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Intense positron beam at KEK
Nuclear Instruments and Methods in Physics Research B 171 (2000) 164±171
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Intense positron beam at KEK Toshikazu Kurihara a,*, Akira Yagishita a, Atsushi Enomoto b, Hitoshi Kobayashi b, Tetsuo Shidara b, Akihiro Shirakawa b, Kazuo Nakahara b, Haruo Saitou c, Kouji Inoue c, Yasuyuki Nagashima c, Toshio Hyodo c, Yasuyoshi Nagai d, Masayuki Hasegawa d, Yoshi Inoue e, Yoshiaki Kogure e, Masao Doyama e a
c
Institute of Materials Structure Science (IMSS), High Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan b Accelerator Laboratory, High Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan Department of Basic Science, Graduate School of Arts and Sciences, University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan d Institute for Material Research, The Oarai Branch, Tohoku University, Oarai, Ibaraki 311-1313, Japan e Teikyo University of Science and Technology, Uenohara, Yamanashi 409-0193, Japan Received 7 December 1999; received in revised form 8 February 2000
Abstract A positron beam is a useful probe for investigating the electronic states in solids, especially concerning the surface states. The advantage of utilizing positron beams is in their simpler interactions with matter, owing to the absence of any exchange forces, in contrast to the case of low-energy electrons. However, such studies as low-energy positron diraction, positron microscopy and positronium (Ps) spectroscopy, which require high intensity slow-positron beams, are very limited due to the poor intensity obtained from a conventional radioactive-isotope-based positron source. In conventional laboratories, the slow-positron intensity is restricted to 106 e /s due to the strength of the available radioactive source. An accelerator based slow-positron source is a good candidate for increasing the slow-positron intensity. One of the results using a high intensity pulsed positron beam is presented as a study of the origins of a Ps emitted from SiO2 . We also describe the two-dimensional angular correlation of annihilation radiation (2D-ACAR) measurement system with slow-positron beams and a positron microscope. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 01.52; 07.78.+s; 29.17; 78.70.B Keywords: Intense positron beam; Positron annihilation; Positron spectroscopy; Facilities; Linac
1. Introduction *
Corresponding author. Tel.: +81-298-645683; fax: +81298-642801. E-mail address: [email protected] (T. Kurihara).
Positron annihilation as an investigative technique in solid-state physics and atomic physics has a rich forty-year history. Several advances have
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 ( 0 0 ) 0 0 0 7 4 - 4
T. Kurihara et al. / Nucl. Instr. and Meth. in Phys. Res. B 171 (2000) 164±171
contributed to make positron annihilation spectroscopy a useful investigative probe in the abovementioned ®eld. The recent development of a technique to convert a white positron beam, emitted from radioactive nuclei or from pair creation, to a controllable slow and mono-energetic beam has expanded the research targets of this method to studies of the electron/positron states and of atomic defects near surfaces and interfaces. The advantage of utilizing positron beams is in their simpler interactions with matter, owing to the absence of any exchange forces, in contrast to the case of low-energy electrons. However, such studies as low-energy positron diraction, positron microscopy and positronium (Ps) spectroscopy, which require high intensity slow-positron beams, are very limited due to the poor intensity obtained from a conventional radioactive-isotope-based positron source. In conventional laboratories, the slow-positron intensity is restricted to 106 e /s due to the strength of the available radioactive source. An accelerator based slow-positron source is a good candidate for increasing the slow-positron intensity [1±6]. We, therefore, started construction of the KEK-PF slow-positron facility [7,8] from FY1991, aiming to produce more than 2 109 e /s slow-positrons, utilizing our 2.5 GeV electron linac [9,10] as its primary beam source. Our KEK-PF slow-positron facility, built at the third beam switchyard of the PF Injector Linac, had successfully produced a 108 e /s slow-positron beam with a 2 kW primary electron beam. It was used to measure the time-of-¯ight (TOF) spectra of orthopositroniums (o-Ps) emitted from a SiO2 surface [11] to generate a continuous beam utilizing Penning-trap electrodes and to perform brightness-enhancement measurements of slowpositron beams. According to the restructuring of KEK to a new organization, the slow-positron facility now belongs to the Institute of Materials Structure Science (IMSS). Unfortunately for our slow-positron facility, the 2.5 GeV linac is undergoing a reformation process relevant to the KEKB project [12]. With this project, we must relocate our slowpositron facility to the 1.5 GeV point of the KEKB linac.
165
We describe here the KEK slow-positron facility and its performance as well as its recent progress. 2. Layout of the KEK-PF slow-positron facility The KEK-PF slow-positron facility is located at the end of the KEK 2.5 GeV linac. It comprises a beam line for the primary electron beam, a target-moderator assembly, a slow-positron beam transport line and relevant assemblies. The primary electron beam is injected onto a target through an achromatic beam-transport line comprising two 18° de¯ecting magnets and a quadruple magnet. The nominal beam power of the KEK 2.5 GeV linac is 6.25 kW (an energy of 2.5 GeV, a peak current of 50 mA, a pulse length of 1 ls and a pulse repetition rate of 50 pulse/s); an average beam power of 30 kW can be expected from this linac as its maximum beam power [9,10]. The target-moderator assembly comprises a water-cooled tantalum rod of 5 radiation lengths and a moderator with multiple tungsten vanes (thirteen 25 lm thick sheets). The most ecient target thickness for incident electron energy of 2.5 GeV was decided using the EGS4 Code [13,14]. A maximum slow-positron beam intensity of 2 109 e /s can be expected with a full-beam power of 30 kW, according to the calculated energy spectra of positrons emitted from tantalum targets. Electrostatic focusing grids are located just above the moderator. The extracted slow-positron beam was directed by a 30 m long beam-transport line with an axial magnetic ®eld of 100 G to an experimental area at the ground level through a 2.5 m thick radiation shield ¯oor. Twenty sets of steering coils were installed along the slow-positron beam-transport line in order to adjust the slow-positron beam trajectory. A high-voltage station capable of applying 60 kV has been installed in the initial part of the beam-transport line in order to vary the energy of the positron beam, which is useful for depthpro®le measurements. A device controller, combining a personal computer and a programmable sequence controller through optical ®ber, has been adopted to control the monitors and power
166
T. Kurihara et al. / Nucl. Instr. and Meth. in Phys. Res. B 171 (2000) 164±171
supplies at a high-voltage potential. The Penningtrap electrodes have also been installed at this station in order to make a DC beam from a pulsed beam. As for the positron beam monitors, we have introduced retractable positron monitors having a phosphor screen with a tandem micro-channel plate combined with a re¯ecting mirror, with which images of the positron beam were observed. At an experimental area, a slow-positron beam switch system comprising a pair of beam de¯ecting coils and two pairs of Helmholtz coils with magnetic-®eld directions crossing each other was installed. This system has enabled us to direct slowpositron beams to several experimental stations one by one without breaking the vacuum [15]. At the very initial stage of its performance tests, the observed positron yield was 1/20 of the estimated value [8]. During these tests, the positron intensity at the end of the transport line was estimated by detecting annihilation c-rays utilizing a BGO scintillator with a photo-multiplier tube (HAMAMATSU H2611). This discrepancy was thought to be due to the condition of the moderator. An excellent improvement in the positron
yield was achieved by annealing the moderator assembly (tungsten foils) at 2270 K for 10 min under ultra-high-vacuum conditions (see Fig. 1). A slow-positron ¯ux of 1 108 e /s was successfully achieved with a 2.0 GeV, 2 kW primary electron beam power. The typical size of the 400 eV beam was 6 12 mm2 measured by the above-mentioned positron monitor. The achieved conversion eciency has almost reached our designed goal. We can therefore expect a slow-positron intensity of the order of 109 e /s with a maximum beam power of 30 kW. The energy of the positron beam was successfully varied from 50 eV to 40 keV by applying a voltage to the high-voltage station at the initial part of the slow-positron beam-transport line. Although radiation from the target chamber, under kW primary beam operation, caused severe damage to the programmable sequence controller at the high-voltage station (RAM bit error); reinforcement of the radiation shield for the target chamber cured this problem. A slow-positron beam with a beam energy of 50 eV to 9 keV was successfully switched from one direction to another utilizing the slow-positron
Fig. 1. Modi®ed electron±positron converter and positron moderator assembly. The electron±positron converter comprising a tantalum rod was changed to that of a plate in order to increase the eective eciency.
T. Kurihara et al. / Nucl. Instr. and Meth. in Phys. Res. B 171 (2000) 164±171
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beam switch system. We can therefore supply slow-positron beams to several experimental stations without any waste of time. One of the results using a high intensity pulsed positron beam is presented as a study of the origin of positronium (Ps) emitted from SiO2 .
3. Positronium TOF experiment As an example experiment at the KEK-PF slow-positron facility, we brie¯y describe here energy-distribution measurements of the Ps emitted from a single-crystal quartz [11]. The Ps is known to form in the interior of many insulators with wide band gap energy. The energy loss and slowing down process of positrons in matter are becoming well-understood. For positrons with energies of less than the band gap, the production of Ps and phonon excitation have become the dominant energy-loss mechanism. To date, very little has been studied about the kinetics of the formation and diusion of Ps, which is in the state immediately after the production and before its delocalization. We obtained the energy distribution of Ps by adopting the TOF method (see Fig. 2) to emitted Ps. The TOF was determined by measuring the time interval between the arrival time of a pulsedpositron beam and the detection of radiated c-ray from annihilated Ps, as shown in Fig. 3. Since the
Fig. 2. Schematic view of the Ps-TOF chamber.
Fig. 3. Relation between the sample and the detectors. The positron beam comes from the right hand side with the magnetic ®eld guide, injected into the sample. A Ps formed in the sample or on the surface is emitted from the surface of the right. We can detect any dierence in the velocity of the Ps when the three c-ray annihilation event is measured.
lifetime of Ps is well known, we can easily deduce the energy distribution of Ps from the TOF spectra, which were measured by changing the distance between the sample surface and the annihilation cray detector. The pulse width of the injected positron beam is 20 ns (FWHM) in the present case. Fig. 4 shows the result of a positronium TOF spectrum measured with a distance between the sample surface and the c-ray detector of 90 mm. Two energy peaks are clearly resolved, which
Fig. 4. One of the TOF spectrum detected by the TOF measurement system. The injected energy of the positron beam was 0.5 keV. The length from the sample to the detector was 90 mm.
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correspond to Ps energies of 3 and 1 eV, respectively. Although the 3 eV peak had already been reported by Sferlazzo et al. [16], 1 eV peak was identi®ed for the ®rst time by the present measurements. We have measured the TOF of the Ps emitted from single-crystal and amorphous SiO2 using a high-intensity pulsed positron beam [17]. The data indicate that a Ps is emitted by two mechanisms; the emission of Ps formed in the bulk, and Ps formation on the surface with energies of 1 and 3 eV, respectively. 4. Layout of the KEK IMSS slow-positron facility The KEK-PF slow-positron facility was relocated to the 1.5 GeV point of the upgraded electron/positron injector linac relevant to the KEKB project. There are two primary electron beam sources for the relocated slow-positron facility: the 1.5 GeV beam of the KEKB linac and the 45 MeV beam from a test linac. The nominal beam power of the 1.5 GeV beam is 0.75 kW (an energy of 1.5 GeV, charge of 10 nC, a pulse length of 10 ps and a pulse repetition rate of 50 pulse/s). Since the injection interval of the J-linac for the KEKB rings might be relatively short, a dedicated linac for slow-positron use only (the slow-positron generator linac named TEST LINAC) was installed utilizing the remnants of the J-linac upgrade plan. Fig. 5 shows a schematic view of the slow-positron
generator linac. An average beam power of 1 kW can be expected from the latter linac. The measurements of the positron beam characteristics will be done after the commissioning of the slow-positron generator linac. Figs. 6 and 7 show the relocated new slowpositron facility, which is located at the 1.5 GeV point of the KEKB linac. It comprises beam lines for the primary electron beams, an electron±positron converter±moderator assembly, a slow-positron beam-transport line and several experimental stations. The primary electron beam is injected into an electron±positron converter. The electrically extracted slow-positron beam is directed by a 30 m long beam-transport line with an axial magnetic ®eld of 100 G to an experimental hall. Ion pumps and SORB-AC cartridge pumps, which sorb active gases with a non-evaporable gutter material, were used to attain an ultra-high-vacuum condition. A high-voltage station capable of applying 60 kV was installed in order to vary the energy of the positron beam. A device controller, combining a personal computer and a programmable sequence controller through optical ®ber, has been adopted to control the monitors and power supplies at a high-voltage potential. The new experimental hall is now under construction. (1) Positron-beam two-dimensional angular correlation of annihilation radiation (2DACAR) measurements, (2) positron-beam TOF experiments, (3) a construction of a positron microscope will be performed in this hall. A
Fig. 5. Schematic view of the slow-positron generator linac. ML: magnetic lens; CM: current monitor; GV: gate valve; PB: prebuncher; SM: screen monitor; ACC: accelerator tube; STC: steering coils; QM: quadruple magnet; IP: ion pump.
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Fig. 6. Schematic view of the slow-positron facility. It comprises beam lines for the primary-electron beams, an electron±positron converter±moderator assembly, a slow-positron beam-transport line and several experimental stations.
Fig. 7. Schematic view of an experimental hall. (1) Positron beam 2D-ACAR measurements, (2) positron beam TOF experiments, (3) positron microscope.
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T. Kurihara et al. / Nucl. Instr. and Meth. in Phys. Res. B 171 (2000) 164±171
schematic view of the new experimental hall is shown in Fig. 7. Here, we describe the new experimental method, in which the conventional positron-annihilation method with the newest slow-positron beam will be employed. The 2D-ACAR measurements combined with the slow-positron beam technique provides useful information about the surface, defects near the surface, the interface, positron and/or PS diusion in materials, and so on from the aspect of the momentum distribution. The essential parts in the 2D-ACAR apparatus are a pair of positionsensitive c-ray detectors. The sizes of the small BGO crystals are 2:2 2:2 15 mm3 . The crystals of 25 21 pieces are arranged in 2.4 mm pitch and optically coupled to a position sensitive photo-multiplier tube (PS-PMT) HAMAMATSU R3941, which has a rectangular sensitive area of 60 55 mm2 . The position signals from the anodes are converted into voltage signals by preampli®ers in the photo-multiplier bases, and are fed into a CAMAC 16-channel 12-bit analogue to digital converter (ADC), which is controlled by a personal computer (PC). The outputs from the re¯ecting plane (last) dynodes of the PS-PMTs, after being ampli®ed by fast preampli®ers in the photo-multiplier bases and energy selected by discriminators, are used to take coincidence between the two 511 keV c-rays. The rise time of the outputs of the fast preampli®ers is 30 ns. About 30 coincidence counts per second can be obtained with a momentum resolution of 1:2 10ÿ3 mc if 108 slow positrons per second are available. We have measured the TOF spectra of o-Ps emitted from a SiO2 surface, and demonstrated the surface sensitivity of positrons. The above-mentioned 2D-ACAR equipment combined with positron beams will be a powerful tool to investigate surface electronic structures. Fig. 8 shows a side view of the experimental hall. The positron microscope beam line is presented. Magnetically guided slow-positrons are injected into the brightness-enhancement stage through the magnetic-®eld termination section. After this section, the positron beam is enhanced twice, and injected into the positron microscope.
Fig. 8. Side view of the conjunctive section between the magnetic transportation beam line and the positron microscope.
5. Future plans In this FY1999, the construction of a room for an RF power source of slow-positron generator linac will be undertaken. After that, we will increase the number of the klystrons from one to three in order to raise the beam power. We expect a beam intensity of about 109 e /s slow positrons in the near future. Based on discussions at the Positron Workshop 1999, we will examine a dierential pumping system and an interlock system with a fast closing valve. Not only for atomic scattering experiments but for solid-state physics experiments, these systems are essential to a positron facility. 6. Summary The KEK-PF slow-positron facility has successfully produced 108 e /s slow positrons, 6 12 mm2 square beam with a 2.0 GeV, 2 kW primary electron beam power. The energy of the slow-positron beam could be easily varied from 50 to 40 keV. This enables us to measure the depthpro®le, which is very useful for locating any defects in materials. The slow-positron beams have been smoothly supplied to several experimental stations without breaking the vacuum by the aid of the slow-positron beam switch system. One of the
T. Kurihara et al. / Nucl. Instr. and Meth. in Phys. Res. B 171 (2000) 164±171
results of using an intense positron beam was presented as a study of the origins of Ps emitted from SiO2 . A dedicated linac for slow-positron use only was installed.
[4] [5]
Acknowledgements The authors are greatly indebted to Director General, Professor H. Sugawara, as well as to the sta of the KEK administration department for their encouragement and continuous support of this slow-positron project. They wish to express their gratitude to Professor Y. Kimura, the Director of Institute of Materials Structure Sciences for his encouragement and support. They also wish to express their gratitude to Professor M. Kihara, the Director of Accelerator Laboratory, for his continuous encouragement. The sta of the KEK electron linac is also gratefully acknowledged for machine operation and support. References [1] R.H. Howell, R.A. Alvarez, M. Stanek, Appl. Phys. Lett. 40 (1982) 751. [2] R. Ley, K.D. Niebling, A. Osipowicz, A. Picard, G. Werth, in: P.C. Jain, R.M. Singru, K.P. Gopinathan (Eds.), Proceedings of the Seventh International Conference on Positron Annihilation, New Delhi, World Scienti®c, Singapore, 1985, p. 996. [3] L.D. Hulett Jr., T.A. Lewis, D.L. Donohue, A. Pendyala, in: L. Dorikens-Vanpraet, M. Dorikens, D. Segers (Eds.), Proceedings of the Eighth International Conference on
[6]
[7] [8] [9] [10] [11]
[12] [13] [14] [15]
[16] [17]
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Positron Annihilation, Gent, 1988, World Scienti®c, Singapore, 1989, p. 586. T. Akahane, T. Chiba, N. Shiotani, S. Tanigawa, T. Mikado, R. Suzuki, M. Chiwaki, T. Yamazaki, T. Tomimasu, Appl. Phys. A 51 (1990) 146. R. Suzuki, Y. Kobayashi, T. Mikado, H. Ohgaki, M. Chiwaki, T. Yamazaki, in: Proceedings of Ninth Symposium on Accelerator Science and Technology, Tsukuba, 1993, p. 237. Y. Itoh, M. Hirose, S. Takamura, O. Sueoka, I. Kanazawa, K. Mashiko, A. Ichimiya, Y. Murata, S. Okada, M. Hasegawa, T. Hyodo, Nucl. Instr. and Meth. A 305 (1991) 269. A. Asami, A. Enomoto, H. Kobayashi, T. Kurihara, K. Nakahara, T. Shidara, Mater. Sci. Forum 105±110 (1992) 1833. T. Kurihara, A. Shirakawa, A. Enomoto, T. Shidara, H. Kobayashi, K. Nakahara, Appl. Surf. Sci. 85 (1995) 178. J. Tanaka, Nucl. Instr. and Meth. 177 (1980) 101. I. Sato, Nucl. Instr. and Meth. 177 (1980) 91. Y. Morinaka, Y. Nagashima, Y. Nagai, T. Hyodo, T. Kurihara, T. Shidara, K. Nakahara, in: Proceedings of the 11th International Conference on Positron Annihilation, Kansas City, USA, 1997, Mater. Sci. Forum 255±257 (1997) 689. KEKB B-Factory Design Report, KEK Report, 95-7, 1995. W.R. Nelson, H. Hirayama, D.W.O. Rogers, SLAC-265, 1985. H. Hirayama, KEK Internal, 89-17, 1990. T. Kurihara, A. Enomoto, H. Kobayashi, T. Shidara, A. Shirakawa, K. Nakahara, in: Proceedings of the 11th International Conference on Positron Annihilation, Kansas City, USA, 1997, Mater. Sci. Forum 255±257 (1997) 757. P. Sferlazzo, S. Berko, K.F. Canter, Phys. Rev. B 35 (1987) 5315. Y. Nagashima, Y. Morinaka, T. Kurihara, Y. Nagai, T. Hyodo, T. Shidara, K. Nakahara, Phys. Rev. B 58 (1998) 12676.
www.elsevier.nl/locate/nimb
Intense positron beam at KEK Toshikazu Kurihara a,*, Akira Yagishita a, Atsushi Enomoto b, Hitoshi Kobayashi b, Tetsuo Shidara b, Akihiro Shirakawa b, Kazuo Nakahara b, Haruo Saitou c, Kouji Inoue c, Yasuyuki Nagashima c, Toshio Hyodo c, Yasuyoshi Nagai d, Masayuki Hasegawa d, Yoshi Inoue e, Yoshiaki Kogure e, Masao Doyama e a
c
Institute of Materials Structure Science (IMSS), High Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan b Accelerator Laboratory, High Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan Department of Basic Science, Graduate School of Arts and Sciences, University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan d Institute for Material Research, The Oarai Branch, Tohoku University, Oarai, Ibaraki 311-1313, Japan e Teikyo University of Science and Technology, Uenohara, Yamanashi 409-0193, Japan Received 7 December 1999; received in revised form 8 February 2000
Abstract A positron beam is a useful probe for investigating the electronic states in solids, especially concerning the surface states. The advantage of utilizing positron beams is in their simpler interactions with matter, owing to the absence of any exchange forces, in contrast to the case of low-energy electrons. However, such studies as low-energy positron diraction, positron microscopy and positronium (Ps) spectroscopy, which require high intensity slow-positron beams, are very limited due to the poor intensity obtained from a conventional radioactive-isotope-based positron source. In conventional laboratories, the slow-positron intensity is restricted to 106 e /s due to the strength of the available radioactive source. An accelerator based slow-positron source is a good candidate for increasing the slow-positron intensity. One of the results using a high intensity pulsed positron beam is presented as a study of the origins of a Ps emitted from SiO2 . We also describe the two-dimensional angular correlation of annihilation radiation (2D-ACAR) measurement system with slow-positron beams and a positron microscope. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 01.52; 07.78.+s; 29.17; 78.70.B Keywords: Intense positron beam; Positron annihilation; Positron spectroscopy; Facilities; Linac
1. Introduction *
Corresponding author. Tel.: +81-298-645683; fax: +81298-642801. E-mail address: [email protected] (T. Kurihara).
Positron annihilation as an investigative technique in solid-state physics and atomic physics has a rich forty-year history. Several advances have
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 ( 0 0 ) 0 0 0 7 4 - 4
T. Kurihara et al. / Nucl. Instr. and Meth. in Phys. Res. B 171 (2000) 164±171
contributed to make positron annihilation spectroscopy a useful investigative probe in the abovementioned ®eld. The recent development of a technique to convert a white positron beam, emitted from radioactive nuclei or from pair creation, to a controllable slow and mono-energetic beam has expanded the research targets of this method to studies of the electron/positron states and of atomic defects near surfaces and interfaces. The advantage of utilizing positron beams is in their simpler interactions with matter, owing to the absence of any exchange forces, in contrast to the case of low-energy electrons. However, such studies as low-energy positron diraction, positron microscopy and positronium (Ps) spectroscopy, which require high intensity slow-positron beams, are very limited due to the poor intensity obtained from a conventional radioactive-isotope-based positron source. In conventional laboratories, the slow-positron intensity is restricted to 106 e /s due to the strength of the available radioactive source. An accelerator based slow-positron source is a good candidate for increasing the slow-positron intensity [1±6]. We, therefore, started construction of the KEK-PF slow-positron facility [7,8] from FY1991, aiming to produce more than 2 109 e /s slow-positrons, utilizing our 2.5 GeV electron linac [9,10] as its primary beam source. Our KEK-PF slow-positron facility, built at the third beam switchyard of the PF Injector Linac, had successfully produced a 108 e /s slow-positron beam with a 2 kW primary electron beam. It was used to measure the time-of-¯ight (TOF) spectra of orthopositroniums (o-Ps) emitted from a SiO2 surface [11] to generate a continuous beam utilizing Penning-trap electrodes and to perform brightness-enhancement measurements of slowpositron beams. According to the restructuring of KEK to a new organization, the slow-positron facility now belongs to the Institute of Materials Structure Science (IMSS). Unfortunately for our slow-positron facility, the 2.5 GeV linac is undergoing a reformation process relevant to the KEKB project [12]. With this project, we must relocate our slowpositron facility to the 1.5 GeV point of the KEKB linac.
165
We describe here the KEK slow-positron facility and its performance as well as its recent progress. 2. Layout of the KEK-PF slow-positron facility The KEK-PF slow-positron facility is located at the end of the KEK 2.5 GeV linac. It comprises a beam line for the primary electron beam, a target-moderator assembly, a slow-positron beam transport line and relevant assemblies. The primary electron beam is injected onto a target through an achromatic beam-transport line comprising two 18° de¯ecting magnets and a quadruple magnet. The nominal beam power of the KEK 2.5 GeV linac is 6.25 kW (an energy of 2.5 GeV, a peak current of 50 mA, a pulse length of 1 ls and a pulse repetition rate of 50 pulse/s); an average beam power of 30 kW can be expected from this linac as its maximum beam power [9,10]. The target-moderator assembly comprises a water-cooled tantalum rod of 5 radiation lengths and a moderator with multiple tungsten vanes (thirteen 25 lm thick sheets). The most ecient target thickness for incident electron energy of 2.5 GeV was decided using the EGS4 Code [13,14]. A maximum slow-positron beam intensity of 2 109 e /s can be expected with a full-beam power of 30 kW, according to the calculated energy spectra of positrons emitted from tantalum targets. Electrostatic focusing grids are located just above the moderator. The extracted slow-positron beam was directed by a 30 m long beam-transport line with an axial magnetic ®eld of 100 G to an experimental area at the ground level through a 2.5 m thick radiation shield ¯oor. Twenty sets of steering coils were installed along the slow-positron beam-transport line in order to adjust the slow-positron beam trajectory. A high-voltage station capable of applying 60 kV has been installed in the initial part of the beam-transport line in order to vary the energy of the positron beam, which is useful for depthpro®le measurements. A device controller, combining a personal computer and a programmable sequence controller through optical ®ber, has been adopted to control the monitors and power
166
T. Kurihara et al. / Nucl. Instr. and Meth. in Phys. Res. B 171 (2000) 164±171
supplies at a high-voltage potential. The Penningtrap electrodes have also been installed at this station in order to make a DC beam from a pulsed beam. As for the positron beam monitors, we have introduced retractable positron monitors having a phosphor screen with a tandem micro-channel plate combined with a re¯ecting mirror, with which images of the positron beam were observed. At an experimental area, a slow-positron beam switch system comprising a pair of beam de¯ecting coils and two pairs of Helmholtz coils with magnetic-®eld directions crossing each other was installed. This system has enabled us to direct slowpositron beams to several experimental stations one by one without breaking the vacuum [15]. At the very initial stage of its performance tests, the observed positron yield was 1/20 of the estimated value [8]. During these tests, the positron intensity at the end of the transport line was estimated by detecting annihilation c-rays utilizing a BGO scintillator with a photo-multiplier tube (HAMAMATSU H2611). This discrepancy was thought to be due to the condition of the moderator. An excellent improvement in the positron
yield was achieved by annealing the moderator assembly (tungsten foils) at 2270 K for 10 min under ultra-high-vacuum conditions (see Fig. 1). A slow-positron ¯ux of 1 108 e /s was successfully achieved with a 2.0 GeV, 2 kW primary electron beam power. The typical size of the 400 eV beam was 6 12 mm2 measured by the above-mentioned positron monitor. The achieved conversion eciency has almost reached our designed goal. We can therefore expect a slow-positron intensity of the order of 109 e /s with a maximum beam power of 30 kW. The energy of the positron beam was successfully varied from 50 eV to 40 keV by applying a voltage to the high-voltage station at the initial part of the slow-positron beam-transport line. Although radiation from the target chamber, under kW primary beam operation, caused severe damage to the programmable sequence controller at the high-voltage station (RAM bit error); reinforcement of the radiation shield for the target chamber cured this problem. A slow-positron beam with a beam energy of 50 eV to 9 keV was successfully switched from one direction to another utilizing the slow-positron
Fig. 1. Modi®ed electron±positron converter and positron moderator assembly. The electron±positron converter comprising a tantalum rod was changed to that of a plate in order to increase the eective eciency.
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beam switch system. We can therefore supply slow-positron beams to several experimental stations without any waste of time. One of the results using a high intensity pulsed positron beam is presented as a study of the origin of positronium (Ps) emitted from SiO2 .
3. Positronium TOF experiment As an example experiment at the KEK-PF slow-positron facility, we brie¯y describe here energy-distribution measurements of the Ps emitted from a single-crystal quartz [11]. The Ps is known to form in the interior of many insulators with wide band gap energy. The energy loss and slowing down process of positrons in matter are becoming well-understood. For positrons with energies of less than the band gap, the production of Ps and phonon excitation have become the dominant energy-loss mechanism. To date, very little has been studied about the kinetics of the formation and diusion of Ps, which is in the state immediately after the production and before its delocalization. We obtained the energy distribution of Ps by adopting the TOF method (see Fig. 2) to emitted Ps. The TOF was determined by measuring the time interval between the arrival time of a pulsedpositron beam and the detection of radiated c-ray from annihilated Ps, as shown in Fig. 3. Since the
Fig. 2. Schematic view of the Ps-TOF chamber.
Fig. 3. Relation between the sample and the detectors. The positron beam comes from the right hand side with the magnetic ®eld guide, injected into the sample. A Ps formed in the sample or on the surface is emitted from the surface of the right. We can detect any dierence in the velocity of the Ps when the three c-ray annihilation event is measured.
lifetime of Ps is well known, we can easily deduce the energy distribution of Ps from the TOF spectra, which were measured by changing the distance between the sample surface and the annihilation cray detector. The pulse width of the injected positron beam is 20 ns (FWHM) in the present case. Fig. 4 shows the result of a positronium TOF spectrum measured with a distance between the sample surface and the c-ray detector of 90 mm. Two energy peaks are clearly resolved, which
Fig. 4. One of the TOF spectrum detected by the TOF measurement system. The injected energy of the positron beam was 0.5 keV. The length from the sample to the detector was 90 mm.
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correspond to Ps energies of 3 and 1 eV, respectively. Although the 3 eV peak had already been reported by Sferlazzo et al. [16], 1 eV peak was identi®ed for the ®rst time by the present measurements. We have measured the TOF of the Ps emitted from single-crystal and amorphous SiO2 using a high-intensity pulsed positron beam [17]. The data indicate that a Ps is emitted by two mechanisms; the emission of Ps formed in the bulk, and Ps formation on the surface with energies of 1 and 3 eV, respectively. 4. Layout of the KEK IMSS slow-positron facility The KEK-PF slow-positron facility was relocated to the 1.5 GeV point of the upgraded electron/positron injector linac relevant to the KEKB project. There are two primary electron beam sources for the relocated slow-positron facility: the 1.5 GeV beam of the KEKB linac and the 45 MeV beam from a test linac. The nominal beam power of the 1.5 GeV beam is 0.75 kW (an energy of 1.5 GeV, charge of 10 nC, a pulse length of 10 ps and a pulse repetition rate of 50 pulse/s). Since the injection interval of the J-linac for the KEKB rings might be relatively short, a dedicated linac for slow-positron use only (the slow-positron generator linac named TEST LINAC) was installed utilizing the remnants of the J-linac upgrade plan. Fig. 5 shows a schematic view of the slow-positron
generator linac. An average beam power of 1 kW can be expected from the latter linac. The measurements of the positron beam characteristics will be done after the commissioning of the slow-positron generator linac. Figs. 6 and 7 show the relocated new slowpositron facility, which is located at the 1.5 GeV point of the KEKB linac. It comprises beam lines for the primary electron beams, an electron±positron converter±moderator assembly, a slow-positron beam-transport line and several experimental stations. The primary electron beam is injected into an electron±positron converter. The electrically extracted slow-positron beam is directed by a 30 m long beam-transport line with an axial magnetic ®eld of 100 G to an experimental hall. Ion pumps and SORB-AC cartridge pumps, which sorb active gases with a non-evaporable gutter material, were used to attain an ultra-high-vacuum condition. A high-voltage station capable of applying 60 kV was installed in order to vary the energy of the positron beam. A device controller, combining a personal computer and a programmable sequence controller through optical ®ber, has been adopted to control the monitors and power supplies at a high-voltage potential. The new experimental hall is now under construction. (1) Positron-beam two-dimensional angular correlation of annihilation radiation (2DACAR) measurements, (2) positron-beam TOF experiments, (3) a construction of a positron microscope will be performed in this hall. A
Fig. 5. Schematic view of the slow-positron generator linac. ML: magnetic lens; CM: current monitor; GV: gate valve; PB: prebuncher; SM: screen monitor; ACC: accelerator tube; STC: steering coils; QM: quadruple magnet; IP: ion pump.
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Fig. 6. Schematic view of the slow-positron facility. It comprises beam lines for the primary-electron beams, an electron±positron converter±moderator assembly, a slow-positron beam-transport line and several experimental stations.
Fig. 7. Schematic view of an experimental hall. (1) Positron beam 2D-ACAR measurements, (2) positron beam TOF experiments, (3) positron microscope.
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schematic view of the new experimental hall is shown in Fig. 7. Here, we describe the new experimental method, in which the conventional positron-annihilation method with the newest slow-positron beam will be employed. The 2D-ACAR measurements combined with the slow-positron beam technique provides useful information about the surface, defects near the surface, the interface, positron and/or PS diusion in materials, and so on from the aspect of the momentum distribution. The essential parts in the 2D-ACAR apparatus are a pair of positionsensitive c-ray detectors. The sizes of the small BGO crystals are 2:2 2:2 15 mm3 . The crystals of 25 21 pieces are arranged in 2.4 mm pitch and optically coupled to a position sensitive photo-multiplier tube (PS-PMT) HAMAMATSU R3941, which has a rectangular sensitive area of 60 55 mm2 . The position signals from the anodes are converted into voltage signals by preampli®ers in the photo-multiplier bases, and are fed into a CAMAC 16-channel 12-bit analogue to digital converter (ADC), which is controlled by a personal computer (PC). The outputs from the re¯ecting plane (last) dynodes of the PS-PMTs, after being ampli®ed by fast preampli®ers in the photo-multiplier bases and energy selected by discriminators, are used to take coincidence between the two 511 keV c-rays. The rise time of the outputs of the fast preampli®ers is 30 ns. About 30 coincidence counts per second can be obtained with a momentum resolution of 1:2 10ÿ3 mc if 108 slow positrons per second are available. We have measured the TOF spectra of o-Ps emitted from a SiO2 surface, and demonstrated the surface sensitivity of positrons. The above-mentioned 2D-ACAR equipment combined with positron beams will be a powerful tool to investigate surface electronic structures. Fig. 8 shows a side view of the experimental hall. The positron microscope beam line is presented. Magnetically guided slow-positrons are injected into the brightness-enhancement stage through the magnetic-®eld termination section. After this section, the positron beam is enhanced twice, and injected into the positron microscope.
Fig. 8. Side view of the conjunctive section between the magnetic transportation beam line and the positron microscope.
5. Future plans In this FY1999, the construction of a room for an RF power source of slow-positron generator linac will be undertaken. After that, we will increase the number of the klystrons from one to three in order to raise the beam power. We expect a beam intensity of about 109 e /s slow positrons in the near future. Based on discussions at the Positron Workshop 1999, we will examine a dierential pumping system and an interlock system with a fast closing valve. Not only for atomic scattering experiments but for solid-state physics experiments, these systems are essential to a positron facility. 6. Summary The KEK-PF slow-positron facility has successfully produced 108 e /s slow positrons, 6 12 mm2 square beam with a 2.0 GeV, 2 kW primary electron beam power. The energy of the slow-positron beam could be easily varied from 50 to 40 keV. This enables us to measure the depthpro®le, which is very useful for locating any defects in materials. The slow-positron beams have been smoothly supplied to several experimental stations without breaking the vacuum by the aid of the slow-positron beam switch system. One of the
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results of using an intense positron beam was presented as a study of the origins of Ps emitted from SiO2 . A dedicated linac for slow-positron use only was installed.
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Acknowledgements The authors are greatly indebted to Director General, Professor H. Sugawara, as well as to the sta of the KEK administration department for their encouragement and continuous support of this slow-positron project. They wish to express their gratitude to Professor Y. Kimura, the Director of Institute of Materials Structure Sciences for his encouragement and support. They also wish to express their gratitude to Professor M. Kihara, the Director of Accelerator Laboratory, for his continuous encouragement. The sta of the KEK electron linac is also gratefully acknowledged for machine operation and support. References [1] R.H. Howell, R.A. Alvarez, M. Stanek, Appl. Phys. Lett. 40 (1982) 751. [2] R. Ley, K.D. Niebling, A. Osipowicz, A. Picard, G. Werth, in: P.C. Jain, R.M. Singru, K.P. Gopinathan (Eds.), Proceedings of the Seventh International Conference on Positron Annihilation, New Delhi, World Scienti®c, Singapore, 1985, p. 996. [3] L.D. Hulett Jr., T.A. Lewis, D.L. Donohue, A. Pendyala, in: L. Dorikens-Vanpraet, M. Dorikens, D. Segers (Eds.), Proceedings of the Eighth International Conference on
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