacuum. An ion with mass of m and charge of q will experience a Lorentz force 8 < F x ¼ qvBy ¼ l0 2qvNI x; r20 ð3Þ : F y ¼ qvBx ¼ l0 2qvNI y: r2 0
The equation of motion ions may be expressed as ( x00 þ k 2 x ¼ 0; y 00 k 2 y ¼ 0;
ð4Þ
where k is the a parameter of the magnetic quadrupole lens 2qNI r2 mv
4. Beam monitoring and data acquisition
k 2 ¼ l0
The control and data acquisition system of the new microprobe facility is running on a PC under MS Windows 98. The detectors of STIM, RBS, and PIXE are connected to OM1000, which convert the analog signals to digital signals. Multiple detectors can operate synchronously (maxim eight detectors) and the computer controls the scanning size (range 2 mm · 2 mm) with the software OMDAQ. Data analysis of PIXE is performed using the software package GUPIX; data from STIM is analyzed by a program based on the energy median method. The beam from our accelerator tends to fluctuate, resulting in difficulties with beam tuning. In order to optimize the feedback of the beam position to the accelerator control room, CCD cameras were installed at the observation windows and behind the target chamber. The signals were fed to two monitors, one in the accelerator control room, and the other in the microbeam control room. The camera behind the observation window of the target chamber is used to locate the beam in the target chamber. However, often the STIM detector is installed in this position, therefore we plan to solve this problem by mounting the STIM detector on a movable plate. It can then be moved away during beam adjustments, and moved back during measurements.
v is velocity of the ions. Solution to this equation is 8 x0 > xðlÞ ¼ x0 cos kl þ k0 sin kl; > > > < x0 ðlÞ ¼ x k sin kl þ x0 cos kl; 0 0 y 00 > > yðlÞ ¼ y chkl þ shkl; > 0 k > : 0 y ðlÞ ¼ y 0 kshkl þ y 00 chkl:
In order to compare with the old microbeam system, beam optics calculations were carried out with respect to the geometry conditions of the new Fudan SPM [3–5]. The usual way to focus an MeV ion beam is to use quadrupole magnets [6]. The magnetic scalar potential field W can be calculated by 2NI x y; r20
The transportation matrix of quadrupole lens in position– slope phase space is ! cosðklÞ sinðklÞ=k Tx ¼ ; k sinðklÞ cosðklÞ ð6Þ ! chðklÞ shðklÞ=k Ty ¼ : kshðklÞ chðklÞ If the beam is focused in x plane, it is defocused simultaneously in y plane. Therefore at least two quadrupole magnets are needed to focus a beam [6]. The whole transportation matrix of the triplet quadrupole lens which is used in our system is: R11x;y R12x;y Rx;y ¼ : ð7Þ R21x;y R22x;y Thus the relationship between object distance dobject and image distance dimage is given by
5. Ion optical calculation
Wðx; yÞ ¼
ð5Þ
ð1Þ
d image ¼
R12 þ R11 d obj R22 þ R21 d obj
ð8Þ
and the demagnification b could be expressed as b ¼ R11 þ d image R21 :
ð9Þ
In Fig. 4, the beam envelopes of the Oxford triplet quadrupole are shown. To obtain the minimum beam spot size, it is necessary to make both x plane and y plane focused on the target.
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L. Zhong et al. / Nucl. Instr. and Meth. in Phys. Res. B 260 (2007) 109–113
still kept as 3.5 m, which is the value of our system. The results are shown in Fig. 5(a)–(b). It is important that when the value of excitations (k value) is chosen, the ranges in which the image distance shifts sharply should be avoided, otherwise the system would lose the focus after a slight change of the magnetic quadrupole current. According to the equations above and corresponding to our microprobe facility (image distance and object distance is 0.15 m and 3.5 m, respectively), demagnification of the
Fig. 4. Schematic of optical path of the Oxford triplet quadrupole.
All the calculations for a 3.0 MeV proton beam were done by the MATLAB package under Linux. Table 1 shows the image distance dependence on the object location and k coefficient. The results indicate that image distance does not change greatly for an object distance shift from 3.0 to 4.0 m, with the same k value, so that the object distance will not influence the focus conditions. The image distance is 0.15 m, is close to our triplet package’s arrangement. Calculations performed with the computer programs PRAM [7] were also carried out. Table 2 shows that when object distance increases from 3.0 to 4.0 m while the image distance is still kept at 0.15 m, the magnetic field intensity of the quadrupole lens does not change significantly. The PRAM result is concordant with our near axial approximation. The variation in the image distance has been investigated by varying the k value, while the object distance is Table 1 Changes of dimage when dobject shifts from 3.0 to 4.0 m, with same k1, k2 k1, k2 (m1)
dobject (m) 3.0
3.1
3.2
3.3
3.4
3.5
(9.6, 8.9) (10.4, 11.2)
0.656 0.151
0.654 0.151
0.652 0.151
0.651 0.151
0.650 0.151
0.648 0.151
(9.6, 8.9) (10.4, 11.2)
3.6
3.7
3.8
3.9
4.0
0.647 0.151
0.646 0.151
0.645 0.151
0.644 0.150
0.643 0.150
Fig. 5. Relation between image distance and quadrupole lens parameters.
Table 2 Relationship between object distance and magnetic field intensity of quadrupole lens (image distance = 0.15 m) Object distance (m)
Magnetic field intensity B1a (T)
Magnetic field intensity B2 (T)
3.0 3.2 3.4 3.5 3.6 3.8 4.0
0.204308 0.204189 0.204049 0.203958 0.203883 0.203821 0.203767
0.236111 0.236005 0.235883 0.235804 0.235740 0.235686 0.235641
a The first and second quadrupole lens were coupled, they had the same excitation.
Fig. 6. Elemental map of copper grid (size 25 lm · 25 lm).
L. Zhong et al. / Nucl. Instr. and Meth. in Phys. Res. B 260 (2007) 109–113
113
Fig. 7. Elemental maps of flying ash (size 250 lm · 250 lm).
new microprobe is 50 · 14.3 (when k1 = 10.4 and k2 = 11.2). The demagnification of the old doublet quadrupole lens used at Fudan University was 14.3 · 3.3. Compared to the old one, the performance of the new facility is much better under the same geometric conditions. Since the length of our beamline is restricted by the space of the lab hall, it is important to know that the new system can get larger demagnifications with same beamline length. Calculations performed with PRAM code give demagnifications of 46 · 13.3, close to the calculations mentioned above. The results also show that the demagnification increases for larger object distances when the image distance is kept unchanged. Demagnifications change from 39.2 · 11.4 to 59.7 · 16.9, when object distance increase from 3.0 to 4.0 m (image distance = 0.15 m). The demagnification is very sensitive to the object distance, while the image distance is quite insensitive to the object distance. In the future, if the object distance could be extended, a better demagnification would be obtained without much change of the focus system.
is undergoing intensive testing with various ions and energies. So far PIXE, RBS and STIM systems have been installed. Now we are in the process of optimizing the beam intensity, installing a secondary electron imaging detector and a goniometer for STIM-CT measurements. The new facility will be mainly devoted to the environmental and biological studies. Acknowledgements This work was supported by the Chinese National Science Foundation under the Grant (Nos. 10675033 and 10490180). The authors wish to thank Dr. Jeroen Anton van Kan for commissioning the facility and thank accelerator group for good beam condition during experiments. We also would like to thank Prof. David N. Jamison for permission to use the PRAM code. For the revision of the manuscript with regard to the language, Dr. Thomas Osipowicz is gratefully acknowledged. References
6. Performance of the microprobe The performance of the new microprobe has been tested by scanning a copper grid. Present minimum beam spot size is about 1.5 lm · 2 lm obtained with a 3 MeV proton beam with a current about 10 pA. Fig. 6 shows a typical image of a copper grid with 12.5 lm repeat distance. As an example, Fig. 7 shows images of a flying ash particle. 7. Conclusions and prospect The new microprobe facility has been constructed at I.M.P, Fudan University. At the moment, the microprobe
[1] D. Zou, S. Zhou, C. Ren, F. Yang, J. Tang, Nucl. Instr. and Meth. B 104 (1995) 238. [2] G.W. Grime, F. Watt, Principles and Applications of High-energy Ion Microbeam, Adam Hilger Ltd., British, 1987, p. 150. [3] G.R. Moloney, D.N. Jamieson, G.J.F. Legge, Nucl. Instr. and Meth. B 130 (1997) 97. [4] M.B.H. Breese, D.N. Jamieson, Nucl. Instr. and Meth. B 158 (1999) 236. [5] G.W. Grime, F. Watt, Beam Optics of Quadrupole Probe-Forming System, Adam Hilger, British, 1984, p. 20. [6] S. Asad, A. Vaida, E. Mikael, K. Per, M. Klas, N. Christer, Nucl. Instr. and Meth. B 231 (2005) 1. [7] www.ph.unimelb.edu.au/marco.
Nuclear Instruments and Methods in Physics Research B 260 (2007) 109–113 www.elsevier.com/locate/nimb
The Fudan nuclear microprobe set-up and performance L. Zhong, W. Zhuang, H. Shen *, Y. Mi, Y. Wu, B. Liu, M. Yang, H. Cheng Institute of Modern Physics, Applied Ion Beam Physics Laboratory, Fudan University, Shanghai 200433, PR China Available online 14 February 2007
Abstract A new scanning nuclear microprobe has been constructed at the Institute of Modern Physics in Fudan University, to replace the old microbeam system which had been running for more than ten years. The key parts were purchased from Oxford Microbeams Ltd., including triplet quadrupole lens (model OM-150), collimator slits, scanning system, target chamber, and data acquisition system. Ion beams are provided from a NEC 9SDH-2 Tandem accelerator. Three CCD cameras and multiple monitors were installed to assist beam adjust. The design of beam line and beam monitors is described. Beam optics calculations were carried out based on the specific Fudan microprobe system geometry, and the results regarding beam line performance and limitations of the spacial resolution are presented and discussed here. A comparison with experimental results is given as well. About 1.5 lm beam spot size could be achieved with a 3 MeV proton beam at a current of around 10 pA. Recently, the new microprobe is applied to obtain information of fly ash particle, algae cell and otoliths. 2007 Elsevier B.V. All rights reserved. PACS: 07.79.v; 41.75.Ak; 41.85.p Keywords: Nuclear microprobe; Beam optics; Quadrupole lens
1. Introduction With the rapid development of the nuclear microprobe technique in 1970’s, research based on aperture-collimated nuclear microprobe was carried out in the early 1980’s at Fudan University. A scanning proton microprobe (SPM), essentially a doublet quadrupole lens, was established with the cooperation of SUNY/Albany and research work was carried out in the fields of materials science, biological science and geological science [1]. However, the performance of the system was not good enough for the increasing research requirements, and most of the instrumentation was outdated. There were only a Si (Li) and an annular Au (Si) surface barrier detector (SSB) installed because the target chamber had insufficient space for other detectors. The length of the beam line was limited by geometry of the lab hall, so there was no possibility to increase *
Corresponding author. Tel.: +86 021 65642176; fax: +86 021 65642787. E-mail address: [email protected] (H. Shen). 0168-583X/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.01.283
demagnification by extending the object distance. In 2000, supported by the 211 project, the Institute of Modern Physics in Fudan University imported key parts of a new microprobe system from Oxford Microbeams Ltd., and a new microbeam system was set-up. Detectors for PIXE, RBS and STIM techniques have been installed. From beam optical calculations it was found that the demagnification of the new microprobe with a triplet quadrupole lens was better than the old system with the same length of beamline [2].
2. Beam line In the new microprobe facility, ion beams are produced using an Alpha-Tross (for a beams) or a SINCS ion source. Then the ions are injected into a NEC 9SDH-2 Tandem Accelerator. The operating voltage of the accelerator ranges from 0.3 to 3 MV with a proton beam current of 10 lA at the exit of the ion source. Fig. 1 is a layout of the Tandem accelerator and the SPM beamline. Ions are mass/energy analyzed by the
110
L. Zhong et al. / Nucl. Instr. and Meth. in Phys. Res. B 260 (2007) 109–113
Fig. 1. Layout of tandem and SPM beamline.
high-energy analyzing magnet and injected into the microprobe facility installed at the 45 line. Fig. 2 is a schematic of the SPM at Fudan University. In order to minimize vibrations, the object slits (4) and the target chamber are supported by separate systems, a solid mount and a concrete platform. Bellows (2) are used for vibration isolation between the parts. There is an observation window (5) placed downstream from the object slits and before anti-scattering slits. The ions travel through the energy stabilization slits (3) placed before the deflection coil (8). Another set of slits (3) was installed just before the object slits (4) to limit the beam current impinging on the object slits. Two valves (6) divide the microprobe system between object slits and target chamber into three parts. There is a separate vacuum system for each part. In the first part, there are a pre-pump and a turbo molecular pump (450 l/s). The vacuum system of the middle part is constructed with a prepump, a turbo molecular pump (110 l/s) and a sputter ion pump (100 l/s). The vacuum of the target chamber is provided by a pre-pump, a turbo molecular pump (450 l/s) and a sputter ion pump (100 l/s), these pumps allow the quick resumption of measurements after sample changes. The vacuum in the target chamber is about 107 torr which can be achieved in less than 3 h. The turbo molecular pump of the target chamber is shut down during experiments to reduce vibration in the SPM target chamber. To avoid beam interference by magnetic fields, the distance between the beamline and the ion pump is enlarged, and the pipe of beamline is covered with magnetic shielding material.
3. Target chamber A schematic of the target chamber is shown in Fig. 3. The target chamber is based on the OM70 design (Oxford Microbeams Ltd.), with an x–y–z target manipulator (range x, y = ± 12.5 mm, z = 0–50 mm, step size = 5 lm). Three detectors are already installed. Particle induced X-ray emission (PIXE) for detection of light, medium
Fig. 3. Schematic of the target chamber.
Fig. 2. Schematic of the new microprobe facility.
L. Zhong et al. / Nucl. Instr. and Meth. in Phys. Res. B 260 (2007) 109–113
and heavy elements; Rutherford back scattering (RBS) in order to detect lighter elements (C, O, N, etc.) as well as depth and thickness distributions of heavier elements, and scanning transmission ion microscope (STIM). A Si (Li) detector (Sirius80, Gresham Ltd.) with an energy resolution of 150 eV at 5.9 keV for PIXE is set at 135 with respect to the direction of incident ion beam. The distance between target holder and detector is adjustable for various measurements. An annular Au (Si) surface barrier detector is situated at about 170 of the beam direction for RBS measurements. For STIM, another Si surface barrier detector is placed behind the target holder. A microscope is set at 135, connected to a CCD video camera. To illuminate the target holder, a LED was installed, which produced a low thermal load. With the help of CCD and LED, it is easy to observe the samples and the position of the beam spot.
111
where I is excitation of the coil, r0 is radius of the lens opening, N is the number of turns of the coil. The magnetic induction intensity B is then 8 < Bx ¼ l0 oWðx;yÞ ¼ l0 2NI y; ox r20 ð2Þ : By ¼ l0 oWðx;yÞ ¼ l0 2NI2 x: oy r 0
l0 is permeability in vacuum. An ion with mass of m and charge of q will experience a Lorentz force 8 < F x ¼ qvBy ¼ l0 2qvNI x; r20 ð3Þ : F y ¼ qvBx ¼ l0 2qvNI y: r2 0
The equation of motion ions may be expressed as ( x00 þ k 2 x ¼ 0; y 00 k 2 y ¼ 0;
ð4Þ
where k is the a parameter of the magnetic quadrupole lens 2qNI r2 mv
4. Beam monitoring and data acquisition
k 2 ¼ l0
The control and data acquisition system of the new microprobe facility is running on a PC under MS Windows 98. The detectors of STIM, RBS, and PIXE are connected to OM1000, which convert the analog signals to digital signals. Multiple detectors can operate synchronously (maxim eight detectors) and the computer controls the scanning size (range 2 mm · 2 mm) with the software OMDAQ. Data analysis of PIXE is performed using the software package GUPIX; data from STIM is analyzed by a program based on the energy median method. The beam from our accelerator tends to fluctuate, resulting in difficulties with beam tuning. In order to optimize the feedback of the beam position to the accelerator control room, CCD cameras were installed at the observation windows and behind the target chamber. The signals were fed to two monitors, one in the accelerator control room, and the other in the microbeam control room. The camera behind the observation window of the target chamber is used to locate the beam in the target chamber. However, often the STIM detector is installed in this position, therefore we plan to solve this problem by mounting the STIM detector on a movable plate. It can then be moved away during beam adjustments, and moved back during measurements.
v is velocity of the ions. Solution to this equation is 8 x0 > xðlÞ ¼ x0 cos kl þ k0 sin kl; > > > < x0 ðlÞ ¼ x k sin kl þ x0 cos kl; 0 0 y 00 > > yðlÞ ¼ y chkl þ shkl; > 0 k > : 0 y ðlÞ ¼ y 0 kshkl þ y 00 chkl:
In order to compare with the old microbeam system, beam optics calculations were carried out with respect to the geometry conditions of the new Fudan SPM [3–5]. The usual way to focus an MeV ion beam is to use quadrupole magnets [6]. The magnetic scalar potential field W can be calculated by 2NI x y; r20
The transportation matrix of quadrupole lens in position– slope phase space is ! cosðklÞ sinðklÞ=k Tx ¼ ; k sinðklÞ cosðklÞ ð6Þ ! chðklÞ shðklÞ=k Ty ¼ : kshðklÞ chðklÞ If the beam is focused in x plane, it is defocused simultaneously in y plane. Therefore at least two quadrupole magnets are needed to focus a beam [6]. The whole transportation matrix of the triplet quadrupole lens which is used in our system is: R11x;y R12x;y Rx;y ¼ : ð7Þ R21x;y R22x;y Thus the relationship between object distance dobject and image distance dimage is given by
5. Ion optical calculation
Wðx; yÞ ¼
ð5Þ
ð1Þ
d image ¼
R12 þ R11 d obj R22 þ R21 d obj
ð8Þ
and the demagnification b could be expressed as b ¼ R11 þ d image R21 :
ð9Þ
In Fig. 4, the beam envelopes of the Oxford triplet quadrupole are shown. To obtain the minimum beam spot size, it is necessary to make both x plane and y plane focused on the target.
112
L. Zhong et al. / Nucl. Instr. and Meth. in Phys. Res. B 260 (2007) 109–113
still kept as 3.5 m, which is the value of our system. The results are shown in Fig. 5(a)–(b). It is important that when the value of excitations (k value) is chosen, the ranges in which the image distance shifts sharply should be avoided, otherwise the system would lose the focus after a slight change of the magnetic quadrupole current. According to the equations above and corresponding to our microprobe facility (image distance and object distance is 0.15 m and 3.5 m, respectively), demagnification of the
Fig. 4. Schematic of optical path of the Oxford triplet quadrupole.
All the calculations for a 3.0 MeV proton beam were done by the MATLAB package under Linux. Table 1 shows the image distance dependence on the object location and k coefficient. The results indicate that image distance does not change greatly for an object distance shift from 3.0 to 4.0 m, with the same k value, so that the object distance will not influence the focus conditions. The image distance is 0.15 m, is close to our triplet package’s arrangement. Calculations performed with the computer programs PRAM [7] were also carried out. Table 2 shows that when object distance increases from 3.0 to 4.0 m while the image distance is still kept at 0.15 m, the magnetic field intensity of the quadrupole lens does not change significantly. The PRAM result is concordant with our near axial approximation. The variation in the image distance has been investigated by varying the k value, while the object distance is Table 1 Changes of dimage when dobject shifts from 3.0 to 4.0 m, with same k1, k2 k1, k2 (m1)
dobject (m) 3.0
3.1
3.2
3.3
3.4
3.5
(9.6, 8.9) (10.4, 11.2)
0.656 0.151
0.654 0.151
0.652 0.151
0.651 0.151
0.650 0.151
0.648 0.151
(9.6, 8.9) (10.4, 11.2)
3.6
3.7
3.8
3.9
4.0
0.647 0.151
0.646 0.151
0.645 0.151
0.644 0.150
0.643 0.150
Fig. 5. Relation between image distance and quadrupole lens parameters.
Table 2 Relationship between object distance and magnetic field intensity of quadrupole lens (image distance = 0.15 m) Object distance (m)
Magnetic field intensity B1a (T)
Magnetic field intensity B2 (T)
3.0 3.2 3.4 3.5 3.6 3.8 4.0
0.204308 0.204189 0.204049 0.203958 0.203883 0.203821 0.203767
0.236111 0.236005 0.235883 0.235804 0.235740 0.235686 0.235641
a The first and second quadrupole lens were coupled, they had the same excitation.
Fig. 6. Elemental map of copper grid (size 25 lm · 25 lm).
L. Zhong et al. / Nucl. Instr. and Meth. in Phys. Res. B 260 (2007) 109–113
113
Fig. 7. Elemental maps of flying ash (size 250 lm · 250 lm).
new microprobe is 50 · 14.3 (when k1 = 10.4 and k2 = 11.2). The demagnification of the old doublet quadrupole lens used at Fudan University was 14.3 · 3.3. Compared to the old one, the performance of the new facility is much better under the same geometric conditions. Since the length of our beamline is restricted by the space of the lab hall, it is important to know that the new system can get larger demagnifications with same beamline length. Calculations performed with PRAM code give demagnifications of 46 · 13.3, close to the calculations mentioned above. The results also show that the demagnification increases for larger object distances when the image distance is kept unchanged. Demagnifications change from 39.2 · 11.4 to 59.7 · 16.9, when object distance increase from 3.0 to 4.0 m (image distance = 0.15 m). The demagnification is very sensitive to the object distance, while the image distance is quite insensitive to the object distance. In the future, if the object distance could be extended, a better demagnification would be obtained without much change of the focus system.
is undergoing intensive testing with various ions and energies. So far PIXE, RBS and STIM systems have been installed. Now we are in the process of optimizing the beam intensity, installing a secondary electron imaging detector and a goniometer for STIM-CT measurements. The new facility will be mainly devoted to the environmental and biological studies. Acknowledgements This work was supported by the Chinese National Science Foundation under the Grant (Nos. 10675033 and 10490180). The authors wish to thank Dr. Jeroen Anton van Kan for commissioning the facility and thank accelerator group for good beam condition during experiments. We also would like to thank Prof. David N. Jamison for permission to use the PRAM code. For the revision of the manuscript with regard to the language, Dr. Thomas Osipowicz is gratefully acknowledged. References
6. Performance of the microprobe The performance of the new microprobe has been tested by scanning a copper grid. Present minimum beam spot size is about 1.5 lm · 2 lm obtained with a 3 MeV proton beam with a current about 10 pA. Fig. 6 shows a typical image of a copper grid with 12.5 lm repeat distance. As an example, Fig. 7 shows images of a flying ash particle. 7. Conclusions and prospect The new microprobe facility has been constructed at I.M.P, Fudan University. At the moment, the microprobe
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