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Ionization efficiencies for highly charged stable and radioactive ions in the AECR-U ion source
Nuclear Instruments and Methods in Physics Research B 168 (2000) 117±124
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Ionization eciencies for highly charged stable and radioactive ions in the AECR-U ion source Z.Q. Xie, D. Wutte *, C.M. Lyneis Lawrence Berkeley National Laboratory, University of California, 1 Cyclotron Road MS 88, Berkeley, CA 94720, USA Received 10 August 1999
Abstract Ionization eciencies for high charge state ions have been measured with the LBNL AECR-U ion source for ion beams produced from stable and radioactive gases. Various calibrated gas leaks from stable CO, CO2 , O2 , Ne, Ar, CHF3 , Kr and Xe were used in the measurements. Ionization eciencies as high as 25% or higher were measured for 16 6 O and 12 C4 ion beams and more than 10% for high charge state stable ion beams of Ar, Kr and Xe. In addition, ionization eciency measurements for radioactive species of 11 C and 14 O were carried out in which an eciency of more than 10% for 11 C4 was achieved. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 07.77.K; 29.25.L Keywords: High charge state ECR ion source; Electron cyclotron resonance ion sources; Plasma ion source; Radioactive ion beams; Ionization eciency
1. Introduction There are signi®cant eorts worldwide to produce radioactive ion beams (RIB) for science research. Because of the low production yield of radioactive species, eciency of every part of the whole system is the key to the success of RIB production. In the area of ion sources, there are many R&D studies currently undergoing to investigate and optimize the ionization eciencies
*
Corresponding author. Tel.: +510-486-7814; fax: +510486-7983. E-mail address: [email protected] (D. Wutte).
of various types of ion source [1]. Electron cyclotron resonance (ECR) ion sources are attractive for radioactive ion beams because of good ionization eciency, long lifetime, and ¯exibility as demonstrated by a number of investigations reported in recent years [2±5]. However, those ECR ion sources were designed and used for low or intermediate charge states, so the ionization eciency for the highly charged ions has not yet been well explored. This paper presents a recent study of the ionization eciency of the highly charged ion beams with a well optimized high charge state ECR ion source, the LBNL AECRU [6], at the Lawrence Berkeley National Laboratory.
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 6 3 2 - 1
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Z.Q. Xie et al. / Nucl. Instr. and Meth. in Phys. Res. B 168 (2000) 117±124
2. The AECR-U ion source Since the pioneering development began in the early 1970s [7] the ECR ion source technology has matured although without a full understanding of the detailed physics processes involved in the ECR plasma. In the past decade, tremendous progress has been achieved on the charge states and on beam intensities by increasing the magnetic con®nement, the microwave frequency and specialized techniques. These techniques include aluminum oxide coatings to provide high yield of the secondary cold electrons, two-frequency plasma heating, and high magnetic mirror ratios [8±10]. The AECR-U ion source is optimized for the production of high charge state ions and combines all of the current ECR ion source techniques. Shown in Fig. 1 is a cross-sectional view of the AECR-U ion source. It was built in 1990 [11] and was upgraded in 1996 to further enhance its performance. Its maximum peak ®elds on axis are 1.7 and 1.1 T at the injection and extraction regions,
respectively. The maximum radial ®eld at the inner surface of the aluminum plasma chamber is 0.85 T. The aluminum plasma chamber is 30 cm in length with an inner diameter of 7.6 cm. Six radial slots and an external turbo pump give about 150 l/s pumping speed. The radial slots also provide easy oven access in the plasma chamber. The AECR-U plasma is driven by microwaves of two-frequency (14 and 10 GHz) launched through two of the three o-axis wave-guides terminated at a bias plate in the injection region. The working gases are axially bled into the source through one of the oaxis wave-guides. Although its overall magnetic ®eld strength is about 60% of the highest ®eld strength ECR ion sources in operation, the AECR-U holds many records for the ion charge states and beam intensities [6]. For example, a low intensity U64 beam was produced with the AECR-U ion source and accelerated by the 88-in. Cyclotron to 2.06 GeV. Table 1 lists some of the high charge state ion beams produced by the AECR-U ion source.
Fig. 1. A cross-sectional view of the LBNL AECR-U ion source. Only one of the three waveguides is shown. The cross section of the aluminum plasma chamber is shown on the far right side.
Z.Q. Xie et al. / Nucl. Instr. and Meth. in Phys. Res. B 168 (2000) 117±124 Table 1 A few high charge state ion beams produced by the LBNL AECR-U Ion
I (elA)
O6 O7 Ne9 Ar13 Co13 Kr18 Ar16 Co20 Kr26 Xe30 Au36 U38 Ar17 Kr28 Xe36 Au46 Bi47 U48 Ar18 Ca19 Xe38 Au50 Bi50 U52
840 360 110 120 116 100 25 13 18 10 13 11 2 2 1 1 1 1 0.12 0.25 0.25 0.15 0.15 0.10
3. Measurements Various calibrated gas leaks, CO, CO2 , O2 , Ne, Ar, CHF3 , Kr and Xe, were used in the study. The leak rates of these ranged from a few to tens of plA. The quoted uncertainty is 15%, except for
119
the Ne leak, which has a much higher uncertainty of )100% due to its age (Table 2). The radioactive species of 11 C and 14 O are being developed for the Berkeley experiments with accelerated radioactive species (BEARS), an initiative to develop RIB capability at the 88-Inch Cyclotron Facility, LBNL [12,13]. The gases were axially injected into the source through one of the o-axis wave-guides. Helium or oxygen support plasma was used to optimize the ion production. The goal was to evaluate the eciency of the AECR-U ion source, not to investigate the in¯uence of a particular source tuning parameter on the ionization eciency of a speci®c ion beam. Therefore, the intensity of each analyzed ion beam was optimized with respect to all source-tuning parameters, such as magnetic ®eld, support gas ¯ow, and microwave power. Plasma was heated by either 14 GHz frequency microwaves with a total power of up to 1.5 kW or by both 14 and 10 GHz frequency microwaves with total power up to 2.1 kW. In these tests, the extracted ion beams of energy of 15 keV ´ q (extraction voltage times the ion charge state q) were analyzed by a 90° magnet and measured by a Faraday cup with secondary electron suppression. The total beam transport eciency of the AECRU beam analysis system is de®ned as the dierence between the sum of all the analyzed beams measured at the Faraday cup and the source net discharge current. Typical beam de®ning slits of 20 mm width were used before and after the analyzing magnet in the measurements. As noted in the data
Table 2 List of calibrated gas leaks and plasma sustaining support gases used Calibrated leak
Uncertainty (%)
Leak rate (scc/s)
Iequi: (plA at 20°C)
Support gas
CO CO2
15 15
2.3 ´ 10ÿ6 2.4 ´ 10ÿ6
He He
CHF3 O2
15 15
2.5 ´ 10ÿ6 2.1 ´ 10ÿ5
Ar Ne Kr 129 Xe (75%)
15 )100 15 15
1.7 ´ 10ÿ5 2.5 ´ 10ÿ6 2.5 ´ 10ÿ6 1.3 ´ 10ÿ6
9.2 9.6 (for O 2) (19.2 for O ) 10 (30 for F ) 76.3 (for O 2) (152.5 for O ) 68.2 10 10 5.2
He He O2 He He, O2 He, O2
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table, narrower slits were used in certain cases, to clearly separate some of the high charge state heavy ion beams. The total beam transport eciency ranges from 60% to 80%, depending upon the width of the beam de®ning slits for the studied ion beams. The background contamination after closing the calibrated leak was subtracted from the measured current in the Faraday cup to determine the net current for a speci®c ion beam. The ionization eciency was calculated with the net current, divided by the leak rate but without considering the leak rate uncertainty. The eciencies given below represent the overall system eciencies (ion source and transport line).
Table 3 Ionization eciencies and mean decay times of the extracted ion beams for selected charge states of noble gases % Ne Ne6 Ne8 Ne9
3.1. Noble gases Ionization eciencies of various high charge state ion beams for noble gases of Ne, Ar, Kr and Xe are summarized in Table 3. As an illustration, Fig. 2 shows the ionization eciencies for a number of argon charge states with each charge state optimized individually. Eciencies up to about 13% were measured for the Ar8 and Ar9 and about 9% for Ar12 . For the higher charge states, the eciency was 5% for Ar14 and 1.2% for Ar16 . The ionization eciency of each ion charge state evaluated from two argon spectra (optimized on 9+ and 12+) is shown in Fig. 3. The overall ionization eciency for argon, determined by summarizing all of the contributions from each charge state, varied from 58% to 66%. Optimization for Ar16 resulted in the lowest total ionization eciency of 58%, while the optimization for Ar11 lead to the highest total ionization of 66%. These results indicate that an average total ionization of 60% for argon was achieved with the LBNL AECR-U ion source. The total ionization eciency for Xe can be estimated between 80% and 85% by summarizing over the current contribution from charge 18 to 31. For Kr and Ne there are too many mixed species to determine the total ionization eciencies. Measurement of the beam current decay time after the closure of the calibrated leak can provide an estimate of the system hold-up time for a particular element. The beam current decay can be described by a mean decay time s with an expo-
a b
2.7a 6.1 1.5a
Ar Ar8 Ar9 Ar11 Ar12 Ar13 Ar14 Ar16
12.7 12.4 10.4 9 6.9 5.4 1.2
Kr Kr17 Kr19 Kr19 Kr20 Kr20 Kr22 Kr23 Kr25
12.9a 10.2a 13.7b 11.2a 12.3b 6.5a 6.3a 4.2a
Xe Xe20 Xe21 Xe22 Xe23 Xe25 Xe26 Xe27 Xe29 Xe30 Xe31
20.4b 13.8b 14.9b 16.0b 19.0b 17.8b 14.0b 7.3b 4.1b 2.0b
s (s) 2.3 2.3 2.3
3.6 3.2 3.6 3.6 4.4
7.3 7.7 7.7
7.9 9.3 9.9
12 ´ 12 mm resolving slits. 16 ´ 16 mm resolving slits.
nential ®t A exp()t/s). The mean ion current decay time with the AECR-U ranges from about 2 to 10 s for the ion beams studied as tabulated in Table 3. There is an indication that the higher charge state ion beams seem to have a slightly longer decay time. That may be due to the non-equilibrium changes in the charge production and extraction process during the decay after the leak valve was closed. Nevertheless, within certain percentage range, the mean decay times seem to be categorically the same for all ion beams of a speci®c
Z.Q. Xie et al. / Nucl. Instr. and Meth. in Phys. Res. B 168 (2000) 117±124
Fig. 2. Ionization eciency for argon for the medium and high charge states. The ionization eciency was optimized for each charge state separately.
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Fig. 4. Normalized beam current decay for selected charge states for a few noble gases with the LBNL AECR-U ion source.
indicative of the ion trapping times in the AECRU plasma is a question for future investigations. 3.2. Non-noble gases
Fig. 3. Ionization eciency distribution for argon spectra optimized on Ar9 and Ar12 with oxygen as support gas. The open symbols indicate the mixed ion species.
element. As listed in Table 3, the category decay times for Ne, Ar, Kr and Xe are in the order of 2.3, 3.6, 7.7 and 9.5 s, respectively. Fig. 4 shows a comparison of the decays of Ne8 , Ar11 , Kr20 and Xe25 ion beams. The measured decay times increase with ion mass for neon to xenon and slightly with increasing charge state. Whether the ion decay times are a
There are two radioactive ion beam projects under development: BEARS and the low energy on-line 14 O experiment at the 88-Inch Cyclotron [14]. The activities are produced as CO2 and N2 O for BEARS (see Section 3.3) or as CO for the online low energy 14 O experiment. In order to optimize the on-line eciencies of the ion source, oline ionization eciency studies were carried out with oxygen and carbon. Listed in Table 4 are the ionization eciencies and decay times of various high charge state ion beams of carbon, oxygen and ¯uorine from nonnoble gases of CO, CO2 , O2 and CHF3 . These gases (or their dissociated components) may react with or stick to the plasma chamber surface made of aluminum. The eciencies are similar for different gases for a particular ion beam, but the beam current decay times vary among dierent gases. Eciencies of up to 25% were achieved for C4 and 33% for O6 and more than 10% for C5 and O5 . Shown in Figs. 5 and 6 are the decay times of 16 O6 and 12 C4 produced from CO, CO2 , CHF3 and O2 . A conclusion, which can be drawn
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Table 4 Ionization eciencies and hold up times for selected charge states of oxygen, carbon and ¯uorine ions produced by the LBNL AECR-U ion source with CO, CO2 , O2 and CHF3 calibrated leaks
a
%
sfast (s)
CO O5 O6 O7 C4 C5
11.5 26.3 7.5 23.7 14.1
3.2 3.2 3.2 2.9 2.9
CO2 O5 O6 O7 C4 C5
12.5 33 7.44 23.4 15.4
O2 O2 O3 O5 O6 O7
3.9 8 11.3 16.0 5.6
CHF3 F5 F7 F9 C4
2.8 4.5a 0.7a 18
7.1 5.6
4.7
4.5 4.3
8 ´ 8 mm resolving slits.
Fig. 6. Normalized current decay for C4 ion beams from CO, CO2 and CHF3 calibrated leaks with the AECR-U ion source.
from these comparisons, is that the CO gas is the least reactive gas among the four studied non-noble gases. The fast component of the holdup time for nonnoble ion beams is similar as for the noble gases. There is a dierence in the decay time for the same ion produced from dierent gases, which is likely due to the chemical properties of the these gases. The experimental gas hold-up time is now described by an exponential ®t A exp()t/sfast ) + B exp()t/sslow ). The fast component describes the holdup time of the ions in the plasma, the slow component is related to the wall sticking. About 70% of the signal is dropped within sfast , which is listed in Table 4. The high ionization eciency and the only slightly longer current decay of the nonnoble gaseous ion beams, indicate that a good portion of the incoming gases may be ionized and trapped in the AECR-U plasma on the ®rst path. Therefore, the beam decay is still dominated by the plasma, which indicates that the AECR-U plasma is a good pump. 3.3. Radioactive gases
Fig. 5. Normalized current decay for O6 ion beams from CO and O2 calibrated leaks with the AECR-U ion source.
The radioactive isotopes were produced in a pressurized N2 gas target with a 10 MeV proton beam of intensities up to 30 lA provided from either the 88-Inch Cyclotron or a medical cyclo-
Z.Q. Xie et al. / Nucl. Instr. and Meth. in Phys. Res. B 168 (2000) 117±124
tron in Building 56. After the target production the activity, believed in the form of 11 CO2 and N2 14 O was transported in batch mode through a cryogenic trap and released o-axially into the AECRU ion source [13]. In this batch mode operation, the 11 C activity was measured at the trap before it was released to the ion source. Due to its shorter lifetime, the trapped activity of the 14 O in each test run was not measured before releasing it to the ion source. Therefore, the o-line measurement of about 50% trapping eciency of the target yield of 14 O was assumed to determine the ionization eciency. This assumption of the 14 O trapped activity leads to a large uncertainty for the ionization ef®ciency number. The ionization eciency of each ion species is calculated by normalizing the measured activity of the particular charge state at a time t back to the trap at t 0. The activity was released from the trap in typically about 1 minute for the 14 O (half-life of 71 s) and a few minutes for 11 C (half-life of 20 min) with total trapped radioactivity of up to a few hundred mCi. The short release time and the amount of non-radioactive preclude optimizing the tuning for each charge state, as it would be done for constant mass ¯ow in stable beam operation. With thus batch mode operation, the measured ionization eciency with the AECR-U for the high charge state 11 C and 14 O ions are lower, but peak on the same charge state. High charge states up to fully stripped 11 C (2%) and 14 O (0.4%) ion beams were extracted. The detailed eciencies of the 11 C and 14 O are listed in Table 5. The highest eciency for a single radioactive ion species is 11% for 11 C4 and 3.6% for 14 O6 which are about a factor of 2 and 7, respectively, lower than the case of 12 C4 and 16 O6 . This indicates that there may be still room for further improvement for the ionization of the 11 C and 14 O isotopes. As in the case of natural CO2 , the gas holdup time has a slow component and a fast component. Table 5 Ionization eciencies of source Ion
11
%
4
C3
11
C4
11
11
5
11
C and
C5
11
2
14
123
The fast component describes the holdup time of the ions in the plasma; the slow component is related to the wall sticking time. Figs. 7 and 8 show 11 6 C ion beam decay times after the feeding valve was closed, measured in a non-suppressed Faraday cup after the beam was extracted from the cyclotron. The 11 C4 ion beam was fully stripped after extraction and mass-separated to eliminate any 11 4 B acontamination coming from the ECR ion source. Since there is no background for 11 C, the
Fig. 7. Current decay for a 11 C6 ion beam after extraction from the cyclotron during the ®rst minute of activity release from the cryogenic trap.
O with the AECR-U ion
C6
14
O6
3.6
14
O7
1.2
14
O8
0.4
Fig. 8. Current decay for a 11 C6 ion beam after extraction from the cyclotron after 12 min of continuous release from the cryogenic trap.
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wall-sticking component can be clearly measured. Fig. 7 shows the ion beam decay after the ®rst minute of the release time. The initial fast ion beam decay time of about 24 s is consistent with the gas hold-up time for the non-radioactive species. Fig. 8 shows the ion beam decay after 12 min of continuous release of radioactive 11 CO2 . After the initial fast decay, there is a slow beam current decay of 11 C coming o the chamber walls showing a mean decay time of about 6 min. 4. Remarks The high ionization eciencies measured with the AECR-U ion source demonstrate that ECR plasmas are very good electron strippers. It would be feasible to build a radioactive ion beam injector, where the radioactive species diuses through a thermal transport line into a high eciency ECR ion source. With careful manipulation of the line temperature distribution, the thermal transfer line can at the same time reduce the isobar contaminants with good eciency, which is a crucial for the RIB production. Such a system will provide an alternative and less complex injection system to the post-accelerators of the future ISOL facilities. Acknowledgements The authors wish to thank Dr J. Nolen of Argonne National Lab for loaning the Ne and CHF3 calibrated leaks used in the measurements. This work was supported by the Director, Oce of Energy Research, Oce of High Energy and Nuclear Physics, Nuclear Physics Division of the US Department of Energy under Contact DE AC0376SF00098.
References [1] H.L. Ravn, Nucl. Instr. and Meth. B 70 (1992) 107. [2] M. Gaelens, P. Decrock, M. Huyse, M. Loiselet, G. Ryckwaert, G. Vancrawynest, P. Van Duppen, in: Proceedings of the 11th International Workshop on ECR on Sources, KVI, The Netherlands, 1993, p. 31. [3] L. Buchmann, J. Vincent, H. Springer, M. Dombsky, J.M. D'Auria, P. McWesly, G. Roy, Nucl. Instr. and Meth. B 62 (1992) 521. [4] R. Leroy et al., in: Proceedings of the 12th International Workshop on ECR Ion Sources, RIKEN, Japan, 1995, p. 57. [5] B.V. Dohrmann, S.A. Sheik, in: Proceedings of the 7th International Workshop on ECR Ion Sources, Juelich, Germany, 1986, p. 248 . [6] Z.Q. Xie, C.M. Lyneis, in: Proceedings of the 13th International Workshop on ECR Ion Sources, Texas A&M, College Station, USA, 1997, p. 16. [7] R. Geller, IEEE Trans. 26 (2) (1979) 2120. [8] T. Nakagawa, Jpn. J. Appl. Phys. 30 (1991) 930. [9] Z.Q. Xie, C.M. Lyneis, Rev. Sci. Instr. 65 (1994) 2947. [10] T.A. Antaya, S. Gammino, Rev. Sci. Instr. 65 (1994) 1723. [11] C.M. Lyneis, Z.Q. Xie, D.J. Clark, R.S. Lam, S.A. Lundgren, in: Proceedings of the 10th International Workshop on ECR Ion Sources, Oak Ridge, ORNL CONF-9011136, 1990, p. 47. [12] J. Powell, F.Q. Guo, R. Joosten, R.M. Larimer, C.M. Lyneis, P. McMahan, E.B. Norman, J.P. OÕNeil, M.W. Rowe, H.F. VanBrocklin, D. Wutte, X.J. Xu, Z.Q. Xie, J. Cerny, in: J.L. Duggan, I.L. Morgan (Eds.), Proceedings of the 15th International Conference on the Application of Accelerator to Research and Industry (ICAARI), CP475, American Institute of Physics 1-56396-825-8, 1999, p. 318. [13] Z.Q. Xie, J. Cerny, F.Q. Guo, R. Joosten, R.M. Larimer, C.M. Lyneis, P. McMahan, E.B. Norman, J.P. OÕNeil, J. Powell, M.W. Rowe, H.F. VanBrocklin, D. Wutte, X.J. Xu, in: K.W. Shephard (Ed.), Proceedings of the Heavy Ion Accelerator Technology Conference (HIATÕ98), CP473, American Institute of Physics 1-56396-806-1, 1999, p. 312. [14] D. Wutte, J. Burke, B. Fujikawa, P. Vetter, S.J. Freedman, R.A. Gough, C.M. Lyneis, Z.Q. Xie, in: K.W. Shephard (Ed.), Proceedings of the Heavy Ion Accelerator Technology Conference (HIATÕ98), CP473, American Institute of Physics 1-56396-806-1, 1999, p. 566.
www.elsevier.nl/locate/nimb
Ionization eciencies for highly charged stable and radioactive ions in the AECR-U ion source Z.Q. Xie, D. Wutte *, C.M. Lyneis Lawrence Berkeley National Laboratory, University of California, 1 Cyclotron Road MS 88, Berkeley, CA 94720, USA Received 10 August 1999
Abstract Ionization eciencies for high charge state ions have been measured with the LBNL AECR-U ion source for ion beams produced from stable and radioactive gases. Various calibrated gas leaks from stable CO, CO2 , O2 , Ne, Ar, CHF3 , Kr and Xe were used in the measurements. Ionization eciencies as high as 25% or higher were measured for 16 6 O and 12 C4 ion beams and more than 10% for high charge state stable ion beams of Ar, Kr and Xe. In addition, ionization eciency measurements for radioactive species of 11 C and 14 O were carried out in which an eciency of more than 10% for 11 C4 was achieved. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 07.77.K; 29.25.L Keywords: High charge state ECR ion source; Electron cyclotron resonance ion sources; Plasma ion source; Radioactive ion beams; Ionization eciency
1. Introduction There are signi®cant eorts worldwide to produce radioactive ion beams (RIB) for science research. Because of the low production yield of radioactive species, eciency of every part of the whole system is the key to the success of RIB production. In the area of ion sources, there are many R&D studies currently undergoing to investigate and optimize the ionization eciencies
*
Corresponding author. Tel.: +510-486-7814; fax: +510486-7983. E-mail address: [email protected] (D. Wutte).
of various types of ion source [1]. Electron cyclotron resonance (ECR) ion sources are attractive for radioactive ion beams because of good ionization eciency, long lifetime, and ¯exibility as demonstrated by a number of investigations reported in recent years [2±5]. However, those ECR ion sources were designed and used for low or intermediate charge states, so the ionization eciency for the highly charged ions has not yet been well explored. This paper presents a recent study of the ionization eciency of the highly charged ion beams with a well optimized high charge state ECR ion source, the LBNL AECRU [6], at the Lawrence Berkeley National Laboratory.
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 6 3 2 - 1
118
Z.Q. Xie et al. / Nucl. Instr. and Meth. in Phys. Res. B 168 (2000) 117±124
2. The AECR-U ion source Since the pioneering development began in the early 1970s [7] the ECR ion source technology has matured although without a full understanding of the detailed physics processes involved in the ECR plasma. In the past decade, tremendous progress has been achieved on the charge states and on beam intensities by increasing the magnetic con®nement, the microwave frequency and specialized techniques. These techniques include aluminum oxide coatings to provide high yield of the secondary cold electrons, two-frequency plasma heating, and high magnetic mirror ratios [8±10]. The AECR-U ion source is optimized for the production of high charge state ions and combines all of the current ECR ion source techniques. Shown in Fig. 1 is a cross-sectional view of the AECR-U ion source. It was built in 1990 [11] and was upgraded in 1996 to further enhance its performance. Its maximum peak ®elds on axis are 1.7 and 1.1 T at the injection and extraction regions,
respectively. The maximum radial ®eld at the inner surface of the aluminum plasma chamber is 0.85 T. The aluminum plasma chamber is 30 cm in length with an inner diameter of 7.6 cm. Six radial slots and an external turbo pump give about 150 l/s pumping speed. The radial slots also provide easy oven access in the plasma chamber. The AECR-U plasma is driven by microwaves of two-frequency (14 and 10 GHz) launched through two of the three o-axis wave-guides terminated at a bias plate in the injection region. The working gases are axially bled into the source through one of the oaxis wave-guides. Although its overall magnetic ®eld strength is about 60% of the highest ®eld strength ECR ion sources in operation, the AECR-U holds many records for the ion charge states and beam intensities [6]. For example, a low intensity U64 beam was produced with the AECR-U ion source and accelerated by the 88-in. Cyclotron to 2.06 GeV. Table 1 lists some of the high charge state ion beams produced by the AECR-U ion source.
Fig. 1. A cross-sectional view of the LBNL AECR-U ion source. Only one of the three waveguides is shown. The cross section of the aluminum plasma chamber is shown on the far right side.
Z.Q. Xie et al. / Nucl. Instr. and Meth. in Phys. Res. B 168 (2000) 117±124 Table 1 A few high charge state ion beams produced by the LBNL AECR-U Ion
I (elA)
O6 O7 Ne9 Ar13 Co13 Kr18 Ar16 Co20 Kr26 Xe30 Au36 U38 Ar17 Kr28 Xe36 Au46 Bi47 U48 Ar18 Ca19 Xe38 Au50 Bi50 U52
840 360 110 120 116 100 25 13 18 10 13 11 2 2 1 1 1 1 0.12 0.25 0.25 0.15 0.15 0.10
3. Measurements Various calibrated gas leaks, CO, CO2 , O2 , Ne, Ar, CHF3 , Kr and Xe, were used in the study. The leak rates of these ranged from a few to tens of plA. The quoted uncertainty is 15%, except for
119
the Ne leak, which has a much higher uncertainty of )100% due to its age (Table 2). The radioactive species of 11 C and 14 O are being developed for the Berkeley experiments with accelerated radioactive species (BEARS), an initiative to develop RIB capability at the 88-Inch Cyclotron Facility, LBNL [12,13]. The gases were axially injected into the source through one of the o-axis wave-guides. Helium or oxygen support plasma was used to optimize the ion production. The goal was to evaluate the eciency of the AECR-U ion source, not to investigate the in¯uence of a particular source tuning parameter on the ionization eciency of a speci®c ion beam. Therefore, the intensity of each analyzed ion beam was optimized with respect to all source-tuning parameters, such as magnetic ®eld, support gas ¯ow, and microwave power. Plasma was heated by either 14 GHz frequency microwaves with a total power of up to 1.5 kW or by both 14 and 10 GHz frequency microwaves with total power up to 2.1 kW. In these tests, the extracted ion beams of energy of 15 keV ´ q (extraction voltage times the ion charge state q) were analyzed by a 90° magnet and measured by a Faraday cup with secondary electron suppression. The total beam transport eciency of the AECRU beam analysis system is de®ned as the dierence between the sum of all the analyzed beams measured at the Faraday cup and the source net discharge current. Typical beam de®ning slits of 20 mm width were used before and after the analyzing magnet in the measurements. As noted in the data
Table 2 List of calibrated gas leaks and plasma sustaining support gases used Calibrated leak
Uncertainty (%)
Leak rate (scc/s)
Iequi: (plA at 20°C)
Support gas
CO CO2
15 15
2.3 ´ 10ÿ6 2.4 ´ 10ÿ6
He He
CHF3 O2
15 15
2.5 ´ 10ÿ6 2.1 ´ 10ÿ5
Ar Ne Kr 129 Xe (75%)
15 )100 15 15
1.7 ´ 10ÿ5 2.5 ´ 10ÿ6 2.5 ´ 10ÿ6 1.3 ´ 10ÿ6
9.2 9.6 (for O 2) (19.2 for O ) 10 (30 for F ) 76.3 (for O 2) (152.5 for O ) 68.2 10 10 5.2
He He O2 He He, O2 He, O2
120
Z.Q. Xie et al. / Nucl. Instr. and Meth. in Phys. Res. B 168 (2000) 117±124
table, narrower slits were used in certain cases, to clearly separate some of the high charge state heavy ion beams. The total beam transport eciency ranges from 60% to 80%, depending upon the width of the beam de®ning slits for the studied ion beams. The background contamination after closing the calibrated leak was subtracted from the measured current in the Faraday cup to determine the net current for a speci®c ion beam. The ionization eciency was calculated with the net current, divided by the leak rate but without considering the leak rate uncertainty. The eciencies given below represent the overall system eciencies (ion source and transport line).
Table 3 Ionization eciencies and mean decay times of the extracted ion beams for selected charge states of noble gases % Ne Ne6 Ne8 Ne9
3.1. Noble gases Ionization eciencies of various high charge state ion beams for noble gases of Ne, Ar, Kr and Xe are summarized in Table 3. As an illustration, Fig. 2 shows the ionization eciencies for a number of argon charge states with each charge state optimized individually. Eciencies up to about 13% were measured for the Ar8 and Ar9 and about 9% for Ar12 . For the higher charge states, the eciency was 5% for Ar14 and 1.2% for Ar16 . The ionization eciency of each ion charge state evaluated from two argon spectra (optimized on 9+ and 12+) is shown in Fig. 3. The overall ionization eciency for argon, determined by summarizing all of the contributions from each charge state, varied from 58% to 66%. Optimization for Ar16 resulted in the lowest total ionization eciency of 58%, while the optimization for Ar11 lead to the highest total ionization of 66%. These results indicate that an average total ionization of 60% for argon was achieved with the LBNL AECR-U ion source. The total ionization eciency for Xe can be estimated between 80% and 85% by summarizing over the current contribution from charge 18 to 31. For Kr and Ne there are too many mixed species to determine the total ionization eciencies. Measurement of the beam current decay time after the closure of the calibrated leak can provide an estimate of the system hold-up time for a particular element. The beam current decay can be described by a mean decay time s with an expo-
a b
2.7a 6.1 1.5a
Ar Ar8 Ar9 Ar11 Ar12 Ar13 Ar14 Ar16
12.7 12.4 10.4 9 6.9 5.4 1.2
Kr Kr17 Kr19 Kr19 Kr20 Kr20 Kr22 Kr23 Kr25
12.9a 10.2a 13.7b 11.2a 12.3b 6.5a 6.3a 4.2a
Xe Xe20 Xe21 Xe22 Xe23 Xe25 Xe26 Xe27 Xe29 Xe30 Xe31
20.4b 13.8b 14.9b 16.0b 19.0b 17.8b 14.0b 7.3b 4.1b 2.0b
s (s) 2.3 2.3 2.3
3.6 3.2 3.6 3.6 4.4
7.3 7.7 7.7
7.9 9.3 9.9
12 ´ 12 mm resolving slits. 16 ´ 16 mm resolving slits.
nential ®t A exp()t/s). The mean ion current decay time with the AECR-U ranges from about 2 to 10 s for the ion beams studied as tabulated in Table 3. There is an indication that the higher charge state ion beams seem to have a slightly longer decay time. That may be due to the non-equilibrium changes in the charge production and extraction process during the decay after the leak valve was closed. Nevertheless, within certain percentage range, the mean decay times seem to be categorically the same for all ion beams of a speci®c
Z.Q. Xie et al. / Nucl. Instr. and Meth. in Phys. Res. B 168 (2000) 117±124
Fig. 2. Ionization eciency for argon for the medium and high charge states. The ionization eciency was optimized for each charge state separately.
121
Fig. 4. Normalized beam current decay for selected charge states for a few noble gases with the LBNL AECR-U ion source.
indicative of the ion trapping times in the AECRU plasma is a question for future investigations. 3.2. Non-noble gases
Fig. 3. Ionization eciency distribution for argon spectra optimized on Ar9 and Ar12 with oxygen as support gas. The open symbols indicate the mixed ion species.
element. As listed in Table 3, the category decay times for Ne, Ar, Kr and Xe are in the order of 2.3, 3.6, 7.7 and 9.5 s, respectively. Fig. 4 shows a comparison of the decays of Ne8 , Ar11 , Kr20 and Xe25 ion beams. The measured decay times increase with ion mass for neon to xenon and slightly with increasing charge state. Whether the ion decay times are a
There are two radioactive ion beam projects under development: BEARS and the low energy on-line 14 O experiment at the 88-Inch Cyclotron [14]. The activities are produced as CO2 and N2 O for BEARS (see Section 3.3) or as CO for the online low energy 14 O experiment. In order to optimize the on-line eciencies of the ion source, oline ionization eciency studies were carried out with oxygen and carbon. Listed in Table 4 are the ionization eciencies and decay times of various high charge state ion beams of carbon, oxygen and ¯uorine from nonnoble gases of CO, CO2 , O2 and CHF3 . These gases (or their dissociated components) may react with or stick to the plasma chamber surface made of aluminum. The eciencies are similar for different gases for a particular ion beam, but the beam current decay times vary among dierent gases. Eciencies of up to 25% were achieved for C4 and 33% for O6 and more than 10% for C5 and O5 . Shown in Figs. 5 and 6 are the decay times of 16 O6 and 12 C4 produced from CO, CO2 , CHF3 and O2 . A conclusion, which can be drawn
122
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Table 4 Ionization eciencies and hold up times for selected charge states of oxygen, carbon and ¯uorine ions produced by the LBNL AECR-U ion source with CO, CO2 , O2 and CHF3 calibrated leaks
a
%
sfast (s)
CO O5 O6 O7 C4 C5
11.5 26.3 7.5 23.7 14.1
3.2 3.2 3.2 2.9 2.9
CO2 O5 O6 O7 C4 C5
12.5 33 7.44 23.4 15.4
O2 O2 O3 O5 O6 O7
3.9 8 11.3 16.0 5.6
CHF3 F5 F7 F9 C4
2.8 4.5a 0.7a 18
7.1 5.6
4.7
4.5 4.3
8 ´ 8 mm resolving slits.
Fig. 6. Normalized current decay for C4 ion beams from CO, CO2 and CHF3 calibrated leaks with the AECR-U ion source.
from these comparisons, is that the CO gas is the least reactive gas among the four studied non-noble gases. The fast component of the holdup time for nonnoble ion beams is similar as for the noble gases. There is a dierence in the decay time for the same ion produced from dierent gases, which is likely due to the chemical properties of the these gases. The experimental gas hold-up time is now described by an exponential ®t A exp()t/sfast ) + B exp()t/sslow ). The fast component describes the holdup time of the ions in the plasma, the slow component is related to the wall sticking. About 70% of the signal is dropped within sfast , which is listed in Table 4. The high ionization eciency and the only slightly longer current decay of the nonnoble gaseous ion beams, indicate that a good portion of the incoming gases may be ionized and trapped in the AECR-U plasma on the ®rst path. Therefore, the beam decay is still dominated by the plasma, which indicates that the AECR-U plasma is a good pump. 3.3. Radioactive gases
Fig. 5. Normalized current decay for O6 ion beams from CO and O2 calibrated leaks with the AECR-U ion source.
The radioactive isotopes were produced in a pressurized N2 gas target with a 10 MeV proton beam of intensities up to 30 lA provided from either the 88-Inch Cyclotron or a medical cyclo-
Z.Q. Xie et al. / Nucl. Instr. and Meth. in Phys. Res. B 168 (2000) 117±124
tron in Building 56. After the target production the activity, believed in the form of 11 CO2 and N2 14 O was transported in batch mode through a cryogenic trap and released o-axially into the AECRU ion source [13]. In this batch mode operation, the 11 C activity was measured at the trap before it was released to the ion source. Due to its shorter lifetime, the trapped activity of the 14 O in each test run was not measured before releasing it to the ion source. Therefore, the o-line measurement of about 50% trapping eciency of the target yield of 14 O was assumed to determine the ionization eciency. This assumption of the 14 O trapped activity leads to a large uncertainty for the ionization ef®ciency number. The ionization eciency of each ion species is calculated by normalizing the measured activity of the particular charge state at a time t back to the trap at t 0. The activity was released from the trap in typically about 1 minute for the 14 O (half-life of 71 s) and a few minutes for 11 C (half-life of 20 min) with total trapped radioactivity of up to a few hundred mCi. The short release time and the amount of non-radioactive preclude optimizing the tuning for each charge state, as it would be done for constant mass ¯ow in stable beam operation. With thus batch mode operation, the measured ionization eciency with the AECR-U for the high charge state 11 C and 14 O ions are lower, but peak on the same charge state. High charge states up to fully stripped 11 C (2%) and 14 O (0.4%) ion beams were extracted. The detailed eciencies of the 11 C and 14 O are listed in Table 5. The highest eciency for a single radioactive ion species is 11% for 11 C4 and 3.6% for 14 O6 which are about a factor of 2 and 7, respectively, lower than the case of 12 C4 and 16 O6 . This indicates that there may be still room for further improvement for the ionization of the 11 C and 14 O isotopes. As in the case of natural CO2 , the gas holdup time has a slow component and a fast component. Table 5 Ionization eciencies of source Ion
11
%
4
C3
11
C4
11
11
5
11
C and
C5
11
2
14
123
The fast component describes the holdup time of the ions in the plasma; the slow component is related to the wall sticking time. Figs. 7 and 8 show 11 6 C ion beam decay times after the feeding valve was closed, measured in a non-suppressed Faraday cup after the beam was extracted from the cyclotron. The 11 C4 ion beam was fully stripped after extraction and mass-separated to eliminate any 11 4 B acontamination coming from the ECR ion source. Since there is no background for 11 C, the
Fig. 7. Current decay for a 11 C6 ion beam after extraction from the cyclotron during the ®rst minute of activity release from the cryogenic trap.
O with the AECR-U ion
C6
14
O6
3.6
14
O7
1.2
14
O8
0.4
Fig. 8. Current decay for a 11 C6 ion beam after extraction from the cyclotron after 12 min of continuous release from the cryogenic trap.
124
Z.Q. Xie et al. / Nucl. Instr. and Meth. in Phys. Res. B 168 (2000) 117±124
wall-sticking component can be clearly measured. Fig. 7 shows the ion beam decay after the ®rst minute of the release time. The initial fast ion beam decay time of about 24 s is consistent with the gas hold-up time for the non-radioactive species. Fig. 8 shows the ion beam decay after 12 min of continuous release of radioactive 11 CO2 . After the initial fast decay, there is a slow beam current decay of 11 C coming o the chamber walls showing a mean decay time of about 6 min. 4. Remarks The high ionization eciencies measured with the AECR-U ion source demonstrate that ECR plasmas are very good electron strippers. It would be feasible to build a radioactive ion beam injector, where the radioactive species diuses through a thermal transport line into a high eciency ECR ion source. With careful manipulation of the line temperature distribution, the thermal transfer line can at the same time reduce the isobar contaminants with good eciency, which is a crucial for the RIB production. Such a system will provide an alternative and less complex injection system to the post-accelerators of the future ISOL facilities. Acknowledgements The authors wish to thank Dr J. Nolen of Argonne National Lab for loaning the Ne and CHF3 calibrated leaks used in the measurements. This work was supported by the Director, Oce of Energy Research, Oce of High Energy and Nuclear Physics, Nuclear Physics Division of the US Department of Energy under Contact DE AC0376SF00098.
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