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positron linac
Nuclear Instruments and Methods in Physics Research B 173 (2001) 104±111
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Experiment of positron generation using a crystal target at the KEK electron/positron linac M. Inoue a, S. Takenaka a, K. Yoshida a,*, I. Endo b, M. Iinuma b, T. Takahashi b, A.V. Bogdanov c, A.M. Kolchuzkin c, A.P. Potylitsin c, I.E. Vnukov c, H. Okuno d, S. Anami e, A. Enomoto e, K. Furukawa e, T. Kamitani e, Y. Ogawa e, S. Ohsawa e b
a Hiroshima Synchrotron Radiation Center, Hiroshima University, 2-313 Kagamiyama, Higashi-Hiroshima 739-8526, Japan Graduate School of Advanced Sciences of Matter, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526, Japan c Nuclear Physics Institute, Tomsk Polytechnic University, 634050 Tomsk, P.O. Box 25, Lenin Ave, Russia d High Energy Accelerator Research Organization, Tanashi Branch, 3-2-1 Midori-cho, Tanashi, Tokyo 188-8501, Japan e High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba-shi 305-0801, Japan
Received 29 December 1999; received in revised form 11 April 2000
Abstract We have performed an experiment of positron generation using a crystal target at an electron/positron linac at KEK. The target consists of a tungsten crystal to emit photons and an amorphous tungsten for pair creation, whose thicknesses are 0.5 and 2.0 radiation length, respectively. When the á1 1 1ñ crystal axis is oriented parallel to the 3 GeV incident electron beam, the positron yield is enhanced by 40% compared to values for the disoriented crystal. No eect of injecting intense ultra-short electron bunches on the crystalline structure is observed. The crystal orientation dependence of the positron yield as well as the enhancement factor can be explained by a simulation assuming a coherent bremsstrahlung process of the electron in the crystal. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Linear accelerator; Positron production; Target; Crystal tungsten; Channeling radiation
1. Introduction It is of great importance to provide an ecient positron source for high-energy electron±positron colliders. Especially in linear colliders, the positron intensity required is close to the electron intensity
*
Corresponding author. Tel.: +81-824-24-6293; fax: +81824-24-6294. E-mail address: [email protected] (K. Yoshida).
for each repetition, if no positron accumulation ring is used. An accelerator-based positron source is also important as a tool to investigate the electron structure of matters through electron±positron annihilation. Positrons for electron±positron colliders have been so far generated through an electromagnetic cascade-shower process where a heavy-metal target is irradiated by high energy electrons from the linear accelerator (linac). Among the positrons, only those which have small emission angles and
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 0 ) 0 0 1 9 5 - 6
M. Inoue et al. / Nucl. Instr. and Meth. in Phys. Res. B 173 (2001) 104±111
momenta within a certain range are accepted by the positron linac. The target thickness is determined to maximize the number of these eective positrons, which depends on the incident electron energy. In a standard con®guration, a powerful solenoid coil is installed at the entrance of the positron linac to extend the acceptable angular range of produced positrons. The positron yield is naturally proportional to the incident electron intensity, but when the intensity exceeds a certain limit, the target will be destroyed due to the excessive heat load. Presently-attainable eciency, the number of eective positrons divided by the number of incident electrons, is not satisfactory as required to meet the requirement from the linear collider. It has been proposed to employ a single crystal instead of normal heavy metals as a target to improve the positron yield [1±4]. The electron can be channeled along the crystal axis or plane when it is injected along the crystal axis or plane within a critical angle (Lindhard angle) [5]. The classical view holds that the electron makes a wiggler motion attracted by an axial potential or a planar potential and emits the so-called channeling radiation. Channeling radiation is known to show a spectrum that is much stronger than the radiation by a Bethe±Heitler process in the low energy region [3]. Channeling thus contributes to enhance the production rate of low-momentum positrons, which directly favours the ®nal positron yield since the positron linac can accept only low-momentum positrons. The intensity of the channeling radiation, and hence the positron yield enhancement is expected to increase markedly with the energy of the incident electron [3], which is the reason why a crystal target is promising as a positron source for linear colliders. We conducted a proof-of principle experiment using a crystal target in 1996 [6,7], injecting a 1.2 GeV electron beam into a tungsten single crystal with a thickness of 1.2 mm (0.34 radiation length, (r.l.)). It was observed that when the h1 0 0i crystal axis was aligned with the electron beam, the positron yield was about three times the yield from the o axis target. There are two approaches to preparing the crystal target [3]: in the ®rst, the crystal works both
105
as a photon emitter and as a material to create electron±positron pairs, as in the experiment quoted above. In the second, the crystal acts only to emit photons and the pair creation takes place in an amorphous material located behind the crystal. Since the electron channeling is maintained only for a short pass length in the crystal, the crystal can be thin, while the material for the pair creation should be thick enough to give intense positrons. For the practical application of the crystal target to existing linac, the latter approach may be more convenient since the positron generator must be placed in a narrow space under a strong magnetic ®eld to focus generated positrons, where it is not easy to adjust a target orientation with a complicated goniometer system. Encouraged by the promising result for the proof-of-principle experiment on the tungsten crystal target, we have tried to apply the crystal target to a working electron/positron linac, the injector for the electron±positron collider B-Factory at High Energy Accelerator Research Organization (KEK). At the KEK linac, electrons are accelerated as a bunch with a width of about 10 ps, whose peak power is of the order of tera watts. The impact of such a short bunch, which has a transverse cross section of only 1 mm2 , on a crystal may initially cause a speci®c kind of crystal lattice excitation rather than simple heating. One of the purposes of the present experiment is to examine whether the crystal preserves its lattice structure under the impact of electron bunches with ultrahigh peak power. In Section 2, the experimental conditions and the simulation are outlined. The experimental results and some concluding remarks are described in Sections 3 and 4, respectively. 2. Experimental conditions and the simulation 2.1. Outline of the KEK electron/positron linac The KEK linac provides 8 GeV electron bunches and 3.5 GeV positron bunches to the electron±positron collider, B-Factory. It is also an injector for the 2.5 GeV synchrotron light source, Photon Factory. The positrons for the B-Factory
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are generated by hitting the tungsten target with 3.7 GeV electrons [8]. Electron bunches with a charge of 10 nC each are delivered at a repetition rate of 50 Hz, the average beam power being 1850 W. The normalized emittance of the electron beam is of the order of 10ÿ3 mrad, and the angular spread is estimated to be of the order of 0.1 mrad, assuming a beam size of 1 mm. The momentum acceptance of the positron linac with a matching section is 8:2 MeV=c < p < 11:6 MeV=c; pt < 2:4 MeV=c; where p is the magnitude of the incident positron momentum and pt is its transverse component. The radius of the incoming positron beam is assumed to be 1.2 mm. The optimum target thickness is determined to be 4 r.l. by the Monte Carlo calculation so as to maximize the yield of the positrons whose emittance is within the acceptance of the positron linac shown above [8]. The heat deposit to the target is estimated to be about 25% of the beam power, 460 W. The heat at the target is conducted to a coolingwater winding through a tungsten±copper alloy. This target assembly is, when the positron beam is required, inserted by a linear motion into the entrance of a pulsed solenoid coil, as shown in Fig. 1.
Fig. 1. The target for positron generation at the KEK electron/ positron linac for electron±positron collider.
The space for the target is a channel with a diameter varying from 22 to 76 mm, surrounded by a ceramic wall. When the present experiment was performed, the KEK linac was being upgraded for the BFactory and the design performance was not attained yet. Thus the operational conditions for the present experiment were limited to the following values: Incident Electron Energy Accelerated Positron Energy Charge per Bunch FWHM Bunch Width Repetition Rate
3 GeV 3 GeV 6 nC 10 ps 5 Hz
Thus the average beam power of the incident electrons was about 90 W, and the heat deposit to the target was estimated to be roughly 25 W if the target thickness is 4 r.l.. The peak beam power, on the other hand, was 1.8 tera W per bunch. 2.2. Target con®guration and the simulation For the reason described in Section 1, we have employed a target system which is composed of a thin crystal to emit photons and a thick amorphous tungsten for the pair creation as shown in Fig. 2. The amorphous tungsten target is placed at the position of the conventional target. A distance of 70 mm at least is needed between the crystal target and the end of the amorphous target in order to install a goniometer for the crystal orientation.
Fig. 2. The crystal target system consisting of a thin tungsten crystal and an amorphous tungsten.
M. Inoue et al. / Nucl. Instr. and Meth. in Phys. Res. B 173 (2001) 104±111
As a target crystal we have selected tungsten because high Z crystals generally produce intense channeling radiation, although the preparation of a perfect single crystal of a reasonable size is not easy. We have prepared a tungsten single crystal with a size of 6.2 ´ 10 mm2 and a thickness of 0.5 r.l. (1.7 mm). The surface is perpendicular to the h1 1 1i axis, along which the electron is to be channeled, and is polished to facilitate rough alignment using laser light. The mosaicity as examined by a Raue picture is about 0.5 mrad. The simulation for the above target arrangement has been made with the following consideration. One remarkable feature of the results of the proof-of-principle experiment on the tungsten crystal is that the width of the rocking curve for the positron yield as well as for the soft photon yield was unexpectedly large [6]. That is to say, the enhancement of the soft photon and positron yields occured in a much broader angular range than the Lindhard angle. This phenomenon including the magnitude of the positron yield enhancement can be explained by the assumption that coherent bremsstrahlung rather than channeling radiation contributes to the enhancement when the crystal includes some mosaicity [7]. The simulation for the present experimental conditions has been made with this assumption. Fig. 3 shows the result of the simulation for the positron yields accepted by the KEK positron
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linac and also the positron yield enhancement due to the eect of the crystal as a function of the thickness of the amorphous tungsten, assuming a 0.5 r.l. tungsten crystal is placed in front of it. Positrons generated in the crystal are neglected in this simulation because they are out of the focusing ®eld and scarcely accepted by the positron linac. As expected, the positron yield increases with increasing target thickness showing a yield maximum at around 4 r.l., while the enhancement factor decreases. This simulation, based on the coherent bremsstrahlung process, shows that the crystal target system gives higher positron yields than the conventional target in the amorphous tungsten thickness range below 4 r.l., but can only give the same maximum yield as the conventional target around 4 r.l.. This result is not discouraging because the merit of the crystal target will be demonstrated at higher energies of the incident electron. For the purpose of this study, it is more important to con®rm the eect of the crystal target than to obtain larger positron yields, since this is the ®rst trial of the application of the crystal target to the working linac. Consequently, the thickness of the amorphous tungsten has been chosen to be 2 r.l., with which the positron yield will be enhanced by about 50% and will reach 70% of the yield from the conventional target with a thickness of 4 r.l.. We have examined by the simulation whether the crystal with a thickness of 0.5 r.l. is suitable for use in combination with the 2 r.l. amorphous target. The positron yields have been calculated for various thicknesses of the crystal keeping the total thickness at 2.5 r.l.. The result of the calculation is shown in Fig. 4. It is seen that the positron yield with the oriented crystal is maximum when the crystal thickness is 0.4±0.5 r.l., justifying our preliminary choice. The crystal orientation dependence of the positron yield (rocking curve) expected from the present simulation will be shown in Section 3. 2.3. Fabrication of the crystal target system
Fig. 3. Simulation results for the positron yields from the crystal target shown in Fig. 2 as a function of the thickness of the amorphous tungsten.
The crystal part is mounted on a three-axis goniometer for the orientation of the crystal axis, while the non-crystal part is supported
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Fig. 4. Simulation results for the positron yields from the crystal target as a function of the thickness of the crystal part keeping the total thickness at 2.5 r.l..
independently from the goniometer as shown in Fig. 2. The vertical arm of the support is situated close to the vacuum-chamber wall separating a focusing solenoid coil in the air and the horizontal arm locates the amorphous tungsten at its end in the focusing ®eld. As described in Section 2.1, the beam power of the electron beam was about 90 W and the heat deposit to the 2 r.l. target was estimated to be less than 10% of this. Since this amount of heat is not
so large, we did not introduce a water cooling channel. Instead, we have made the supports for the crystal as well as for the amorphous target so as to have good heat conduction. The goniometer, designed originally for use in air, has been remodeled for use in a vacuum. The use of steel is avoided, except in electric motors, since it works under a strong magnetic ®eld to focus positrons. It has been con®rmed that the electric motors work normally under a strong magnetic ®eld. The goniometer can rotate the crystal around three axes which are perpendicular with each other. A phosphorous ceramic sheet with a thickness of 0.1 mm is placed in front of the crystal for monitoring the beam position and pro®le. The goniometer for the crystal target and the support of the amorphous target are assembled on a sliding table in a vacuum chamber as shown in Fig. 5. After removing the conventional target, the sliding table is pushed up by an air cylinder to the speci®ed position. In our proof-of-principle experiment using an external electron beam, the crystal orientation was determined by observing the rocking curve of the soft photon yield. At the positron station in the linac, however, observation of the channeling radiation is almost impossible because it is emitted
Fig. 5. The vacuum chamber containing the crystal target system.
M. Inoue et al. / Nucl. Instr. and Meth. in Phys. Res. B 173 (2001) 104±111
just in the forward direction. Then, we intended to determine the crystal orientation by observing the characteristic X-rays from the crystal target, which are emitted almost isotropically and are expected to be enhanced when the crystal axis is oriented [9,10]. For this purpose, we have provided the vacuum chamber with an X-ray window as shown in Fig. 5. A preliminary alignment of the crystal was made against the center line of the vacuum chamber using a conventional laser light before the installation of the vacuum chamber containing the crystal target system in the positron station of the linac. Owing to a mechanism by which the center line of the vacuum chamber can be made to exactly coincide with the beam line, the crystal axis was placed in the beam direction. 2.4. Generation of positrons The positron generation station of the KEK linac is schematically shown in Fig. 6. The electron bunches, after bunch compression, are focused on the target system. The position and pro®le of the electron bunch is monitored by a thin phosphorous ceramic sheet just in front of the target.
Fig. 6. A schematic drawing of the KEK positron generation station.
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Generated positrons are focused by a pulsed solenoid coil. The front section of the positron linac is surrounded by DC solenoid coil. After the contaminating electrons have been rejected with chicane magnets, the positrons are accelerated by regular accelerator tubes. Intensities of incoming electrons and accelerated positrons are measured by an electric probe of button type, which is working also as a beam position monitor. 3. Experimental results Before starting the experiment, the KEK linac was carefully tuned to accelerate positrons using the conventional target. Removing the conventional target, the crystal target was positioned on the beam line. By a rough control of the goniometer, the enhancement of the positron yield was easily found. This means it is eective to make the crystal alignment by re¯ecting laser light from a crystal surface cut perpendicular to the crystal axis. The positron yield was maximized by rotating the crystal around three axes which are perpendicular with each other. At ®rst one axis of the goniometer which is perpendicular to the electron beam was ®xed to give maximum positron yield, then another axis which is also perpendicular to the beam was adjusted to maximize the yield further. The positron yield thus maximized was constant against the rotation of the third axis which is parallel to the electron beam. Using the above process, the h1 1 1i crystal axis was aligned along the electron beam. The positron yield variation when one goniometer axis which is perpendicular to the incident electron direction was rotated is shown in Fig. 7. The positron yield at the peak is about 40% larger than the yields at the base. It can be said that the positron yield is enhanced by 40% due to the eect of the crystal, which deviates little from the result of the simulation, 49%, as shown in Fig. 3. The FWHM width of the rocking curve is 6.9 mrad, which is very large as in the proof-of-principle experiment. p The Lindhard angle wc is given as wc 2U =E, where U is a string potential of
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M. Inoue et al. / Nucl. Instr. and Meth. in Phys. Res. B 173 (2001) 104±111
Fig. 7. The positron yield from the crystal target irradiated by a 3 GeV electron beam as a function of the crystal orientation. The curve is a result of the simulation assuming a bremsstrahlung process in the crystal.
the crystal axis and E is the incident electron energy. For the present experimental conditions, the wc is 0.7 mrad [11], which is smaller than the experimental value by one order of magnitude. Even with the mosaicity of the crystal used, 0.5 mrad, and the angular divergence of the incident electron beam, 0.1 mrad, the experimental angular width cannot be explained. Thus we have checked to see if there is any ®ne structure in the rocking curve by ®ne scan of the goniometer angle. The result shown in Fig. 8 indicates no ®ne structure.
The rocking curve assuming the contribution of the coherent bremsstrahlung instead of the channeling radiation and also assuming a crystal mosaicity of 0.5 mrad is shown in Fig. 7. The curve is normalized to the experimental values at the o axis states. The FWHM width of the curve is 6.7 mrad, which agrees well with the experimental value. We will make a short comment below on the heating of the crystal target system. After starting the experiment with the crystal target, the vacuum pressure increased at the positron generation station. Thus we limited the experimental time to 3 h. This was regarded as due to the heating of the target and/or of the electric motors to drive the goniometer. Since the present experimental setup is only for the testing of the crystal target, and not for practical use, we did not give it any serious consideration. The experiment was carried out twice in an interval of two months. In each experiment, and in every measurement, the enhancement of the positron yield by the crystal eect was almost constant, and we did not observe any degradation of the crystal due to the heating of the crystal by the impact of the intense ultra-short bunch of electrons. A possible structure for the next-step crystal target system is suggested in the following section. 4. Concluding remarks
Fig. 8. Crystal orientation dependence of the positron yield as observed by ®ne control of the crystal orientation.
We have performed the ®rst trial of employing a crystal target for the generation of positrons in a working electron/positron linac as an injector for the electron±positron collider. Electrons with an energy of 3 GeV were injected into the target which consisted of a 0.5 r.l. tungsten crystal and 2.0 r.l. amorphous tungsten. When the h1 1 1i crystal axis was aligned along the electron beam, the positron yield was enhanced by 40% compared to the value for the target with disoriented crystal. The crystal was not degraded by the impact of high-power electron bunches. The crystal surface had been ®nished to be perpendicular to the h1 1 1i axis, and the preliminary orientation of the crystal by laser-light re-
M. Inoue et al. / Nucl. Instr. and Meth. in Phys. Res. B 173 (2001) 104±111
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with the amorphous tungsten, and this combined target will be oriented by a specially designed goniometer out of the vacuum chamber. By this target con®guration, positrons created in the crystal part can also be accepted by the positron linac. We have a plan to investigate the positron yield enhancement varying the incident electron energy and also the charge in one electron-bunch.
Acknowledgements
Fig. 9. Simulation results for the positron yields from the crystal target as a function of the incident electron energy, obtained by assuming coherent bremsstrahlung in the crystal.
¯ection enabled us to ®nd the crystal axis easily with the electron beam. The crystal orientation dependence of the positron yield (rocking curve), which has a width much broader than the Lindhard angle, and the yield enhancement due to the crystal eect can be reproduced fairly well by simulation assuming the coherent bremsstrahlung in a crystal with mosaicity. For incident electrons with higher energies, the eect of the channeling will show up in the positron yield. Keeping it in mind, we have tentatively made a simulation of the positron yield by the present model for the incident electron with higher energies. The result is given in Fig. 9, showing that the enhancement of the positron yield is larger than in the present experiment. In the next-step model of the crystal target, the crystal will be located downstream to be in contact
We are thankful to Mr. T. Fujita for preparing the ®gures to this text. This work is supported by the collaboration program of the High Energy Accelerator Research Organization, KEK.
References [1] R. Chehab, F. Couchot, A.R. Nyaieh, F. Richard, X. Artru, Orsay Report LAL-RT 89-01, 1989. [2] F.J. Decker, SLAC-PUB-5482, 1991. [3] X. Artru, V.N. Baier, R. Chehab, A. Jejcic, Nucl. Instr. and Meth. A 344 (1994) 443. [4] X. Artru et al., Nucl. Instr. and Meth. B 119 (1996) 246. [5] J. Lindhard, K. Dan, Vid. Selsk. Mat. Fys. Medd. 14 (1965) 34. [6] K. Yoshida et al., Phys. Rev. Lett. 80 (1998) 1440. [7] B.N. Kalinin et al., Nucl. Instr. and Meth. B 145 (1998) 209. [8] I. Sato et al., KEK Report 95-18, 1996, p. 46. [9] A.N. Aleinik et al., PisÕma Zh. Tek. Fiz. T. 13 (N22) (1987) 1367. [10] M.Tu. Andreyashkin et al., HSRC Report 99-1, Hiroshima University, 1999. [11] V.K. Bazylev, N.K. Zhevago, Radiation of Fast Particles in Matter and External Fields, Moscow, Nauka, 1987.
www.elsevier.nl/locate/nimb
Experiment of positron generation using a crystal target at the KEK electron/positron linac M. Inoue a, S. Takenaka a, K. Yoshida a,*, I. Endo b, M. Iinuma b, T. Takahashi b, A.V. Bogdanov c, A.M. Kolchuzkin c, A.P. Potylitsin c, I.E. Vnukov c, H. Okuno d, S. Anami e, A. Enomoto e, K. Furukawa e, T. Kamitani e, Y. Ogawa e, S. Ohsawa e b
a Hiroshima Synchrotron Radiation Center, Hiroshima University, 2-313 Kagamiyama, Higashi-Hiroshima 739-8526, Japan Graduate School of Advanced Sciences of Matter, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526, Japan c Nuclear Physics Institute, Tomsk Polytechnic University, 634050 Tomsk, P.O. Box 25, Lenin Ave, Russia d High Energy Accelerator Research Organization, Tanashi Branch, 3-2-1 Midori-cho, Tanashi, Tokyo 188-8501, Japan e High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba-shi 305-0801, Japan
Received 29 December 1999; received in revised form 11 April 2000
Abstract We have performed an experiment of positron generation using a crystal target at an electron/positron linac at KEK. The target consists of a tungsten crystal to emit photons and an amorphous tungsten for pair creation, whose thicknesses are 0.5 and 2.0 radiation length, respectively. When the á1 1 1ñ crystal axis is oriented parallel to the 3 GeV incident electron beam, the positron yield is enhanced by 40% compared to values for the disoriented crystal. No eect of injecting intense ultra-short electron bunches on the crystalline structure is observed. The crystal orientation dependence of the positron yield as well as the enhancement factor can be explained by a simulation assuming a coherent bremsstrahlung process of the electron in the crystal. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Linear accelerator; Positron production; Target; Crystal tungsten; Channeling radiation
1. Introduction It is of great importance to provide an ecient positron source for high-energy electron±positron colliders. Especially in linear colliders, the positron intensity required is close to the electron intensity
*
Corresponding author. Tel.: +81-824-24-6293; fax: +81824-24-6294. E-mail address: [email protected] (K. Yoshida).
for each repetition, if no positron accumulation ring is used. An accelerator-based positron source is also important as a tool to investigate the electron structure of matters through electron±positron annihilation. Positrons for electron±positron colliders have been so far generated through an electromagnetic cascade-shower process where a heavy-metal target is irradiated by high energy electrons from the linear accelerator (linac). Among the positrons, only those which have small emission angles and
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 0 ) 0 0 1 9 5 - 6
M. Inoue et al. / Nucl. Instr. and Meth. in Phys. Res. B 173 (2001) 104±111
momenta within a certain range are accepted by the positron linac. The target thickness is determined to maximize the number of these eective positrons, which depends on the incident electron energy. In a standard con®guration, a powerful solenoid coil is installed at the entrance of the positron linac to extend the acceptable angular range of produced positrons. The positron yield is naturally proportional to the incident electron intensity, but when the intensity exceeds a certain limit, the target will be destroyed due to the excessive heat load. Presently-attainable eciency, the number of eective positrons divided by the number of incident electrons, is not satisfactory as required to meet the requirement from the linear collider. It has been proposed to employ a single crystal instead of normal heavy metals as a target to improve the positron yield [1±4]. The electron can be channeled along the crystal axis or plane when it is injected along the crystal axis or plane within a critical angle (Lindhard angle) [5]. The classical view holds that the electron makes a wiggler motion attracted by an axial potential or a planar potential and emits the so-called channeling radiation. Channeling radiation is known to show a spectrum that is much stronger than the radiation by a Bethe±Heitler process in the low energy region [3]. Channeling thus contributes to enhance the production rate of low-momentum positrons, which directly favours the ®nal positron yield since the positron linac can accept only low-momentum positrons. The intensity of the channeling radiation, and hence the positron yield enhancement is expected to increase markedly with the energy of the incident electron [3], which is the reason why a crystal target is promising as a positron source for linear colliders. We conducted a proof-of principle experiment using a crystal target in 1996 [6,7], injecting a 1.2 GeV electron beam into a tungsten single crystal with a thickness of 1.2 mm (0.34 radiation length, (r.l.)). It was observed that when the h1 0 0i crystal axis was aligned with the electron beam, the positron yield was about three times the yield from the o axis target. There are two approaches to preparing the crystal target [3]: in the ®rst, the crystal works both
105
as a photon emitter and as a material to create electron±positron pairs, as in the experiment quoted above. In the second, the crystal acts only to emit photons and the pair creation takes place in an amorphous material located behind the crystal. Since the electron channeling is maintained only for a short pass length in the crystal, the crystal can be thin, while the material for the pair creation should be thick enough to give intense positrons. For the practical application of the crystal target to existing linac, the latter approach may be more convenient since the positron generator must be placed in a narrow space under a strong magnetic ®eld to focus generated positrons, where it is not easy to adjust a target orientation with a complicated goniometer system. Encouraged by the promising result for the proof-of-principle experiment on the tungsten crystal target, we have tried to apply the crystal target to a working electron/positron linac, the injector for the electron±positron collider B-Factory at High Energy Accelerator Research Organization (KEK). At the KEK linac, electrons are accelerated as a bunch with a width of about 10 ps, whose peak power is of the order of tera watts. The impact of such a short bunch, which has a transverse cross section of only 1 mm2 , on a crystal may initially cause a speci®c kind of crystal lattice excitation rather than simple heating. One of the purposes of the present experiment is to examine whether the crystal preserves its lattice structure under the impact of electron bunches with ultrahigh peak power. In Section 2, the experimental conditions and the simulation are outlined. The experimental results and some concluding remarks are described in Sections 3 and 4, respectively. 2. Experimental conditions and the simulation 2.1. Outline of the KEK electron/positron linac The KEK linac provides 8 GeV electron bunches and 3.5 GeV positron bunches to the electron±positron collider, B-Factory. It is also an injector for the 2.5 GeV synchrotron light source, Photon Factory. The positrons for the B-Factory
106
M. Inoue et al. / Nucl. Instr. and Meth. in Phys. Res. B 173 (2001) 104±111
are generated by hitting the tungsten target with 3.7 GeV electrons [8]. Electron bunches with a charge of 10 nC each are delivered at a repetition rate of 50 Hz, the average beam power being 1850 W. The normalized emittance of the electron beam is of the order of 10ÿ3 mrad, and the angular spread is estimated to be of the order of 0.1 mrad, assuming a beam size of 1 mm. The momentum acceptance of the positron linac with a matching section is 8:2 MeV=c < p < 11:6 MeV=c; pt < 2:4 MeV=c; where p is the magnitude of the incident positron momentum and pt is its transverse component. The radius of the incoming positron beam is assumed to be 1.2 mm. The optimum target thickness is determined to be 4 r.l. by the Monte Carlo calculation so as to maximize the yield of the positrons whose emittance is within the acceptance of the positron linac shown above [8]. The heat deposit to the target is estimated to be about 25% of the beam power, 460 W. The heat at the target is conducted to a coolingwater winding through a tungsten±copper alloy. This target assembly is, when the positron beam is required, inserted by a linear motion into the entrance of a pulsed solenoid coil, as shown in Fig. 1.
Fig. 1. The target for positron generation at the KEK electron/ positron linac for electron±positron collider.
The space for the target is a channel with a diameter varying from 22 to 76 mm, surrounded by a ceramic wall. When the present experiment was performed, the KEK linac was being upgraded for the BFactory and the design performance was not attained yet. Thus the operational conditions for the present experiment were limited to the following values: Incident Electron Energy Accelerated Positron Energy Charge per Bunch FWHM Bunch Width Repetition Rate
3 GeV 3 GeV 6 nC 10 ps 5 Hz
Thus the average beam power of the incident electrons was about 90 W, and the heat deposit to the target was estimated to be roughly 25 W if the target thickness is 4 r.l.. The peak beam power, on the other hand, was 1.8 tera W per bunch. 2.2. Target con®guration and the simulation For the reason described in Section 1, we have employed a target system which is composed of a thin crystal to emit photons and a thick amorphous tungsten for the pair creation as shown in Fig. 2. The amorphous tungsten target is placed at the position of the conventional target. A distance of 70 mm at least is needed between the crystal target and the end of the amorphous target in order to install a goniometer for the crystal orientation.
Fig. 2. The crystal target system consisting of a thin tungsten crystal and an amorphous tungsten.
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As a target crystal we have selected tungsten because high Z crystals generally produce intense channeling radiation, although the preparation of a perfect single crystal of a reasonable size is not easy. We have prepared a tungsten single crystal with a size of 6.2 ´ 10 mm2 and a thickness of 0.5 r.l. (1.7 mm). The surface is perpendicular to the h1 1 1i axis, along which the electron is to be channeled, and is polished to facilitate rough alignment using laser light. The mosaicity as examined by a Raue picture is about 0.5 mrad. The simulation for the above target arrangement has been made with the following consideration. One remarkable feature of the results of the proof-of-principle experiment on the tungsten crystal is that the width of the rocking curve for the positron yield as well as for the soft photon yield was unexpectedly large [6]. That is to say, the enhancement of the soft photon and positron yields occured in a much broader angular range than the Lindhard angle. This phenomenon including the magnitude of the positron yield enhancement can be explained by the assumption that coherent bremsstrahlung rather than channeling radiation contributes to the enhancement when the crystal includes some mosaicity [7]. The simulation for the present experimental conditions has been made with this assumption. Fig. 3 shows the result of the simulation for the positron yields accepted by the KEK positron
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linac and also the positron yield enhancement due to the eect of the crystal as a function of the thickness of the amorphous tungsten, assuming a 0.5 r.l. tungsten crystal is placed in front of it. Positrons generated in the crystal are neglected in this simulation because they are out of the focusing ®eld and scarcely accepted by the positron linac. As expected, the positron yield increases with increasing target thickness showing a yield maximum at around 4 r.l., while the enhancement factor decreases. This simulation, based on the coherent bremsstrahlung process, shows that the crystal target system gives higher positron yields than the conventional target in the amorphous tungsten thickness range below 4 r.l., but can only give the same maximum yield as the conventional target around 4 r.l.. This result is not discouraging because the merit of the crystal target will be demonstrated at higher energies of the incident electron. For the purpose of this study, it is more important to con®rm the eect of the crystal target than to obtain larger positron yields, since this is the ®rst trial of the application of the crystal target to the working linac. Consequently, the thickness of the amorphous tungsten has been chosen to be 2 r.l., with which the positron yield will be enhanced by about 50% and will reach 70% of the yield from the conventional target with a thickness of 4 r.l.. We have examined by the simulation whether the crystal with a thickness of 0.5 r.l. is suitable for use in combination with the 2 r.l. amorphous target. The positron yields have been calculated for various thicknesses of the crystal keeping the total thickness at 2.5 r.l.. The result of the calculation is shown in Fig. 4. It is seen that the positron yield with the oriented crystal is maximum when the crystal thickness is 0.4±0.5 r.l., justifying our preliminary choice. The crystal orientation dependence of the positron yield (rocking curve) expected from the present simulation will be shown in Section 3. 2.3. Fabrication of the crystal target system
Fig. 3. Simulation results for the positron yields from the crystal target shown in Fig. 2 as a function of the thickness of the amorphous tungsten.
The crystal part is mounted on a three-axis goniometer for the orientation of the crystal axis, while the non-crystal part is supported
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Fig. 4. Simulation results for the positron yields from the crystal target as a function of the thickness of the crystal part keeping the total thickness at 2.5 r.l..
independently from the goniometer as shown in Fig. 2. The vertical arm of the support is situated close to the vacuum-chamber wall separating a focusing solenoid coil in the air and the horizontal arm locates the amorphous tungsten at its end in the focusing ®eld. As described in Section 2.1, the beam power of the electron beam was about 90 W and the heat deposit to the 2 r.l. target was estimated to be less than 10% of this. Since this amount of heat is not
so large, we did not introduce a water cooling channel. Instead, we have made the supports for the crystal as well as for the amorphous target so as to have good heat conduction. The goniometer, designed originally for use in air, has been remodeled for use in a vacuum. The use of steel is avoided, except in electric motors, since it works under a strong magnetic ®eld to focus positrons. It has been con®rmed that the electric motors work normally under a strong magnetic ®eld. The goniometer can rotate the crystal around three axes which are perpendicular with each other. A phosphorous ceramic sheet with a thickness of 0.1 mm is placed in front of the crystal for monitoring the beam position and pro®le. The goniometer for the crystal target and the support of the amorphous target are assembled on a sliding table in a vacuum chamber as shown in Fig. 5. After removing the conventional target, the sliding table is pushed up by an air cylinder to the speci®ed position. In our proof-of-principle experiment using an external electron beam, the crystal orientation was determined by observing the rocking curve of the soft photon yield. At the positron station in the linac, however, observation of the channeling radiation is almost impossible because it is emitted
Fig. 5. The vacuum chamber containing the crystal target system.
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just in the forward direction. Then, we intended to determine the crystal orientation by observing the characteristic X-rays from the crystal target, which are emitted almost isotropically and are expected to be enhanced when the crystal axis is oriented [9,10]. For this purpose, we have provided the vacuum chamber with an X-ray window as shown in Fig. 5. A preliminary alignment of the crystal was made against the center line of the vacuum chamber using a conventional laser light before the installation of the vacuum chamber containing the crystal target system in the positron station of the linac. Owing to a mechanism by which the center line of the vacuum chamber can be made to exactly coincide with the beam line, the crystal axis was placed in the beam direction. 2.4. Generation of positrons The positron generation station of the KEK linac is schematically shown in Fig. 6. The electron bunches, after bunch compression, are focused on the target system. The position and pro®le of the electron bunch is monitored by a thin phosphorous ceramic sheet just in front of the target.
Fig. 6. A schematic drawing of the KEK positron generation station.
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Generated positrons are focused by a pulsed solenoid coil. The front section of the positron linac is surrounded by DC solenoid coil. After the contaminating electrons have been rejected with chicane magnets, the positrons are accelerated by regular accelerator tubes. Intensities of incoming electrons and accelerated positrons are measured by an electric probe of button type, which is working also as a beam position monitor. 3. Experimental results Before starting the experiment, the KEK linac was carefully tuned to accelerate positrons using the conventional target. Removing the conventional target, the crystal target was positioned on the beam line. By a rough control of the goniometer, the enhancement of the positron yield was easily found. This means it is eective to make the crystal alignment by re¯ecting laser light from a crystal surface cut perpendicular to the crystal axis. The positron yield was maximized by rotating the crystal around three axes which are perpendicular with each other. At ®rst one axis of the goniometer which is perpendicular to the electron beam was ®xed to give maximum positron yield, then another axis which is also perpendicular to the beam was adjusted to maximize the yield further. The positron yield thus maximized was constant against the rotation of the third axis which is parallel to the electron beam. Using the above process, the h1 1 1i crystal axis was aligned along the electron beam. The positron yield variation when one goniometer axis which is perpendicular to the incident electron direction was rotated is shown in Fig. 7. The positron yield at the peak is about 40% larger than the yields at the base. It can be said that the positron yield is enhanced by 40% due to the eect of the crystal, which deviates little from the result of the simulation, 49%, as shown in Fig. 3. The FWHM width of the rocking curve is 6.9 mrad, which is very large as in the proof-of-principle experiment. p The Lindhard angle wc is given as wc 2U =E, where U is a string potential of
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Fig. 7. The positron yield from the crystal target irradiated by a 3 GeV electron beam as a function of the crystal orientation. The curve is a result of the simulation assuming a bremsstrahlung process in the crystal.
the crystal axis and E is the incident electron energy. For the present experimental conditions, the wc is 0.7 mrad [11], which is smaller than the experimental value by one order of magnitude. Even with the mosaicity of the crystal used, 0.5 mrad, and the angular divergence of the incident electron beam, 0.1 mrad, the experimental angular width cannot be explained. Thus we have checked to see if there is any ®ne structure in the rocking curve by ®ne scan of the goniometer angle. The result shown in Fig. 8 indicates no ®ne structure.
The rocking curve assuming the contribution of the coherent bremsstrahlung instead of the channeling radiation and also assuming a crystal mosaicity of 0.5 mrad is shown in Fig. 7. The curve is normalized to the experimental values at the o axis states. The FWHM width of the curve is 6.7 mrad, which agrees well with the experimental value. We will make a short comment below on the heating of the crystal target system. After starting the experiment with the crystal target, the vacuum pressure increased at the positron generation station. Thus we limited the experimental time to 3 h. This was regarded as due to the heating of the target and/or of the electric motors to drive the goniometer. Since the present experimental setup is only for the testing of the crystal target, and not for practical use, we did not give it any serious consideration. The experiment was carried out twice in an interval of two months. In each experiment, and in every measurement, the enhancement of the positron yield by the crystal eect was almost constant, and we did not observe any degradation of the crystal due to the heating of the crystal by the impact of the intense ultra-short bunch of electrons. A possible structure for the next-step crystal target system is suggested in the following section. 4. Concluding remarks
Fig. 8. Crystal orientation dependence of the positron yield as observed by ®ne control of the crystal orientation.
We have performed the ®rst trial of employing a crystal target for the generation of positrons in a working electron/positron linac as an injector for the electron±positron collider. Electrons with an energy of 3 GeV were injected into the target which consisted of a 0.5 r.l. tungsten crystal and 2.0 r.l. amorphous tungsten. When the h1 1 1i crystal axis was aligned along the electron beam, the positron yield was enhanced by 40% compared to the value for the target with disoriented crystal. The crystal was not degraded by the impact of high-power electron bunches. The crystal surface had been ®nished to be perpendicular to the h1 1 1i axis, and the preliminary orientation of the crystal by laser-light re-
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with the amorphous tungsten, and this combined target will be oriented by a specially designed goniometer out of the vacuum chamber. By this target con®guration, positrons created in the crystal part can also be accepted by the positron linac. We have a plan to investigate the positron yield enhancement varying the incident electron energy and also the charge in one electron-bunch.
Acknowledgements
Fig. 9. Simulation results for the positron yields from the crystal target as a function of the incident electron energy, obtained by assuming coherent bremsstrahlung in the crystal.
¯ection enabled us to ®nd the crystal axis easily with the electron beam. The crystal orientation dependence of the positron yield (rocking curve), which has a width much broader than the Lindhard angle, and the yield enhancement due to the crystal eect can be reproduced fairly well by simulation assuming the coherent bremsstrahlung in a crystal with mosaicity. For incident electrons with higher energies, the eect of the channeling will show up in the positron yield. Keeping it in mind, we have tentatively made a simulation of the positron yield by the present model for the incident electron with higher energies. The result is given in Fig. 9, showing that the enhancement of the positron yield is larger than in the present experiment. In the next-step model of the crystal target, the crystal will be located downstream to be in contact
We are thankful to Mr. T. Fujita for preparing the ®gures to this text. This work is supported by the collaboration program of the High Energy Accelerator Research Organization, KEK.
References [1] R. Chehab, F. Couchot, A.R. Nyaieh, F. Richard, X. Artru, Orsay Report LAL-RT 89-01, 1989. [2] F.J. Decker, SLAC-PUB-5482, 1991. [3] X. Artru, V.N. Baier, R. Chehab, A. Jejcic, Nucl. Instr. and Meth. A 344 (1994) 443. [4] X. Artru et al., Nucl. Instr. and Meth. B 119 (1996) 246. [5] J. Lindhard, K. Dan, Vid. Selsk. Mat. Fys. Medd. 14 (1965) 34. [6] K. Yoshida et al., Phys. Rev. Lett. 80 (1998) 1440. [7] B.N. Kalinin et al., Nucl. Instr. and Meth. B 145 (1998) 209. [8] I. Sato et al., KEK Report 95-18, 1996, p. 46. [9] A.N. Aleinik et al., PisÕma Zh. Tek. Fiz. T. 13 (N22) (1987) 1367. [10] M.Tu. Andreyashkin et al., HSRC Report 99-1, Hiroshima University, 1999. [11] V.K. Bazylev, N.K. Zhevago, Radiation of Fast Particles in Matter and External Fields, Moscow, Nauka, 1987.