- Email: [email protected]
Improvement of AMS system at Kyushu University
Nuclear Instruments and Methods in Physics Research B 223–224 (2004) 135–138 www.elsevier.com/locate/nimb
Improvement of AMS system at Kyushu University S. Mitarai *, Y. Kanegae, K. Tanaka, H. Morikawa, T. Maeda, T. Noro, S. Morinobu Kyushu University Tandem Laboratory, Kyushu University, Hakozaki, Fukuoka 812-8581, Japan
Abstract Beam optics, charging system and voltage control system of the tandem accelerator at Kyushu University were modified for AMS which requests the high beam transmission for the measurements with high precision and reproducibility for the ratio of 14 C/12 C. The transmission of the 12 C beams at the terminal voltage of 6 MV reached 48% using the stripper carbon foil of 7 lg/cm2 . Ó 2004 Published by Elsevier B.V. PACS: 07.75.+h; 07.77.)n; 29.30.)h Keywords: Tandem accelerator; Accelerator mass spectrometry; Pellet chain; GVM control; Down Charge feedback
1. Introduction A previous 36 Cl-AMS system at Kyushu University Tandem Laboratory has identified the Znumber of chlorine by Inverse PIXE method (Projectile X-ray emission) [1]. Mass-36 particles are injected to tandem accelerator and under the same magnetic field of injection magnet; the beam current of 35 Cl was measured at the exit by the off-beam axis Faraday cup. The accelerated 36 Cl particles with 56 MeV hit a Ti foil target, which emits the X-rays of Cl and Ti atoms. The fluctuation in the beam transmission of the tandem accelerator induces the change of the measured ratio of 36 Cl/35 Cl. The sequential injection of the
*
Corresponding author. Tel.: +81-92-642-2707; fax: +81-92642-2710. E-mail address: [email protected] (S. Mitarai). 0168-583X/$ - see front matter Ó 2004 Published by Elsevier B.V. doi:10.1016/j.nimb.2004.04.029
both isotopes to the accelerator is also important for the measurement with the high resolution. The system of 14 C-AMS is popular and well known about the technical method. Therefore, the establishment of 14 C-AMS system with high precision and reproducibility is useful for isotope measurements of 10 Be, 26 Al and 36 Cl.
2. Present status of AMS system Fig. 1 shows the layout of the 14 C-AMS measurement system at Kyushu University Tandem Laboratory. The 14 C beam extracted from a Cesium sputtering ion source was pre-accelerated to 190 kV to improve the beam emittance. An aperture for the injection beam was inserted at the entrance of the injection magnet. One doublet and two triplet electro-static quadrupole lenses transport the most of the beam to the entrance of the
136
S. Mitarai et al. / Nucl. Instr. and Meth. in Phys. Res. B 223–224 (2004) 135–138
Fig. 1. The layout of the 14C-AMS measurement system at Kyushu University Tandem Laboratory. 13 C and 14 C ions are injected into the accelerator alternately.
tandem accelerator and the beam was matched with the acceptance of the accelerator tubes in the tandem by the adjustment of parameters in the lens system. The terminal voltage is controlled within the voltage fluctuations of 3 keV by a Down Charge feedback system with a voltage sensor of capacitive pick-off, coupled to a generating voltmeter (GVM). The accelerator is equipped with a carbon foil stripper of 7 lg/cm2 and a gas stripper. In the 14 C-AMS measurement, 13 C and 14 C ions are alternately placed on the injection line by the amplitude modulation of magnet current. The 13 C4þ current are measured by a Faraday cup (FC3) placed on off-axis at the exit of the 90°analyzing magnet chamber and 14 C ions by a Si detector at the end of the beam line (see Fig. 1).
3. Tandem accelerator The tandem accelerator is a horizontal machine with lots of original devices built at Kyushu University. The main body of the acceleration tube developed at the laboratory consists of Al2 O3 ceramic rings and thin Kovar diaphragms. Eleven titanium electrodes are attached inside each 25 cm length of the tube at each Kovar electrode. The baking procedure by means of the arc discharge [2] has recovered the tube from the damage with micro-discharge which suppress the high voltage
operation. All of the control systems in the tandem accelerator are made of the electrostatic devices. 3.1. Charging device A pellet of the original chain has been adhered on the insulator cord with polyester string coating by polyurethane. The speed of the charging pellet chains were operated at 14 m/s for a long time and recently changed to the half speed to reduce the physical load for the cord. 3.2. GVM control The GVM with high precision has been developed; the care in its mechanical fabrication has been taken to reduce the relative vibration between the rotor and stator. The fluctuation in the rotation has been almost reduced by AC servomotor with stability as high as 0.002%. The width of the voltage fluctuation at the terminal voltage of 6 MV is less than 3 kV by only Down Charge Feedback. 3.3. Comparison between down charge and corona feedback systems Fig. 2 shows the conceptual block diagram of the total feedback system. The GVM is only sensitive for the voltage change in the frequency range of up to no higher than the rotor shutter fre-
S. Mitarai et al. / Nucl. Instr. and Meth. in Phys. Res. B 223–224 (2004) 135–138
137
the SF6 gas in the tank to cause additional voltage instability.
4. Improvement of AMS It was recognized that high transmission of beam is essential to obtain data with high reproducibility of the 14 C/13 C ratio measurements in 14 C-AMS. Therefore, we improved the optics for carbon beam from Cs sputter ion source to a Si detector chamber. The following sentences are summary of the improvements. Fig. 2. Block diagram of the terminal voltage controller involving the GVM and CPO devices.
quency. For the detection of faster components, a part of the liner, which has been used for the feedback purpose, was electrically isolated to form a capacitive pick-off (CPO) for the terminal voltage. The system consists of four types of feedback devices with different time characteristics.
1. A new electro-static quadrupole lens was installed on the beam line downstream of the injection magnet. All of the C ions were transported from the pre-acceleration tube to the entrance of the tandem accelerator. 2. In order to get the high transmission for the accelerator, the software program [3] for beam trajectory calculation searched the condition of the best beam injection into the accelerator tube. 3. A 7 lg/cm2 thick carbon stripper foil is attached to a frame with a large hole of 12 mmf. 4. The beam line downstream of the analyzing magnet was modified for the beams to pass through perfectly. 5. GVM controller of the terminal voltage was improved to reduce the fluctuation of beam trajectory.
1. Up-charge control device delay time ¼ 500 ms 2. Down charge control device delay time ¼ 50 ms 3. Liner feedback device (capacitive feedback) time constant ¼ 10 ms 4. Corona feedback circuit short response time expected
4.1. Optimization of the beam optics
The last device was tentatively installed for examination, since the voltage response for Corona discharges has been known to be short as 10 ms. The Corona feedback system was expected to give a somewhat better performance than DCC system by the fast response. But, both systems showed the same width of the terminal voltage fluctuation at the 6 MV terminal voltage by GVM signal. The same stabilization of the both systems means that the DCC feedback was finally beneficial, since it works even at low terminal voltages where Corona discharge does not occur and since it does not produce any decomposition products of
The calculation of the accelerator acceptance shown in Fig. 3 was performed to see the matching with the emittance of the injection beam. In the calculation, a beam waist was assumed at the charge stripper with a phase ellipse. The phase ellipse was then traced back to the entrance. The largest phase ellipse thus obtained at the entrance was regarded as the acceptance of the accelerator, which was calculated to be about 23 p mm/m-rad at the terminal voltage of 6 MV. Fig. 4 shows the beam envelope in the injection line that calculated to reproduce the beam profile at the position of S1, V6 and V7. In the calculation, a virtual source
138
S. Mitarai et al. / Nucl. Instr. and Meth. in Phys. Res. B 223–224 (2004) 135–138
matching condition between the injection line and the accelerator has been searching from the viewpoint of this calculation. After the calculation, the existing quadrupole lens system in beam injection line was adjusted to match the beam emittance with the acceptance of the accelerator.
5. Performance
Fig. 3. Calculated beam envelops in the acceleration tube from the entrance to the carbon foil stripper at terminal voltage of 6 MV and calculated a phase space acceptance at the waist point F.
Fig. 4. The calculated beam envelopes in the injection line from the pre-acceleration tube to the acceleration tube entrance in the horizontal (upper) and vertical (lower) planes, respectively.
point was assumed at 830 mm upstream from the entrance of the injection magnet and the phase elapses labeled A (5 p mm-mrad) and B (6.5 p mmmrad) in the figure were assumed in the horizontal and vertical planes, respectively. The beam
The 12 C beam extracted from Cs sputter ion source (SNICS II) has an intensity distribution with a peak at the beam center and the outer beam showed the distortion on the viewer. Therefore, an aperture of 5 mmU was located on the beam center line to increase the ratio of the beam with the emittance of low value to the injection negative beam. The injected 12 C beam currents were measured by V7 and the accelerated 12 C4þ beam currents were measured by both FC3 and the detector chamber as a Faraday cup. The fraction of the 12 C4þ ions, which were transported through the accelerator at the terminal voltage of 6 MV, to the injected 12 C ions was observed to be 48% irrespective of thick stripper foil of 7 lg/cm2 and no beam loss was observed for 12 C4þ ions in the beam line between the analyzing magnet and the detector chamber. In near future, we make a plan for the thin stripper foils of 5 lg/cm2 .
References [1] S. Tolmachyov et al., J. Radional. Nucl. Chem. 251 (2) (2002) 217. [2] A. Isoya et al., Proceedings of the Third International Conference on Electrostatic Accelerator Technology, Oak Ridge, 1981, 98. [3] S. Morinobu, A computer code ORBIT2, 1987 (unpublished).
Improvement of AMS system at Kyushu University S. Mitarai *, Y. Kanegae, K. Tanaka, H. Morikawa, T. Maeda, T. Noro, S. Morinobu Kyushu University Tandem Laboratory, Kyushu University, Hakozaki, Fukuoka 812-8581, Japan
Abstract Beam optics, charging system and voltage control system of the tandem accelerator at Kyushu University were modified for AMS which requests the high beam transmission for the measurements with high precision and reproducibility for the ratio of 14 C/12 C. The transmission of the 12 C beams at the terminal voltage of 6 MV reached 48% using the stripper carbon foil of 7 lg/cm2 . Ó 2004 Published by Elsevier B.V. PACS: 07.75.+h; 07.77.)n; 29.30.)h Keywords: Tandem accelerator; Accelerator mass spectrometry; Pellet chain; GVM control; Down Charge feedback
1. Introduction A previous 36 Cl-AMS system at Kyushu University Tandem Laboratory has identified the Znumber of chlorine by Inverse PIXE method (Projectile X-ray emission) [1]. Mass-36 particles are injected to tandem accelerator and under the same magnetic field of injection magnet; the beam current of 35 Cl was measured at the exit by the off-beam axis Faraday cup. The accelerated 36 Cl particles with 56 MeV hit a Ti foil target, which emits the X-rays of Cl and Ti atoms. The fluctuation in the beam transmission of the tandem accelerator induces the change of the measured ratio of 36 Cl/35 Cl. The sequential injection of the
*
Corresponding author. Tel.: +81-92-642-2707; fax: +81-92642-2710. E-mail address: [email protected] (S. Mitarai). 0168-583X/$ - see front matter Ó 2004 Published by Elsevier B.V. doi:10.1016/j.nimb.2004.04.029
both isotopes to the accelerator is also important for the measurement with the high resolution. The system of 14 C-AMS is popular and well known about the technical method. Therefore, the establishment of 14 C-AMS system with high precision and reproducibility is useful for isotope measurements of 10 Be, 26 Al and 36 Cl.
2. Present status of AMS system Fig. 1 shows the layout of the 14 C-AMS measurement system at Kyushu University Tandem Laboratory. The 14 C beam extracted from a Cesium sputtering ion source was pre-accelerated to 190 kV to improve the beam emittance. An aperture for the injection beam was inserted at the entrance of the injection magnet. One doublet and two triplet electro-static quadrupole lenses transport the most of the beam to the entrance of the
136
S. Mitarai et al. / Nucl. Instr. and Meth. in Phys. Res. B 223–224 (2004) 135–138
Fig. 1. The layout of the 14C-AMS measurement system at Kyushu University Tandem Laboratory. 13 C and 14 C ions are injected into the accelerator alternately.
tandem accelerator and the beam was matched with the acceptance of the accelerator tubes in the tandem by the adjustment of parameters in the lens system. The terminal voltage is controlled within the voltage fluctuations of 3 keV by a Down Charge feedback system with a voltage sensor of capacitive pick-off, coupled to a generating voltmeter (GVM). The accelerator is equipped with a carbon foil stripper of 7 lg/cm2 and a gas stripper. In the 14 C-AMS measurement, 13 C and 14 C ions are alternately placed on the injection line by the amplitude modulation of magnet current. The 13 C4þ current are measured by a Faraday cup (FC3) placed on off-axis at the exit of the 90°analyzing magnet chamber and 14 C ions by a Si detector at the end of the beam line (see Fig. 1).
3. Tandem accelerator The tandem accelerator is a horizontal machine with lots of original devices built at Kyushu University. The main body of the acceleration tube developed at the laboratory consists of Al2 O3 ceramic rings and thin Kovar diaphragms. Eleven titanium electrodes are attached inside each 25 cm length of the tube at each Kovar electrode. The baking procedure by means of the arc discharge [2] has recovered the tube from the damage with micro-discharge which suppress the high voltage
operation. All of the control systems in the tandem accelerator are made of the electrostatic devices. 3.1. Charging device A pellet of the original chain has been adhered on the insulator cord with polyester string coating by polyurethane. The speed of the charging pellet chains were operated at 14 m/s for a long time and recently changed to the half speed to reduce the physical load for the cord. 3.2. GVM control The GVM with high precision has been developed; the care in its mechanical fabrication has been taken to reduce the relative vibration between the rotor and stator. The fluctuation in the rotation has been almost reduced by AC servomotor with stability as high as 0.002%. The width of the voltage fluctuation at the terminal voltage of 6 MV is less than 3 kV by only Down Charge Feedback. 3.3. Comparison between down charge and corona feedback systems Fig. 2 shows the conceptual block diagram of the total feedback system. The GVM is only sensitive for the voltage change in the frequency range of up to no higher than the rotor shutter fre-
S. Mitarai et al. / Nucl. Instr. and Meth. in Phys. Res. B 223–224 (2004) 135–138
137
the SF6 gas in the tank to cause additional voltage instability.
4. Improvement of AMS It was recognized that high transmission of beam is essential to obtain data with high reproducibility of the 14 C/13 C ratio measurements in 14 C-AMS. Therefore, we improved the optics for carbon beam from Cs sputter ion source to a Si detector chamber. The following sentences are summary of the improvements. Fig. 2. Block diagram of the terminal voltage controller involving the GVM and CPO devices.
quency. For the detection of faster components, a part of the liner, which has been used for the feedback purpose, was electrically isolated to form a capacitive pick-off (CPO) for the terminal voltage. The system consists of four types of feedback devices with different time characteristics.
1. A new electro-static quadrupole lens was installed on the beam line downstream of the injection magnet. All of the C ions were transported from the pre-acceleration tube to the entrance of the tandem accelerator. 2. In order to get the high transmission for the accelerator, the software program [3] for beam trajectory calculation searched the condition of the best beam injection into the accelerator tube. 3. A 7 lg/cm2 thick carbon stripper foil is attached to a frame with a large hole of 12 mmf. 4. The beam line downstream of the analyzing magnet was modified for the beams to pass through perfectly. 5. GVM controller of the terminal voltage was improved to reduce the fluctuation of beam trajectory.
1. Up-charge control device delay time ¼ 500 ms 2. Down charge control device delay time ¼ 50 ms 3. Liner feedback device (capacitive feedback) time constant ¼ 10 ms 4. Corona feedback circuit short response time expected
4.1. Optimization of the beam optics
The last device was tentatively installed for examination, since the voltage response for Corona discharges has been known to be short as 10 ms. The Corona feedback system was expected to give a somewhat better performance than DCC system by the fast response. But, both systems showed the same width of the terminal voltage fluctuation at the 6 MV terminal voltage by GVM signal. The same stabilization of the both systems means that the DCC feedback was finally beneficial, since it works even at low terminal voltages where Corona discharge does not occur and since it does not produce any decomposition products of
The calculation of the accelerator acceptance shown in Fig. 3 was performed to see the matching with the emittance of the injection beam. In the calculation, a beam waist was assumed at the charge stripper with a phase ellipse. The phase ellipse was then traced back to the entrance. The largest phase ellipse thus obtained at the entrance was regarded as the acceptance of the accelerator, which was calculated to be about 23 p mm/m-rad at the terminal voltage of 6 MV. Fig. 4 shows the beam envelope in the injection line that calculated to reproduce the beam profile at the position of S1, V6 and V7. In the calculation, a virtual source
138
S. Mitarai et al. / Nucl. Instr. and Meth. in Phys. Res. B 223–224 (2004) 135–138
matching condition between the injection line and the accelerator has been searching from the viewpoint of this calculation. After the calculation, the existing quadrupole lens system in beam injection line was adjusted to match the beam emittance with the acceptance of the accelerator.
5. Performance
Fig. 3. Calculated beam envelops in the acceleration tube from the entrance to the carbon foil stripper at terminal voltage of 6 MV and calculated a phase space acceptance at the waist point F.
Fig. 4. The calculated beam envelopes in the injection line from the pre-acceleration tube to the acceleration tube entrance in the horizontal (upper) and vertical (lower) planes, respectively.
point was assumed at 830 mm upstream from the entrance of the injection magnet and the phase elapses labeled A (5 p mm-mrad) and B (6.5 p mmmrad) in the figure were assumed in the horizontal and vertical planes, respectively. The beam
The 12 C beam extracted from Cs sputter ion source (SNICS II) has an intensity distribution with a peak at the beam center and the outer beam showed the distortion on the viewer. Therefore, an aperture of 5 mmU was located on the beam center line to increase the ratio of the beam with the emittance of low value to the injection negative beam. The injected 12 C beam currents were measured by V7 and the accelerated 12 C4þ beam currents were measured by both FC3 and the detector chamber as a Faraday cup. The fraction of the 12 C4þ ions, which were transported through the accelerator at the terminal voltage of 6 MV, to the injected 12 C ions was observed to be 48% irrespective of thick stripper foil of 7 lg/cm2 and no beam loss was observed for 12 C4þ ions in the beam line between the analyzing magnet and the detector chamber. In near future, we make a plan for the thin stripper foils of 5 lg/cm2 .
References [1] S. Tolmachyov et al., J. Radional. Nucl. Chem. 251 (2) (2002) 217. [2] A. Isoya et al., Proceedings of the Third International Conference on Electrostatic Accelerator Technology, Oak Ridge, 1981, 98. [3] S. Morinobu, A computer code ORBIT2, 1987 (unpublished).