POSITRON-PRODUCTION EXPERIMENT IN TUNGSTEN CRYSTALUSING 4 AND 8-GEV CHANNELING ELECTRONS AT THE KEKBINJECTOR LINAC
T. Suwada∗ , S. Anami, R. Chehab† , A. Enomoto, K. Furukawa, K. Kakihara, T. Kamitani, Y. Ogawa,S. Ohsawa, H. Okuno, T. Oogoe, KEK, Tsukuba, JapanT. Fujita, K. Umemori and K. Yoshida, Hiroshima University, Higashi-Hiroshima, Japan
R. Hamatsu, K. Sasahara, Tokyo Metropolitan University, Hachioji, Tokyo, Japan
V. Ababiy, A.P. Potylitsin, I.E. Vnukov, Tomsk Polytechnic University, Tomsk, Russia
Abstract
A series of positron-production experiments with crystal
tungstens hit by 4 and 8-GeV single-bunch electron beams
were carried out at the KEKB 8-GeV injector linac. Threetungsten crystals (<111> axis) with different thicknesses(2.2, 5.3 and 9.0 mm) and those combined with amorphoustungsten plates were tested on a precise goniometer. The
positron-production yields were measured with a magnetic
spectrometer in the positron momentum (P e+ ) range from
5 to 20 MeV/c. The results show that the enhancements ofthe positron yield from crystal targets compared to amorphous targets of the same thickness at P e+ =20 MeV/c arefrom 1.5 to 3.7 and from 1.8 to 5.1, depending upon thetarget thickness for 4 and 8-GeV electrons, respectively.

1

INTRODUCTION

For future e + e− linear colliders and high-luminosity
B-factories, it is critically important to develop a highintensity positron source. In a conventional method using an amorphous heavy-metal target, the target thickness is optimized by taking into account the electromagnetic shower process and the positron capture efficiency inthe succeeding acceleration section. The optimum thickness is 4-5 X0 (radiation length) for a 4-8 GeV electron
beam. In this case, the only possibility to increase thepositron intensity is to increase the incident electron intensity. However, the electron intensity is limited because ofa heat load on the target. One promising method utilizing a crystal target was proposed by Chehab et al. [1] in1989. The benefit of this method is on its high positronproduction efficiency due to channeling radiation (CR) andcoherent bremsstrahlung (CB), since CR and CB increase
low-energy photons in the radiation process. This results ina thinner target compared with the conventional method. It
is also expected that the thin target relaxes the problem of
the heat load, and that the spatial spread of positrons due tomultiple scattering in the target is reduced. Yoshida et al.
demonstrated a clear enhancement of the positron yield in atungsten crystal target using a 1.2-GeV electron beam [2].
This new scheme was tested at the positron station of the∗ Corresponding author. E-mail address:tsuyoshi.suwada@kek.jp.

† On leave of absence from LAL, IN2P3-CNRS, Université de ParisSud, Bat.200-B.P.34, 91898 Orsay, France.

KEKB injector linac. The result indicates that, when a hybrid target made of 1.7-mm-thick tungsten crystal and 7.0mm-thick amorphous plate is used and the <111> crystal axis is oriented along the 3-GeV electron beam direction, the positron yield is enhanced by 40% compared withthat for the disoriented case [3]. Chehab et al. also studied the positron yield from the crystal target for 5-40 GeVelectrons at CERN-SPS [4]. Although a positron enhancement is observed, there have so far been only a few experimental results over a wide energy range of a primaryelectron beam. Thus, more systematic and precise experimental data could help us to understand the complicatedmechanism of these elementary radiation processes and todesign a high-intensity positron source. A series of experiments [5] to investigate the positron yields using various
crystal targets are underway for incident electron energies
lower than 8 GeV.

2 EXPERIMENTAL SETUP
2.1

Electron beam

An electron beam with a pulse width of 10 ps impinged
on a tungsten crystal target at a repetition rate of 25 and 2
Hz with the energy of 4 and 8 GeV, respectively. The beam
charge (0.2 nC/bunch) was measured by a wall-current
monitor for each pulse. The transverse profile of the electron beam at the target was monitored by a 100-µm-thickfluorescent screen monitor during the experiment. The
transverse beam size was 1 to 1.5 mm (FWHM) in diameter. The angular spreads of the electron beam were measured with a wire scanner to be about 123 (23) and 121 (41)µrad in the horizontal and vertical directions to the beamaxis for the 4 (8)-GeV electrons, respectively. While theelectron beam impinged on the target after passing througha vacuum window made of 100-µm-thick stainless steel,the angular spread of the electron beam at the target wasestimated to be 0.2 (0.1) mrad in total at 4 (8) GeV by taking into account multiple scattering. These angular spreads
were less than the critical angles (0.61 and 0.43 mrad) of
the channeling condition at 4 and 8 GeV, respectively.

2.2

Tungsten mono-crystalline target

Three tungsten mono-crystalline targets (W c ) with different thicknesses (2.2, 5.3 and 9.0 mm) were tested eitherdigitized and recorded. The goniometer could rotate thecrystal target around two axes (the horizontal (H) and vertical (V ) axes) by pulse motors. The angular resolutionsof the goniometer were 10.5 and 34.9 µrad/pulse in the H
and V axes, respectively. The <111> crystal axis with respect to the electron beam was determined by changing therelative rotational angles around the two axes with steps of
2 mrad.

3 EXPERIMENTAL RESULTS
3.1

Positron spectrometer

Figure 1 shows a schematic drawing of the experimental setup. This comprises a positron-production target
mounted on a precise goniometer and a positron spectrometer. The positrons emitted from the target in the forward
Goniometer

Positron Detectors
Lead-Glass Cherenkov Calorimeter
Lucite Cherenkov Detector{V
e- Beam

TargetLead Shield

+

H

+

X
-

+

A

e+

+

Targete- Beam

Wall-Current Monitore+e-Screen Monitor

Vacuum Chamber
Analyzer Magnet

Figure 1: Schematic drawing of the experimental setup.
direction were momentum-analyzed by the magnetic field
in a momentum range lower than 20 MeV/c, where the deflection angle was 60 ◦ from the beam axis. The positron
trajectories were determined by five collimators installed
before and behind the momentum-analyzer magnet. They
were kept in a vacuum at a pressure level of 10 −1 Pa.
The momentum-analyzed positrons were detected with a 5mm-thick transmission-type lucite Cherenkov detector anda 20-mm-thick total-absorption-type lead-glass calorimeter with photomultiplier tubes. The lead blocks surrounding the detectors suppressed any background caused byelectromagnetic showers generated upstream of the beamline by off-momentum electrons, and caused by electromagnetic showers generated at the collimators. The acceptance of the positron spectrometer was obtained by using
the detector simulation code GEANT3 [6] at each positron
momentum. The typical geometrical and momentum acceptances are about 1 msr and 2.4% (∆P/P , FWHM) at
Pe+ =20 MeV/c.

2.4

Rocking curves

A rocking curve was measured as a function of the goniometer rotational angle around the H axis while the angle around the V axis was fixed to the angular position
giving the peak yield. Twenty beam-pulse measurements
were performed at the same H angle. The data corrections were made for pedestals in positron charge measurements, for backgrounds, and for the electron-beam intensity. The beam-associated background data were also measured under the condition with the magnetic field off andwith the target away. The background data were carefully
subtracted from the raw data. The results of rocking-curvemeasurements for the W c ’s at Pe+ =20 MeV/c are shownin figs.2. Figures 2 (a) and (b) show the results for the 2.2mm-thick Wc at Ee− =4 and 8 GeV, and for the 5.3 and 9.0mm-thick Wc ’s at Ee− =4 GeV, respectively. These curves
clearly indicate that the width of the rocking-curve peak ismuch larger than the critical angle of the channeling electrons, and broadens with the thickness of the crystal target.
These broad widths of the rocking curves indicate that coherent bremsstrahlung is the predominant process over thechanneling radiation process in this energy region.
(a) 2.2-mm-thick Wc(b) Ee-=4 GeV

35

Ee-=4 GeV & Pe+=20 MeV/c
Ee-=8 GeV & Pe+=20 MeV/c

Relative Positron Yield[arbitrary unit]

2.3

5.3-mm-thick Wc & Pe+=20 MeV/c
9.0-mm-thick Wc & Pe+=20 MeV/c

2.5

Relative Positron Yield[arbitrary unit]

alone or in combination with an amorphous tungsten plate(Wa ). The surface mosaicities of these crystals on bothsides were measured by an X-ray scattering method to be
1.5, 0.5 and 0.5 mrad for 2.2, 5.3 and 9.0-mm-thick crystals, respectively. Wa ’s with different thicknesses from 3 to
18 mm with 3 mm steps were also installed on a horizontal movable stage to the beam axis at 82.5 mm behind the
crystal target. They were used for combined targets. Several other Wa ’s with thicknesses of up to 28 mm were also
mounted on a crystal target holder, which made it possible
to calibrate the positron yield.

2

1.5

1

0.5

0
-80

-60

-40

-20

0

20

40

Rotational Angle [mrad]

60

80

4

3

2

1

0
-80

-60

-40

-20

0

20

40

60

80

Rotational Angle [mrad]

Figure 2: Rocking curves measured at P e+ =20 MeV/c (a)for the 2.2-mm-thick W c at Ee− =4 and 8 GeV, and (b) for
the 5.3 and 9.0-mm-thick W c ’s at Ee− =4 GeV as a functionof the goniometer rotational angle around the H axis.

Data-acquisition system

Since the emitted positrons were shortly bunched, thenumber of positrons per bunch was measured as a pulse
charge from each detector. Signals from the positron detectors and the signal of the wall-current monitor were sent
to a data-acquisition system using a PC/Linux-based CAMAC/ADC, where all signal charges were simultaneously

3.2

Enhancement of the positron yieldAn enhancement of the relative positron yield was obtained from the rocking curves. Here, the enhancement
is defined by the ratio of the peak yield (on-axis) to theyield at the base region (off-axis) 50 mrad apart from thecrystal axis in the H scan. The observed enhancements

positrons to the number of incident electrons. The experimental data points normalized to the calculations at a W athickness of 9 mm are shown in figs. 4 (a) and (b) with
Pe+ =20 MeV/c at Ee− =4 and 8 GeV, respectively. Also
shown in solid curves are the results of calculations, which
reproduce quite well the experimental data points over thewide range of the W a target thicknesses. The maximum
positron yields for the 9.0-mm-thick crystal target were obtained with an enhancement of 20% with respect to theWa targets with the optimum thickness (14 and 18 mm)
at Pe+ =20 MeV/c for E e− =4 and 8 GeV.
0.06(a) Ee-=4 GeV, Pe-=20 MeV/c(b) Ee-=8 GeV, Pe+=20 MeV/c

6

4

3

2

1

0

2.2mmWc
2.2mmWc+Wa
5.3mmWc
9.0mmWc
9.0mmWc+Wa

5

Enhancement

5

Enhancement62.2mmWc
2.2mmWc+Wa
5.3mmWc
5.3mmWc+Wa
9.0mmWc
9.0mmWc+Wa

4

3

2

1

0

5

10

15

20

25

Target Thickness [mm]

30

0

0

5

10

15

20

25

30

Target Thickness [mm]Figure 3: Variations in the enhancement of the positronyield at Pe+ =20MeV/c (a) for E e− =4 GeV and (b) for
Ee− =8 GeV as a function of the target thickness. The solid
curves drawn through the data are only for the eye’s guide.

0.06

Wc's exp.
Wa's exp.
Simulation

Positron Production
Efficiency [%]0.05

Table 1: Momentum dependence of the positron-yield enhancement for the crystal targets
Pe+
Ee− =4 GeV(MeV/c) 2.2mm W c
5.3mm Wc
9.0mm Wc5
3.3 ± 0.1
2.2 ± 0.1
1.5 ± 0.210
3.6 ± 0.3
2.3 ± 0.1
1.5 ± 0.215
3.5 ± 0.1
2.2 ± 0.1
1.7 ± 0.320
3.7 ± 0.1
2.2 ± 0.1
1.5 ± 0.1Pe+
Ee− =8 GeV(MeV/c) 2.2mm W c
5.3mm Wc
9.0mm Wc
55.0 ± 1.5
2.9 ± 0.5
2.1 ± 0.3
10
6.5 ± 0.6
3.4 ± 0.7
2.3 ± 0.415
6.2 ± 0.8
3.2 ± 0.5
2.0 ± 0.220
5.1 ± 0.5
3.0 ± 0.5
1.8 ± 0.2(b) Ee-=8 GeV, Pe-=20 MeV/c(a) Ee-=4 GeV, Pe-=20 MeV/c

0.05

Positron Production
Efficiency [%]at Pe+ =20 MeV/c for both of the W c ’s and the combined
targets as a function of the target thickness are shown in
figs.3. Here, the total thickness of the target was used forthe combined targets. The result shows that the enhancement factor is much reduced with an increase of the total
target thickness, and no crystal effect enhances the positronyield at a target thickness larger than 14 mm in total, whichis almost the same as the optimum thickness of a W a target at Ee− =4 GeV. The momentum dependence of the enhancement for the crystal targets was also measured in the
momentum range of 5-20 MeV/c. The result shows thatthe momentum dependence of the enhancement is not verystrong in the present measurement.

0.04

0.04

0.03

0.03

0.02

0.02

0.01

0

Wc's exp.
Wa's exp.
Simulation

0.01

0

5

10

15

Target Thickness [mm]

20

0

0

5

10

15

20

Target Thickness [mm]Figure 4: Positron production efficiencies measured for the
crystal targets at Pe+ =20 MeV/c (a) for E e− =4 GeV and(b) for Ee− =8 GeV. The solid curves drawn through thedata are based on a simulation.

4 CONCLUSIONSThe positron production efficiencies from the crystal targets by 4 and 8-GeV electrons were measured as a function of the target thickness and as a function of the positron
momentum. The results show that when the <111> crystal axis was aligned along the incident electron beam direction, a large positron yield enhancement was observed.
The enhancement factor decreases as the target thickness
increases. The maximum positron yields for the crystal targets were enhanced by 15 and 18% on the average withrespect to the optimum yields obtained for the amorphoustarget with the momentum range of 5-20 MeV/c at E e− =4
and 8 GeV, respectively. This new scheme using the crystal
target indicates that the heat load in the crystal part of thetarget could be considerably reduced due to a small amountof the energy loss in total.

5

REFERENCES[1] R. Chehab, et al., PAC’89, Chicago, IL, Mar. 1989, p.283.

3.3

Positron production efficiency[2] K. Yoshida, et al., Phys. Rev. Lett. 80, 1437 (1998).
[3] M. Inoue, et al., NIM B173 (2000) p.104.

The absolute values of the measured positron yields
were calibrated by using the data for the W a targets. The
positron production efficiencies for the different W a target
thicknesses were calculated by using the GEANT3 code
in which all of the geometry of the experimental setup
was taken into account. Here, the positron production efficiency is defined as the ratio of the number of detected[4] R. Chehab, et al., RREPS’01, Lake Aya, Altai Mountains,
Russia, September 2001, to be published in NIM B.
[5] H. Okuno, et al., RREPS’01, Lake Aya, Altai Mountains,
Russia, September 2001, to be published in NIM B.
[6] CERN Program Library Long Writeup W5013, CERN,
Geneva, 1994.