Polarized positron source for the linear collider, JLC

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1 Nuclear Instruments and Methods in Physics Research A 455 (2000) 15}24 Polarized positron source for the linear collider, JLC T. Hirose *, K. Dobashi, Y. Kurihara, T. Muto, T. Omori, T. Okugi, I. Sakai, J. Urakawa, M. Washio Department of Physics, Tokyo Metropolitan University, 1-1 Minamiohsawa Hachioji, Tokyo , Japan KEK: High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki , Japan Advanced Research Institute for Science and Technology, Waseda University, Ookubo Shinjuku, Tokyo , Japan Abstract A comprehensive description of a polarized positron project is presented in terms of physics motivations for utilizing a polarized positron in electron}positron collider experiments, a proof-of-principle experiment and a conceptual design of a polarized positron source for the future linear collider JLC. In order to verify a proposed method of creating highly polarized positron beams via successive two fundamental processes, i.e. Compton scattering and pair creation, we have been performing basic experiments both at KEK and BNL. First observation of positrons was made at KEK using an electron beam of 1.26 GeV and a laser of 2.33 ev. High-intensity picosecond X-rays were also generated at BNL using a specially designed Compton chamber. In order to realize polarized positron beams of the JLC which have considerably high intensity, i.e e /pulse and a complicated multi-bunch structure, we have achieved a possible scheme for the Compton scattering system and a positron capture section into an L-band linac Elsevier Science B.V. All rights reserved. PACS: 29.27M; 29.17; 52.40; 11.80C; 52.40F Keywords: Polarized positron; Linear collider; Laser-Compton scattering; Left(right-)-handed helicity; Plasma channeling; E!ective polarization 1. Introduction It is widely accepted that a next-generation accelerator at the high-energy frontier is an electron}positron linear collider (LC) which plays a role complementary to the hadron collider LHC which has been under construction at CERN. Especially, speci"c features of LCs is that it allows to * Corresponding author. Tel.: # ; fax: # address: hirose@comp.metro-u.ac.jp (T. Hirose). make precise observation of interesting and exotic phenomena which usually have a small production rate. In particular, being di!erent from circular machines, the LC can in principle accelerate polarized electron and positron beams without depolarization. The Japan Linear Collider JLC has been designed to start with an initial center-of-mass energy E "500 GeV which is considerably higher than the electron (positron) rest mass [1]. In these high-energy regimes, the helicity may be interpreted as the chirality which is one of the important quantities in the "eld theory. It was also pointed out that a speci"c prescription for an enhancement /00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S ( 0 0 ) LASER COMPTON SCATTERING

2 16 T. Hirose et al. / Nuclear Instruments and Methods in Physics Research A 455 (2000) 15}24 Fig. 1. Schematic illustration of a polarized positron generation via the Compton scattering of circularly polarized laser lights o! relativistic electron beams and the successive pair-creation process. of an exotic process was provided if positrons as well as electrons are transversely polarized [2]. Consequently, the controllability of spins of both electrons and positrons could bring further possibilities for LCs to extract small but signi"cant signals by using spin-dependent characteristics of reaction processes. The polarized positron project started in 1995 when we proposed, as illustrated in Fig. 1, a new method in which highly polarized positrons can be generated via two fundamental processes, namely Compton scattering of circularly polarized laser lights and successive pair creation [3,4]. Since then, a proof-of-principle experiment has been pursued at KEK using an electron beam of 1.26 GeV and a second harmonic laser light with the wavelength of 532 nm generated from Nd:YAG laser. In the "rst stage of the experiment, we had developed basic technologies for creating positrons using unpolarized laser light and then a "rst observation of pair-created positrons was successfully performed [5]. From 1998, the project has been extended over an international collaboration with the BNL}ATF group which has been promoting highly advanced technologies for ultra-bright X-ray production using a 60 MeV linac and a GW CO laser [6]. In parallel with these experimental studies, we have been carrying out a conceptual design of a polarized positron source for the JLC which requires considerably high-intensity positrons and a complicated multi-bunch structure [7,8]. For this purpose, an ultra-high-energy CO laser was adopted owing to its long wave length of λ&10 μm, which, compared to solid-state lasers (λ&1 μm), gives rise to one order of magnitude larger number of photons at a given laser energy. In this paper, a comprehensive description on the polarized positron project is presented both for basic experiments and a conceptual design of the polarized positron source for the JLC. Section 2 describes physics motivations of utilizing a polarized positron beam in linear collider experiments. In Section 3, experimental results of positron production is presented and a design of a positron polarimeter is described. Section 4 discusses technical details of the conceptual design for a laser- Compton system, a positron capture section and an application of plasma channeling. Section 5 is devoted to conclusions and future prospects. 2. Physics motivation For longitudinally polarized electrons (positrons), the helicity h is assigned as the right-handed helicity (denoted as R) or the left-handed helicity (denoted as L) corresponding to the spin vector being parallel or antiparallel to the momentum vector, respectively. Typical reaction processes in high-energy electron and positron collisions are depicted in Fig. 2, where the diagrams (a) and (b) represent a fermion pair production, e e P! (f"e), and (c)}(f) corresponds to those for a W pair production, e e PW W. As seen in these diagrams, electrons and positrons always interact with each other in their combinations of e e or e e. This particular feature of the reaction process expected from the standard model suggests that if only an electron is polarized, the helicity of the positron should be automatically determined to be opposite to the electron helicity. One of the interesting processes is to create exotic particles predicted beyond the standard models, for example, super symmetric particles, Higgs bosons and so on. It is generally indicated that cross-sections of exotic phenomena are, around E "500 GeV, over one order smaller than those of `standard-model processa. Hence, it is crucial to reduce backgrounds caused by standard-model processes such as the W pair production (Fig. 2(c)}(f)). If the electron helicity is selected as right-handed, the process (d) is considerably suppressed and the process (f) is strictly forbidden [8]. Although the right-handed electron plays an

3 T. Hirose et al. / Nuclear Instruments and Methods in Physics Research A 455 (2000) 15}24 17 Fig. 2. Typical fundamental processes in electron}positron collisions expected from the standard model. The su$xes R and L represent the right-handed and left-handed helicities, respectively. important role to extract small signals of new phenomena from the huge amount of backgrounds, obviously the 100% electron polarization cannot be attained. We therefore always su!er backgrounds due to the process caused by the lefthanded electron helicity mixed in the initial electron spin-state which is dominantly lefthanded. It is of great importance to achieve the highest possible electron polarization. Indeed, if positrons as well as electrons are polarized, we can signi"cantly enhance an e!ective polarization de"ned as P "(P!P )/(1!P P ) where P and P represent the electron and positron polarizations, respectively. Here the sign of P and P is positive (negative) for the right-(left-) handed helicity. On the assumption that P "90% and measurement errors of P and P are 1%, the quantity P and its error P /P are calculated as a function of P.As shown in Fig. 3, if we achieve P "80%, P increases up to 99% and P /P is reduced from 0.01 for an unpolarized positron beam down to , thus leading to the improvement of a factor 6. Fig. 4 shows the total cross-section of the W pair production for various cases of electron and positron polarizations as a function of E. At E "500 GeV, the total cross-section of unpolarized electron and positron interactions is re- duced by a factor of about 10, when the electron polarization is P "90% in the right-handed Fig. 3. E!ective polarization and its error as a function of positron polarization calculated based on the assumptions that the degree of polarization is 90% and the measurement error of electron and positron beams is 1%. helicity. Then, when the positron is also polarized to be P "80% in the left-handed helicity, the suppression factor 3 is additionally gained [8]. These results clearly demonstrate that the positron polarization in addition to the electron polarization is a matter of great signi"cance in future collider experiments for accurate determination of certain physical quantities and for e!ective extractions of small signals caused by exotic processes. LASER COMPTON SCATTERING

4 18 T. Hirose et al. / Nuclear Instruments and Methods in Physics Research A 455 (2000) 15}24 Fig. 4. Total cross section of W pair production as a function of the center-of-mass energy calculated under the assumption that the degree of polarization for a right-handed electron is 90% and that for a left-handed positron is 80%. 3. Experiment 3.1. Positron production Fig. 5. (a) Di!erential cross section of pair-creation for the helicities h"$1. normalized by the square of the charge number of nucleus Z. (b) Positorn polarization, as a function of the positron energy. Proof-of-principle experiments so far performed at KEK are based on electron beams of 1.26 GeV/c with the intensity, 6 10 e /pulse, the bunch length, 20 ps extracted every 0.78 s from the KEK}ATF damping ring, and laser beams with a power of 20.3 mj in a pulse duration 6 ns at the wavelength of 532 nm (second harmonic) [5]. Fig. 5 shows the di!erential cross-section and the degree of the polarization for pair-created positrons on a W-target of 1 mm thickness calculated from a Monte-Carlo simulation. It is revealed that the polarization of 80% is achieved when positrons with the energy higher than 23 MeV are selected. In the "rst stage of the experiment, the laser light was guided from the laser generator set outside the radiation shield and the crossing angle between the laser and electron beams was 7 mrad [5]. A special magnet, called a `separation magneta, was designed to yield highly polarized positrons by e!ectively selecting positrons with the energy higher than about 25 MeV, thus yielding a higher polarization. The luminosity is estimated to be 5.2 mb assuming a Gaussian shape for the electron and laser beam distribution, thereby leading to a number of γ-rays generated via the Compton process, i.e /bunch. Table 1 puts together various parameters of the electron and laser beams and numbers of generated γ-rays and positrons. Using a EGS4 simulations [9], a conversion e$ciency from γ-rays to positrons in the W-targett of 1 mm thickness is obtained to be 7.0%, corresponding to the total number of positrons /bunch. Fig. 6 shows the numbers of detected positrons per bunch as a function of the current of the separation magnet. It is veri"ed that the experimental values are in good agreement with the calculated values. Details of the analysis were described in Ref. [5].

5 T. Hirose et al. / Nuclear Instruments and Methods in Physics Research A 455 (2000) 15}24 19 Table 1 Parameters for the "rst- and second-stage experiments Parameter First stage Second stage Electron beam Energy (GeV) Repetition (Hz) No. of electrons (1/bunch) Bunch duration (ps) Normalized emittance (rad m) Horizontal Vertical Momentum spread Horizontal dispersion (m) Horizontal beam size (σ, μm) Vertical beam size (σ, μm) Laser beam Wave length (nm) Energy (mj) Pulse duration (σ, ns) 3 3 Vertical beam size (σ, mm) Horizontal beam size (σ, mm) γ-rays Maximum energy (MeV) Bunch duration (ps) Photon per bunch No. of positrons Fig. 6. Average number of detected positrons per bunch ( ) and calculated values ( ) as a function of the current of the separation magnet. In the basic experiments, we found that the production rate of positrons was so small that the positron polarization might be hardly measured through Bhabha scattering. This is primarily attributed to the large spot size of laser beams caused by the long focal length of the laser optics, 454 cm and the "nite crossing angle, 7 mrad between electron and laser beams. To improve these shortcomings, we have constructed a so-called Compton chamber, in which utilization of o!-axis mirrors considerably shortens the focal length down to 15 cm and realizes the head-on collision [10]. Using this Compton chamber, we were able to attain the luminosity a factor of 20 larger than that obtained in the "rst-stage experiment. For the second-stage experiment, we have been pursuing to achieve further increase of the luminosity by optimizing the collision e$ciency of the Compton process as well as suppressing backgrounds caused by the beam halloo of the extracted electron beams. As shown in Table 1, in the second-stage experiment, the quality of the electron and laser beams will be signi"cantly improved and thus generate 3 10 of γ-rays 106 and 2 10 of positrons. Highly e$cient performance of this chamber was also demonstrated at the BNL}ATF by detection of high-intensity picosecond X-rays [11,12] Polarization measurement For the second-stage experiment at KEK, we are planning the measurement of the positron polarization by Bhabha scattering of a pair-created positron on a magnetized iron foil in which the spins of two free electrons are aligned along an external magnetic "eld. Fig. 7a shows the di!erential crosssection of Bhabha scattering at the positron energy of 40 MeV calculated for the positron and electron spins which are parallel (the dotted line) and antiparallel (the broken line) each other. Then the asymmetry A is derived in Fig. 7b indicating that LASER COMPTON SCATTERING

6 20 T. Hirose et al. / Nuclear Instruments and Methods in Physics Research A 455 (2000) 15}24 example, in the case of positrons with the incident energy of 40 MeV, the angular region is selected as 804θ 4100 which results in A"0.75, and the energy deposit is higher than 12.5 MeV each for electrons and positrons, and the sum energy of the electron and positron is higher than 30 MeV. Fig. 8 shows a schematic view of the whole system of the polarization measurement consisting of the Compton chamber, positron generator and polarimeters. 4. Conceptual design 4.1. Laser-Compton scattering system Fig. 7. (a) Di!erential cross section of Bhabha scattering at the positron energy of 40 MeV as a functio of the center-of-mass angle; the dotted line is for the parallel positron and electron spins and the broken line is for the antiparallel case. (b) Asymmetry obtained by using the results shown in (a). the maximum asymmetry is obtained around 903 of the scattering angle θ in the center-of-mass system. The kinematical aspect thus obtained permits to design an analysing magnet to measure positron (electron) momenta and a detector assembly to achieve a high detection e$ciency of Bhabha events with good momentum resolution. Extensive Monte-Carlo simulation has been carried out to evaluate a detection e$ciency of Bhabha events using the simulation program GEANT [13] for tracing the positron (electron) trajectories in the analysing magnet and POISSON [14] for calculating two-dimensional magnetic "elds. Consequently, the selection criteria are determined as follows: for In the future colliding experiments at the JLC, extremely high-intensity positron beams, e /bunch are required; therefore we have to prepare extremely high-intensity electron and laser beams for Compton scattering. In addition to this severe condition, we encounter another requirement: the time structure of the JLC beam, called a multi-bunch beam, is very complicated as depicted in Fig. 9. This beam has a train structure that a single train which is repeated with 150 Hz contains 85 micro-bunches with the time interval of 2.8 ns. We choose an electron linac of 5.8 GeV with the intensity, 10 e /bunch and with the emittance, 1 10 m-rad and a CO laser with the photon energy ev (the wavelength of 10.6 μm). This combination of the e and CO laser beams yields back scattered γ-rays with the maximum energy of 60 MeV (comparable to the energy obtained in our current experiment at KEK) and gives rise to large cross-sections, 658 mb for the Compton scattering and 22 b for the pair creation on a tungsten (W) target at the γ-ray energy of 30 MeV. The Monte-Calro study reveals that the energy of a single laser-pulse should be about 10 J to produce the required numbers of positrons for the JLC [7,8]. However, if the laser energy 10 J is supplied by a single laser pulse with a diameter of 30 μm at the collision point, large non-linear e!ects cause inevitably a wider energy spread and lower polarization of generated γ-rays. Hence, the laser energy is split into 40 pieces of CO laser pulses

7 T. Hirose et al. / Nuclear Instruments and Methods in Physics Research A 455 (2000) 15}24 21 Fig. 8. Schematic view of the experimental system consisting of the Compton chamber, positron generator and polarimeter. Fig. 9. Time structure of the JLC multi-bunch beam. Fig. 10. Schematic diagram of the Compton scattering system to create multi-pulse laser lights. and each pulse, containing 250 mj, collides 40 times successively with one electron bunch, as illustrated in Fig. 10. We install 40 units of o!-axis parabolic mirrors like the Compton chamber described in Section 3. The distance between two adjacent mirrors is 5 cm, resulting in a total collisiondistance of 2 m. In this con"guration, one electron collides with 5.5 laser photons, so that one electron bunch yields γ-rays/bunch. To slice a CO laser pulse with the time span of about 200 ns into 85 micro-pulses, we designed, as shown in Fig. 11, a laser system consisting of a CO oscillator, an optical switching system and ampli- "ers [15]. Re#ection and transmission of the original CO laser pulse are controlled on a Ge Brewster plate by pico-second Nd:YLF laser pulses which have the same time structure as that of the electron beam. After the re#ection switch, the original CO laser pulse is sliced into 85 pulses, each having the time span of 150 ps corresponding to a relaxation time constant of the Ge switch. Then, using a transmission switch, we can achieve further reduction of the pulse width down to about 10 ps. Finally, each pulse is regulated to the same energy level by means of an electro-optical (EO) power modi"er. Since the energy of a single pulse LASER COMPTON SCATTERING

8 22 T. Hirose et al. / Nuclear Instruments and Methods in Physics Research A 455 (2000) 15}24 Fig. 11. Schematic illustration of the laser system composed of a CO oscillator, an optical switch and ampli"er. generated in this manner is about 1 mj, we have to amplify this by a factor of through two stages of ampli"cation systems Capture section Extensive Monte-Carlo simulation has been made to design a capture section in which positrons pair-created in a W target of 1.5 mm thickness are collected and guided into subsequent accelerators [7,8]. Fig. 12 shows a schematic illustration of the capture section including an L-band linac followed by another linac and a damping ring. The L-band linac with the "eld strength, 30 MeV/m and a large aperture, i.e. the diameter of 5.2 cm is installed in solenoid coils with a magnetic "eld of 4 T. The programs used for the calculation of various quantities in each step of the Compton scattering, paircreation and capture process are as follows: CAIN [16] for a spin-dependent Compton scattering in which non-linear e!ects are taken into account, GRACE [17] for the cross-section of pair creation, EGS4 [9] for multiple scatterings of positrons produced in the W target and a Runge}Kutta method specially designed for tracking each positron in the Fig. 12. Capture section of pair created positrons including the W target, and the L-band linac followed by a linac and a damping ring. L-band linac. Conditions of beam acceptance into the damping ring is that a momentum spread is smaller than 17 MeV at 1.98 GeV and the normalized emittance is smaller than 0.06 rad m. Keeping these requirements, we optimized an RF phase of the L-band linac so as to maximize the intensity and polarization of the positron beams. Fig. 13 shows the momentum distributions of the captured positrons together with the total positrons measured at the exit of the L-band linac. It is found that we can attain the beam intensity, e / bunch and the polarization, 73%, which almost meet the JLC requirements.

9 T. Hirose et al. / Nuclear Instruments and Methods in Physics Research A 455 (2000) 15}24 23 If the pulse duration of the laser is elongated in the plasma channel up to 1 ns, we attain the long interaction length of 15 cm, whereas the waist length in the current design is 9 mm. Thus, keeping the average power of the pulse as 4 kw (&250 mj 85 pulses 150 Hz), we may increase the pulse energy in the plasma channel at least one order of magnitude, i.e. 2.5 J without su!ering nonlinear e!ects: This can signi"cantly simplify the sophisticated Compton scattering system originally designed. One of possible schemes for Compton scattering in which plasma channeling is employed is given in Ref. [18]. 5. Conclusion and discussion Fig. 13. Momentum distributions of captured positrons (shaded) and total positrons (histogram) measured at the entrance of the L-band linac Plasma channeling To put our conceptual design mentioned above to practical use, we have to overcome the most serious technical problem of realizing a picosecond laser with high average-power and high repetition. The short pulse of 10 ps necessarily demands a high-pressure (10 atm) CO laser system. However, the high-pressure, high-repetition laser does not exist yet, and multiple slicing and ampli"cation have not been demonstrated to date. In order to ease these technological constraints on the laser system, we have been investigating technical feasibilities of con"ning laser}electron interactions in an extended plasma channel [18]. This approach permits to use nanosecond laser pulses instead of picosecond pulses used in the current design. The use of the long laser pulse can avoid a crucial di$culty in operating the laser system at 10 atm and makes it possible to introduce a high-repetition TEA CO laser operating at atmospheric pressure. This would also help to suppress undesirable nonlinear e!ects and thus allows the utilization of higher-energy laser beams. With regard to plasma channels, there are numbers of proposed schemes and demonstrations for high-intensity laser pulses [19}21]. In the past few years, the polarized positron project has been achieving rapid progress, both in the basic experiments and in the conceptual design of the positron source for a linear collider. On the basis of the proposed scheme [3,4], the positron production was successfully observed at KEK and we will soon start a polarization measurement through Bhabha scattering of a positron in a magnetized iron foil. To study fundamental aspects of laser-compton scattering, the Japan}US joint team has been challenging development of highly advanced technologies including plasma channeling [22] and the grade-up of the GW CO laser to the TW one [6]. We have been performing the conceptual design of the polarized positron beam at the JLC on the basis of the extensive Monte-Carlo simulations and accomplished a possible scheme which seems to meet the requirements of the JLC in terms of the intensity and multibunch structure [7,8]. Nevertheless, we are still studying various possibilities of taking advantage of new technologies in order to relax major di$culties involved in the original design. Finally, it should be noted that the transverse polarization in collider experiments is of great importance to reveal various new phenomena predicted by models beyond the standard model [2] and is uniquely achieved by both of electron and positron polarizations. It has been indicated that the laser-compton scattering is also applicable in LASER COMPTON SCATTERING

10 24 T. Hirose et al. / Nuclear Instruments and Methods in Physics Research A 455 (2000) 15}24 determining both longitudinal and transverse polarizations by measuring the angular distributions of Compton scattered γ-rays [23]. Acknowledgements Authors would like to thank all collaborators of the polarized positron project both in Japanese institutes and BNL for their important contribution in di!erent stages of the polarized positron project. This research was partially supported by a Grant-in-Aid for Scienti"c Research, (C) , (A) , (B) , Research fund of Japan}US Cooperation in the Field of High-Energy Physics and those of Cooperative Developments by KEK and Universities. References [1] JLC Design Study, KEK Report 97-1, [2] K. Hikasa, Phys. Rev. D 33 (1986) [3] T. Hirose, in: A. Miyamoto, Y. Fujii, T. Matsui, S. Iwata (Eds.), Proceedings of International Workshop on physics and experiments with linear colliders, Morioka, Iwate, Japan, September 8}12, 1995, World Science, Singapore, 1996, pp. 748}756. [4] T. Okugi, Y. Kurihara, M. Chiba, A. Endo, R. Hamatsu, T. Hirose, T. Kumita, T. Omori, Y. Takeuchi, M. Yoshioka, Jpn. J. Appl. Phys. 35 (1996) [5] K. Dobashi, T. Hirose, T. Kumita, Y. Kurihara, T. Muto, T. Omori, T. Okugi, K. Sugiyama, J. Urakawa, Nucl. Instr. and Meth. A 437 (1999) 169. [6] I.V. Pogorelski, Nucl. Instr. and Meth. A 411 (1998) 172. [7] T. Hirose, Proceedings of International Conference on Laser'98, Tuson, Arizona, USA, December 7}11, 1998, SRS Press, McLean, Vol. 748, [8] T. Omori, The "rst ACFA Workshop on Physics/Detector at the Linear Collider Tsinghua University, Beijing, The People's Republic of China, November 26}27, 1998, KEK preprint 98}237. [9] W.R. Nelson, H. Hirayama, D.W.O. Rogers, SLAC-R-265. [10] K. Dobashi, A. Higurashi, T. Hirose, T. Kobuku, T. Kumita, Y. Kurihara, T. Muto, T. Omori, T. Okugi, J. Urakawa, M. Washio, Nucl. Instr. and Meth. A, to be published. [11] S. Kashiwagi et al., Nucl. Instr. and Meth., to be published. [12] I.V. Pogorelsky, I. Ben-Zvi, T. Hirose, S. Kashiwagi, V. Yakimenko, K. Kushe, P. Siddons, J. Skaritka, A. Tsunemi T. Omori, J. Urakawa, M. Washio, T. Okugi, Phys. Rev. E, to be submitted. [13] CERN Program Library Long Writeup W5013. [14] User's guide for the POISSON/SUPERFISH group of codes, LA-UR [15] A. Tsunemi, A. Endo, M. Washio, T. Hirose, Y. Kurihara, T. Omori, J. Urakawa, Proceedings of International Conference on Laser'97, New Orleans, Louisiana, USA, SRS Press, McLean, VA, Vol. 838, [16] K. Yokoya, CAIN2.lb contact yokoya@kekvax.kek.jp. [17] Minamitateya-Collaboration, KEK Report 92-19, [18] I.V. Pogorelski, I. Ben-Zvi, T. Hirose, BNL Report 65907, October [19] Y. Ehrlich, C. Cohen, A. Zigler, J. Krall, P. Sprangle, E. Esarey, Phys. Rev. Lett. 77 (1996) [20] C.D. Durfee, H.M. Milchberg, Phys. Rev. Lett. 71 (1993) [21] I.V. Pogorelsky, W.D. Kimura, Y. Liu, Sixth Workshop on Advanced Accelerator Concept June 12}18, 1994, Lake Geneva WI, Vol. AIP-335, 1995, p [22] I.V. Pogorelsky, I. Ben-Zvi, X.J. Wang, T. Hirose, Nucl. Instr. and Meth. A 455 (2000) 177, these proceedings. [23] H. Ishiyama, T. Hirose, T. Kumita, Y. Kurihara, T. Okugi, T. Omori, KEK Preprint , Preprint of Tokyo Metropolitan University, TMU HEP/EXP 96-10, 1996.