CO11-1

Progress of Terahertz-wave Spectrophotometry by Compton Backscatteringof Coherent Synchrotron Radiations at KURRI-LINAC

N. Sei and T. Takahashi1
1.2

INTRODUCTION: Terahertz-wave spectrophotometryby Compton backscattering (THz-SCB) has been developed with an L-band electron linear accelerator at theKyoto
UniversityResearchReactor
Institute(KURRI-LINAC) [1, 2]. We have already demonstrated
the THz-SCB with using coherent transition radiation(CTR) as a THz wave source [1]. However, optical transition radiation generated with the CTR at wavelengths ofthe Compton backscattered photons (CBPs) in theTHz-SCB. Because the intensity of the optical transition
radiation was much stronger than that of the CBP, an error caused in a spectrum measurement of the CBPs wasconsiderable. Then, we started to develop THz-SCB experiments with using coherent synchrotron radiation(CSR) as the THz-wave source. Because a critical wavelength of synchrotron radiation for the low-energy electron beam at the KURRI-LINAC is in a far-infrared region, visible photons which become backgrounds in theTHz-SCB experiments are not generated from the synchrotron radiation.
EXPERIMENTS: To generate CSR for THz-SCB experiments, two permanent bending magnets with a lengthof 50 mm was installed in a vacuum chamber dislocatedfrom the accelerator by titanium windows. The bending
magnet had a gap of 40 mm, and its magnetic field was0.155 T. A distance between the two bending magnets
was 216 mm. The CSR beam emitted from the bending
magnets was reflected by two flat mirrors and was converted into the parallel beam with a concave lens of focus
distance 1.5 m. It was necessary to return the CSR beam
to the electron beam so that the CSR beam performed
Compton backscattering. Then, a hole-coupled flat mirror
was inserted in the parallel beam to separate a CBP beamfrom the CSR beam. The hole-coupled flat mirror wasdeposited aluminum onto a quartz window except a center circle with a diameter of 20 mm.
We observed the CBP with using the CSR at a chargeof 0.32nC per micropulse. To shift a peak of a CBP spectrum to the visible region, the electron beam energy wasset to be 18.9 MeV. The relativistic energy spread of theelectron beam was measured 10.8% in full width at halfmaximum. To isolate the effect of the CBPs, a millimeter-wave absorber with the same cross section as thehole-coupled flat mirror was installed in front of the mirror. The intensity of the CBPs could be calculated as thedifference with and without the millimeter-wave absorber.
The measured CBP spectrum had maxima at wavelengthsof around 400 and 500 nm, and it was in agreement with1.0

Transmisstion

Research Institute of Instrumentation Frontier, National
Institute of Advanced Industrial Science and Technology
1Research Reactor Institute, Kyoto University0.8
0.6
0.4
0.2
0.023456789-1

Wavenumber [cm ]Fig.1. Transmission spectra measured by
THz-SCB (circle) and interferometer (line).

thethe calculated one. A ratio of the CBPs to the backgroundphotons was approximately 1/4 with using the CSR, andit was improved than that with using the CTR in the previous experiment, which was 1/13.
Because the CBP spectrum could be measured inspite of high background level, we performed theTHz-SCB experiment with using CSR. A hole-coupledpolystyrene film with a thickness of 0.3 mm was inserted
instead of the millimeter-wave absorber. The effective
thickness of the polystyrene film was 0.6 mm because theCSR beam passed through the polystyrene film twice.
The transmission spectrum measured by the THz-SCB
was roughly in agreement with that measured by theMartin-Puplett-type interferometer in the whole of theobserving wavelength region as shown in Fig. 1.
CONCLUSIONS: We achieved the observation of theCBP spectrum with using the CSR in KURRI-LINAC. It
was similar to the spectrum calculated using the CSRspectrum. The transmission spectrum of the polystyrene
film was measured by THz-SCB, and it roughly agreed
with that measured by the interferometer. Using the CSR
as a THz-wave source, we could reduce the backgroundphotons in the visible region where the CBPs were emitted. However, the CSR reflected in the vacuum chamber
generated intense background photons between thebending magnets by Compton backscattering. We will
study a technique to eliminate the background photons bycontrolling the reflected CSR.
This work was supported
ACKNOWLEDGMENT:by JSPS Grant-in-Aid for Challenging Exploratory Research 2365696.
REFERENCES:[1] N. Sei and T. Takahashi, Appl. Phys. Exp., 3 (2010)
052401.
[2] T. Takahashi and K. Takami, Infrared Phys. Technol.,
51 (2008) 363-366.

採択課題番号 24008 コヒーレント放射を用いたテラヘルツ波電子線分光高度化の研究 共同通常（産総研）清 紀弘（京大・原子炉）高橋 俊晴