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

Spectral Narrowing ofBackscattered Photons in Electron-Laser
Collisions under a Strong Magnetic Field
Ichiro Sakai, and ffironari Yamada
Faculty of Science and Engineering, ltitsumeikan University
1-1 ~ 1 Nojihigashi, Kusatsu City, 525 Japan
Abstract

We studied an electron-laser collisions, forthe first time, at a bending section of a compact
storage ring to study an effect of the magnetic
The observed backscattered photonfield.
spectrum showed an unexpectedly narrow band
width without any collimation. We were only
able to reproduce this phenomenon by assumingthat the scattering occurs within the electron
orbital plane. Conservation of the total angularmomentum may become important when a strong
magnetic field is applied to an electron-laser
collision.
1 Introduction

The Compton backscattering method isadopted as a quasi-monochromatic and tunablegamma- or x-ray source. AURORA is acompact electron storage ring that has no straightsections [1]. We have studied electron-laser
collisions in a strong magnetic field.
Figure 1 shows our experimental setup, theelectron-laser interaction region was set in thestrong magnetic field of AURORA, which wasoperated at various electron energies. The 50-W
C02 laser was set so the polarization would beparallel to the electron orbital plane of the ring.
The laser beam was focused by a set of ZnSe
lenses on the electron orbit. A 4-mrn thick Si
mirror was used to reflect laser light and to extract
x-ray beams. No x-ray collimating apparatus
was used except an opening for the 1 X 0.5 em
vacuum chamber and 1.23 X 1.23 em a hole on
the Si mirror mount in both horizontal andvertical directions, respectively: X-rays weredetected by a 2"$ x2" Nal scintillator with a 50-ll
m thick Be window. In this setup, &e materials
that absorb x-rays were the 4-mrn thick Si mirrorand 2-mrn thick Ah03 vacuum window. In thisexperiment, to keep analog digital converter deadtime below 10 %, we limited the electron beam
current to the order of 10 rnA.

In order to make a quantitative comparison,
we nade the following corrections on thetheor~tically calculated spectrum: the opening
hole :m the Si mirror mount, the absorption ofxrays by the vacuum window, Be window and Si
mirrcr, and the response function of the Nalscintillator. In order to estimate the absolute
number of collisions, we have experimentallyobtai:led the density distribution of a laser beamat the focusing point, and carefully estimated theoverl1p volume of the photon and electron beams.
In the calculation we have taken a picture that thelaser density is distributed uniformly along thelaser trajectory, and that the electrons passperioiically inside the laser beam. Effects ofabsmption and detector response on the energy
spectrum were corrected. The convolution oftheor~tical spectrum due to the response functionof th·:! Nal scintillation detector was made withclose approximation by Gaussian distribution.
storage ring AURORA

Interaction
regionelectronorbit

slit
Si mirror

Al203 vacuum wln<tlow-.aNal scintilation detector
Multi channel analyzer

Fig. 1. Schematic view of the electron-laser
collision experimental setup using the exact
circular electron storage ring.
Figure 2 shows the observed energy
spectra of the backscattered photons when theelectron energies were 288, 315, and 366 MeV,respectively. Here that x-ray peak energy-113-The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

increases as electron.energy increases. These
peak energies are quite consistent withcalculations from kinematics of the head-oncollision. Contrary to our expectations, however,
the observed energy spectra all exhibited very
narrow band widths. We think the collimation
effect that is usually introduced to select scatteredphoton energy is useless to explore thisnarrowmg.

:iOO

Electron Energy=288 MeV---data- - - ca !elation

:~50

en

§ ··oo0aElectron Energy=:315 MeV:150

20

Electron Energy=366 MeV

photon energy(keV)

Fig. ~. Corrected theoretical calculations arecompared with observed spectra in absoluteinten:;ity.
3 Effect of the vertical slit

photon energy(keV)

Fig. 2. Observed scattered x-ray energy spectra
corresponding to electron energies of 288, 315
and366MeV.
2 Quantitative conparison of energy
spectrum

Figure 3 shows corrected theoretical
calculations and observed spectra. In this figure
these corrected theoretical spectra(dashed lines)
are compared with the observed ones(solid line)
in an absolute value. The calculated spectra
reproduce the increasing Compton edge as theelectron energy increases. The edge in the low
energy side appears due to the absorption and slit.
However, great deviations from the obtained data
are apparent. None of these corrections wasable to introduce band width narrowing. We
think the collimation effect that is usually
introduced to select scattered photon energy is
In theuseless to explore this narrowing.
following discussion, we will focus on how
narrow the observed x-ray energy spread iscompared to a standard calculation known to give
good agreement with experiments performed at
the straight section of storage rings[2-5].

In order to figure out the unknown effectin this band width narrowing, weresultedthat. have studied the effect of the vertical slit on theThis examination
theoetical calculation.
corre;ponds to confining the scattering angle tonear the electron orbital plane. When wenarrow the spacing of the vertical slit, the band
width narrows, as shown in this Figure, but it wasimpossible to reproduce present experimental
From
distriJution even with 0.46 mrad slit.
this examination we can easily imagine that somekind of intrinsic selection mechanism is involvedin this phenomenon. We think the collimation
effect that is usually introduced to select scatteredphoton energy is useless to explore thisnarrowmg.

10(0 ~-----r---~---,-----,0.8 ;;::"'c:-§ 5(0.cQ.

""· .: :' .·/.:'. ·3.19m.rad\m~(\.

:.. ·''\_///i:

I,:,'_.,

100\200

photon energy(keV)

Fig. 4. Spectral shapes calculated for various
vertical acceptance half angles.

-114-The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

due to a strong magnetic field.

4 Extreme case spectrum· 80

The cross-section ofbackscattered photons
was calculated according to Compton
backscattering theory [6-7]. The laboratory
differential cross-section is given by transfonningto the laboratory system in the scattered photon
energy and by integrating it over an azimuthal
angle with regard to the head-on collision,

Electron Energy=288 MeV. 2060(/)

c0:~400a·2030

photon energy(keV)

Fig. 5. The corrected theoretical spectra basedon Eq. (2) and the observed spectra in absoluteintensity.
We have calculated an extreme case in which 'instead of integrating the azimuthal scattering
angle from 0 to 2 Jr as ordinary Laboratory
spectrum, we have limited the scattering angle tothe electron orbital plane. We have obtained thefonnula:

Figure 5 shows the corrected spectra(dashedlines) are again compared with the observed
spectra in an absolute intensity. When x-ray
absorption and response function corrections are
made to the extreme case theoretical spectrum,
most of the low energy peak disappears and alittle peak, caused by the absorption, remains.
We think here rather good agreement between
theoretical and experimental spectra In thehigh energy section, the spectral shapes are
Even the absolute
beautifully reproduced.
intensities are in a good agreement. However,in the low energy section, the peaks are not seenin the experimental spectra, although the amountis small. We think carefully that the low energy
peak near the discrimination level can easily
disappear due to a decrease of detection
efficiency caused by a nature of electronics.
This correction is not introduced to the theoretical
As a consequence, the newspectrum.
fonnalism, extreme case cross section is usefulfor reproducing the present experimental results.
This implies that the photon scattering angle isconfined to the electron orbital plane, apparently

5 Summary

The Observed energy spectra all exhibitedvery :1arrow band widths without any collimaton.
If we assume the scattering occur within a orbital
plane, we obtained rather good agreement. The
electron-photon interaction must occur within theThis narrowing would
electron orbital plane.
be caused by a strong magnetic field. A
conservation of the total angular momentum may
take ~m imporumt role in this phenomena
Ackowledgements

Yamada as a fanner member of PRESTO
grate::Ully acknowledges the financial support by
the Research and Development Corporation ofJapart throughout this study.
Reference
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[7]

-115-

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