- Email: [email protected]
Improvements to the PKUAMS injection system
Nuclear Instruments and Methods in Physics Research B 123 (1997) 84-86
Beam Interactions with Materials 8 Atoms
ELSEVIER
Improvements to the PKUAMS injection system Lu Xiangyang a**, Li Kun a, Zhao Qiang a, Li Bin a, Liu Kexin a, Guo Zhiyu b, Wang Jianjun b, Yuan Jinglin b a Department ofTechnical Physics, Peking University. Beijing 100871, China b Institute of Heavy Ion Physics, Peking University. Beijing 100871, China
Abstract In the PKUAMS injection system upgrade project, we adjusted smallest emittance and we also redesigned the sample changing sample change process. The other improvement is that we have personal computer. It is more stable, reliable and easy to use. With of the beam optics, now, PKUAMS has the ability to measure the
1. Introduction The Peking University Accelerator Mass Spectrometry (PKUAMS) facility started routine operation in 1993. Since that time we have mainly measured 14C and “Be for some applications in geology, archaeology, oceanology, environment and biomedicine and we have obtained a series of valuable results[l-31. In the past three years, we also found some problems during measurements, which are concentrated on the injection section. In order to obtain higher precision and throughput, we decided to improve the injection system, including the ion source, physical and mechanical, and the electrical devices control system.
2. Ion source improvement At the beginning of PKUAMS project, we used a modified Hiconex 834 sputter ion source, with only 12 target positions on the target wheel. We have replaced this with a high current sputter source - a modified General Ionex Corporation (GIG) 860, which was made by the Shanghai Institute for Nuclear Research (SINR) especially for PKUAMS. This new source has 20 target positions on the target wheel. The performance of the new ion source has been reported in Ref. [4]. To avoid cross contamina-
* Corresponding author. Fax: [email protected].
+ 86-01 O-6275- 1875;
email:
the ion source parameters to get better Cs focus and the mechanical structure. This provides for a more reliable rebuilt injection control system by using an industrial the improvement of sample preparation and optimization sample which is less than 100 p,gC.
tion, the SINR ion source is designed to operate so that only the sample to be measured is pushed into the sputter point, all other targets are kept in the target changer wheel and this is 10 cm from the sputter point. The target insertion and retraction is operated by a four-finger bar. There are two leaf spring sets to help the four-finger bar to hold the target tight when the target is inserted. The leaf springs should not hinder the target retraction process. We find that it is necessary to change the leaf springs after 100 h of operation. Unless this is done, incorrect operation of the springs causes a target to fall down from the four-finger target holder both in the inserted and retraction modes. The most serious occurrence was when a target was dropped into the vacuum lock valve. When this accident happened, we had to interrupt the measurement and it took more than four hours to solve the problem. We redesigned the target-changing mechanical device and replaced the four-finger structure by a plug-socket structure. On the target wheel, we use a pinch-ball instead of the leaf springs to ensure the pushing bar is locked with the sample holder. The new target changer mechanism is shown in Fig. 1. There is a small stick fixed to sample holder. When sample holder is placed on the target wheel in the correct position, the stick is behind the pinch-ball. During the insert process, the pushing bar is inserted into the sample holder and the pinch-ball is about 0.8 mm higher than the stick. This prevents the sample holder from moving forward. It provides enough force to lock the pushing bar with the sample holder. Then, the pinch-ball is pressed down by the insertion driving force from the
0168-583X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved PII SOl68-583X(96)00744-6
Lu Xiangyang er al./Nucl.
Instr. and Meth. in Phys. Res. B 123 (1997) 84-86
stepper motor, and the sample holder is left the target wheel (see Fig. 2a). In the retraction process, the pinch-ball hinders the stick again, but it does not affect the interlock. When the sample holder passes over the pinch-ball and reaches the end of target wheel, the target wheel stops the sample holder sleeve. The pushing bar is moved back and the sample holder sleeve will be pressed back and shifted 1 mm, then the lock is released (Fig. 2b). Compared with the old one, this new design is much more reliable. In many different applications, the researcher can only provide a limited amount of sample. The ability to measure such small samples less than the normal size (- 1 mg Cl is important. In our experience, one important problem for the measurement of small sample is reduction of beam current intensity and the limited counting time. We find that, under normal conditions, the target surface will be sputtered to a deep, small pit with the well-focused Cs beam. Most of the sample is not used. For more efficient use of samples, we modified the ion source operation parameters, and allowed the Cs focus spot to cover the majority of the sample surface area. This had the result that the beam current is not obviously different from the finely-focussed Cs spot, but the beam life time, for a 100 pgC sample, is doubled.
3. Industrial personal computer injection system
85
(IPC) control for the
In our injector section, equipments are separated on four different potential levels. That is the ground, preacceleration, beam extraction and Cs sputter source float at different potentials. Control of these different units is a problem. Our original remote control used Plexiglas bars, driven by motors, to tune variable resistors[5]. In this method, it is really not easy to get high precision tunings and also high stability. On the other hand, we have to use a camera/monitor couple to read the instrument indicators. In order to enhance stability, precision, and convenience of operation, we decided to use a computer control system to upgrade the injection control system. A block diagram of the new control system is shown in Fig. 3. Most of the equipment is controlled by the 12 Bit ADC/DAC except the injection magnet, which requires 16-Bit control precision. The local controller is a 486 DX/66 IPC which is floated on the pre-acceleration potential. In the control room, there is another 486 PC. Both computers are Windows-driven. The communication between the computers is by an interface IEEE 422. The target change is also controlled by computer. The period between changing
The target changer structure Fig. 1. Schematic
drawing of PKUAMS new target changer divce.
111. EXISTING
FACILITIES
Lu Xiangyang et al./Nucl.
86
Instr. and Meth. in Phys. Res. B 123 (1997) 84-86
targets has been reduced from 40 s to 20 s. This provides the possibility of incorporating the target changing system with control by the data acquisition system to achieve the total automatic measurement. This improvement will be made in future.
4. Summary
The lock is solved m
The improvements discussed give more reliable and better performance of our injection system. With the improvement of sample preparation and the optimization of the beam optics, PKUAMS has the ability to measure samples of less than 100 pg C. To improve higher precision and throughput, we continue to improve our AMS. The recent upgrade is a new start. In the next three years, we expect our measurement precision can be improved to better than 5 X 10p3, and the throughput of samples can be greater than 1500 samples per year.
References
The rod is locked with a target Fig. 2. (a) The bar is pushed in and locked with target. (b) The interlock is released when the target slops at the end of target wheel.
opticiulu6ce nlrGND
Fig. 3. Block diagram of the new injection control system.
111 Chen Chia-erh, Li Kun, et al., Nucl. Instr. and Meth. B 52 (1990) 306. [2] Chen Chia-erh, Guo Zhiyu, et al., Nucl. Instr. and Meth. B 92 t 1994) 47. (31 Liu Yuanfang and Guo Zhiyu, et al., Pure Appl. Chem. 66 ( 1994) 305. [41 Si Houzhi and Zhang Weizhong, et al., Rev. Sci. Instr. 63 ( 1992) 2472. [51 Lu Xiangyang and Zhang Ruju, et al., Collect. Ocean. Works 16 (1993) 14.
Beam Interactions with Materials 8 Atoms
ELSEVIER
Improvements to the PKUAMS injection system Lu Xiangyang a**, Li Kun a, Zhao Qiang a, Li Bin a, Liu Kexin a, Guo Zhiyu b, Wang Jianjun b, Yuan Jinglin b a Department ofTechnical Physics, Peking University. Beijing 100871, China b Institute of Heavy Ion Physics, Peking University. Beijing 100871, China
Abstract In the PKUAMS injection system upgrade project, we adjusted smallest emittance and we also redesigned the sample changing sample change process. The other improvement is that we have personal computer. It is more stable, reliable and easy to use. With of the beam optics, now, PKUAMS has the ability to measure the
1. Introduction The Peking University Accelerator Mass Spectrometry (PKUAMS) facility started routine operation in 1993. Since that time we have mainly measured 14C and “Be for some applications in geology, archaeology, oceanology, environment and biomedicine and we have obtained a series of valuable results[l-31. In the past three years, we also found some problems during measurements, which are concentrated on the injection section. In order to obtain higher precision and throughput, we decided to improve the injection system, including the ion source, physical and mechanical, and the electrical devices control system.
2. Ion source improvement At the beginning of PKUAMS project, we used a modified Hiconex 834 sputter ion source, with only 12 target positions on the target wheel. We have replaced this with a high current sputter source - a modified General Ionex Corporation (GIG) 860, which was made by the Shanghai Institute for Nuclear Research (SINR) especially for PKUAMS. This new source has 20 target positions on the target wheel. The performance of the new ion source has been reported in Ref. [4]. To avoid cross contamina-
* Corresponding author. Fax: [email protected].
+ 86-01 O-6275- 1875;
email:
the ion source parameters to get better Cs focus and the mechanical structure. This provides for a more reliable rebuilt injection control system by using an industrial the improvement of sample preparation and optimization sample which is less than 100 p,gC.
tion, the SINR ion source is designed to operate so that only the sample to be measured is pushed into the sputter point, all other targets are kept in the target changer wheel and this is 10 cm from the sputter point. The target insertion and retraction is operated by a four-finger bar. There are two leaf spring sets to help the four-finger bar to hold the target tight when the target is inserted. The leaf springs should not hinder the target retraction process. We find that it is necessary to change the leaf springs after 100 h of operation. Unless this is done, incorrect operation of the springs causes a target to fall down from the four-finger target holder both in the inserted and retraction modes. The most serious occurrence was when a target was dropped into the vacuum lock valve. When this accident happened, we had to interrupt the measurement and it took more than four hours to solve the problem. We redesigned the target-changing mechanical device and replaced the four-finger structure by a plug-socket structure. On the target wheel, we use a pinch-ball instead of the leaf springs to ensure the pushing bar is locked with the sample holder. The new target changer mechanism is shown in Fig. 1. There is a small stick fixed to sample holder. When sample holder is placed on the target wheel in the correct position, the stick is behind the pinch-ball. During the insert process, the pushing bar is inserted into the sample holder and the pinch-ball is about 0.8 mm higher than the stick. This prevents the sample holder from moving forward. It provides enough force to lock the pushing bar with the sample holder. Then, the pinch-ball is pressed down by the insertion driving force from the
0168-583X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved PII SOl68-583X(96)00744-6
Lu Xiangyang er al./Nucl.
Instr. and Meth. in Phys. Res. B 123 (1997) 84-86
stepper motor, and the sample holder is left the target wheel (see Fig. 2a). In the retraction process, the pinch-ball hinders the stick again, but it does not affect the interlock. When the sample holder passes over the pinch-ball and reaches the end of target wheel, the target wheel stops the sample holder sleeve. The pushing bar is moved back and the sample holder sleeve will be pressed back and shifted 1 mm, then the lock is released (Fig. 2b). Compared with the old one, this new design is much more reliable. In many different applications, the researcher can only provide a limited amount of sample. The ability to measure such small samples less than the normal size (- 1 mg Cl is important. In our experience, one important problem for the measurement of small sample is reduction of beam current intensity and the limited counting time. We find that, under normal conditions, the target surface will be sputtered to a deep, small pit with the well-focused Cs beam. Most of the sample is not used. For more efficient use of samples, we modified the ion source operation parameters, and allowed the Cs focus spot to cover the majority of the sample surface area. This had the result that the beam current is not obviously different from the finely-focussed Cs spot, but the beam life time, for a 100 pgC sample, is doubled.
3. Industrial personal computer injection system
85
(IPC) control for the
In our injector section, equipments are separated on four different potential levels. That is the ground, preacceleration, beam extraction and Cs sputter source float at different potentials. Control of these different units is a problem. Our original remote control used Plexiglas bars, driven by motors, to tune variable resistors[5]. In this method, it is really not easy to get high precision tunings and also high stability. On the other hand, we have to use a camera/monitor couple to read the instrument indicators. In order to enhance stability, precision, and convenience of operation, we decided to use a computer control system to upgrade the injection control system. A block diagram of the new control system is shown in Fig. 3. Most of the equipment is controlled by the 12 Bit ADC/DAC except the injection magnet, which requires 16-Bit control precision. The local controller is a 486 DX/66 IPC which is floated on the pre-acceleration potential. In the control room, there is another 486 PC. Both computers are Windows-driven. The communication between the computers is by an interface IEEE 422. The target change is also controlled by computer. The period between changing
The target changer structure Fig. 1. Schematic
drawing of PKUAMS new target changer divce.
111. EXISTING
FACILITIES
Lu Xiangyang et al./Nucl.
86
Instr. and Meth. in Phys. Res. B 123 (1997) 84-86
targets has been reduced from 40 s to 20 s. This provides the possibility of incorporating the target changing system with control by the data acquisition system to achieve the total automatic measurement. This improvement will be made in future.
4. Summary
The lock is solved m
The improvements discussed give more reliable and better performance of our injection system. With the improvement of sample preparation and the optimization of the beam optics, PKUAMS has the ability to measure samples of less than 100 pg C. To improve higher precision and throughput, we continue to improve our AMS. The recent upgrade is a new start. In the next three years, we expect our measurement precision can be improved to better than 5 X 10p3, and the throughput of samples can be greater than 1500 samples per year.
References
The rod is locked with a target Fig. 2. (a) The bar is pushed in and locked with target. (b) The interlock is released when the target slops at the end of target wheel.
opticiulu6ce nlrGND
Fig. 3. Block diagram of the new injection control system.
111 Chen Chia-erh, Li Kun, et al., Nucl. Instr. and Meth. B 52 (1990) 306. [2] Chen Chia-erh, Guo Zhiyu, et al., Nucl. Instr. and Meth. B 92 t 1994) 47. (31 Liu Yuanfang and Guo Zhiyu, et al., Pure Appl. Chem. 66 ( 1994) 305. [41 Si Houzhi and Zhang Weizhong, et al., Rev. Sci. Instr. 63 ( 1992) 2472. [51 Lu Xiangyang and Zhang Ruju, et al., Collect. Ocean. Works 16 (1993) 14.