POSITRON-PRODUCTION EXPERIMENT USING SILICON AND
DIAMOND CRYSTALS IN THE KEKB 8-GEV INJECTOR LINAC
Tsuyoshi Suwada†, KEK, Tsukuba, Ibaraki, 305-0801 Japan.

Abstract
Intense positron sources are widely investigated forthe next-generation of linear colliders and B-factories. A
new method utilizing an axially-oriented crystal as apositron-production target is one of the bright schemes
since it provides a powerful photon source throughchanneling and coherent bremsstrahlung processes whenhigh-energy electrons penetrate the target. A series of
positron-production experiments with heavy and light
crystals hit by 4 and 8-GeV single-bunch electron beams
have been carried out at the KEKB 8-GeV injector linac.
This report gives the brief summary on the positronproduction experiment using silicon and diamond
crystals carried out in December 2002 at KEK. Later thereport based on the more precise analysis will be
published elsewhere.

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 is
optimized by taking into account the electromagnetic
shower process and the positron capture efficiency in the
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 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. in 1994[1].
The benefit of this method is on its high positronproduction efficiency due to channeling radiation(CR)[2] and coherent bremsstrahlung (CB), since CR
and CB increase low-energy photons in the radiation
process. This results in a thinner target compared withthe conventional method. It is also expected that the thintarget relaxes its heat load, and that the spatial spread of
positrons due to multiple scattering in the target is
reduced. Yoshida et al. demonstrated a clear
enhancement of the positron yield in a tungsten crystal
target using a 1.2-GeV electron beam[3]. This new
scheme has been tested at the positron-production stationand the end station of the KEKB injector linac[4].
Chehab et al. also studied the positron yield from acrystal target for 5-40 GeV electrons at CERN-SPS[5].
Although a positron enhancement is observed, there______________________________________________†

E-mail address: tsuyoshi.suwada@kek.jp.

have so far been only a few experimental results over awide energy range of a primary electron beam.
On the theoretical side, various simulation studies havebeen carried out by various authors. Among them, Baier,
Katkov and Strakhovenko have developed the simulation
code of the electromagnetic shower formation at axialalignment of a crystal by using the semiphenomenological radiation spectrum[6]. This scheme
allows one to consider a positron-yield enhancement at
the relatively low energy range (a few GeV) of the initial
electrons, which is suitable for the B-factory injector.
The substantial enhancement of the positron yield fromcrystal targets is expected due to the increase in thenumber of relatively soft photons in comparison withthat from an amorphous target. Thus, more systematicand precise experimental data could help us tounderstand the complicated mechanism of theseelementary radiation processes and to design a highintensity positron source. A series of experiments[7,8] toinvestigate the positron yields using various crystal
targets are underway for an incident electron energy
lower than 8 GeV. The following table shows a historicalview of the positron-production experiments carried out
at KEK-Tsukuba and KEK-Tanashi branch.
Table 1: Historical view of the positron-production
experiments at KEK.
Month/Year Accelerator
E
Positron Target
[GeV]
[mm]
May/1997
Tanashi/ES
1.2
Crystal W (Wc)[1.2]Apr, Jun
KEK/Linac3
Wc[1.7] + Amor. W/1998(Wa)[7]
Nov/1998
Tanashi/ES0.6,
Wc[0.4, 1.2, 2.2],
0.8, 1
GaAs[0.36],
Diamond[1.1]
Sep, Oct
KEK/Linac
8
Wc[2.2],/2000
Wc[2.2]+Wa[5,10,15]
Apr/2001
KEK/Linac
8
Wc[2.2], Wc[9]Wc[9]+Wa[2, 4]
Sep/2001
KEK/Linac
8
Wc[2.2], Wc[5.3],
Wc[9]Combined targets
Jan/2002
KEK/Linac4Wc[2.2], Wc[5.3],
Wc[9]Combined targets
Aug-Sep
KEK/Linac
8
Si [2.6, 30, 48]/2002
Diamond[4.57]
Combined targets
Dec/2002
KEK/Linac
8
Si [10, 30, 48]
Diamond[4.57]
Combined targets

2 EXPERIMENTAL SETUP

detectors are installed in a vacuum chamber kept at a
vacuum pressure of 10-1 Pa.

2.1 Beam Line
Our experiment was performed in the beam switchyardof the KEKB 8-GeV injector linac (see Fig.1).

Figure 2: Schematic drawing of the experimental setup.

Figure 1: Photograph of the beam line and theexperimental setup viewed from the electron beam.
An electron beam with a pulse width of 10 ps (S-band
single bunch) and with an energy of 8 GeV impinged ona tungsten target at a repetition rate of 25 Hz. The beam
charge (~0.1 nC/bunch) was measured by a wall-current
monitor for each pulse. The transverse profile of theelectron beam at the target was monitored by a screen
monitor during the experiment. The transverse beam size
was 0.8 mm (FWHM) in diameter. The angular spreadsof the electron beam were expected to be about 22 and44 mrad in the horizontal (H) and vertical (V) directions
for the 8-GeV electrons, respectively. However, since theelectron beam impinged on the target after passing
through a vacuum window made of 30mm-thick stainlesssteel (SUS304), the angular spreads (H and V) of theelectron beam at the target were estimated to be 59 and70 mrad in total by taking into account the multiple
scattering at 8 GeV, respectively. The multiple scattering
effect of the vacuum window was investigated by using
SUS foils with several thicknesses. These angularspreads were less than the critical angles (170 mrad for Si
crystal and 130 mrad for Diamond crystal) of thechanneling condition at 8 GeV.

2.2 Experimental Setup
Figure 2 shows a schematic drawing of theexperimental setup. This comprises a positronproduction target mounted on a precise goniometer, amagnetic spectrometer, collimators, and two kinds ofpositron detectors (a lead-glass calorimeter and anacrylic Cherenkov counter). All of the collimators andIn this experiment, silicon and diamond crystals of<110> axis with different thicknesses were tested either
alone or in combination with an amorphous tungsten
plate (Wa). These Was’ with different thicknesses from 3
to 18 mm by 3mm steps are installed on a horizontal
movable stage 82.5 mm after the crystal target. Thesetungsten plates were also used to calibrate the positronyield without the crystal targets. The specification of thecrystal targets is summarized in Table 2.
Table 2: Tested crystal specification.
Elem. Denom. Thickness
X0[mm]
DiamondC
5mmDia4.57
0.0372
SiliconSi
2.5mmSi
2.55
0.0272
SiliconSi
10mmSi9.9
0.1058
SiliconSi
30mmSi
29.9
0.319
SiliconSi
50mmSi
48.15
0.514*1X0=123mm (Diamond), 1X0=93.6mm (Silicon). It
should be noted that the radiation length of anamorphous carbon (2.3 g/cm3) is different from that of adiamond crystal (3.5 g/cm3). This was pointed out by A.
Potylitsyn.
CrystalThe positrons emitted from the target in the forward
direction were momentum-analyzed in a momentum
range lower than 30 MeV/c by the magnetic field, where
the deflection angle was 60o from the beam axis. The
positron trajectory was determined by five collimators
installed before and behind the magnetic spectrometer.
The geometrical acceptance and momentum acceptanceis summarized in Table 3, which were calculated byusing the simulation code GEANT3.
The momentum-analyzed positrons were detected witha 3mm-thick acrylic Cherenkov counter and a lead-glass
calorimeter shielded 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 per bunch was measured as a pulse charge fromeach detector. Signals from the positron detectors and thesignal of the electron beam charge were sent to a dataacquisition system using a PC-based CAMAC/ADC
system, where all signal charges were simultaneously
digitized. The goniometer could rotate the crystal target
around two axes (H and V axes) by a pulse motor. The
angular resolutions of the goniometer were 10.5 and 34.9
mrad/pulse in the H and V axes, respectively. The crystal
axis, <110>, was determined by changing the relativerotational angles around the two axes with a step of 2 (or1) 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.
Table 3: Acceptance of the positron spectrometer.
Pe+
Acceptance(MeV/c)(10-4 x (MeV/c∑sr))
5
1.08 ± 0.03
10
2.47 ± 0.07
15
3.80 ± 0.10
20
4.81 ± 0.12

3 EXPERIMENTAL RESULTS
3.1 Two-Dimensional Crystal Axis Scan
We developed a computer software which quickly
scans a crystal axis in two dimensional plane. The
scanning time (10~20 min depending on the step sizes)
was much reduced compared with that of the previous
one-dimensional scanning. Figures 3(a) and 3(b) show
the typical results of the crystal axis scanning measured
for the diamond and 30mm-thick silicon crystal,respectively, at the positron momentum of 20 MeV/c.
You can clearly see main sharp peaks for both the
crystals generated from the <110> axis orientation
together with many weak peaks. The main sharp peak
was formed as the intersecting point of the several lines(see the density distribution in the projected plane in
Fig.3). These weak peaks or lines may come from planechanneling in the crystal. It is interesting that the numberof the weak peaks for the diamond crystal is larger thanthat of the silicon crystal.

3.2 Rocking Curves
Figures 4(a) (for diamond) and 4(b) (for 30mm-thicksilicon) show the measured rocking curves at thepositron momentum of 20 MeV/c. You can see very
narrow peak widths for both the crystals. The peakwidths were obtained to be 0.7 (for diamond) and 1 mrad(for 30mm-thick silicon) in one sigma by the leastsquares fitting procedures with two gaussian functions.
Figure 5 shows the variation of the peak width as afunction of the crystal thickness in unit of X0. The peakwidth increases gradually with the increase of the crystal
thickness and its value is about 1 mrad level dependingon the crystal thickness.

3.3 Positron-Yield Enhancement
Figure 6 shows the result of the positron-yieldenhancement measurement for the crystal alone as afunction of the crystal thickness in unit of X0 at thepositron momentum of 20 MeV/c. The enhancement of
the silicon crystal changes by a factor of 8-14 dependingon the crystal thickness, and for the diamond it is about
17 which is clearly larger than that of the silicon crystals.
It means that the crystal effect of the diamond is larger
than that of the silicon.
Figure 7 shows the result of the momentum
dependence of the positron-production enhancement.
The enhancement is slightly dependent on the positron
momentum in the measured region.
Figure 8 shows the summarized result of theenhancement measurement together with the tungsten
crystal data, which were obtained from the previous
experiment. The result shows that although in the regionof the thin thickness the enhancement obtained for thelight crystals is a little bit high, in the region of the large
thickness the enhancement obtained for the tungsten
crystals is clearly higher. It is presumed that thechanneling photons generated from the light crystal aresoft, and thus, the positrons generated in the later
amorphous tungsten are absorbed and multiply scatteredin the target because they are also soft.

3.4 Positron Yields and Crystal Effect
The detected positron yields were calibrated by using
the data from the amorphous tungsten plates withdifferent thicknesses. Figure 9 (a) shows the variations of
the positron yield depending on the thickness of theamorphous tungsten plates alone, and they were obtained
as the positron-yield calibration data in each crystal
measurement. For all the amorphous tungsten data, thepositron yield was normalized by using the data of the
9mm-thick amorphous tungsten obtained in the 30mmthick silicon crystal measurement. Figure 9 (b) shows thevariations of the positron yield depending on the total
thickness in unit of X0, where the crystal axis was set in
the direction of off axis. These results show that thepositron-yield normalization was performed quite wellwithin the experimental accuracy.
Figure 10 shows the summarized result of thenormalized positron yield obtained for all the crystal
measurements. It is clearly found several following
things:∑∑The maximum positron yield is almost the samelevel compared with that of the amorphoustungsten except for the data of the 10mm-thicksilicon crystal.
The shower maximum thickness of each crystal isreduced compared with that of the amorphous∑

tungsten, which means that the radiation thickness
is effectively reduced.
The reduction of the shower maximum thicknessof the diamond crystal is relatively large
compared with other crystals although the
thickness of the diamond is thin.

Figure 11 shows the result of the crystal effect. If weplot the shower maximum peak thickness as a function of
the crystal thickness, we can estimate how large thecrystal effect is. For the silicon crystal, the effect issaturated around the thickness of 30 mm, and for the
diamond crystal, although we have only one data point,
the effect is larger than the silicon crystal. This resultshows that the maximum positron yield from the 5mmthick diamond crystal is almost the same level as that of
the 30mm-thick silicon crystal, which gives larger crystal
effect than that of the diamond even with a 5mmthcikness. We can expect higher positron-production
yield if we use a thicker diamond crystal.

3.5 Effect of the Vacuum Window
Figure 12 shows the result of the positronenhancement measurement depending on the thicknessof the vacuum window, where the diamond crystal alone
was used in this test. From the result, the enhancement isalmost constant up to the window thickness of 200 mm,
and the rocking-curve peak width slightly increases with
the increase of the window thickness. This may comefrom the increase of background generated at thewindow. It is noted that the window thickness of 100 mm
has been used in the previous channeling experiment.

4 SUMMARY
The positron-production experiment using diamondand silicon crystal targets has been successfully carriedout at the KEKB 8-GeV injector linac. The obtainedrocking peak widths are very narrow less than 1 mrad in
one sigma for both the crystals. For the crystal target
alone, the enhancements of the positron yield are 9.3±0.5(10mmSi), 9.9±0.5 (30mmSi), 6.4±0.3 (50mmSi), and
16±0.8 (5mmDiamond) at the positron momentum of 20
MeV/c. The enhancement is much reduced with theincrease of the total target thickness. The maximum
positron yields are the same level as the maximum yieldobtained for amorphous tungsten targets.

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] I. Abe, et al.,”The KEKB Injector Linac”; KEKPreprint-2001-157 (2001); To be published in NIM A.
[5] R. Chehab, et al., RREPS01, Lake Aya, Russia,
September 2001, published in NIM B103 (2003) 243.
[6] V.N. Baier, V.N. Katkov and V.M. Strakhovenko,
NIM B103 (1995) 147.
[7] H. Okuno, et al., RREPS01, Lake Aya, Russia,
September 2001, published in NIM B103 (2003) 259.
[8] T.Suwada, et al., Phys. Rev. E67, 016502 (2003).

(a)

(b)

(b)

Figure 4: Rocking curves measured for (a) the 5mmthick diamond and (b) 30mm-thick silicon crystal at thepositron momentum of 20 MeV/c.

Figure 3: Results of the two-dimensional axis scan for (a)the diamond and (b) silicon crystal. The software of theaxis scan was developed by Tokyo Metropolitan Univ.

(a)s)[mrad]Rocking-Curve Peak Width (1s2Preliminary

Si Crystal
Diamond Crystal+

Pe =20MeV/c
1.5

1

0.5

0
0

0.1

0.2

0.3

X

0.4

0.5

0.6

0

Figure 5: Variations of the rocking-curve peak width
measured for (a) the diamond and (b) silicon crystal as afunction of the crystal thickness at the positron
momentum of 20 MeV/c.

Thickness (in Total) vs.Enhancement

Crystal Thickness vs. Enhancement
20

20

Preliminary

Preliminary

SiliconDiamond16+

Pe =20MeV/c+

15

Enhancement

Enhancemnet

Pe =20MeV/c
12

8

2 .2mmWCrystal
5.3mmWCrystal
9mmWCrystal
Reference

10

5

4

0

0

0.1

0.2

0.3

X

0.4

0.5

0

0.6

1

2

3

X
0

Figure 6: Variations of the positron yield enhancement
depending on the crystal alone thickness in unit of X0 at
the positron momentum of 20 MeV/c.

Positron Momentum Dependence

4

5

6

0

Figure 8: Variations of the positron-yield enhancement
depending on the target thickness in total in unit of X0 at
the positron momentum of 20 MeV/c. The crystal
tungsten data with three different thicknesses are also
added.
(a)25

Preliminary+
Pe =20MeV/c

1 0mm Si
3 0mm Si
50mm Si
5mm Diamond

20+

Normalized e Yield from W Target
Preliminary

15

10

5

0
5

10

15

20

25

30

35

+

Pe [MeV/c]

Figure 7: Variations of the enhancement depending on
the positron momentum.

Positron Yield [Arbitrary Unit]

Enhancement

400

+

350

Pe =20MeV/c

300
250
200

Normalization

150

10mmSi
30mmSi
50mmSi
5mmDiamond

100
50
0
0

5

10

15

W Thickness [mm]

20

(b)
Crystal Effect+Normalized e Yield from Off-Axis Combined Target

6400

Preliminary+

Pe =20MeV/c

300
250

0

200
150

10mmSi
30mmSi
50mmSi
5mmDiamond

100
50
0
0

1

2

3

X

4

5

iliconiamond+Pe =20MeV/c

5.5

at Shower Maximum

350

Thickness[X ] of a Combined Target

Positron Yield [Arbitrary Unit]

Preliminary

5

Thickness at Shower Maximum (W base)

4.5

4Palabora Fitting

3.5

3

6

0

0.1

0.2

0.3

X

0

Figure 9: Variations of the shower development curve(positron yield) for (a) the amorphous tungsten plate
alone and (b) off-axis combined target depending on the
thickness of the combined target at the positron
momentum of 20 MeV/c.

0.4

0.5

0.6

0

Figure 11: Variations of the crystal effect for the diamondand silicon crystal depending on the crystal thickness.

Multiple Scattering Effect
5mm Diamond Crystal
120
Width[mm]Enhancement

Enhancement

400

Positron Yield [Arbitrary Unit]

Preliminary350+

Pe =20MeV/c

300
250

15
0.8

0.7
10

0.6

200

Preliminary+
Pe =20MeV/c

150

10mmSi
30mmSi
50mmSi
5mmDiamond
W Base e+ Yield

100
50

5
0

1

2

3

X

4

5

0.2

0.3

0.4

0.5

0.5
0.6

Thickness of Vacuum Window (SUS304) [mm]

0
0

0.1

Rocking-Curve Peak Width [mm]0.9+Normalized e Yield from On-Axis Combined Target

60

Figure 10: Shower development curves (positron yields)
measured for the diamond and silicon crystal as afunction of the combined target thickness in total at thepositron momentum of 20 MeV/c.

Figure 12: Vacuum-window thickness effect. The effectwas estimated by using two parameters “Enhancement”
and “Peak width” of the diamond crystal data as afunction of the thickness of the vacuum window.