- Email: [email protected]
Current status of the AMS system at the University of Tokyo
Nuclear Instruments
and Methods in Physics Research B 123 ( 1997) lO7- I I I
Beam Interactions with Materials &IAtoms
ELSEVIER
Current status of the AMS system at the University of Tokyo K. Kobayashi a.* , S. Hatori a, H. Nagai b, K. Yoshida ‘, M. Imamura d, H. Matsuzaki e, M. Murayama f, M. Sakamoto g, Y. Aramaki f, M. Tanikawa e, S. Shibata d, H. Ohashi h, U. Zoppi a, C. Nakano a, Y. Sunohara a, T. Saitoh g a Research Center ji)r Nuclenr Science and Technology, The University of Tokyo. Yuyoi Z-11-16, Bunkyo-ku. Tokyo 113. Japun b College of Humanities and Sciences, Nihon University, Tokyo 156. Japun ’ The University Museum, The University of Tokyo. Tokyo 113, Japan ’ Institute for Nuclear Study, The University of Tokyo. Tokyo 188, Jopun e Faculty of Science, The University of Tokyo, Tokyo 113. Jupan t Graduate School of Environmental Eurth Science, Hokkaido University. Sapporo 060. Japan g National Institute of Japanese History. Chiha 285, Japan h The department of Liberal Arts. Tokyo University of Fisheries. Tokyo 108, Jupon
Abstract AMS has been applied to many application researches at the new tandem accelerator facility of the University of Tokyo since April of 1995. The reproducibility of the measurements has not been good enough especially for 14C-AMS, although, the larger acceptance of the stripper canal is found useful for it to be better. The minimum detection sensitivity is on the level of practical use.
1. Introduction The tandem accelerator facility MALT (Micro Analysis Laboratory, Tandem accelerator) was installed in the Research Center for Nuclear Science and Technology at the University of Tokyo. MALT was designed with special emphasis on micro-analytical studies using techniques like AMS, PIXE, RBS, ERD, NRA and nuclear microprobe analysis [I]. It was completed in 1994 and has been open to the University scholars since April of 1995. The new facility is equipped with a 5 MV tandem accelerator, two ion sources for gas and solid materials, nine beam lines dedicated to micro-analytical researches and many special items designed for AMS experiments [2]. Since MALT has been responsible for many research projects other than AMS applications, only about 40% of its total machine time has been used for AMS programs, including research and development studies. In the last one year, roughly 400 and 200 samples were measured by “Be-AMS and “AlAMS, respectively, while only a few samples have been measured by 14C-AMS, because there have still remained reproducibility problems. In the case of “C-AMS, the larger relative mass-difference of the isotopes is found to have a larger effect on the beam transmission than in the case of 26A1-AMS.
* Corresponding author. Tel./Fax: [email protected].
(81)3-5802-3361; email:
In this paper, we report the outline of the MALT-AMS system and its performance, putting a stress on the mass effect for the beam transport.
2. AMS system at the University of Tokyo; MALT-AMS Fig. 1 shows a schematic drawing of the AMS system of MALT. As the details of the AMS system and MALT were described in the other papers [l,2], only a brief description of the AMS system follows. The ion source MC-SNICS can be loaded with 40 samples on a rotating cathode wheel. The sample is pressed into each hole of a sample cathode with diameter of I mm. Carbon negative ion current from the source can be larger than 100 pA from a graphite cathode, and is controlled down to roughly 30 pA for the use of 14C-AMS. BeOand Al- ion currents are typically 1 FA and 0.1 pA. respectively. The electrostatic spherical analyzer (45”, r = 30 cm, gap = 5 cm) is used in order to select the ion source and to energy-analyze the ion beam to be injected into the accelerator, with the energy resolution of about 100. The 90” injection magnet can analyze ions with a mass-energy product of 15 with a mass resolution of 200. The internal clearance in the magnet chamber is designed to be wide enough (3.8 cm) for 100% transmission. By biasing the insulated magnet chamber with appropriate voltage in the fixed magnetic field, we can select quickly
0168.583X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved PII SO 16%583X(96)007 I6- I
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of scattered ions. At the down stream position of the magnet near the focal line, three sets of off-axis Faraday cup are installed, the positions of which are remotely adjustable. The Faraday cups measure the stable ion beam currents and the current difference measured by a set of slit jaws in front of one of the cups is used to stabilize the terminal voltage. This multi-Faraday cup system is used for both the beam monitor and sequential injection method. AMS beam line is installed on the 20” line of the switching magnet. The electrostatic cylindrical analyzer (ECA; r = 6 m, gap = 3 cm> on this line deflects the ion beam by 20”. The configuration of the ECA and the triplet quadrupole lens gives an energy resolution of about 450 at the heavy ion detector position. Two types of heavy ion detection system are provided for the particle identification. One is for 14C and 26Al measurements and consists of a silicon surface-barrier detector @SD) covered with a Havar foil absorber with thickness of 2.2 pm. Another is for “Be detection by an SSD with a gas absorber. These two detectors align along the beam line. A A E-E gas ionization chamber is planned for use in the near future. A gas-filled magnet (GFM) [7-91 is now under construction to be installed soon. The radius of the main ions is 63 cm. At the exit of the magnet, an X-ray detection system [lo,1 l] with a proper target foil will be installed. The X-ray measurements are expected to improve the
the ions to be injected into the accelerator. This process is referred to as “sequential injection” [3,4]. By using three offset Faraday cups installed near the focal line of the magnet, we measure the ratio of the negative ion current from the ion source when the “internal beam monitor method” [5,6] is used. The accelerator is a vertically mounted tandem Pelletron with designed terminal voltage of 5 MV. Both gas and foil strippers are provided in the high voltage terminal for the charge exchange system. Two sets of turbo-molecular pump are installed to concentrate the stripper gas (argon) into the stripper canal in order to reduce the background caused by the gas flow to the accelerator tubes. Fluctuation of the terminal voltage had been as high as k 1.5 kV at 5 MV. However, the voltage fluctuation has been reduced to f.500 V after applying fluorocarbonloaded grease to the surface of the charging chains, although it is difficult to maintain this stability under all conditions. After being focused by an electrostatic quadrupole lens, ions are analyzed by the positive ion 90” bending magnet (r = 1.270 m, gap = 3.4 cm). The magnet can analyze the ions with mass-energy product of up to 150, which enables us to analyze lz916+ ions with the energy of 35 MeV. The pole piece of the magnet is designed to transmit simultaneously ions having a wide mass range. The inner wall of the magnet chamber has baffles for the reduction 45’Electrostatic spherical analyzer@ = 305 mm)
Pre-accelerator MC-SNICS (40 samples)
Electrostatic quadmpole triplet XandY steerers
90”Analyzer magnet (p = 1270 mm, ME/Z=1
Fig. 1.Schematic drawing of MALT-AMS system. Although only the AMS course is shown in the figure, MALT has nine heam courses for micro analytical researches. The gas-filled magnet will he installed by the end of May, 1996. The TOF system has heen set already and is ready for use.
K. Kobayashi et al./Nucl.
Instr. and Meth. in Phys. Rex B 123 (19971 107-11 I
analyzing power of the magnet. The GFM system will be used to distinguish 36C1 ions from 36S ions and “Ni from 59co. A TOF system for ‘291-AMS has been already installed in the AMS beam line and is ready for use.
3. Performance oping stage
of the MALT-AMS
in the initial devel-
We have made substantial progress on AMS in these 12 months despite initial failures in both the soft- and hardware. The largest problem still remaining is poor beam transmission which may be caused by poor alignment, faults in the optical elements, or small acceptance of the stripper canal. For ‘“C using 5 MV acceleration, as an example, the particle transmission of ‘*C ions has only been about 50-70%. and it is not constant. This instability appears to be the main cause of the reproducibility and precision problems in the AMS-measurements. In the sequential injection method, by quickly changing the voltages applied to the chamber of the injection magnet and the steerers for the correction of the trajectory, the ions of interest with different masses are injected sequentially into the accelerator. The gaps near the object and image points of the magnet, where the acceleration and decelera-
double
slit
109
tion take place, naturally steer ions which are not on axis. Since the gap voltages differ from each other for the injected isotopes, each isotope may have a different trajectory in the magnet, if the ion beam was not on the exact center of the gap lenses. This effect, combined with beam loss in the stripper canal, will produce different transmission for the different isotopes. Fig. 2 illustrates the problem. In this case, the ion injection elements were first tuned so that both j2C3+ and 13C3+ ions are transmitted through the accelerator with maximum transmission efficiency. At this stage every slit was kept wide open. Then, with a view to stabilizing and increasing the transmission efficiency through the accelerator, the width of the object slits of the injection magnet was gradually reduced. Fig. 2 shows the different tendencies in the change of the transmission of “C and 13C ions as one jaw of each slit was closed. This phenomenon is apparently caused by the off-axis and imperfect transmission of each isotope through the accelerator, probably through the stripper canal. The different voltage applied to the magnet chamber for each isotope (= 5 kV for 13C, = 11 kV for ‘*C) might not be able to reproduce the same trajectory as 14C ions, i.e., the trajectory for each carbon isotope is different. If the stripper canal cannot accept all of the ion beams transported along me different trajectories, then the trans-
double
-X 05
,
slit
+X
q
I
/+13c/ J
0 -10
-8
-6 position
-4
-2
of the jaw
double
slit
0
-2
0
2 position
-Y
01 -12
2
(mm)
4 of the
double
slit
2
4
6
8
10
8
10
jaw (mm)
+Y
I -10
-8 position
-6 of the jaw
-4 (mm)
-2
0
-2
0
position
of the jaw
6 (mm)
Fig. 2. Changes of the transmission through the accelerator of ‘*C and 13C ions. The transmission of each isotope changed with different tendency as one jaw of the object slits of the injection magnet was closed at a time. The upper two figures are for the horizontal slit ( +X and -X slit jaw) and the lower two arc for the vertical slit (+ Y and - Y jaw). When one slit jaw was closed the other three jaws were kept open.
III. EXISTING FACILITIES
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Instr. and Meth. in Phys. Res. B 123 (1997) 107-111
mission will easily be affected by small changes of the position and direction of the ion beam. The ion beam quality and its position or direction inevitably changes considerably due to cratering of the sample cathode by cesium ion sputtering in the ion source. The situation described above may explain the poor reproducibility in the measurements of the isotope abundance ratio. Table 1 shows the performance of the AMS system of MALT in the initial developing stage in these 12 month. In the case of sequential injection method, reproducibility of 14C-AMS measurements is worse than that of 26AI-AMS, as is shown in Table 1. The bias voltage applied to the chamber is determined for each isotope to pass along the same trajectory in the fixed magnetic field, i.e., to have the same mass-energy product. Therefore, the gap-lens effect caused by the voltage depends on the mass-ratio of the isotopes. The ratios are “C/ 14C and 13C/ 14C in 14CAMS, but only 26A1/ *‘Al in 26A1-AMS.The relative mass difference for 14C-AMS is therefore much larger than that for 26A1-AMS, so the gap-lens effect is much larger in 14C-AMS than in 26Al-AMS. This may be why the reproducibility of 14C-AMS measurements is worse than that of 26A1-AMS. In the sequential injection method, we have to tune the injection line for the two or three different isotopes. Alternatively, using the internal beam monitor method (static operation method) only one ion beam is injected, for example, m/e = 14 beam for ‘“C or m/e = 26 beam for “Be. Using the internal beam monitor method, it was expected that the beam transmission problem would not negatively affect the reproducibility. As is shown in Table 1, the reproducibility measured by the beam monitor method is better compared to those measured by the sequential injection method. Unfortunately, the values are not any better than those obtained by using the old tandem accelerator system in the University of Tokyo before 1991 [4]. In the internal beam monitor method, we inject not only elemental ions but also molecular ions. For example, in “C-measurements, ‘“C- and 13CH- are injected, and in “Be-measurements, ‘“Be’60- and 9Be’70-. These molecular ions break into the elemental ions, which are used for the monitor, in the stripper canal. The stripping collisions result in the spread of the beam in the canal, causing loss of some portion of the beam from hitting the wall of the canal. Then, a change of the ion beam qualities
can change the transmission of the molecular ions, although less than in the sequential case. The typical reproducibility of “Be- and 14C-AMS measurements performed with the beam monitor method has been 2-3% with occasional fluctuation to 4-S%. On the other hand, we have recently been able to obtain a reproducibility of about 1% for 26A1-AMS measurements. The worst reproducibility was for 14C-AMS performed by the sequential injection method as is shown in Table 1. It is possible that, in order to obtain higher precision and better reproducibility, a replacement of the gas stripper canal with one having larger acceptance supported by a good differential pumping system is required. The origin of the background counts that limit the minimum detection sensitivity is classified into two categories. One comes from the sample itself, i.e. contamination or the nuclide produced by cosmic ray or by environmental radioactivity. The other comes from the machine, i.e. cross-contamination in the ion source or residual gas in the vacuum, or poor resolution of the heavy ion counting system. The minimum background levels for each AMS measurement are also shown in Table 1. Although we have not yet tried to reduce them, the minimum background for 14Cis already excellent, at the level of 60000 y, and those for “Be and 26A1 are also very low, within the range needed for practical use.
4. Summary MALT-AMS has been contributing significantly to research and development programs since April of 1995 while still in the developing stage. Although “Be-AMS (internal beam monitor method) and 26Al-AMS (sequential injection method) have been applied to many natural samples, 14C-AMS (sequential injection) has not attained the level of practical use since it requires precision measurement of better than 1%. In the case of AMS on the isotopes with larger relative mass difference, the transmission of each isotope ion through the accelerator can change and fluctuate significantly compared to that of the isotopes with smaller relative mass difference. In the case of AMS performed by the beam monitor method, the reproducibility of both “BeAMS and 14C-AMS measurements has not fluctuated as
Table 1 The performance of MALT-AMS in the initial developing stage
Nuclide
Typical ion current from the ion source @A)
AMS method
Reproducibility(%)
Background X */X =
“Be ‘k
BeO-; 1 C-; 30 (graphite) C- ; 5 (amorphous) Al-; 0.1
beam monitor
2-4 2-5 4-7 l-4
< 5 x lo- I4 <5x 10-16
%A1
beam monitor sequential injection sequential injection ,
< 5 x lo- I4
K. Kobayashi et al./Nucl.
Instr. and Meth. in Phys. Res. B 123 (1997) 107-11 I
much although it is still less than ideal. A stripper canal with larger acceptance might be useful in obtaining better reproducibility for all AMS isotopes, even when the ion beam quality changes in the ion source. The minimum background levels of MALT-AMS are low enough for the practical use. Acknowledgements Many thanks are due to the staffs of National Electrostatic Corporation in USA and Hakuto Co. in Japan for their sustained supports.
References [I] K. Kobayashi,
S. Hatori and C. Nakano, Nucl. Instr. and Meth. B 79 (1993) 742. 121 K. Kobayashi, S. Hatori and C. Nakano, Nucl. Instr. and Meth. B 92 (1994) 31. [3] K.H. Purser and P.R. Hartley, Proc. First Conf. on Radiocarbon Dating with Accelerators, ed. H.E. Gove (University of Rochester, Rochester, 1978) p. 165.
111
141 M. Suter, R. Balzer, G. Bonari, Ch. Staller, W. WBlfli, J. Beer, H. Oescheger and B. Stauffer, Proc. Symp. on Accelerator Mass Spectrometry, ed. W. Kutschera (ANL, kgOMe, 1982) p. 87. I.51 M. Imamura, Y. Hashimoto, K. Yoshida, 1. Yamane, Y, Yamashita, T. Inoue, S. Tanaka, H. Nagai, M. Honda, K. Kobayashi, N. Takaoka and Y. Ohba, Nucl. Instr. and Meth. B 5 (1984) 211. [61 K. Kobayashi, M. Imamura, H. Nagai, K. Yoshida, H. Ohashi, H. Yoshikawa and H. Yamashita, Nucl. Instr. and Meth. B 52 (1990) 254. [7] M. Paul, Nucl. Instr. and Meth. B 52 (1990) 315. [8] U. Zoppi, P.W. Kubik, M. Suter, H.A. Synal. H.R. Von Gunten and D. Zimmermamt, Nucl. Instr. and Meth. B 92 (1994) 142. [9] G. Korschinek, T. Faestermanu, S. Kastel, K. Knie, H.J. Maier, J. Femandez-Niello, M. Rothenberger and L. Zerle, Nucl. In&. and Meth. B 92 (1994) 146. [IO] H. Artigalas, J.L. Debrun, L. Kilius, X.L. Zhao, A.E. Litherland, J.L. Pmault, C. Fouillac and C.J. Maggiore, Nucl. Instr. and Meth. B 92 (1994) 227. [I l] J.E. McAninch, G.S. Bench, S.P.H.T. Freeman, M.L. Roberts, J.R. Southon, J.S. Vogel and I.D. Proctor, Nucl. Instr. and Meth. B 99 (1995) 541.
III. EXJSTING FACILITIES
and Methods in Physics Research B 123 ( 1997) lO7- I I I
Beam Interactions with Materials &IAtoms
ELSEVIER
Current status of the AMS system at the University of Tokyo K. Kobayashi a.* , S. Hatori a, H. Nagai b, K. Yoshida ‘, M. Imamura d, H. Matsuzaki e, M. Murayama f, M. Sakamoto g, Y. Aramaki f, M. Tanikawa e, S. Shibata d, H. Ohashi h, U. Zoppi a, C. Nakano a, Y. Sunohara a, T. Saitoh g a Research Center ji)r Nuclenr Science and Technology, The University of Tokyo. Yuyoi Z-11-16, Bunkyo-ku. Tokyo 113. Japun b College of Humanities and Sciences, Nihon University, Tokyo 156. Japun ’ The University Museum, The University of Tokyo. Tokyo 113, Japan ’ Institute for Nuclear Study, The University of Tokyo. Tokyo 188, Jopun e Faculty of Science, The University of Tokyo, Tokyo 113. Jupan t Graduate School of Environmental Eurth Science, Hokkaido University. Sapporo 060. Japan g National Institute of Japanese History. Chiha 285, Japan h The department of Liberal Arts. Tokyo University of Fisheries. Tokyo 108, Jupon
Abstract AMS has been applied to many application researches at the new tandem accelerator facility of the University of Tokyo since April of 1995. The reproducibility of the measurements has not been good enough especially for 14C-AMS, although, the larger acceptance of the stripper canal is found useful for it to be better. The minimum detection sensitivity is on the level of practical use.
1. Introduction The tandem accelerator facility MALT (Micro Analysis Laboratory, Tandem accelerator) was installed in the Research Center for Nuclear Science and Technology at the University of Tokyo. MALT was designed with special emphasis on micro-analytical studies using techniques like AMS, PIXE, RBS, ERD, NRA and nuclear microprobe analysis [I]. It was completed in 1994 and has been open to the University scholars since April of 1995. The new facility is equipped with a 5 MV tandem accelerator, two ion sources for gas and solid materials, nine beam lines dedicated to micro-analytical researches and many special items designed for AMS experiments [2]. Since MALT has been responsible for many research projects other than AMS applications, only about 40% of its total machine time has been used for AMS programs, including research and development studies. In the last one year, roughly 400 and 200 samples were measured by “Be-AMS and “AlAMS, respectively, while only a few samples have been measured by 14C-AMS, because there have still remained reproducibility problems. In the case of “C-AMS, the larger relative mass-difference of the isotopes is found to have a larger effect on the beam transmission than in the case of 26A1-AMS.
* Corresponding author. Tel./Fax: [email protected].
(81)3-5802-3361; email:
In this paper, we report the outline of the MALT-AMS system and its performance, putting a stress on the mass effect for the beam transport.
2. AMS system at the University of Tokyo; MALT-AMS Fig. 1 shows a schematic drawing of the AMS system of MALT. As the details of the AMS system and MALT were described in the other papers [l,2], only a brief description of the AMS system follows. The ion source MC-SNICS can be loaded with 40 samples on a rotating cathode wheel. The sample is pressed into each hole of a sample cathode with diameter of I mm. Carbon negative ion current from the source can be larger than 100 pA from a graphite cathode, and is controlled down to roughly 30 pA for the use of 14C-AMS. BeOand Al- ion currents are typically 1 FA and 0.1 pA. respectively. The electrostatic spherical analyzer (45”, r = 30 cm, gap = 5 cm) is used in order to select the ion source and to energy-analyze the ion beam to be injected into the accelerator, with the energy resolution of about 100. The 90” injection magnet can analyze ions with a mass-energy product of 15 with a mass resolution of 200. The internal clearance in the magnet chamber is designed to be wide enough (3.8 cm) for 100% transmission. By biasing the insulated magnet chamber with appropriate voltage in the fixed magnetic field, we can select quickly
0168.583X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved PII SO 16%583X(96)007 I6- I
III. EXISTING
FACILITIES
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er al./Nucl.
Insrr. and Meth. in Phys. Res. B 123 (1997) 107-111
of scattered ions. At the down stream position of the magnet near the focal line, three sets of off-axis Faraday cup are installed, the positions of which are remotely adjustable. The Faraday cups measure the stable ion beam currents and the current difference measured by a set of slit jaws in front of one of the cups is used to stabilize the terminal voltage. This multi-Faraday cup system is used for both the beam monitor and sequential injection method. AMS beam line is installed on the 20” line of the switching magnet. The electrostatic cylindrical analyzer (ECA; r = 6 m, gap = 3 cm> on this line deflects the ion beam by 20”. The configuration of the ECA and the triplet quadrupole lens gives an energy resolution of about 450 at the heavy ion detector position. Two types of heavy ion detection system are provided for the particle identification. One is for 14C and 26Al measurements and consists of a silicon surface-barrier detector @SD) covered with a Havar foil absorber with thickness of 2.2 pm. Another is for “Be detection by an SSD with a gas absorber. These two detectors align along the beam line. A A E-E gas ionization chamber is planned for use in the near future. A gas-filled magnet (GFM) [7-91 is now under construction to be installed soon. The radius of the main ions is 63 cm. At the exit of the magnet, an X-ray detection system [lo,1 l] with a proper target foil will be installed. The X-ray measurements are expected to improve the
the ions to be injected into the accelerator. This process is referred to as “sequential injection” [3,4]. By using three offset Faraday cups installed near the focal line of the magnet, we measure the ratio of the negative ion current from the ion source when the “internal beam monitor method” [5,6] is used. The accelerator is a vertically mounted tandem Pelletron with designed terminal voltage of 5 MV. Both gas and foil strippers are provided in the high voltage terminal for the charge exchange system. Two sets of turbo-molecular pump are installed to concentrate the stripper gas (argon) into the stripper canal in order to reduce the background caused by the gas flow to the accelerator tubes. Fluctuation of the terminal voltage had been as high as k 1.5 kV at 5 MV. However, the voltage fluctuation has been reduced to f.500 V after applying fluorocarbonloaded grease to the surface of the charging chains, although it is difficult to maintain this stability under all conditions. After being focused by an electrostatic quadrupole lens, ions are analyzed by the positive ion 90” bending magnet (r = 1.270 m, gap = 3.4 cm). The magnet can analyze the ions with mass-energy product of up to 150, which enables us to analyze lz916+ ions with the energy of 35 MeV. The pole piece of the magnet is designed to transmit simultaneously ions having a wide mass range. The inner wall of the magnet chamber has baffles for the reduction 45’Electrostatic spherical analyzer@ = 305 mm)
Pre-accelerator MC-SNICS (40 samples)
Electrostatic quadmpole triplet XandY steerers
90”Analyzer magnet (p = 1270 mm, ME/Z=1
Fig. 1.Schematic drawing of MALT-AMS system. Although only the AMS course is shown in the figure, MALT has nine heam courses for micro analytical researches. The gas-filled magnet will he installed by the end of May, 1996. The TOF system has heen set already and is ready for use.
K. Kobayashi et al./Nucl.
Instr. and Meth. in Phys. Rex B 123 (19971 107-11 I
analyzing power of the magnet. The GFM system will be used to distinguish 36C1 ions from 36S ions and “Ni from 59co. A TOF system for ‘291-AMS has been already installed in the AMS beam line and is ready for use.
3. Performance oping stage
of the MALT-AMS
in the initial devel-
We have made substantial progress on AMS in these 12 months despite initial failures in both the soft- and hardware. The largest problem still remaining is poor beam transmission which may be caused by poor alignment, faults in the optical elements, or small acceptance of the stripper canal. For ‘“C using 5 MV acceleration, as an example, the particle transmission of ‘*C ions has only been about 50-70%. and it is not constant. This instability appears to be the main cause of the reproducibility and precision problems in the AMS-measurements. In the sequential injection method, by quickly changing the voltages applied to the chamber of the injection magnet and the steerers for the correction of the trajectory, the ions of interest with different masses are injected sequentially into the accelerator. The gaps near the object and image points of the magnet, where the acceleration and decelera-
double
slit
109
tion take place, naturally steer ions which are not on axis. Since the gap voltages differ from each other for the injected isotopes, each isotope may have a different trajectory in the magnet, if the ion beam was not on the exact center of the gap lenses. This effect, combined with beam loss in the stripper canal, will produce different transmission for the different isotopes. Fig. 2 illustrates the problem. In this case, the ion injection elements were first tuned so that both j2C3+ and 13C3+ ions are transmitted through the accelerator with maximum transmission efficiency. At this stage every slit was kept wide open. Then, with a view to stabilizing and increasing the transmission efficiency through the accelerator, the width of the object slits of the injection magnet was gradually reduced. Fig. 2 shows the different tendencies in the change of the transmission of “C and 13C ions as one jaw of each slit was closed. This phenomenon is apparently caused by the off-axis and imperfect transmission of each isotope through the accelerator, probably through the stripper canal. The different voltage applied to the magnet chamber for each isotope (= 5 kV for 13C, = 11 kV for ‘*C) might not be able to reproduce the same trajectory as 14C ions, i.e., the trajectory for each carbon isotope is different. If the stripper canal cannot accept all of the ion beams transported along me different trajectories, then the trans-
double
-X 05
,
slit
+X
q
I
/+13c/ J
0 -10
-8
-6 position
-4
-2
of the jaw
double
slit
0
-2
0
2 position
-Y
01 -12
2
(mm)
4 of the
double
slit
2
4
6
8
10
8
10
jaw (mm)
+Y
I -10
-8 position
-6 of the jaw
-4 (mm)
-2
0
-2
0
position
of the jaw
6 (mm)
Fig. 2. Changes of the transmission through the accelerator of ‘*C and 13C ions. The transmission of each isotope changed with different tendency as one jaw of the object slits of the injection magnet was closed at a time. The upper two figures are for the horizontal slit ( +X and -X slit jaw) and the lower two arc for the vertical slit (+ Y and - Y jaw). When one slit jaw was closed the other three jaws were kept open.
III. EXISTING FACILITIES
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Instr. and Meth. in Phys. Res. B 123 (1997) 107-111
mission will easily be affected by small changes of the position and direction of the ion beam. The ion beam quality and its position or direction inevitably changes considerably due to cratering of the sample cathode by cesium ion sputtering in the ion source. The situation described above may explain the poor reproducibility in the measurements of the isotope abundance ratio. Table 1 shows the performance of the AMS system of MALT in the initial developing stage in these 12 month. In the case of sequential injection method, reproducibility of 14C-AMS measurements is worse than that of 26AI-AMS, as is shown in Table 1. The bias voltage applied to the chamber is determined for each isotope to pass along the same trajectory in the fixed magnetic field, i.e., to have the same mass-energy product. Therefore, the gap-lens effect caused by the voltage depends on the mass-ratio of the isotopes. The ratios are “C/ 14C and 13C/ 14C in 14CAMS, but only 26A1/ *‘Al in 26A1-AMS.The relative mass difference for 14C-AMS is therefore much larger than that for 26A1-AMS, so the gap-lens effect is much larger in 14C-AMS than in 26Al-AMS. This may be why the reproducibility of 14C-AMS measurements is worse than that of 26A1-AMS. In the sequential injection method, we have to tune the injection line for the two or three different isotopes. Alternatively, using the internal beam monitor method (static operation method) only one ion beam is injected, for example, m/e = 14 beam for ‘“C or m/e = 26 beam for “Be. Using the internal beam monitor method, it was expected that the beam transmission problem would not negatively affect the reproducibility. As is shown in Table 1, the reproducibility measured by the beam monitor method is better compared to those measured by the sequential injection method. Unfortunately, the values are not any better than those obtained by using the old tandem accelerator system in the University of Tokyo before 1991 [4]. In the internal beam monitor method, we inject not only elemental ions but also molecular ions. For example, in “C-measurements, ‘“C- and 13CH- are injected, and in “Be-measurements, ‘“Be’60- and 9Be’70-. These molecular ions break into the elemental ions, which are used for the monitor, in the stripper canal. The stripping collisions result in the spread of the beam in the canal, causing loss of some portion of the beam from hitting the wall of the canal. Then, a change of the ion beam qualities
can change the transmission of the molecular ions, although less than in the sequential case. The typical reproducibility of “Be- and 14C-AMS measurements performed with the beam monitor method has been 2-3% with occasional fluctuation to 4-S%. On the other hand, we have recently been able to obtain a reproducibility of about 1% for 26A1-AMS measurements. The worst reproducibility was for 14C-AMS performed by the sequential injection method as is shown in Table 1. It is possible that, in order to obtain higher precision and better reproducibility, a replacement of the gas stripper canal with one having larger acceptance supported by a good differential pumping system is required. The origin of the background counts that limit the minimum detection sensitivity is classified into two categories. One comes from the sample itself, i.e. contamination or the nuclide produced by cosmic ray or by environmental radioactivity. The other comes from the machine, i.e. cross-contamination in the ion source or residual gas in the vacuum, or poor resolution of the heavy ion counting system. The minimum background levels for each AMS measurement are also shown in Table 1. Although we have not yet tried to reduce them, the minimum background for 14Cis already excellent, at the level of 60000 y, and those for “Be and 26A1 are also very low, within the range needed for practical use.
4. Summary MALT-AMS has been contributing significantly to research and development programs since April of 1995 while still in the developing stage. Although “Be-AMS (internal beam monitor method) and 26Al-AMS (sequential injection method) have been applied to many natural samples, 14C-AMS (sequential injection) has not attained the level of practical use since it requires precision measurement of better than 1%. In the case of AMS on the isotopes with larger relative mass difference, the transmission of each isotope ion through the accelerator can change and fluctuate significantly compared to that of the isotopes with smaller relative mass difference. In the case of AMS performed by the beam monitor method, the reproducibility of both “BeAMS and 14C-AMS measurements has not fluctuated as
Table 1 The performance of MALT-AMS in the initial developing stage
Nuclide
Typical ion current from the ion source @A)
AMS method
Reproducibility(%)
Background X */X =
“Be ‘k
BeO-; 1 C-; 30 (graphite) C- ; 5 (amorphous) Al-; 0.1
beam monitor
2-4 2-5 4-7 l-4
< 5 x lo- I4 <5x 10-16
%A1
beam monitor sequential injection sequential injection ,
< 5 x lo- I4
K. Kobayashi et al./Nucl.
Instr. and Meth. in Phys. Res. B 123 (1997) 107-11 I
much although it is still less than ideal. A stripper canal with larger acceptance might be useful in obtaining better reproducibility for all AMS isotopes, even when the ion beam quality changes in the ion source. The minimum background levels of MALT-AMS are low enough for the practical use. Acknowledgements Many thanks are due to the staffs of National Electrostatic Corporation in USA and Hakuto Co. in Japan for their sustained supports.
References [I] K. Kobayashi,
S. Hatori and C. Nakano, Nucl. Instr. and Meth. B 79 (1993) 742. 121 K. Kobayashi, S. Hatori and C. Nakano, Nucl. Instr. and Meth. B 92 (1994) 31. [3] K.H. Purser and P.R. Hartley, Proc. First Conf. on Radiocarbon Dating with Accelerators, ed. H.E. Gove (University of Rochester, Rochester, 1978) p. 165.
111
141 M. Suter, R. Balzer, G. Bonari, Ch. Staller, W. WBlfli, J. Beer, H. Oescheger and B. Stauffer, Proc. Symp. on Accelerator Mass Spectrometry, ed. W. Kutschera (ANL, kgOMe, 1982) p. 87. I.51 M. Imamura, Y. Hashimoto, K. Yoshida, 1. Yamane, Y, Yamashita, T. Inoue, S. Tanaka, H. Nagai, M. Honda, K. Kobayashi, N. Takaoka and Y. Ohba, Nucl. Instr. and Meth. B 5 (1984) 211. [61 K. Kobayashi, M. Imamura, H. Nagai, K. Yoshida, H. Ohashi, H. Yoshikawa and H. Yamashita, Nucl. Instr. and Meth. B 52 (1990) 254. [7] M. Paul, Nucl. Instr. and Meth. B 52 (1990) 315. [8] U. Zoppi, P.W. Kubik, M. Suter, H.A. Synal. H.R. Von Gunten and D. Zimmermamt, Nucl. Instr. and Meth. B 92 (1994) 142. [9] G. Korschinek, T. Faestermanu, S. Kastel, K. Knie, H.J. Maier, J. Femandez-Niello, M. Rothenberger and L. Zerle, Nucl. In&. and Meth. B 92 (1994) 146. [IO] H. Artigalas, J.L. Debrun, L. Kilius, X.L. Zhao, A.E. Litherland, J.L. Pmault, C. Fouillac and C.J. Maggiore, Nucl. Instr. and Meth. B 92 (1994) 227. [I l] J.E. McAninch, G.S. Bench, S.P.H.T. Freeman, M.L. Roberts, J.R. Southon, J.S. Vogel and I.D. Proctor, Nucl. Instr. and Meth. B 99 (1995) 541.
III. EXJSTING FACILITIES