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A flat distribution in the energy spectrum and a spatial distribution with a small beam size, Flat-Laser Compton Scattering Gamma-ray (F-LCS) beam, has been generated by exciting a circular motion of an electron beam for a multi-isotope CT Imaging application through Nuclear Resonance Fluorescence (NRF) method. A proof-of-principle experiment to generate F-LCS beam has been carried out at the beamline BL1U in UVSOR-III where a helical undulator is installed. The electron beam with 746 MeV and about 6 mA collided head-on with the laser beam from a Tm-fiber laser system (TLR-50-AC-Y14, IPG Laser GmbH) with around 1 W CW power. A high-purity germanium detector with a relative efficiency of 120% was used to measure the energy spectra of the generated gamma-rays for different K-value of the helical undulator from 0 to 0.3. Using a 2-mm collimator placed about 8.7 m downstream of the center of BL1U, the energy bandwidth of the standard LCS beam (K=0) was 2.7% FWHM. When we excited the undulator K=0.2, the energy bandwidth became wider, 7.2% FWHM. At the same time, the peak energy shifted from 5.53 MeV to 5.50 MeV. These results showed a good agreement with EGS5 simulation [1]. We also measured the F-LCS beam's space distribution and obtained reasonable results.
Next, we tried to take a multi-isotope imaging by using F-LCS beam in UVSOR. The F-LCS beam with K=0.2 irradiated $^{206,207,208}$Pb enriched targets simultaneously and NRF peaks from all three isotopes were observed. The yields of the F-LCS excited NRF peaks below 5300 keV were larger than those by the standard LCS beam, which was consistent with the ratio of the energy spectrum of the F-LCS beam to the standard LCS beams.
This research is partially supported by the Japanese Sociality for the Promotion of Science (JSPS) KAKENHI under Grants 21H01859 and 22H01239. This experimental work was performed at the BL1U of UVSOR Synchrotron Radiation Facility with the approval of Institute for Molecular Science (IMS), National Institute of Natural Science (NINS) (Proposal No. 21-603, 21-806, 22IMS6605, 22IMS6806, 23IMS6607).
[1] H. Ohgaki et al., “GENERATION OF FLAT-LASER COMPTON SCATTERING GAMMA-RAY BEAM IN UVSOR,” Proc. of 13th Int. Particle Acc. Conf, 3070, (2022).
