Proceedings of the 1st Annual Meeting of Particle Accelerator Society of Japanand the 29th Linear Accelerator Meeting in Japan (August 4 - 6, 2004, Funabashi Japan)

POLARIZED HIGH-ENERGY PHOTON BEAM PRODUCTION WITHLASER-COMPTON BACKSCATTERING
K.Hosono, K.Aoki, K.Kinoshita, M.Fukada, T.Fujimoto, T.Hiramatsu, A.Miyahara
Graduate School of Engineering, University of Hyogo, 2167 Shosha Himeji, Hyogo 671-2201

Abstract
We have produced a beam of high-energy gamma-raysby Compton backscattering of 1064 nm laser photons
from 1 GeV electrons circulating in NewSUBARUstorage ring. The detected photon yields were measured
at about 3×104 photons s-1mA-1W-1 for the 20 mmΦ
collimator. The photon energy width from the collimator
corresponds to 6.6-17.6 MeV. The fundamental
characteristics well agree with the theoretical ones. We
have tried to measure the polarization of the produced
gamma-rays using a nuclear fluorescence experiment.
The preliminary results are described in this paper.

collision area and about 1 m behind the lead collimator.
The gamma-ray energies were calibrated with the 1.461
MeV gamma-rays from the 40K and several standard
gamma-ray sources.

1 BACKSCATTERIG FACILITY
In order to carry out photonuclear experiments on theelectromagnetic interaction in the energy range of several
MeV to tens of MeV, a laser-Compton backscattering
facility has been installed at NewSUBARU[1] storage
ring. The angle-energy correlation method is used in thatsystem. These gamma-rays can be used also for electron
beam diagnoses at the storage ring. An 11-m long straightline (BL1), was used for the laser-Compton
backscattering experiment. Fig.1 shows a schematic
overview of the laser-Compton backscattering facility.
The laser source used is a CW Nd:YVO4 laser (λ=1064 nm, EL=1.168 eV) and both the horizontal and thevertical waist diameters of the laser source are 0.42 mm
and both the full divergences are 3.4 mrad. The laser
photons are injected into the interaction area with fourmirrors and a lens. The laser beam waist is located at thecenter of the interaction area

2 PRODUCTION OF PHOTON BEAM
We conducted experiments at an electron energy of 1GeV, a current of 5-20 mA and laser power of 0.5-1.0 W.
The backscattering photon beam was collimated in one of0.67 mrad(20 mmΦ), 0.335 mrad (10 mmΦ) or 0.067
mrad (2 mmΦ) 10 cm long lead collimators. The photon
energy widths correspond to 6.6-17.6 MeV, 12.4-17.6
MeV, 17.3-17.6 MeV for 20, 10 and 2 mmΦcollimators,respectively. The collimator was placed on the gammaray beam axis approximately 14 m from the center of theinteraction region.
We monitored the produced photon flux with a 180
cm3 coaxial-type HPGe or 3”×3” NaI detector. Thedetector shielded by lead blocks was placed on photon
beam axis approximately 20 m from the center of theFigure 1: Schematic overview of laser-Compton backscattering facility
The energy spectra of the backscattered photons weremeasured at an electron energy of 1 GeV. The solid lines
in Fig. 2 show the energy spectra of the backscattering
photons measured with the various collimators. The
Bremsstrahlung photons have been subtracted from theoriginal spectra. When using the 2mmΦcollimator, thephoto peak and single escape peak can be seen as shownin Fig. 2, though the peaks are very small due to thevolume of the detector.
We compared the obtained energy spectra with thesimulated model calculations using the Monte-Carloelectron-gamma shower simulation code, EGS4[2]. The
calculated energy spectra are shown with the dotted lines
in Fig. 2. The shapes of the calculated energy spectra are
reproduce the measured ones. The photon yield can be
estimated from the normalization between the measured
and calculated spectra. The detected photons are about 3×104, 5×103 and 5×102 s-1mA-1W-1 for the 20, 10 and2 mm Φ collimators, respectively. The details of theproduction of the photon beam with the Compton
backscattering are described in Ref.[3]3POLARIZATION OF PHOTON BEAM

In the head-on collision, the photon beam generated by180°Compton backscattering of polarized laser beam ispolarized completely. We tried to measure the gamma-183

Proceedings of the 1st Annual Meeting of Particle Accelerator Society of Japanand the 29th Linear Accelerator Meeting in Japan (August 4 - 6, 2004, Funabashi Japan)

shows the gamma-ray polarization is 〜 98 ％ for theincident energy of 15.11 MeV.

Figure 2: Energy spectra (solid line) of the backscattering photons measured various collimators andthe energy spectra (dotted lines) calculated using
simulation code, EGS4.
beam polarization using nuclear fluorescence experiment.
We measure the asymmetry,In this equation, θis the scattering angle, σ(θ,0°)is the yield for elastically scattered photons in the plane
parallel to the electric vector of the incident photons andσ(θ,90°) is that in the perpendicular plane. P is thepolarization of the incident photons. The intensity
distribution function of a 0+→1π→0+ photon scattering
can be derived within angular correlation formalism[4]and is given by

whereπis parity. The asymmetry A to be +1 for positive
parity state and –1 for the negative parity state under thecondition of θ=90°and an even-even nucleus(J=0).
Elastically scattered photons from 12C(15.11 MeV,1+,T=1) were measured with the linearly polarized photons.
The energy width of the incident photons is 6.6 –17.6
MeV. The polarization axis of the laser photons can berotated by 90°by using a λ/2 wave–plate.
Figure 3 shows the preliminary energy spectra of natC
(γpol,γ) reaction. The upper part of this figure showsthe elastically scattered photons in the plane parallel tothe electric vector of the incident polarized gamma-rays
and the lower one shows that in the perpendicular plane.
We can find the elastically scattered photons only in theupper energy spectrum. The measured asymmetry islarger than 0.98 after background subtraction. ThisFigure 3: Energy spectra of the 12C(γpol,γ) reaction atincident energy of 6.6 to 17.5 MeV. The upper part of
the figure shows the elastically scattered photons in theplane parallel and the lower shows that in theperpendicular plane.

4 SUMMARY
We produced a beam of high energy gamma-rays byCompton Backscattering of 1064 nm laser photons from
1 GeV electrons circulating in a storage ring,
NewSUBARU. The photon energy width from the 20mm φ correspondst0 6.6-17.6 MeV. By using thesephotons, we are measuring the polarization of the
produced gamma-ray beam by Nuclear Resonancefluorescence(NRF), 12C(γpol,γ) in order to check theperformances of the facility.

REFERENCES[1] A. Ando, Y. Fukuda, S. Hashimoto, Nucl. Instr. andMeth. A481(2002)43.
[2] W.R. Nelson, H. Hirayama, D.W.O.Rogers, The
EGS4 code system, Stanford Linear Accelerator
Center Report SLAC-265, Stanford University, 1985.
[3] K.Aoki, K.Hosono, T.Hadame, H.Munenaga,M.Toda, K.Kinoshita, S. Amano, S.Miyamoto,T.Mochizuki, M.Aoki, D.Li, Nucl. Instr. and Meth.
A516(2004)228.
[4] L.W.Fagg and S.S.Hanna; Rev.Mod.Phys. 31(1959)
711.

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