こTungsten mono-crystalline target fora high-intensity positron source•••

T.Suwada (tsuyoshi.suwada@kek.jp)
Accelerator Laboratory, KEK••こthe 10th KEKB Acc. Rev. Committe,Feb. 21-23, 2005

1

Motivation
 High-intensity positron sources are required
for future linear colliders and B-factories.
 Conventional methods using amorphousheavy metals limit to increase the intensity ofprimary electron beams due to the heat loadon the target.
 New method using a mono-crystalline target
for positron production is expected to be oneof the bright schemes for high-intensity e+sources.
the 10th KEKB Acc. Rev. Committe,Feb. 21-23, 2005

2Introduction
 New method utilizing a crystal target was proposedby Chehab, et al. in 1989.
(R. Chehab, et al., PAC’89, Chicago, IL, USA, Mar. 1989, p.283)

 Yoshida, et al., demonstrated a clear enhancement of
the e+ yield in a tungsten crystal target using a 1.2GeV electron beam of INS/Tokyo.
(K. Yoshida, et al., Phys. Rev. Lett. 80, 1437, 1988)

 However, any theoretical studies taking into accountboth processes of Channeling Radiation (CR) andCoherent Bremsstrahlung (CB) has not yet been
established on the simulation.
 More experimental data are expected to clearly
understand the physical interaction processes of theCR and CB.
the 10th KEKB Acc. Rev. Committe,
Feb. 21-23, 2005

3

Channeling Radiation & Coherent
Bremsstrahlung Processes

the 10th KEKB Acc. Rev. Committe,
Feb. 21-23, 2005

4

New Positron Production Schemes

the 10th KEKB Acc. Rev. Committe,
Feb. 21-23, 2005

5

Experimental Setup

60o

the 10th KEKB Acc. Rev. Committe,
Feb. 21-23, 2005

6Linac Beam Line at the 3rd switch yard

the 10th KEKB Acc. Rev. Committe,
Feb. 21-23, 2005

7

Experimental Setup (cont’d):Positronspectrometerthe 10th KEKB Acc. Rev. Committe,
Feb. 21-23, 2005

8

Acceptance of the Positron Spectrometer The acceptance (ΔP ΔΩ) was obtained byusing the simulation
code (GEANT3).
 Typical acceptance
Momentum:
ΔP/P=2.4% (FWHM)&Geometrical:
ΔΩ=1msr
at Pe+=20MeV/c.
the 10th KEKB Acc. Rev. Committe,
Feb. 21-23, 2005

9

Experimental Condition
Electron Beam: S-band single bunch• Beam Energy = 4 (8) GeV• Angular Spread ~123(23) µrad (H), ~121(41) µrad (V)• Transverse Beam Size 1 ~1.5mm (FWHM) in diameter• Beam Charge = 0.2 nC/bunch• Bunch Length (Single Bunch) ~10 ps (FWHM)• Beam Repetition = 25(2)Hz
Angular Spread of the Electron Beam at the Positron Target• Φ ~ 0.2(0.1) mrad < Φc (due to multiple scattering by a vacuum
window(100µm-thick SUS))
Critical Angle for the Channeling Condition at the Positron Target• Φc ~ 0.61(0.43) mrad @4 and 8 GeV (Linhard Angle)

the 10th KEKB Acc. Rev. Committe,Feb. 21-23, 200510

Experimental Condition (cont.)
Positron-Production Targets:• Crystal Tungsten Target : 2.2, 5.3 and 9mm thickness• Amorphous Tungsten Target : 2-28mm thick (for the e+
production yield calibration)• Amorphous Tungsten Target: 3-18mm (3mm step) thickness (forthe purpose of hybrid targets)
Detected Momentum Range• 5 MeV/c ≤ Pe+ ≤ 20 MeV/c
Positron Detectors•Lead-Glass Calorimeter:Measurement of total energy of e+•Acrylic Cherenkov Counter:Measurement of number of e+
Beam Monitors•Wall-current monitor for the electron beam-charge measurement•Screen monitor for the beam-profile measurement

the 10th KEKB Acc. Rev. Committe,Feb. 21-23, 2005

11Experimental Results:
Rocking Curves (Crystal Axis <111>) at Ee-=4and 8 GeV (Pe+=20MeV/c)(f) Ee-=8 GeV(d) Ee-=8 GeV&'

9.0mm-thick Wc

on-axisRelative Positron Yield[arbitrary unit]

Relative Positron Yield[arbitrary unit]

2.2mm-thick Wc!"&!"%!"$off-axis!"#!
(&!

(%!

($!

(#!

!

#!

$!

%!

&!

on-axis%"#%$"#$off-axis

!"#!
'(!

')!

Rotational Angle [mrad]the 10th KEKB Acc. Rev. Committe,
Feb. 21-23, 2005

'*!

'%!

!

%!

*!

)!

(!

Rotational Angle [mrad]

12

Experimental Results:
Variations in the width of the rocking-curve peakfor Ee-=4 and 8 GeV (Pe+=20MeV/c)'!

Ee-=8 GeV
Ee-=4 GeV

Peak Width [mrad]&!

Pe + =20 MeV/c

%!

$!

#!

"!

!

!

#%'("!

Target Thickness [mm]the 10th KEKB Acc. Rev. Committe,
Feb. 21-23, 200513

Experimental Results: Variations in theenhancement (Ne+@peak/ Ne+@base) of the e+ yield at
Ee-=4 and 8 GeV (Pe+=20MeV/c)(b) Ee-=8 GeV, Pe+=20 MeV/c(a) Ee-=4 GeV, Pe+=20 MeV/c''Enhancement

&%

$

#%

$

#

"

"

!

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

Enhancement

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

!

!

&"!

"&

#!

#&

Target Thickness [mm]

$!

!

&"!

"&

#!

#&

$!

Target Thickness [mm]the 10th KEKB Acc. Rev. Committe,
Feb. 21-23, 200514

Experimental Results:
Momentum dependence of the positron-yieldenhancement

the 10th KEKB Acc. Rev. Committe,
Feb. 21-23, 200515

e+ production efficiencies for the crystal tungsten
at Ee-=4 and 8 GeV (Pe+=20MeV/c)
Ee-=4 GeV

&!"#

%'!

%(!

%)!

!

)!

(!

'!

$off-axis
#"

!
'(!

&!

Rotational Angle [mrad]-on-axis')!

'%!

'#!

!

#!

%!

)!

(!

!"%

!"$!"#(%!

e+ base yield

(#!

!

#!

$!

-Ee =4GeV, Pe =20 MeV/c

%!

%"#%

$"#

$!"#!
'(!

&!

')!

'*!

'%!

!

%!

*!

)!

(!

Rotational Angle [mrad]+Ee =8GeV, Pe =20 MeV/c!"!(!"!(Tungsten crystal
GEANT3
Amorphous tungsten!"!'!"!&

Positron Production Efficiency [%]Positron Production Efficiency [%]($!

Rotational Angle [mrad]

Rotational Angle [mrad]+&9.0mm-thickWc!"&!
(&!

9mmWc

Ee-=8 GeV

2.2mm-thickWc

Relative Positron Yield[arbitrary unit]$'

9.0mm-thickWc%

2.2mmWc

Ee-=8 GeV

Relative Positron Yield[arbitrary unit]

2.2mmWc

Relative Positron Yield[arbitrary unit]

Relative Positron Yield[arbitrary unit]

2.2mm-thickWc

!
%&!

9mmWc

Ee-=4 GeV$"#Crystal W~30%!"!%!"!$

Amorphous W!"!#!

~30%

Crystal W!"!'!"!&!"!%

Amorphous W!"!$

Tungsten crystal
GEANT3
Amorphous tungsten

!"!#!

!

'#!

#'

$!

!

'#!

#'

Target Thickness [mm]Target Thickness [mm]the 10th KEKB Acc. Rev. Committe,Suwada, et al., Phys.Rev.E 67, 016502 (2003) Feb. 21-23, 2005$!

16

Experimental Results:
e+ production efficiencies for the crystal andcombined targets at Ee-=4 and 8 GeV(Pe+=20MeV/c)(a) Ee-=4 GeV, Pe+=20 MeV/c(b) Ee-=8 GeV, Pe+=20 MeV/c!"!(Wc's
2.2mmWc+Wa
5.3mmWc+Wa
9mmWc+Wa
Simulation
Wa's

Positron Production
Efficiency

Shower max.
of crystal W!"!&!"!'!"!&

Shower max.
of amorphous W!"!%!"!$

Positron Production
Efficiency [%]!"!'!"!(!"!$!"!#!

!"!%

W c's
2.2mmWc+Wa
9.0mmWc+Wa
Simulation
W a's!"!#!

'#!

#'

$!

$'

Target Thickness [mm]

%!

!

!

'#!

#'

$!

$'

%!

Target Thickness [mm]the 10th KEKB Acc. Rev. Committe,
Feb. 21-23, 200517

Conclusions♠ Rocking curves•••

The obtained widths of the rocking-curve peak is much larger thanthe critical angle, and broaden with the thickness of the crystal target.
The broad widths of the rocking curves indicate that the CB process
may be predominant over CR process in this energy region.
The increase of the peak width depending on the target thickness may
come from the multiple scattering of the incident electrons in thetarget.

♠ Enhancement of the e+ yield @Pe+=20MeV/c••

4GeV En=3.7 ±0.1 (2.2mm), 2.2 ±0.1 (5.3mm), 1.5 ±0.1 (9mm)8GeV En=5.1 ±0.5 (2.2mm), 3.0 ±0.5 (5.3 mm), 1.8 ±0.2 (9mm)♠ Positron production efficiency for the crystal targets••

The absolute e+ yields were enhanced by ~26% with Pe+=20MeV/c by
15 and 18% on the average with the momentum range of 5-20MeV/c
at Ee-=4 and 8GeV, respectively, compared with the maximum e+yield obtained for the amorphous target.
We have a new plan to install a tungsten mono-crystalline target atthe present e+ source in this summer.
the 10th KEKB Acc. Rev. Committe,Feb. 21-23, 2005

18