The 13th Symposium on Accelerator Science and Technology, Suita, Osakn, Japan, October 2001

GAMMA-RAYS BY COMPTON BACKSCATTERING IN A STORAGE RING,NEWSUBARU
K.Aoki, K.Hosono, K.Tanimoto, Faculty ofEngineering, Himeji Institute of Technology, Himeji
Hyogo 671-2201
S.Amano, S.Miyamoto, T.Mochizuki, A.Ando, T.Hashimoto, YShoji, LASTI, Himeji Institute ofTechnology, Kamigori-Kouto, Hyogo 678-1205

Abstract
Linearly polarized gamma rays have been produced
via the Compton backscattering. The beam of 17. 6
MeV gamma rays was obtained by backscattering
1064 nm laser photons from 1 GeV electrons
circulating in a ring named NewSUBARU. Spectra of
the backscattering photons have been measured with aNal scintillation detector. The maximum energy andthe energy spectra measured with the detector show
agreements with simulation calculations. A detailed
description of the gamma ray beam and the outlook
for future improvements are presented.

Experiments have been made with the electronenergy of 1.0 GeV and YAG laser (1,064nm, 1.168
eV). The stored electron current was 10 mA and YAG
laser power was 0.36 W. The backscatterep photon
beam was collimated in 0.66 mrad. by a leadcollimator with a length of 90 mm. The produced
photon flux was monitored by a 3" X 3" Nal(Tl)scintillator. The Nal scintillation detector was placedon the gamma-ray beam axis approximately 20 mfrom the center collision point and at about 1 m
behind the lead collimator. The response function ofNal detector was calculated by code EGS4[3).

1 INTRODUCTION
Backward Compton scattering of laser from ahigh-energy electron beam has been a promising
method of producing useful yields of high energypolarized photons. The gamma ray beam will haveproperties, which open up many new possibilities forbasic research and applications. The first real gamma
ray beam for nuclear physics research was developed
at the 1. 5 GeV ADONE storage ring [ 1). There are atpresent a number of facilities that produce polarized
gamma-rays for nuclear physics studies. In order tocarry the photonuclear experiments in the energy
range of several MeV to tens of MeV, the Compton
backscattering system has been installed at a storage
ring, NewSUBARU[2] at Laboratory of Advanced
Science and Technology for Industry (LASTI) in
Himeji Institute of Technology. The method of theangle-energy correlation has been used in the system.
In this paper, we will give some results obtained bythe experiments

Figure 1: A plane view ofNewSUBARU.

2 SETUP AND CALCULATIONS
A plan of the NewSUBARU facility is shown inFig.l. The racetrack shaped ring can operate at energy
1. 0 GeV to 1.5 GeV and has the ability to store anaverage current of 100 mA at injection energy of 1
GeV A line (BL-1 ), which is 14 m straight line, wasused for the experiment of the Compton
backscattering. The experimental arrangement isschematically displayed in Fig.Z.

-131-(em)Figure 2: Schematic experimental arrangement.

The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

We calculated the properties of the gamma rays
expected from the facility by the simulated
calculations. Figme 3 shows the calculated results.

3EXPERUWENTALRESULTSFirst, we meastrred the room background for 3053
sec in the tunnel where the ring is installed. Figure 4shows an energy spectrwn of the room background inthe tunnel. The spectrwn consists of the 1.461 MeV
gamma rays from the 4°K, of the 2.4 MeV gamma raysfrom ThC' and low energy gamma rays. We did notobserve the gamma rays of higher energy than 2.5
MeV The gamma ray energies were calibrated withthe 1.461 MeV and the several standard gamma ray
somces.
There are two potential background sources ofphotons, which are bremsstrahlung flux that originatesfrom high-energy electrons interacting with residual
atoms in the evacuated beam line and the synchrotron
radiation. The synchrotron radiation energies are allless than keV We measured bremsstrahlung flux of 1
GeV electrons and 10 mA electron current on theresidual gas atoms in the BL-1 beam line using theN ai detector. The degree of the vacuum in the beamline was 1 X 10-7 Pa. A measured spectrum of thebremsstrahlung gamma rays is shown in Fig. 5.
The spectra of the backscattered photons weremeasured at the electron energy of 1 GeV The
produced maximum photon energy of 17.6 MeV at thewavelength of the primary laser photon of 1064 nm isexpected for the head-on collision. Figure 5 shows ameasured energy spectrwn. The stored electron
current was 10 mA and the laser power was 0.36 W.
The spectrwn subtracted the bremsstrahlung spectrum
is the spectrwn of the Compton backscattering and isalso shown in Fig. 5. The maximum photon energy
measmed shows a good agreement with the calculatedenergy.
Room Background4°K (1.461 MeV)

1000gj......
"'§ 100uEeBr
Ey=•e•

34

Energy (MeV)-····--··· ..

·--------------..-- --05101520
Energy (MeV)

2530

Figure 4: Energy spectrum of the room background.

( 8«1 )

1+(y8) 2 + 4yEL /(me 2)·~~:~...:+:~·.>

2EL

0"v~~~~~--~~~~._~~~~Laser Photon

4 Y2 ELI•

10 :f:

10~£~"'' Pho<oo0\,0Laser Compton Backscattering y-ray

Electron Beam

1000 \·~ - - - - - - - ~ 100~/ ThC" (2.615 MeV)

'y=Ee/mc 2

Spectra of the Laser Compton andBremsstrahlung y-rays
using the 3"$x3" Nal(Tl) detector

Electron current : 1Om A ( 1Ge V)
Laserpower: CW0.36W( 1064nm Nd:YAG)
Collimator radius : 10mm ( 0.66mrad)

1o' ["'T"""~...,....,.~..,.,~~,.,--~.-,-~,_,....,~-,--,--,
Measured Compton y-ray spectrum

I'I

Bremsstrahlung··l-·-~! -r-Compton backscattered y-ray spectrum
after background subtraction. ·-j--r·----

0oi-----=o'-='.,-1~.o------:'-:-----::
Scattering Angle(mrad)

10'

0.5

1.S

1.0

2.0

Scattering Angle(mrad)

1000

100 '--'~'--'--'~~-'-'..~...__L~~J....._,'-"-'
00.2

0.4

0.6

0.8

500

1000

1500

2000

25003000

ChannelScattering cone (mrad)

Figure 3: Calculations of the properties of the gammarays.

Figure 5: Energy spectra of Bremsstrahlung andCompton backscattering. Spectrwn after subtractionof Bremsstrahlung spectrwn is also shown.

-132-

The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

The· total yields ·of the baokscattered photons weremeasured with the Nai detector. The geometry of thepresent system requires the gamma rays to penetrate
several media between the interaction, P9~t and thedetector. The pr«iuced gamma rays penetrate a silicon
dioxide mirror for the inJection of the laser photons
and. a. koval. window that s~parates the high .. vacmum
from air pressure. The total yields of photons wereestimated by taking account of the detector efficiency
and the attenuation by the several media. We obtained
the.photons .ofabout 3Xl04 photons/sec .inthe energy
range of 7 MeV to ·1 7.6 MeV The difference between
the experimenta l and calculated values is very small
as seen in Fig. 3.
A more detailed calculation of the results wasperformed using the E-GS4(3] code. The simulated
spectrum was created by assuming an ideal energy
spe.ctrum. at the collision point and by c.onvoluting. itwith a simulated response function of Nal detector.
The simulated and measured· spectra are shown inFig,6. The agreement of the shapes between. calculation and experiment is good.

being develope& We are preparing a 4 W laser source
to obtain more high flux of gamma rays and we are
performing the Compton backscattering with theelectron beam current of 100 rnA.

5 REFEREN CES[1] L.Federici, G.Giorda.no,. G.M~tone et al., Nuovo·Cimento 59Bp.247(1 980).
[2] A.Ando et at, J: Synchrotron R:ad: Vol.5, p342
(1998)[3] W,R..Nelson, RHira.yama and D.W,Q.R.ogers, theEGS4 code system; Stanford Linear Accelerator
Center Report SLAC-265, Stanford University (1985).

Comparison of Experiment al andcalculated Compton backscattere d y-ray spectra.
Electron current : I OmA ( 1GeV)
Laser power: 0.36W (1064nrn)
Collimator radius: 1 Ornm (0.66mrad)
Detector : 3 "<jl x3" Nal(Tl)~0u100 c._._~.._,L~~..J......~_.__j_~~--'--'~-"25201510030

Photon Energy (MeV)
Figure 6: Comparison of the measured spectrum andthe .simulated gamma ray spectrum.

4SUMMA RY
In this paper; we demonstrate d the Compton
backscatteri ng in NewSUBAR U. The gamma ra('yields were measur~ci t() ~ ~Q photons mA"1W 1sec· ..
The fimdamental ·characteristics well agree with thecalculations. At present, a·full program of the study is-133-