POSITRON-PRODUCTION EXPERIMENT USING 8-GEV CHANNELING
ELECTRONS IN A CRYSTAL-TUNGSTEN TARGET
T.Suwada, S.Anami, A.Enomoto, K.Furukawa, K.Kakihara, T.Kamitani,Y.Ogawa, S.Ohsawa,and T.Oogoe, Accelerator Laboratory, KEK, Tsukuba, Japan
H.Okuno, Institute of Particle and Nuclear Studies, KEK, Tsukuba, Japan
T.Fujita, K.Umemori, and K.Yoshida, Hiroshima University, Higashi-Hiroshima, Japan
R.Hamatsu and K.Sasahara, Tokyo Metropolitan University, Hachioji, Japan
V.Ababiy, A.P.Potylitsyn, and I.E.Vnukov, Tomsk Polytechnic University, Tomsk, Russia
Abstract
An experiment to study the positron-production
efficiency by 8-GeV electrons hitting an axially oriented
tungsten monocrystalline target was carried out using asingle-bunch beam (10 ps width) from the KEKB injector
linac. In this experiment 2.2mm-thick and 9mm-thickcrystal tungsten targets were tested either alone or in
combination with an amorphous tungsten target (2.0 and4.0mm-thick). Positrons emitted from the target in theforward direction were momentum-analyzed by amagnetic spectrometer in a momentum range lower than
20 MeV/c and were detected with a 3mm-thick acrylic
Cherenkov counter and a lead-glass calorimeter. Since theproduced positrons were shortly bunched, the number of
positrons per bunch was measured as a pulse charge fromeach detector. The positron yields have been measured foreach target as a function of the rotational angle of agoniometer and as a function of the positron momentum.
For the 2.2mm-thick and 9mm-thick crystal targets, 5.1times and 1.7-times enhancements of the positron yield
were observed, respectively, when the crystal axis, <111>,
was oriented to the incident electron beam. This paper
describes the results of this series of experiments.

1 INTRODUCTION
For future e+e- linear colliders and high-luminosity Bfactories, it is critically important to develop a highintensity positron source. In a conventional method using
an amorphous heavy-metal target, the target thickness isoptimized by taking into account the electromagnetic
shower process and the positron capture efficiency in thesucceeding 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 the positron
intensity is to increase the incident electron intensity.
However, the electron intensity is limited due to a heatload on the target. One promising method utilizing acrystal target was proposed by Chehab et al.[1] in 1994.
The benefit of this method is its high positronproduction efficiency due to channeling radiation (CR)[2]and coherent bremsstrahlung (CB), since CR and CB______________________________________________*E-mail address: tsuyoshi.suwada@kek.jp (T. Suwada).

increase low-energy photons in the radiation process. This
results in a thinner target compared with the conventional
method. It is also expected that the thin target relaxes itsheat 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 ina crystal tungsten target using a 1.2-GeV electron
beam[3]. This new scheme was tested at the positron
station of the KEKB injector linac[4]. The result indicates
that, when a hybrid target made of 1.7mm-thick crystal
tungsten and 7.0mm-thick amorphous one is used and thecrystal axis, <111>, is oriented to the 3-GeV electron
beam, the positron yield is enhanced by 40% comparedwith that for the disoriented case[5]. Chehab et al. alsostudied the positron yield from the crystal target for 5-40
GeV electrons at CERN-SPS[6]. Although a positronenhancement is expected, there have so far been only a
few experimental studies over a wide energy range ofprimary electron beams. Theoretically unified treatments
by taking into account both processes of CR and CB have
not yet been established. More precise experimental data
will help us to understand the complicated mechanism ofthese elementary radiation processes and to design a highintensity positron source.
A series of experiments[7-9] to investigate the positronyields under various crystal target conditions areunderway for an incident electron energy of 8 GeV.

2 EXPERIMENTAL SETUP
2.1 Beam Line
Our experiment was performed at the energy analyzer
line in the beam switchyard of the KEKB 8-GeV injector
linac (see Fig.1). An electron beam with a pulse width of
10 ps and with an energy of 8 GeV impinged on atungsten target at a repetition rate of 2 Hz. The beam
intensity in this experiment was 0.2 nC/bunch, which waslimited mainly by the radiation safety condition. The
beam charge was measured by a wall-current monitor foreach pulse. The transverse profile of the electron beam at
the target was monitored by a screen monitor during theexperiment. The transverse beam size was about 1 mm(FWHM) in diameter. The angular spreads of the electron
beam were expected to be about 72 and 15 mrad in thehorizontal and vertical directions, respectively. However,since the electron beam impinged on the target after
passing through a vacuum window made of 100mm-thickstainless steel (SUS304), the angular spread of theelectron beam at the target was estimated to be 0.1 mradin total by taking into account the multiple scattering.
This angular spread was less than the critical angle (0.43mrad) of the channeling condition at 8 GeV.

system using a PC-based CAMAC/ADC system, where
all signal charges were simultaneously digitized. Thegoniometer could rotate the crystal target around two axes
(the horizontal(H) and vertical(V) axes) by a pulse motor.
The angular resolutions of the goniometer were 10.5 and34.9 mrad/pulse in the H and V axes, respectively. The
crystal axis, <111>, was determined by changing therelative rotational angles around the two axes with a stepof 2 mrad. The positron yields were measured for eachtarget as a function of the rotational angle of thegoniometer and as a function of the positron momentum.

Beam

Rotational Axes ofthe Goniometer

Lead-Glass Calorimeter
Acrylic Cherenkov Counter
Lead-Block Shields

V Axis
H AxisCollimator(40mmf)
Collimator(30mmf)
8-GeV e- Beam

Target/Goniometere+e-e+

60oe-Figure 1: Photograph of the beam line and theexperimental setup viewed from the electron beam.

2.2 Experimental Setup
Figure 2 shows a schematic drawing of the experimental
setup. This comprises a positron-production target
mounted on a precise goniometer, a magnetic
spectrometer, collimators, and two kinds of positrondetectors (a lead-glass calorimeter and an acrylic
Cherenkov counter). All of the collimators and detectors
are installed in a vacuum chamber kept at a vacuum
pressure of 10-3 Torr. In this experiment, 2.2mm-thick and
9mm-thick crystal tungstens (Wc) were tested either alone
or in combination with an amorphous tungsten target (Wa)(2.0 and 4.0mm-thick). The positrons emitted from thetarget in the forward direction were momentum-analyzed
in a momentum range lower than 20 MeV/c by themagnetic field, where the deflection angle was 60o fromthe beam axis. The positron trajectory was determined by
five collimators installed before and behind the magnetic
spectrometer. The geometrical acceptance and momentum
acceptance were about 1 msr and 3% at a positron
momentum of 20MeV/c, respectively. The momentumanalyzed positrons were detected with a 3mm-thickacrylic Cherenkov counter and a lead-glass calorimetershielded by lead blocks. The lead blocks suppressed any
background caused by electromagnetic showers generated
upstream of the beam line due to the off-momentum
electrons, and caused by electromagnetic showers
generated at the collimators. Since the emitted positrons
were also shortly bunched, the number of positrons perbunch was measured as a pulse charge from each detector.
Signals from the positron detectors and the signal of theelectron beam charge were sent to a data-acquisition

Vacuum Window
Screen Monitor
Wall-Current Monitor

Collimators(20mmf)Magnetic Spectrometer

Figure 2: Schematic drawing of the experimental setup.

3 EXPERIMENTAL RESULTS
3.1 Results for the Tungsten Crystals
The enhancement of the positron yield was measured
by changing the V and H angles of the goniometer for thetwo crystal tungsten targets. Figure 3 shows the results((a) 2.2mm-thick Wc and (b) 9mm-thick Wc) of therelative positron yield (rocking curve) measured by theacrylic Cherenkov counter at a momentum of 20 MeV/c.
Each data point shows the average value and its errors
obtained by twenty beam-pulse measurements at the samerotational angle. Corrections were made for pedestals in
the charge measurements, for backgrounds, and for theelectron-beam intensity. The beam-associated backgrounddata were also measured under the condition without amagnetic field. The data measured by the lead-glass
calorimeter and the acrylic Cherenkov counter wereconsistent with each other within the experimental
uncertainty.
The enhancement of the relative positron yield isdefined by the ratio of the peak yield (on-axis) to the yieldat the base region 50 mrad apart from the crystal axis in
the H scan. The observed enhancements and the angularwidths (FWHM) of the rocking curve were 5.1 ± 0.1 and
9.0 mrad, and 1.7 ± 0.1 and 39.2 mrad for the 2.2mmthick and 9mm-thick Wc, respectively. The momentum
dependence of the positron enhancement was alsomeasured in the same way in the momentum range of 1020 MeV/c. The results are summarized in Table 1.

Table 1: Measured enhancements of the positron yields
depending upon the positron momentum.

including CR and CB radiation processes is underdevelopment.

Momentum[MeV/c]10Enhancement(2.2mm Wc)
6.6 ± 0.1

Enhancement(9mm Wc)
1.9 ± 0.1

15

6.1 ± 0.1

1.7 ± 0.120

5.1 ± 0.1

1.7 ± 0.1

Positron-Production Efficiency (%)

0.08

4On Axis(a)3.5

2.2mm-thick Wc

Relative Positron Yield[arbitrary unit]+

Pe =20MeV/c3+

Pe =20MeV/c
0.07
0.06

9mm-thick Wc

9mm-thick Wc+2mm-thick Wa
9mm-thick Wc+4mm-thick Wa

0.05
0.04
0.03

2.2mm-thick Wc

0.02
0.01
0

2.50

FWHM=9.0 mrad

21
0.5
0-60-40-2002040

101520W Thickness [mm]

Off Axis1.5560

Rotational Angle [mrad]

Figure 4: Positron-production efficiency measured forcrystal and amorphous tungsten targets at the momentumof 20 MeV/c. The positron-production efficiency isdefined as the ratio of the number of the detected
positrons to an electron injection. A solid curve based on
the EGS4 simulation drawn through data points is alsoshown.

4(b)Relative Positron Yield[arbitrary unit]

3.59mm-thick Wc
On Axis4 CONCLUSIONS+Pe =20MeV/c

32.5

FWHM=39.2 mrad

Off Axis

21.5
10.50-60-40-200204060

Rotational Angle [mrad]

Figure 3: Relative positron yields (a) for the 2.2mm-thickWc and (b) for the 9mm-thick Wc as a function of thegoniometer rotational angle around the H axis while theangle around the V axis is fixed to the position giving theyield peak. The solid curves are only for the eye’s guide.

3.2 Calibration for the Positron Yield
The measured positron yields were calibrated by using
amorphous tungsten targets with different thicknesses of 3
mm to 18 mm. Figure 4 shows the positron-production
efficiency with a momentum of 20 MeV/c. The data forthe crystal targets are also shown for a comparison. The
experimental data were normalized by using the simulated
data point of 8mm-thick Wa. The thickness-dependence ofthe positron yield from Wa in the momentum range of 1020 MeV/c agrees with simulation results using the EGS4code[10] within the experimental uncertainty. A full
simulation with the GEANT3 code[11] taking into
account all geometry of the experimental setup showed
good agreement with that of the EGS4 simulation[8]. For
the crystal target, a more sophisticated simulation code

Positron-production experiments by 8-GeV electrons
hitting an axially-oriented tungsten monocrystalline target
were carried out using single-bunch beams at the KEKB
injector linac. The enhancements of the positron yields
were 5.1 ± 0.1 times and 1.7 ± 0.1 times for targets withthe 2.2mm-thick and 9mm-thick crystal tungsten at amomentum of 20 MeV/c, respectively, when the crystal
axis, <111>, was oriented to the incident electron beam.
It is interesting to note that the measured positron yieldfor 9mm-thick crystal is slightly larger than the maximum
yield attainable for the 15mm-thick amorphous target by
the 8-GeV electron beam.

5 REFERENCES[1] R.Chehab, Orsay Report No. LAL/RT94-09, 1994.
[2] A.H.Sorensen and E.Uggerhoj, Scientific American June(1989) 70.
[3] K.Yoshida, et al., Phys. Rev. Lett. 80, 1437 (1998).
[4] Design Report on PF Injector Linac Upgrade for KEKB,KEK Report 95-18, 1996.
[5] M.Inoue, et al., NIM B 173 (2000) p.104.
[6] R.Chehab, et al., LINAC2000, Monterey, CA, USA, August
2000, p.143.
[7] T.Suwada, et al., HEACC2001, Tsukuba, Ibaraki, Japan,March 2001.
[8] K.Sasahara, et al., Proc. the 21st ICFA Beam Dynamics
Workshop on Laser-Beam Interactions, Stony Brook, NY,U.S.A., June 2001.
[9] K.Furukawa, et al., Proc. the 26st Linear Accelerator Meetingin Japan, Tsukuba, August 2001, p.138.
[10] W. R. Nelson, H. Hirayama and D. W. O. Rogers, SLAC
Report No.265 (1985).
[11] CERN Program Library Long Writeup W5013, CERN,Geneva, 1994.