Fig. 1. Overview of the LEPS system. (a) Collision point between 8 GeV electrons and laser photons. (b) Laser hutch for laser light injection. (c) Experimental hutch consisting of a dipole magnet, wire chambers, and plastic scintillation counters. Upper-right inset: principle of inverse Compton scattering used to generate LEPS. 82 STUDIES IN QUARK NUCLEAR PHYSICS AT LEPS Synchrotron radiation is generated when electrons are bent in a magnetic field. This radiation is used in research in the atomic, molecular, organic, material and medical sciences to probe materials on a molecular level. However, t h e r e i s a n o t h e r w a y t o p r o b e t h e s m a l l e r microscopic world; nuclear and nucleon structure. High - energy real photons produced by means of inverse Compton scattering are very useful for such a purpose. The most powerful way to investigate hadrons is to use electroweak probes due to the reduced theoretical complexity of this method. Using a stored electron beam with energy of 8 GeV, an intense γ -ray beam in the GeV energy region can be produced. The LEPS (Laser-Electron-Photons) facility was constructed at beamline BL33LEP for nuclear and hadron physics experiments with primary support from Monbusho (the Ministry of Education) and STA. Construction of the beamline began in 1998 with the modification of accelerator components. A laser injection system was constructed in the first quarter of 1998 and the experimental hall was completed in March 1999. Figure 1 shows an overall view of the LEPS system. The purpose of the beamline is to inject a laser light beam which collides with the electron beam, allowing the extraction of high- energy photons produced via inverse Compton scattering. High - energy photons are produced in the direction of the 8 GeV stored-electron beam. A bending magnet in the storage ring after to the collision point components is used to measure the energies of knocked-on electrons. Electrons with 4.5 - 6.5 GeV are tagged in this manner, corresponding to 1.5 - 3.5 GeV γ -rays. γ -rays extracted from the ring a r e g u i d e d i n t h e v a c u u m c h a m b e r t o t h e experimental hall. In the experimental hall, there Laser light Collision 8 GeV electron Recoil electron Electron tagging (b) Laser hutch (c) Experimental hutch Inverse Compton γ -ray (a) SPring-8 SR ring Laser light 8 GeV electron beam Lorentz contraction Recoil electron LEPS collision are two hutches along the beamline. The first one , at the exit of the beam tunnel , is the laser hutch and is used mainly for laser injection. The second one is for experimental devices including a spectrometer dipole magnet, drift chambers and plastic scintillation counters. The first and second hutches are connected with a beam transport pipe. A laser beam injection was made for the first time at this site, and the high- energy photons (LEPS) from the inverse Compton scattering were observed in the laser hutch using a n e w l y - d e v e l o p e d L e a d T u n g s t a t e ( P W O) scintillation counter. The γ -rays were detected by the calorimetric m e t h o d . A c a l o r i m e t e r c o n s i s t i n g o f n i n e rectangular PWO crystal cells [1] was placed behind a sweep magnet. Energy calibration of each PWO scintillation counter was carried out with gas Bremsstrahlung [2]. The measured energy spectra for beam exposures centered on each crystal were convoluted using a known radiation spectrum and a resolution function of the crystal . 83 Mamoru Fujiwara Research Center of Nuclear Physics E-mail: fujiwara@rcnp.osaka-u.ac.jp Photon Energy (GeV) Counts 0.5 1 1.5 2 2.5 3 0 500 0 2000 2500 1500 1000 The energy spectra obtained from nine PWO scintillation counters were summed to generate the total γ -ray spectrum. Figure 2 shows the energy spectrum of LEPS, which clearly demonstrates that high-energy photons up to 2.4 GeV are created by inverse Compton scattering between 8 GeV electrons and laser light. The maximum energy calibrated with the photon spectrum from the gas Bremsstrahlung agrees with that calculated using special relativity theory. The emittance of the γ -ray beam was also measured using the PWO detector. The horizontal and vertical widths of the γ -ray (LEPS) beam were f o u n d t o b e σ x = 3 m m a n d σ y = 2 m m , respectively, at a distance of 40 m from the collision point. These values are consistent with those expected using a Lorentz factor of E e /m e c 2 = 16,000, demonstrating the extremely good precise emittance of the 8 GeV electron beam in the storage ring at SPring-8. References [1] H. Shimizu et al. , Nucl. Instrum. and Meth. in Phys. Research A, to be published. [2] A. Asano, to be published. Fig. 2. Energy spectrum of the γ -rays produced by the inverse Compton scattering between 8 GeV electrons and laser light.
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