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Proceedings of PAC07, Albuquerque, New Mexico, USA

ENHANCEMENT OF THE POSITRON INTENSITY BY A TUNGSTENSINGLE-CRYSTAL TARGET AT THE KEKB INJECTOR LINAC
T. Suwada*, M. Satoh, T. Sugimura, T. Kamitani, K. Furukawa, K. Umemori, and H. Okuno,KEK, Tsukuba, Japan
T. Haruna, R. Hamatsu, T. Sumiyoshi, Tokyo Metropolitan University, Hachioji, Japan
K.Yoshida, SAGA Light Source, Tosu, Japan
A.P. Potylitsin, I.S. Tropin, Tomsk Polytechnic University, Tomsk, Russia
R. Chehab, IPNL, IN2P3-CNRS, Universite Claude Bernald 1, France
Abstract
A new tungsten single-crystal target has beensuccessfully employed at the positron source of theKEKB injector linac. The crystal thickness wasdetermined to be 10.5 mm based on the previous
systematic measurements of the positron-production
efficiency. The crystal axis, <111>, was precisely aligned
to the direction of the 4-GeV primary electron beam. The
positron yield increased by ~25% compared to that for aconventional tungsten plate with a thickness of 14 mm.
On the contrary, the heat load on the crystal target
decreased by ~20%. No degradation of the positronproduction efficiency was observed since the crystal
target was installed in September 2006, and it has beenstably operating at the KEK B-factory.

INTRODUCTION
In order to achieve high luminosities in high-energy
e+e- collider experiments, positron sources must bereinforced, especially towards next generation of Bfactories and e+e- linear colliders. In a conventional
positron source, positrons are produced by high-energy
electrons hitting a heavy-metal target, and lowmomentum positrons are captured and accelerated in thesucceeding positron capture section. The maximum
positron-production efficiencies are obtained with an
optimized target thickness of 4-5X0 and with a typicalmomentum acceptance of 5-25 MeV/c for 4-8 GeV
incident electrons. In this case, a possible scheme toincrease the positron intensity is to increase the incident
electron intensity and energy. However, the allowable
heat load on the target limits the beam power of theincident electrons.
One promising approach to improve the positronproduction efficiency is to use a crystal-assisted positronsource first proposed by Chehab, et al. [1] in 1989. Whenhigh-energy electrons hit a single crystal in the directionof the crystal axis, intense low-energy photons areproduced due to channeling radiation and coherent
bremsstrahlung. These photons create e+e- pairs in thesame crystal target. Thus, it is expected that the crystal
target could increase the obtainable maximum positrons
compared with a conventional heavy-metal target.
Intensive theoretical investigations [2,3] have indicatedthat the optimum thickness is less for the crystal target___________________________________________*E-mail address: tsuyoshi.suwada@kek.jp.

and that the energy deposition in the crystal target isconsiderably reduced compared with that of aconventional target. Also, they have shown that thecrystal effect is much larger at higher incident electronenergies.
Encouraged by these stimulating results, we proceeded
to systematic studies on the positron-production
efficiencies with tungsten crystals having various
thicknesses using 4- and 8-GeV electron beams [4,5].
Based on these results, we optimized the thickness of thetungsten crystal at a 4-GeV primary electron beam. Afterdeveloping a crystal-axis alignment technique, weinstalled a tungsten crystal target at the positron source ofthe KEKB injector linac.

THE KEKB POSITRON SOURCEThe KEKB positron source comprises a positronproduction target and a positron capture section. The
target is composed of a conventional tungsten plate, andthe capture section is employed for a so-called quarterwave transformer. The design and its performance aredescribed elsewhere [6]. The positrons are generated bythe 4-GeV primary electron beam impinging on thetungsten target. The average beam power is 2 kW at amaximum repetition rate of 50 Hz. The typical transverse
beam size is 0.7 mm (rms) in radius, and the typicalhorizontal and vertical normalized emittances are 660mm·mrad and 360 mm·mrad (rms) on average at thetarget. The horizontal (vertical) angular spread at thetarget is estimated to be 0.2 (0.1) mrad (rms). Theseangular spreads need to be controlled within the criticalangle for axial channeling, which is 0.61 mrad at 4 GeVin a tungsten crystal.
The previously used 14-mm-thick (4X0) conventional
tungsten plate was replaced by a 10.5-mm-thick tungsten
crystal in September 2006 without any significant
modification of the accelerator layout.

TUNGSTEN CRYSTAL TARGET
Optimization of the Target Thickness
For practical applications of the crystal target, we
systematicallystudied
thepositron-production
efficiencies as a function of the crystal thickness at anincident electron energy of 4 GeV at the KEKB injector
linac. The positrons produced in the forward direction
were measured by a magnetic spectrometer with an

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Proceedings of PAC07, Albuquerque, New Mexico, USA

angular acceptance of about 1 msr and a typicalmomentum acceptance of 2.4% (at the momentum of 20
MeV/c) in the momentum range of 5-20 MeV/c. Tungsten
crystal targets with various thicknesses were tested andthe optimum thickness was investigated with an incident
electron energy of 4 GeV. Figure 1 shows the results ofthe positron-production efficiencies measured at a
momentum of 20 MeV/c.

fabricated so that the central axis of the cylindrical copper
body corresponded exactly to the crystal axis, <111>. Inorder to precisely align these two axes, the relative
inclination angles between them were obtained by setting
the copper body on a positioning jig and by measuring thecrystal axis with an X-ray Bragg reflection method. Basedon the results, the copper body was mechanicallymachined so as to correct for any misalignment.

0.04

(a)
Tungsten Crystal
Standard Tungsten Plate

0.035

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e-0.030.0250.02

e+0.015e-0.01
0.005
005

101520(b)Target Thickness [mm]Figure 1: Positron-production efficiencies measured forthe tungsten crystal as a function of the crystal thickness(see [5]). The solid curves through the data are gammafunction fits of the data.
The optimum thickness giving the maximum positronproduction efficiency was obtained to be about 10 mm.
As a comparison, it was about 14 mm for theconventional tungsten plate. From these data, it wasconcluded that the maximum positron yield for the 10mm-thick tungsten crystal target was about 26% larger
than that for the conventional tungsten plate at amomentum of 20 MeV/c. The peak width of the rocking
curve was also obtained to be 40 mrad in FWHM (see [5]).
The relatively large peak width helps us to relax thetolerance of the target axis alignment with respect to thedirection of the primary electron beam.

Target Fabrication
Figure 2 (a) shows a mechanical drawing of the target
assembly with a tungsten crystal target. The crystal target
with a thickness of 10.5 mm and a cross section of 5 mm
× 5 mm is fixed at the center of a cylindrical copper bodyfor water cooling with a hot isostatic pressing technique.
Figure 2 (b) also shows the target assembly installed in a
vacuum chamber. The heat at the target is conducted
through a cooling water channel wound around a copper
body of 50-mm diameter. Two thermocouples aremounted near the tungsten crystal in order to measure thetemperature rise. The target assembly linked with amechanical linear actuator is mounted in a vacuum
chamber. It is inserted into the beam line by the linear
actuator for an injection request of the positron beam.
Since there was no room to install a goniometer forcontrolling the crystal axis with respect to the incident
electron beam, the target assembly was carefully

Figure 2: (a) Mechanical drawing of the target assembly.
Primary electrons (blue arrow) impinge the tungsten
crystal target and are converted to electrons (blue arrow)and positrons (red arrow). (b) Target assembly installed ina vacuum chamber seen from downstream.

PERFORMANCE OF THE CRYSTALTARGET
Positron-production Efficiency
Beam tests were carried out by adjusting the incident
angles of the primary electron beam at the crystal target
with two sets (horizontal and vertical) of upstream
steering magnets. The charges of the primary electron
beam was 7.5 nC/bunch on average during the beam tests.
After optimization of the two sets of steering magnets,
both the positron and primary electron charges weresimultaneously measured with the upstream anddownstream beam-position monitors. Figure 3 shows thedistributions of the positron-production efficiencies(Ne+/Ne-) of the 1st bunch measured for each beam pulse,
where Ne+ is the number of positrons captured in thepositron capture section and Ne- is the number of theprimary electrons.

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Proceedings of PAC07, Albuquerque, New Mexico, USA

1200

Tungsten Crystal Target
Previously-used Tungsten Target

GeV. The present result is consistent with their theoreticalestimation.
2.5

1000

Tungsten Crystal Target
Previously-used Tungsten Plate

800

2600

1.54001200
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0.50.12 0.14 0.16 0.180.20.22 0.24 0.26

Positron-Production Efficiency

Figure 3: Positron-production efficiencies of the 1stbunch measured for each beam pulse. For the sake ofcomparison, the data of the previous tungsten plate areplotted. The solid lines are Gaussian-function fits of thedata.
For the sake of comparison, the data with thepreviously used conventional tungsten plate obtained inJune 2006 are also shown in this figure. The results showthat the positron-production efficiencies of the first(second) bunch are 0.25±0.01 (0.26±0.01) and 0.2±0.01(0.2±0.01) on average for the tungsten crystal target andthe previously used tungsten plate, respectively. The
increase in the positrons for the 1st (2nd) bunch from thecrystal tungsten is 25±2% (28±2%) on average, where theerrors indicate one standard-deviation uncertainties. The
positron-production efficiencies of both bunches areconsistent with each other within the experimental errors.
These results are quantitatively in agreement with ourprevious results obtained by experimental studies
described before.

02030

405060Figure 4: Variations of the temperature rise of thetungsten crystal target as a function of the beam repetition
rate in one-bunch operation.

SUMMARY
We have successfully applied a new tungsten crystal
target for generating an intense positron beam at thepositron source of the KEKB injector linac. The positron
intensity increased by ~25% compared with that from thepreviously used conventional tungsten plate, and on thecontrary, the heat load on the crystal target decreased by~20%. It has boosted the positron intensity to its
maximum since the beginning of the KEKB operation in1999. This is the first application of a crystal target tohigh-energy electron/positron linacs. Useful information
concerning radiation damage and stability of the crystal
target will be obtained through the long-term KEKB
operation.

ACKNOWLEDGEMENT
This work was supported by the collaborative R&D
program at KEK, by the Grant-in-Aid of Ministry ofEducation, Culture, Sports, Science and Technology(MEXT) of Japan (Grant No.17540377), and by the grantof Ministry of Education and Science of the RussianFederation ”Advancement of the Scientific Potential ofHigh Education” (Grant No.2.1.1.889).

REFERENCES[1] R. Chehab, et al., PAC’89, Chicago, IL, U.S.A., 1989,p. 283.
[2] V.N. Baier and V.M. Strakhovenko, Phys. Rev. STAccel. Beams 5, 121001 (2002).
[3] X. Artru, et al., Phys. Rev. ST Accel. Beams 6,091003 (2003).
[4] T. Suwada, et al., Phys. Rev. E 67 (2003) 016502.
[5] T. Suwada, et al., LINAC'06, Knoxville Convention
Center, TN, U.S.A., 2006, p. 797.
[6] A. Enomoto, et al., EPAC’92, vol.1, Berlin, Germany,
1992, p. 524.

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Beam Repetition Rate [Hz]Heat Load on the Crystal Tungsten
The temperature rise of the crystal target was alsomeasured with two thermocouples mounted inside thecopper body. The temperature rise is defined by theaverage temperature of the two thermocouples. The
temperature rise was ΔT≈13.2 oC at a beam repetition rateof 50 Hz in one-bunch operation with a bunch charge of
7.8 nC, on average. Figure 4 shows the variations in thetemperature rise of the crystal target normalized by thecharges of the primary electron beam as a function of thebeam repetition rate along with the data of the previously
used tungsten plate.
These results confirm that the heat load on the crystal
target clearly decreased by about 20% compared with thatof the previously used tungsten plate. The energy deposits
on the crystal and conventional targets has been
investigated theoretically by Artru, et al. [3] at an incident
electron energy of 2-10 GeV. Based on their results, thetotal energy deposit on a 10-mm-thick crystal is estimatedto be ~20% lower than that of a 14-mm-thickconventional tungsten at an incident electron energy of 40T19 Collimation and Targetry

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