NewSUBARU 153 NewSUBARU Laser Compton scattering γ -ray (LCS γ ) beamline BL01 [1-3] at NewSUBARU has been upgraded. A new irradiation hutch for the γ -ray beam has been added in collaboration with Konan University. The user use of γ -ray beams with photon energy of up to 76.3 MeV has begun with maximum γ -ray beam power of 0.33 mW. The LCS γ 's are unique photon source generated by scattering of laser photon by relativistic electron beam. Schematic of LCS γ is shown in Fig. 1. The maximum γ -ray photon energy at head on collision is calculated by kinematics as E γ = 4 E L γ 2 / (1+ γ 2 θ 2 + 4 γ E L / mc 2 ) , where γ = 1+ E e / mc 2 is the Lorentz factor, E e is an electron energy, mc 2 is the rest energy of electrons, E L is a laser photon energy, θ is the scattering angle of γ -ray relative to the electron beam axis, and 4 γ E L / mc 2 is the recoil effect. When the Nd:YVO 4 laser (wavelength of 1064 nm) is scattered by a 1-GeV electron, the maximum scattered photon energy is E γ =17.5 MeV. The angular distribution of the γ -ray photon energy is calculated by the above equation, as shown in Fig. 2. A quasi-monochromatic γ -ray beam is obtained by extracting small angle scattering using an axial collimator. Polarization of the laser beam is also conserved at the beam axis. Due to these special features of the LCS γ , this γ -ray source was used for nuclear physics and an astro-nuclear physics research, as well as generation of useful isotopes, non-destructing inspection of bulk material by γ -ray radiography and positron annihilation. In brief, the experimental setup is as follows: a beam of 974-MeV electrons is injected from the SPring-8 linear accelerator into the NewSUBARU storage ring. The energy of the storage electrons is a relative value with ~1% uncertainty based on the magnetic field strength and the beam optics of the storage ring [4], hereafter referred to as the "nominal" energy in this paper. We performed absolute calibration of electron beam energies of the storage ring NewSUBARU in the nominal energy range of 0.55 - 1.0 GeV [5]. The technique of laser Compton backscattering has been developed to accurately determine electron beam energies [6] as an alternative to the technique of resonant spin depolarization [7], which is limited to high energy electrons because of spin depolarization time [8]. We al s o p er for m e d t he ener gy c ali b rat i o n systematically with the following steps, (1) production of low-energy LCS γ beams in collisions of CO 2 laser photons with electrons at ten nominal energies, from 974 MeV to 550 MeV; (2) measurements of LCS γ beams with a high-purity germanium (HPGe) detector; (3) energy calibration of the HPGe detector with standard γ -ray sources. Figure 3 indicates the experimental setup for the measurement in the γ -ray beamline BL01 of the NewSUBARU synchrotron light facility. A grating- fixed CO 2 laser (INFRARED INSTRUMENTS, IR-10- WS-GF-VP) oscillated at a single line of the strongest master transition P(20). The central wavelength of the P(20) transition is known (=10.5915 μ m ± 3 Å) with a bandwidth of 1.3 Å in FWHM. The accuracy of the wavelength of CO 2 laser is 4.1 × 10 −5 . The CO 2 laser photons produced outside the storage ring vault were led through four mirrors and one lens into the vacuum tube of the ring to a collision Fig. 1. Schematic of laser Compton scattering γ -ray generation at head-on collision. Fig. 2. Dependence of the γ -ray photon energy on the scattering angle θ . Laser Compton scattering γ -ray beamline at NewSUBARU electron beam laser beam gamma-ray beam E e E L E γ scattered angle θ Scattered Photon Energy E γ Scattering Angle θ (rad) 4 E L γ 2 0 0 1/ γ 2/ γ 3/ γ 4/ γ γ = E e /mc 2 NewSUBARU 154 point P1 in the straight section of the storage ring. The collision point P1 for the CO 2 laser is located at the distance of 8.78 m from a collimator set in the experimental Hutch 1. The collision point P2 for the Nd:YVO 4 laser is located at 18.47 m from the collimator. A coaxial HPGe detector (64 mm in diameter × 60 mm in length) was mounted in Hutch 2 and aligned with synchrotron radiation to measure the low-energy LCS γ 's. The HPGe detector was calibrated with the standard γ -ray sources, 60 Co including the sum peak, 133 Ba, 137 Cs, and 152 Eu and a natural radioactivity 40 K. After the injection of electrons into the NewSUBARU storage ring, the electron beam was decelerated to the nominal energy of 950 MeV, and subsequently down to 550 MeV in steps of 50 MeV, followed by a production of the LCS γ beam and measurements with the HPGe detector at every energy. The difference between the calibrated energy E(measure), and the nominal energy E(nominal), Δ E = E(measure) – E(nominal), is shown in Fig. 4. The 4th-order polynomial fit to the data gives Δ E = –4.6949 × 10 −10 (E n ) 4 + 1.3017 × 10 −6 (E n ) 3 – 1.3596 × 10 −3 (E n ) 2 + 0.63854(E n ) – 103.94 Here Δ E, E(measured) and E n = E(nominal) are given in MeV. The difference between the nominal and calibrated energies is 10.92 MeV (1.36%) at the nominal energy of 800 MeV. The electron beams in the storage ring NewSUBARU have been systematically calibrated in the nominal energy range of 550 - 974 MeV by using low-energy LCS γ beams produced with a CO 2 laser. For more information about this experiment and analysis, please see Ref. [5]. S. Miyamoto a, *, H. Utsunomiya b , T. Shima c and S. Amano a a NewSUBARU, LASTI, University of Hyogo b Department of Physics, Konan University c Research Center for Nuclear Physics, Osaka University *E-mail: miyamoto@lasti.u-hyogo.ac.jp References [1] S. Miyamoto et al .: Radiat. Meas. 41 (2007) S179. [2] S. Amano et al .: Nucl. Instrum. Meth. Phys. Res. A 602 (2009) 337. [3] K. Horikawa et al .: Nucl. Instrum. Meth. Phys. Res. A 618 (2010) 209. [4] A. Ando et al .: J. Synchrotron Rad. 5 (1998) 342. [5] H. Utsunomiya, T. Shima, K. Takahisa, D.M. Filipescu, O. Tesileanu, I. Gheorghe, H.-T. Nyhus, T. Renstrom, Y.-W. Lui, Y. Kitagawa, S. Amano, S. Miyamoto: IEEE Trans. Nucl. Sci. 61 (2014) 1252. [6] L.C. Hsu et al .: Phys. Rev. E 54 (1996) 5657. [7] Ya. S. Derbenev et al .: Part. Accel. 10 (1980) 177. [8] R. Klein et al .: Nucl. Instrum. Meth. Phys. Res. A 384 (1997) 293. Fig. 3. Two γ -ray irradiation hutches are shown. Layout of laser Compton scattering γ -ray beam source in NewSUBARU BL01. Two collision points and two γ -ray irradiation hutches are shown. Fig. 4. The difference Δ E of the calibrated energy E(measured) from the nominal energy E(nominal) of the electron beams, Δ E = E(measured) – E(nominal), at the NewSUBARU storage ring. Hutch-1 Hutch-2(GACKO) P2 P1 Electron beam Laser beam Collimator Laser Dump NewSUBARU Synchrotron Light Facility Parameters of electron storage ring Electron energy 0.5 - 1.5 GeV Electron current 300-500 mA Circumference BL01 HPGe 8.78 m 118.7 m 12 11 10 9 8 7 6 Δ E=E (measured) –E (nominal) (MeV) 1000 900 800 700 600 500 N ominal Electron Energy E (nominal) (MeV)
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