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Static operation of an AMS system using the beam monitor method
254
Nuclear
Instruments
and Methods
in Physics
Research
B52 (1990) 254-258 North-Holland
Static operation of an AMS system using the beam monitor method Koichi Kobayashi ‘) Mineo Imamura ‘) Hisao Nagai 3), Kunio Yoshida 4,, Hideo Ohashi ‘)9 Hideki Yoshikawa 6f and Hiroshi Yamaihita ‘) ” Research Center for Nuclear Science ond Technology. University of Tokyo, Bunkyo-ku, Tokyo 113, Japun -‘IInsritute for Nuclear SKI&, University of Tokyo, Tanushi. Tokyo 188, Japan ” College of Humanities and Sciences. Departmenr of Chemisrry Nihon Universily, Setagaya-ku. Tokyo 156, Japun ” Deparrment of Chemistry University of Tokyo, Bunkyo-ky Tokyo 113, Japan ” Insrirure for Cosmic Ray Research, University of Tokyo, Tunashi. Tokyo 188, Japan ” Laboratory for Radiopharmaceutical Chemistry, Kyoriru College of Pharmacy, Minaro-ky Tokyo 105, Japan 7’ Depurtment of Physics, University of Tokyo, Bunkyo-ku. Tokyo 113, Japan
The AMS system with a beam monitor method using the tandem accelerator of Tokyo University has been improved. The negative ion beam currents of two isotopes (or molecular ions) can be measured by two Faraday cups in order to monitor the fluctuation of the ion currents during the injection of the ions of interest into the tandem accelerator. The Faraday cups can be moved semiautomatically using computer-controlled stepping motors which search for the current peaks. Using these negative ion beam monitors and another one for accelerated positive ion currents, static operation of the whole system has become possible. The recision of measurements obtained by this method has been improved to *0.6% (la) for ‘“Be and 26Al-AMS and 50.8% (la) for P, C-AMS.
1. Introduction Accelerator mass spectrometry (AMS) is a technique for measuring isotope ratios that are usually below 10-12. The amounts of the rarer isotopes are so small that to identify them among the numerous other isotopes is the essential technique for AMS. The rarer isotopes are accelerated to energies high enough for them to be identified and counted using various particle identification techniques, such as a AE-E counter telescope, an energy absorber using a gas cell or a metal foil, an electrostatic deflector, a TOF technique and a Wien-filter, etc. [4-71. Meanwhile the amounts of the abundant isotopes are measured by various methods before or after the acceleration of the ions, mainly using a Faraday cup. The ion beam current from the ion source can fluctuate at any time. and the transmission efficiency of the ions through the accelerator and the detecting efficiency of the particle identifiers are not usually constant and change during the measurements. Therefore, for precise AMS it is necessary to take measures to reduce the variations mentioned above. One of the typical AMS techniques using a tandem accelerator is the sequential method, in which the ions of each isotope are injected alternately, while rapidly varying the magnetic field of the analyzing magnet for negative ions or varying the applied voltage to the chamber which is located in the field of the analyzing magnet. In this case, as the ion currents of both isotopes are measured after acceleration through the tandem 0168-583X/90/$03.50
accelerator, almost all of the effect of the slow variation of the transmission efficiency can be cancelled, so that it becomes possible to measure the isotopic abundance fairly precisely [5-81.
2. Accelerator mass spectrometry system
with a beam monitor
One of the AMS methods developed at the University of Tokyo, making use of the 5 MV tandem accelerator at the Research Center for Nuclear Science and Technology, has been called the “internal beam monil-31. In this method, aimed ion beams of tor method” 5 14C, “Be or 6Al together with molecular ion beams of the same mass to charge ratio as the aimed ions are injected and accelerated simultaneously. As the latter beams are used to monitor fluctuations of the output current from the ion source, changes in the parameters of the injection system and the transmission efficiency through the accelerator, they may be called monitor beams. The current of the monitor beam is measured by a Faraday cup with a slit system which is located on the focal plane of the positive beam analyzing magnet, while the number of aimed ions is counted by a silicon surface barrier heavy-ion detector which follows an electrostatic deflector and an absorber. The position of the Faraday cup and the slit can be adjusted manually from outside the vacuum for ion species such as “Be,
0 1990 - Elsevier Science Publishers B.V. (North-Holland)
255
K. Kobayashi et al. / AMS using the beam monitor method Einzel lens Cs sputtering ion source
Cf\ VA
(HI CONEX834)
Einrel lens
9O’Analysing magnet
l--u----
:
(3D’Magnet)
Negative
ion
(
monitor
Pre-acceleration
‘d u
tube
(O-80kV) /
Electrostatic O-lens
TANDEM ACCELERATOR (l-4.5MV)
Terminal voltage stabilizer
L................l
Stripper
)_I/
(Ar
gas)
Current amplifier
(
Magnetic Q-lens
Monitor slit
current
1
Absorber SHD
Electrostatic deflector
SO’Analysing magnet
(p
=80cm)
magnet
Fig. 1. Schematic diagram of the AMS system using the beam monitor method at the RCNST of the University of Tokyo. In addition to the positive ion beam monitor, a negative ion beam monitor is installed, which consists of two Faraday cups controlled by a microcomputer.
14C or %l which are to be measured by AMS. Furthermore, the terminal voltage of the tandem accelerator is stabilized by a slit feedback method using the monitor beam, while the aimed ion beam of very weak l”Be3+, 14c3+ or 26~13+ is allowed to pass through another sets of slits in the main beam course. Fig. 1 is a schematic drawing of the improved internal beam monitor system with newly installed negative
configuration and ion monitors. The instrumental method of the new system are almost the same as those of the old system, which have been reported in detail elsewhere [l-3]. In this section an outline of the old system is given using the figure. Before injection into the tandem accelerator the abundance ratio of the negative molecular ion beams is measured by an analyzing magnet, then the field is fixed I. PROGRESS REPORTS
K. Kobayashi et al. / AMS using the beam monitor melhod
256
scripts m, d and s indicate the beam monitor, the detector and the ion source, respectively. R, is the ratio of negative ion currents of m/e = 26 over m/e = 25 ions which should be measured during injection of the m/e = 26 ions into the accelerator.
to an appropriate value for the aimed ions to be injected and travel straight through the accelerator. As is listed in table 1, in the case of “Be-AMS the injected ions are “Be”‘O-, 9Be’70and 9Be’60H(m/e = 26). 14Cand “CH(m/e = 14) ions for 14C-AMS, and “Al(m/e = 26) for 26AI-AMS. The injected and ‘“B’% negative ions are accelerated towards the high-voltage terminal and pass through a gas stripper canal where the charge state of the ions is converted from negative to positive with some charge-state distribution, depending on the gas pressure and the terminal voltage. At the same time, the injected molecular ions are fragmented into atomic ions with positive charge. The terminal voltage is set to 3.5 MV for “Be-AMS, 3.0 MV for 14C-AMS and 2.5 MV fo 26A1-AMS, and the selected ions with proper charge state and energy are “Be3+ with an energy of 12 MeV, 14C3+ of 12 MeV and 26A13+ of 10 MeV. From the fragment ions originating from the molecular ions, 9Be3+, 13C3+ and 1602+ ion beams are used as monitor beams for “Be-. 14C- and ‘6Al-AMS, respectively. Typical currents for each of the ions are also listed in table 1. As shown in the table, the monitor currents are usually as low as 1 nA. Two sets of fast amplifiers, which convert the current of 1 nA to about 1 V, are used for slit feedback amplifiers. Using the number of aimed ions counted by the heavy-ion detector and the intepted ion current of the monitor beam, the ratios of oBe/9Be, 14C/‘3C and ‘“AI/*‘Al in the samples are expressed as follows: (“Be),
(“Be),
(‘“C), C4C)d
o,=(‘“C),Rq
1
where
and z is the transmission efficiency through the same path as in the case of ‘“Be, the value of which is usually greater than 0.9 and is also determined using a standard sample. R, is the ratio of CH- to C- which varies from sample to sample, depending mainly on the chemical treatment procedure. I,,/I,, is the ratio of the negative ion currents of m/e = 13 over m/e = 12, which should also be measured durin the injection of m/e = ‘8 14 ions into the accelerator. ( .C)/(12C) is the isotopic abundance of carbon in the sample, instead of which the ratio of natural carbon can be used (1.12 x lo-*). (26A1)5 (*‘Al’+), (“A1>5 = (‘602+),~c
z, 1 r’
where R=
(‘“B’60-)r (27A1-),
1
’
c is the same as defined
above, and c’ and c2 are the efficiencies through the accelerator for 2bA13+ and ‘bo2+, respectively. R is the ratio of the negative ion currents of m/e = 26 over m/e = 27, which should also be measured at the ion source during injection of m/e = 26 ions into the accelerator. The transmission efficiency through the tandem accelerator for “‘Be- and 14C-AMS can be cancelled in the beam monitor method, although this is not the case for 26Al-AMS. However, even for 26AI-AMS. the beam monitor method works well if samples are sufficiently
o,=o,Ro;3
transmission
where
R = (yBe’70-+9Be’60H-), 0 (9Be’60-),
’
and c is the transmission efficiency between the beam monitor and the ion detector, the value of which is about 0.8 and changes depending on the parameters of the beam handling elements. Its value is determined using standard samples of known “Be/‘Be ratios. SubTable 1 List of the molecular species of negative and positive for “Be-, 14C- and 26A1-AMS are also typical values AMS nuclide
Negative ion monitor beam
“Be
I,, = 10400
‘4
C
26Al
Injected
nA 9Be’60-
monitor
ion beams
ion currents.
The data of the performance
Positive ion monitor beam
beam 10
I 26 =l-1onA Be’60+ 9Be’70- + 9Be’60H(“0: 2% enriched)
I,, =l-3 PA “CI,X=1-50nA’3C-+‘2CH-
I,,=O.Ol-1.5 nA14C
I 27 = lo-60
I,,=lO-3OnA 26A1- +‘“B”O(“B: 92% enriched)
nA 2’Al -
and their typical
+‘%ZH-
Background level (reproducibility)
9Be3+: 11.7 MeV, 0.03-0.5
nA
“C3’:
nA
11.8 MeV,0.05-0.5
“Be/‘Be ( + 0.6%)
= 5 x lo-l4
“C/‘2C=3~10-‘6 (*lx)
1602’:
6.5 MeV, 0.5-3
nA
26A1/27Al = 1 x 10 - I3 ( * 0.5%)
K. Kobayashi et al. / AMS
251
using the beam monitor method
well mixed to be uniform and the gas flow rate in the charge stripper is kept constant [3].
3. Static accelerator mass spectrometry In the beam monitor method, as described above, it is essential to measure the mass ratio of the negative ions concerned. As the intensity ratio of the molecular beams is not very stable, this ratio should be measured as many times as possible, even during acceleration of the aimed ions. In our previous system, without the negative ion monitoring Faraday cups, the ratio was measured only twice by varying the magnetic field before and after acceleration of the directing ions, since this process took too much time compared to the acceleration time. In fig. 1 the negative ion monitoring Faraday cups (FC) are also shown, which are located close to the focal plane of the negative ion analyzing magnet. The system consists of two sets of FCs with slits, in addition to the ordinary retractable FC positioned at the center of the main beam course. The monitor FCs are supported by a pair of rails and can be moved perpendicularly to the beam line. Two stepping motors which turn
0
IO
No. of measurements Fig. 3. “Be- and 26A1-AMS data on standard samples obtained by repeated measurements. The standard errors for the weighted mean values are less than about +0.6% (lo) with a standard deviation of about +2% (lo). The error bars show only the counting statistical errors.
screws of micrometer heads connected to the FCs are controlled by a microcomputer through a GP-IB controller. The position of each FC can be adjusted separately to search for the peak of the ion beam intensity. For example, in the case of i4C-AMS, first of all the I.9 mean = I.670*10-12 Std 1 : weighted strength of the magnetic field is set to the most ap(&0.7X) propriate value for mass 14 to be injected, and next, the two FCs are moved back and forth several times to search for the peaks of the ion currents of mass 12 and mass 13 semiautomatically, Then the position of each FC is fixed at the peak of each ion beam. The currents measured during counting of the 14C ions are averaged weighted mean = I.676*10-12 or accumulated. After adjusting the positions of the FCs, all of the parameters of the ion optical elements, including the magnetic and electrostatic field strengths of the ion analyzer and, of course, the operating parameters of the tandem accelerator, are completely fixed throughout the measurements without GVM feedback. This system is called static AMS. During 14C-AMS I.9 mean = 1.653*10-l2 Std 3 : weighted measurements, for example, all negative and positive (f0.9%) ion currents concerned are measured and integrated simultaneously, which is expected to improve the accu_............. I.7 __.*__m__~__, ______. . . . . . . $...p.f ___...._______...________ racy. In fact, however, it was found that during i4C-AMS I.5 a little mass discrimination occurred in the negative ion FIlIII II”“““14 mass analyzer, because of the thin analyzing chamber IO 0 through which more than 10% of the ions were lost by No. of measurements hitting the walls along the ion beam path. Further, the Fig. 2. Reproducibility test obtained by repeated measureion path varies depending on its projected position in ments on three standard samples. The dotted lines indicate the the ion source, so that the mass discrimination varies as weighted mean values with a standard error of la. The error the parameters change during measurements and from bars show only counting statistical errors. The weighted mean sample to sample. As shown in fig. 2, the precision and values of the three samples agree well with each other to within reproducibility are less than & 1% (la), which is a slight k-0.8%.
I
FX
1
1
I. PROGRESS
REPORTS
258
K. Kobayashi ei al. / AMS using the beam monitor method
improvement compared to the early system with no ion monitor. The figure shows the compiled data of three standard samples measured alternately among the measurements of other samples under the same machine conditions. Each averaged value is the weighted mean value, and the weights refer to the 14C counts which range from about 1000 to 7000 counts. Since we could not get much ion beam from the ion source (usually less than 2 PA of 12C- ion current), we could not get good counting statistics. The error bars are only lo counting statistical errors. The weighted mean values of the three samples agreed well with each other to within *0.8%. For *‘Be-AM& one FC of the negative ion monitor is used to measure the current of mass 25. The FC is set in much the same way as in the case of 14C-AMS. Once the position of the FC is fixed, the current of mass 25 is measured all throughout the time of counting the “Be3+ ions, while monitoring the 9Be3+ ion current. The integrated current of the negative ions reflects the total ion current extracted from the ion source during the measurement. The ratio of the integrated ion current measured by the negative and the positive ion monitors should reflect the transmission efficiency through the tandem accelerator, if the ratio of mass 25 over mass 26 of negative ions is kept constant. However, since the ratio of negative ions usually changes during measurement and it is impossible to measure the ion current of mass 26 at the time of injection, we usually measure the ratio twice before and after the injection and the averaged value is used as the ratio. The procedure for ‘?U-AMS is practically the same as that of “Be-AM& except for the fact that the 26A13+ ion has no relation to the monitor beam of 1602+ ions which reflects the transmission efficiency and output intensity from the ion source. In both “Be and 26A1measurements, as the mass discrimination is much smaller than in the case of 14C-AMS, it is not necessary to move the FC each time from sample to sample, keeping the magnetic field fixed negative
at a constant value. Therefore, the ion beam path through the tandem accelerator can be fixed to some
extent, and the beam handling procedure for negative
ions becomes very simple, which results in very short measurement times. Fig. 3 shows typical data on standard samples of “Be and 26A1,which were taken alternately among measurements of other samples. As is shown in the figure, the standard error of the “Be data is about f0.6W (la) with a standard deviation of k 1.8% (la) which has been improved slightly compared to a typical value previously of about *3% (lo). The corresponding error in 26Al-AMS is +0.5% (la), with nearly the same standard deviation as that of “Be-AMS, which has been significantly improved to about *2% (lo), compared to f5% (10) using the old system. Acknowledgement This work was supported by a Grant-in-Aid for Scientific Research (B) from the Ministry of Education, Science and Culture, Japan. References 111M. Imamura, Y. Hashimoto, K. Yoshida, I. Yamane, Y. Yamashita, T. Inoue, S. Tanaka, H. Nagai, M. Honda, K. Kobayashi, N. Takaoka and Y. Ohba, Nucl. Instr. and Meth. BS (1984) 211. PI K. Kobayashi, K. Yoshida, M. Imamura, H. Nagai, H. Yoshikawa, H. Yamashita, S. Okizaki and M. Honda, Nucl. Instr. and Meth. B29 (1987) 173. 131H. Nagai, T. Kobayashi, M. Honda, M. Imamura, K. Kobayashi, K. Yoshida and H. Yamashita, Nucl. Instr. and Meth. B29 (1987) 266. [41 H.E. Gove (ed.), 1st Proc. Conf. on Radiocarbon Dating with Accelerators, University of Rochester, USA (1978). 151 W. Henning, W. Kutschera and J.L. Yntema (eds.). Prcc. Symp. on Accelerator Mass Spectrometry, Argonne National Laboratory Report ANL/PHY-81-1 (1981). PI W. Wblfi, H.A. Polach and H.H. Andersen (eds.), Proc. 3rd Int. Symp. on Accelerator Mass Spectrometry, Ztirich, Switzerland, 1984, Nucl. Instr. and Meth. B5 (1984). I71 H.E. Gove, A.E. Litherland and D. Elmore (eds.), Proc. 4th Int. Symp. on Accelerator Mass Spectrometry, Niagara-onthe-Lake, Canada, 1987, Nucl. Instr. and Meth. B29 (1987). 181G. Bonani, J. Beer, H. Hoffman, H. Synal, M. Suter, W. Wi)lfli, C. Pfleiderer, B. Kramer, C. Junghans and K.O. Miinnich, Nucl. Instr. and Meth. B29 (1987) 87.
Nuclear
Instruments
and Methods
in Physics
Research
B52 (1990) 254-258 North-Holland
Static operation of an AMS system using the beam monitor method Koichi Kobayashi ‘) Mineo Imamura ‘) Hisao Nagai 3), Kunio Yoshida 4,, Hideo Ohashi ‘)9 Hideki Yoshikawa 6f and Hiroshi Yamaihita ‘) ” Research Center for Nuclear Science ond Technology. University of Tokyo, Bunkyo-ku, Tokyo 113, Japun -‘IInsritute for Nuclear SKI&, University of Tokyo, Tanushi. Tokyo 188, Japan ” College of Humanities and Sciences. Departmenr of Chemisrry Nihon Universily, Setagaya-ku. Tokyo 156, Japun ” Deparrment of Chemistry University of Tokyo, Bunkyo-ky Tokyo 113, Japan ” Insrirure for Cosmic Ray Research, University of Tokyo, Tunashi. Tokyo 188, Japan ” Laboratory for Radiopharmaceutical Chemistry, Kyoriru College of Pharmacy, Minaro-ky Tokyo 105, Japan 7’ Depurtment of Physics, University of Tokyo, Bunkyo-ku. Tokyo 113, Japan
The AMS system with a beam monitor method using the tandem accelerator of Tokyo University has been improved. The negative ion beam currents of two isotopes (or molecular ions) can be measured by two Faraday cups in order to monitor the fluctuation of the ion currents during the injection of the ions of interest into the tandem accelerator. The Faraday cups can be moved semiautomatically using computer-controlled stepping motors which search for the current peaks. Using these negative ion beam monitors and another one for accelerated positive ion currents, static operation of the whole system has become possible. The recision of measurements obtained by this method has been improved to *0.6% (la) for ‘“Be and 26Al-AMS and 50.8% (la) for P, C-AMS.
1. Introduction Accelerator mass spectrometry (AMS) is a technique for measuring isotope ratios that are usually below 10-12. The amounts of the rarer isotopes are so small that to identify them among the numerous other isotopes is the essential technique for AMS. The rarer isotopes are accelerated to energies high enough for them to be identified and counted using various particle identification techniques, such as a AE-E counter telescope, an energy absorber using a gas cell or a metal foil, an electrostatic deflector, a TOF technique and a Wien-filter, etc. [4-71. Meanwhile the amounts of the abundant isotopes are measured by various methods before or after the acceleration of the ions, mainly using a Faraday cup. The ion beam current from the ion source can fluctuate at any time. and the transmission efficiency of the ions through the accelerator and the detecting efficiency of the particle identifiers are not usually constant and change during the measurements. Therefore, for precise AMS it is necessary to take measures to reduce the variations mentioned above. One of the typical AMS techniques using a tandem accelerator is the sequential method, in which the ions of each isotope are injected alternately, while rapidly varying the magnetic field of the analyzing magnet for negative ions or varying the applied voltage to the chamber which is located in the field of the analyzing magnet. In this case, as the ion currents of both isotopes are measured after acceleration through the tandem 0168-583X/90/$03.50
accelerator, almost all of the effect of the slow variation of the transmission efficiency can be cancelled, so that it becomes possible to measure the isotopic abundance fairly precisely [5-81.
2. Accelerator mass spectrometry system
with a beam monitor
One of the AMS methods developed at the University of Tokyo, making use of the 5 MV tandem accelerator at the Research Center for Nuclear Science and Technology, has been called the “internal beam monil-31. In this method, aimed ion beams of tor method” 5 14C, “Be or 6Al together with molecular ion beams of the same mass to charge ratio as the aimed ions are injected and accelerated simultaneously. As the latter beams are used to monitor fluctuations of the output current from the ion source, changes in the parameters of the injection system and the transmission efficiency through the accelerator, they may be called monitor beams. The current of the monitor beam is measured by a Faraday cup with a slit system which is located on the focal plane of the positive beam analyzing magnet, while the number of aimed ions is counted by a silicon surface barrier heavy-ion detector which follows an electrostatic deflector and an absorber. The position of the Faraday cup and the slit can be adjusted manually from outside the vacuum for ion species such as “Be,
0 1990 - Elsevier Science Publishers B.V. (North-Holland)
255
K. Kobayashi et al. / AMS using the beam monitor method Einzel lens Cs sputtering ion source
Cf\ VA
(HI CONEX834)
Einrel lens
9O’Analysing magnet
l--u----
:
(3D’Magnet)
Negative
ion
(
monitor
Pre-acceleration
‘d u
tube
(O-80kV) /
Electrostatic O-lens
TANDEM ACCELERATOR (l-4.5MV)
Terminal voltage stabilizer
L................l
Stripper
)_I/
(Ar
gas)
Current amplifier
(
Magnetic Q-lens
Monitor slit
current
1
Absorber SHD
Electrostatic deflector
SO’Analysing magnet
(p
=80cm)
magnet
Fig. 1. Schematic diagram of the AMS system using the beam monitor method at the RCNST of the University of Tokyo. In addition to the positive ion beam monitor, a negative ion beam monitor is installed, which consists of two Faraday cups controlled by a microcomputer.
14C or %l which are to be measured by AMS. Furthermore, the terminal voltage of the tandem accelerator is stabilized by a slit feedback method using the monitor beam, while the aimed ion beam of very weak l”Be3+, 14c3+ or 26~13+ is allowed to pass through another sets of slits in the main beam course. Fig. 1 is a schematic drawing of the improved internal beam monitor system with newly installed negative
configuration and ion monitors. The instrumental method of the new system are almost the same as those of the old system, which have been reported in detail elsewhere [l-3]. In this section an outline of the old system is given using the figure. Before injection into the tandem accelerator the abundance ratio of the negative molecular ion beams is measured by an analyzing magnet, then the field is fixed I. PROGRESS REPORTS
K. Kobayashi et al. / AMS using the beam monitor melhod
256
scripts m, d and s indicate the beam monitor, the detector and the ion source, respectively. R, is the ratio of negative ion currents of m/e = 26 over m/e = 25 ions which should be measured during injection of the m/e = 26 ions into the accelerator.
to an appropriate value for the aimed ions to be injected and travel straight through the accelerator. As is listed in table 1, in the case of “Be-AMS the injected ions are “Be”‘O-, 9Be’70and 9Be’60H(m/e = 26). 14Cand “CH(m/e = 14) ions for 14C-AMS, and “Al(m/e = 26) for 26AI-AMS. The injected and ‘“B’% negative ions are accelerated towards the high-voltage terminal and pass through a gas stripper canal where the charge state of the ions is converted from negative to positive with some charge-state distribution, depending on the gas pressure and the terminal voltage. At the same time, the injected molecular ions are fragmented into atomic ions with positive charge. The terminal voltage is set to 3.5 MV for “Be-AMS, 3.0 MV for 14C-AMS and 2.5 MV fo 26A1-AMS, and the selected ions with proper charge state and energy are “Be3+ with an energy of 12 MeV, 14C3+ of 12 MeV and 26A13+ of 10 MeV. From the fragment ions originating from the molecular ions, 9Be3+, 13C3+ and 1602+ ion beams are used as monitor beams for “Be-. 14C- and ‘6Al-AMS, respectively. Typical currents for each of the ions are also listed in table 1. As shown in the table, the monitor currents are usually as low as 1 nA. Two sets of fast amplifiers, which convert the current of 1 nA to about 1 V, are used for slit feedback amplifiers. Using the number of aimed ions counted by the heavy-ion detector and the intepted ion current of the monitor beam, the ratios of oBe/9Be, 14C/‘3C and ‘“AI/*‘Al in the samples are expressed as follows: (“Be),
(“Be),
(‘“C), C4C)d
o,=(‘“C),Rq
1
where
and z is the transmission efficiency through the same path as in the case of ‘“Be, the value of which is usually greater than 0.9 and is also determined using a standard sample. R, is the ratio of CH- to C- which varies from sample to sample, depending mainly on the chemical treatment procedure. I,,/I,, is the ratio of the negative ion currents of m/e = 13 over m/e = 12, which should also be measured durin the injection of m/e = ‘8 14 ions into the accelerator. ( .C)/(12C) is the isotopic abundance of carbon in the sample, instead of which the ratio of natural carbon can be used (1.12 x lo-*). (26A1)5 (*‘Al’+), (“A1>5 = (‘602+),~c
z, 1 r’
where R=
(‘“B’60-)r (27A1-),
1
’
c is the same as defined
above, and c’ and c2 are the efficiencies through the accelerator for 2bA13+ and ‘bo2+, respectively. R is the ratio of the negative ion currents of m/e = 26 over m/e = 27, which should also be measured at the ion source during injection of m/e = 26 ions into the accelerator. The transmission efficiency through the tandem accelerator for “‘Be- and 14C-AMS can be cancelled in the beam monitor method, although this is not the case for 26Al-AMS. However, even for 26AI-AMS. the beam monitor method works well if samples are sufficiently
o,=o,Ro;3
transmission
where
R = (yBe’70-+9Be’60H-), 0 (9Be’60-),
’
and c is the transmission efficiency between the beam monitor and the ion detector, the value of which is about 0.8 and changes depending on the parameters of the beam handling elements. Its value is determined using standard samples of known “Be/‘Be ratios. SubTable 1 List of the molecular species of negative and positive for “Be-, 14C- and 26A1-AMS are also typical values AMS nuclide
Negative ion monitor beam
“Be
I,, = 10400
‘4
C
26Al
Injected
nA 9Be’60-
monitor
ion beams
ion currents.
The data of the performance
Positive ion monitor beam
beam 10
I 26 =l-1onA Be’60+ 9Be’70- + 9Be’60H(“0: 2% enriched)
I,, =l-3 PA “CI,X=1-50nA’3C-+‘2CH-
I,,=O.Ol-1.5 nA14C
I 27 = lo-60
I,,=lO-3OnA 26A1- +‘“B”O(“B: 92% enriched)
nA 2’Al -
and their typical
+‘%ZH-
Background level (reproducibility)
9Be3+: 11.7 MeV, 0.03-0.5
nA
“C3’:
nA
11.8 MeV,0.05-0.5
“Be/‘Be ( + 0.6%)
= 5 x lo-l4
“C/‘2C=3~10-‘6 (*lx)
1602’:
6.5 MeV, 0.5-3
nA
26A1/27Al = 1 x 10 - I3 ( * 0.5%)
K. Kobayashi et al. / AMS
251
using the beam monitor method
well mixed to be uniform and the gas flow rate in the charge stripper is kept constant [3].
3. Static accelerator mass spectrometry In the beam monitor method, as described above, it is essential to measure the mass ratio of the negative ions concerned. As the intensity ratio of the molecular beams is not very stable, this ratio should be measured as many times as possible, even during acceleration of the aimed ions. In our previous system, without the negative ion monitoring Faraday cups, the ratio was measured only twice by varying the magnetic field before and after acceleration of the directing ions, since this process took too much time compared to the acceleration time. In fig. 1 the negative ion monitoring Faraday cups (FC) are also shown, which are located close to the focal plane of the negative ion analyzing magnet. The system consists of two sets of FCs with slits, in addition to the ordinary retractable FC positioned at the center of the main beam course. The monitor FCs are supported by a pair of rails and can be moved perpendicularly to the beam line. Two stepping motors which turn
0
IO
No. of measurements Fig. 3. “Be- and 26A1-AMS data on standard samples obtained by repeated measurements. The standard errors for the weighted mean values are less than about +0.6% (lo) with a standard deviation of about +2% (lo). The error bars show only the counting statistical errors.
screws of micrometer heads connected to the FCs are controlled by a microcomputer through a GP-IB controller. The position of each FC can be adjusted separately to search for the peak of the ion beam intensity. For example, in the case of i4C-AMS, first of all the I.9 mean = I.670*10-12 Std 1 : weighted strength of the magnetic field is set to the most ap(&0.7X) propriate value for mass 14 to be injected, and next, the two FCs are moved back and forth several times to search for the peaks of the ion currents of mass 12 and mass 13 semiautomatically, Then the position of each FC is fixed at the peak of each ion beam. The currents measured during counting of the 14C ions are averaged weighted mean = I.676*10-12 or accumulated. After adjusting the positions of the FCs, all of the parameters of the ion optical elements, including the magnetic and electrostatic field strengths of the ion analyzer and, of course, the operating parameters of the tandem accelerator, are completely fixed throughout the measurements without GVM feedback. This system is called static AMS. During 14C-AMS I.9 mean = 1.653*10-l2 Std 3 : weighted measurements, for example, all negative and positive (f0.9%) ion currents concerned are measured and integrated simultaneously, which is expected to improve the accu_............. I.7 __.*__m__~__, ______. . . . . . . $...p.f ___...._______...________ racy. In fact, however, it was found that during i4C-AMS I.5 a little mass discrimination occurred in the negative ion FIlIII II”“““14 mass analyzer, because of the thin analyzing chamber IO 0 through which more than 10% of the ions were lost by No. of measurements hitting the walls along the ion beam path. Further, the Fig. 2. Reproducibility test obtained by repeated measureion path varies depending on its projected position in ments on three standard samples. The dotted lines indicate the the ion source, so that the mass discrimination varies as weighted mean values with a standard error of la. The error the parameters change during measurements and from bars show only counting statistical errors. The weighted mean sample to sample. As shown in fig. 2, the precision and values of the three samples agree well with each other to within reproducibility are less than & 1% (la), which is a slight k-0.8%.
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K. Kobayashi ei al. / AMS using the beam monitor method
improvement compared to the early system with no ion monitor. The figure shows the compiled data of three standard samples measured alternately among the measurements of other samples under the same machine conditions. Each averaged value is the weighted mean value, and the weights refer to the 14C counts which range from about 1000 to 7000 counts. Since we could not get much ion beam from the ion source (usually less than 2 PA of 12C- ion current), we could not get good counting statistics. The error bars are only lo counting statistical errors. The weighted mean values of the three samples agreed well with each other to within *0.8%. For *‘Be-AM& one FC of the negative ion monitor is used to measure the current of mass 25. The FC is set in much the same way as in the case of 14C-AMS. Once the position of the FC is fixed, the current of mass 25 is measured all throughout the time of counting the “Be3+ ions, while monitoring the 9Be3+ ion current. The integrated current of the negative ions reflects the total ion current extracted from the ion source during the measurement. The ratio of the integrated ion current measured by the negative and the positive ion monitors should reflect the transmission efficiency through the tandem accelerator, if the ratio of mass 25 over mass 26 of negative ions is kept constant. However, since the ratio of negative ions usually changes during measurement and it is impossible to measure the ion current of mass 26 at the time of injection, we usually measure the ratio twice before and after the injection and the averaged value is used as the ratio. The procedure for ‘?U-AMS is practically the same as that of “Be-AM& except for the fact that the 26A13+ ion has no relation to the monitor beam of 1602+ ions which reflects the transmission efficiency and output intensity from the ion source. In both “Be and 26A1measurements, as the mass discrimination is much smaller than in the case of 14C-AMS, it is not necessary to move the FC each time from sample to sample, keeping the magnetic field fixed negative
at a constant value. Therefore, the ion beam path through the tandem accelerator can be fixed to some
extent, and the beam handling procedure for negative
ions becomes very simple, which results in very short measurement times. Fig. 3 shows typical data on standard samples of “Be and 26A1,which were taken alternately among measurements of other samples. As is shown in the figure, the standard error of the “Be data is about f0.6W (la) with a standard deviation of k 1.8% (la) which has been improved slightly compared to a typical value previously of about *3% (lo). The corresponding error in 26Al-AMS is +0.5% (la), with nearly the same standard deviation as that of “Be-AMS, which has been significantly improved to about *2% (lo), compared to f5% (10) using the old system. Acknowledgement This work was supported by a Grant-in-Aid for Scientific Research (B) from the Ministry of Education, Science and Culture, Japan. References 111M. Imamura, Y. Hashimoto, K. Yoshida, I. Yamane, Y. Yamashita, T. Inoue, S. Tanaka, H. Nagai, M. Honda, K. Kobayashi, N. Takaoka and Y. Ohba, Nucl. Instr. and Meth. BS (1984) 211. PI K. Kobayashi, K. Yoshida, M. Imamura, H. Nagai, H. Yoshikawa, H. Yamashita, S. Okizaki and M. Honda, Nucl. Instr. and Meth. B29 (1987) 173. 131H. Nagai, T. Kobayashi, M. Honda, M. Imamura, K. Kobayashi, K. Yoshida and H. Yamashita, Nucl. Instr. and Meth. B29 (1987) 266. [41 H.E. Gove (ed.), 1st Proc. Conf. on Radiocarbon Dating with Accelerators, University of Rochester, USA (1978). 151 W. Henning, W. Kutschera and J.L. Yntema (eds.). Prcc. Symp. on Accelerator Mass Spectrometry, Argonne National Laboratory Report ANL/PHY-81-1 (1981). PI W. Wblfi, H.A. Polach and H.H. Andersen (eds.), Proc. 3rd Int. Symp. on Accelerator Mass Spectrometry, Ztirich, Switzerland, 1984, Nucl. Instr. and Meth. B5 (1984). I71 H.E. Gove, A.E. Litherland and D. Elmore (eds.), Proc. 4th Int. Symp. on Accelerator Mass Spectrometry, Niagara-onthe-Lake, Canada, 1987, Nucl. Instr. and Meth. B29 (1987). 181G. Bonani, J. Beer, H. Hoffman, H. Synal, M. Suter, W. Wi)lfli, C. Pfleiderer, B. Kramer, C. Junghans and K.O. Miinnich, Nucl. Instr. and Meth. B29 (1987) 87.