The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

Present Status of the Polarized-Photon Generation System with the Laser-ComptonBackscattering of the ETL Electron Storage Ring TERAS
Hiroyuki TOYOKAWA, Hideaki OHGAKI, Katsuhisa KUDO, Naoto TAKEDA, Tetsuo YAMAZAKI, Tomohisa MIKADO,Suguru SUGIYAMA, Kawakatsu YAMADA, Ryoichi SUZUKI, Toshiyuki OHDAIRA and Norihiro SEIElectrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, IBARAKI 305,Japan

Abstract
Since the first success of the storage of electrons, the800 MeV electron storage ring, TERAS, has !been used formany experiments. In one of the straight sections, we installeda polarized-photon facility with the laser-Comptonbackscattering method. The project started in 1981, when
TERAS was constructed, for the development of a high--energy,
quasi-monochromatic, and highly polarized photons field forstudies of nuclear physics, measurements of photo-induced
cross sections, and detector calibration. We, here, introduce
our polarized-photons generation system with a laserCompton-backscattering method and show the present statusof the facility together with some of the results from recentstudies.

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YAG LASER BEAM/lbJ!

'\..-SCATTERED PHOTONS

Fig.l Layout of TERAS and the beam lines [5].
1. Introduction
In 1%3, Milburn pointed out that highly-polarized highenergy photon beams could be obtained with a Compton
backscattering of the laser light quanta from relativistic
electrons [1]. His idea for the production of the polarized
photon beams was based on the work by Feenburg andPrimakoff [2], who pointed out that the energy degradation ofthe high energy electrons were due to the Compton scatterings
with sunlight and starlight. Some facilities to generate thephoton beams were constructed using the high energy electrons
from a linear accelerator [3], and a electron storage ring [4].

Fig. 2 Experimental arrangement [6].

2. Laser-Compton-Backscattered photon beam FacilityofETL
The first storage of electrons in Tsukuba Electron Ring
for Accelerating and Storage (IERAS) was achieved on Oct.
7, 1981. Figure 1 shows the layout of TERAS [5] and thebeam lines for experiments and the injection straight sectionof the electrons from a linear accelerator. The main parametersof TERAS are summarized in Table 1. The lie life time isabout 3.8 hours when the laser beam of 12 W was introduced
into the storage ring
Figure 2 shows the layout of the ETL Iaser-Comptonbackscattered photon beam facility. Five laser lights of different
wavelength are currently available: the first and the second
harmonic lights of Q-switched Nd:YAG laser (A.= 1064 nm),·and the second, the third and the fourth harmonic lights of Qswitched Nd:YLF laser (A.= 1053 nm). The laser beams areguided to the collision region around the center of the 1.8 m
injection straight section between DSR-1 and DSR--8, througha focusing lens L1 (f = 1000 mm), a mirror and a second
focusing lens L., (f =2000 mm). The backscattered high--energy
photons are collimated with a lead collimator of 100 X 100
mm2 and 150 mm long with holes varying from 0.9 to 3.9mmS-1, after passing through the glass window of a vacuum
chamber, the second focusing lens, and the mirror. The distance
between the collision point of electrons with the laser photons

Table 1 The ring parameters for TERAS.
Max. energy
800 MeVStored current
400 rnA
Circumference
31.45 mRadius of curvature 2.0 m
Latticeo/2BdQr~QfBp/2
Betatron freq.
x: 2.2z: 1.3
RFfreg.
171.690 MHz
and the first collimator is 5 m. The second collimator which isa 150 mm thick lead wall with a 20 mmS-1 hole, reduces
background bremsstrahlung photons originating fromcollisions of the circulating electrons with the residual gas
molecules within the ring. Because the electron orbit slightly
changes as the electron energy of the storage ring, the centerand the rotational angle in the horizontal plane of the collimator
are movable and are adjusted to give the highest photon flux
at the detector position. The photon flux is continuously-110-The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

monitored with a plastic scintillation detector with a thicknessof5mm.
The size of the laser beam is about 24 mm~ just before
the entrance window of the vacuum chamber and about l1
mm height and 13 mm wide at the exit window because of thecurved shape of the entrance window. The spectrum of thelaser light was measured using a monochromator and a silicon
photodiode [6].
3. Photon beam characteristics
The photon energy spectra were measured, at first, witha 33.9 cm3 HP-Ge detector [6]. The measured pulse heightspectrum for the laser-Compton-backscattered photons
incident on the detector with the energy of 2.09 MeV is shownin fig. 3. The continuum found in a low energy region is due
to multi-Compton scattered photons within the detector. Figure
4 shows the pulse height spectra for the 1.67 MeV photons [7]measured with (a) the HP-Ge detector and (b) an anti-Compton
HP-Ge spectrometer with Nai(Tl) crystals as an anticoincidence detector. The spectrum obtained with the antiCompton spectrometer showed a sharp full energy peak witha reduced continuum. As the energy of the incident photons
increase, the number of photons that deposit part of their energy

Fig. 3 Pulse height spectrum for 2.09 MeV photons [6].

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becomes greater, because the anti-Compton method becomesless effective and the energy resolution of the HP-Ge detector
also becomes worse.
Recently, a large volume HP-Ge detector (520 cm3) was
installed at our laser-Compton-backscattered photon beamfacility. Figure 5 shows the pulse height spectra for the 5.97
MeV photons measured with the new HP-Ge detector (dots)
and the calculated ones (lines) with the Monte Carlo code EGS4 [8]. A sharp cut-off of the incident photon energy at 5.'17
MeV, a Compton-continuum due to the 5.97 MeV full energy
peak, and the single- and the double-escape peaks are seen inthe figure. The continua extending from the maximum photonenergy cut-off to the higher energy region is dueto two-photoninduced events, in which two laser photon quanta, collided
with the electrons, are backscattered and are incident on thedetector within a resolving time of the counting system. It
should be pointed out that the timing property of thebackscattered photons are governed by that of the circulating
electrons in TERAS, because the pulse width of the laser beamis 150 ns which is 25.8 times longer than the RF frequency of
TERAS (171.69 MHz). So, 25.8 electrons bunches, each ofwhich is separated 5.82 ns, sequentially collide with acontinuous laser photon quanta of 150 ns pulse width, andgenerate backscattered photons.
We are trying to improve the photon yield by inteQSifying
the laser power and the electron beam currents, and alsoaligning precisely the laser beam and the electron orbit. The
alignment is, however, very sensitive to the movements of thelens and the mirror installed along the path of the laser beam,
and also to the electron orbit, which changes slightly as theenergy of the stored electrons. So, it takes more than a few
minutes to align them every time before experiments. It is,now, our interests to find a method to align the laser beam andthe electron orbit, precisely, in a relatively short time.
4. Applications using the Laser-Compton-Backscatteredphotons
The laser beams can be polarized with inserting a Glanlaser prism, and the polarization axis can be rotated by 90° byusing a iJ2 wave-plate. Therefore, the polarization axis of thelaser-Compton-backscattered photons can be changed by 90°
because the scattered photons retain the polarization of theoriginal laser beam photons to a high degree, up to almost
100% [1]. Using the polarized high-energy photon beams,
measurements of photon absorption cross sections and nuclear
resonance fluorescence are being undergone [9,10].
4.1 Measurement of Nuclear Resonance Fluorescence:::J0u£0.....

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ENERGY (keV)

Fig. 4 Pulse height spectra for 1.67 MeV photons [7].

-111-Fig. 5 Pulse height spectrum for 5.97 MeV photons.

The 11th Symposium on Accelerator Science and Technology; Harima Science Garden City, 1997

The photon source is used for a measurement of a nuclearresonance fluorescence from the 208Pb (y ,y) reaction. In theexperiment, the energy and the yield of~e laser-Comptonbackscattered photons was 5.52 MeV and 2 X 10" s-t at a stored
current of 300 rnA and a laser power of 3 W, respective~. The
upper part of fig. 6 shows the energy spectrum of elasticallyscattered photons in the plane parallel to the electric vector of
the incident photons on the target, and the lower part showsthat in the perpendicular plane. The 5.514 MeV 1' level wasclearly seen in the lower part of the energy spectra. Theasymmetry:
1 o(O") - a(90°)
A=p o(O") +a(90°)
for the negative parity state is -1 and that for the positive parity
is +1, under the condition of the scattering angle of 90° andthe polarization of the incident photons of 100 % for a J =0target.. In this expression cr(0°) represents the counts for theelastically scattered photons in the plane parallel to the electric
vector of the incident photons, and o(90°) represents those in
the plane perpendicular to the electric vector. The asymmetry
was measured -0.950 ± 0.047, which confirm the parity of the5.514MeV level to be negative.
The upper part of fig. 7 shows the energy spectrum ofphotons from the nuclear resonance fluorescence of 208Pb(yrd'y), elastically scattered in the plane parallel to the electric
vector of the incident photons on the target, and the lower part_shows that in the perpendicular plane. In the experiment, theenergy and the yield of the laser-Compton-backscatteredphotons were 7.8 MeV with 13.4% energy spread and -1~ s·1, respectively. The polarization of the photons
was calculated
98.2 %. From the figure, we can easily assign the parity of the7.332 MeV to be negative.
4.2 Development of a Photoneutron Source
One of the outstanding advantage of the laser-Compton
backscattered photons over the tagged bremssttahlung photons[11,12] is that the maximum photon energy falls very sharply
compared with that of the tagged photons. With the sharp cutoff of the maximum photon energy, a new system to control
photo-induced reactions that have threshold energy is made.
We are now developing a quasi-monoenergetic neutron source

10o(O")
IllPositive Paritya(911")
Negative Parity

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Fig. 7 Energy spectra of the nuclear resonancefluorescence for 7.8 MeV photons [10].
using the laser-Compton-backscattered photons as a photonswitch. When the maximum energy of the incident photons
exceeds the threshold energy for the photoneutron productions,
neutrons whose energy spread was determined by theoverlapping width of the photon energy and the reaction crosssections will be generated. If the width is slnall, as is the casefor the laser-Compton-backscattered photons incident on atarget, the neutrons will be of quasi-monoenergetic. Then, thephoton-switched photoneutron source can be made bycontrolling the energy of the photons slightly lower and higher
than the threshold energy. A wired Beryllium metal target isused to generate low and middle energy neutrons.

5. Conclusion
ElL laser-Compton-backscattered photon beam facili_ty
was introduced and the present status was presented together
with some of the results from the recent studies usi~g thephoton source. The facility has now been operated periodicallyfor many users for studies of nuclear physics, measurementsof photo-induced cross sections, and the development of thephotoneutron source.

References[1]30208Pb(ypo ,y), E
11>~ 20.,...

l0u=5.52 MeV
0'(90",0")10·'f'l'0lO.a(90",90")
Single Escape~-"'= 20lQ

211Pb I',S.SI4 MeV~:I0 10u

7.7

PhaiDn Energy(MeV)

R. H. Milburn. Phys. Rev. Lett. 10(1963)75.
(2]E. Feenburg and H. Primalroff, Phys. Rev. 73 (1948) 449.
[3]J. J. Munay andP. R. Klein, SI.ACReport1N- 67-19 (1967).
[4] L. Federichi, G. Giordano, G. Malone, G. Pasquariello and P.
G. Picozza, Nuovo Cimento 59B (1980) 247.
[5]
T. Tomimasu et al., IEEE Trans. Nucl. S~. 30 (1983) 3133.
[6]
T. Yamazaki et al.,IEEE Trans. Nucl. Sci. 32 (1985) 3406.
[7]
H. Ohgaki et al.,IEEE Trans. Nucl. Sci. 38 (1991) 386.
[8]
W. R. Nelson, H. Hirayama and W. 0. Roger, SIAC Report~(1985).
[9]
H. Ohgaki et al., Nucl. lnstr. and Meth. A3S3 (1994) 384.
[10] H. Ohgaki et al., "Search for Bound State Ml Transitions in:Dpb", Proc. NSRf,1997.

[11] J-O.Adler etal., Nucl. Instr. andMeth. A294(1990) 15.
[12]

Photon Energy (MeV)

Fig. 6 Energy spectra of the nuclear resonancefluo~ce for 5.52 MeV photons [9].

-112-B-E. Andersson et al., Phys. Rev. CSl (1995) 51.