Proc. Int. Workshop NASCES11 J. Phys. Soc. Jpn. 80 (2011) SB025 c 2011 The Physical Society of Japan SB025- The Fermi Chopper Spectrometer 4SEASONS at J-PARC Ryoichi Kajimoto *† , Mitsutaka Nakamura, Yasuhiro Inamura, Fumio Mizuno, Kenji Nakajima, Seiko Ohira-Kawamura, Tetsuya Yokoo, Takeshi Nakatani, Ryuji Maruyama, Kazuhiko Soyama, Kaoru Shibata, Kentaro Suzuya, Setsuo Sato, Kazuya Aizawa, Masatoshi Arai, Shuichi W akimoto 1 , Motoyuki Ishikado 1 , Shin-ichi Shamoto 1 , Masaki Fujita 2 , Haruhiro Hiraka 2 , Kenji Ohoyama 2 , Kazuyoshi Y amada 3 , and Chul-Ho Lee 4 Materials and Life Science Division, J-PARC Center, Tokai, Ibaraki 319-1195 1 Quantum Beam Science Directorate, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195 2 Institute for Materials Research, Tohoku University, Sendai 980-8577 3 WPI-AIMR, Tohoku University, Sendai 980-8577 4 National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Ibaraki 305-8568 (Received May 13, 2011) 4SEASONS is a Fermi chopper spectrometer in operation at the Materials and Life Science Experimental Facil- ity (MLF) in the Japan Proton Accelerator Research Complex (J-PARC). 4SEASONS is expected to facilitate high- efficiency measurements of weak inelastic signals on novel spin and lattice dynamics using thermal neutrons. The spec- trometer is equipped with a coupled moderator and sophisticated components such as an elliptical converging neutron guide with high-critical-angle supermirrors, long (2.5 m) position-sensitive detectors, and a Fermi chopper appropriate for multiple-incident-energy (multi-E i ) measurements by the repetition-rate multiplication technique. Herein, we dis- cuss in detail the design and performance of the spectrometer, and present some examples of the measurements obtained using this spectrometer. KEYWORDS: neutron-scattering instrumentation, chopper spectrometer, J-PARC 1. Introduction Currently, there are three chopper-type neutron inelastic in- struments at the Materials and Life Science Experimental Fa- cility (MLF) in the Japan Proton Accelerator Research Com- plex (J-PARC). Each spectrometer has its own characteristics. Among them, the Fermi chopper spectrometer 4SEASONS (or SIKI in Japanese) on the BL01 beamline at MLF is de- signed for high-intensity and high-efficiency measurements using thermal neutrons. The main target of the spectrometer is the observation of weak inelastic signals from novel spin and lattice dynamics in strongly correlated electron systems such as high-T c superconductors. For this purpose, the spec- trometer is designed to provide a high counting rate for mea- surement with E < ∼100 meV and an energy resolution of ΔE/E i > 5% (E is energy transfer and E i is incident en- ergy). 1) The development of 4SEASONS started in 2005 as part of a project to study high-T c superconductivity by neu- tron scattering. The spectrometer received its first neutron beam in September 2008, and then started commissioning us- ing neutron beam. At that time, however, the monochromat- ing chopper (Fermi chopper) had not been installed because of a delay in its development. The first inelastic measurement was performed in June 2009 after the installation of the Fermi chopper. 2) Now, we are implementing user programs on the spectrometer while continually improving its performance. In this paper, we show the detailed design of the spectrom- eter, an evaluation of its performance, and some examples of measurements, as good references for users of the spec- trometer. The Monte Carlo simulations shown here were per- * Present address: Research Center for Neutron Science and Technology, Comprehensive Research Organization for Science and Society (CROSS), Tokai, Ibaraki 319-1106 † E-mail address: ryoichi.kajimoto@j-parc.jp 15.8 L 2 = 2.5 m 16.3 18 20.5 m 3 He detectors Fermichopper 2.5 m sam ple sam ple 2.3 4.3 7.5 L 1 = 18 m shutter 0 biological shield coupled m oderator guide tube Floor 1.452 m diskchopper T 0 chopper top view side view 12 vacuum scattering cham ber 9 8.45 L 3 = 1.7 m Fig. 1. Schematic view of 4SEASONS. formed using the simulation package McStas. 3) If not explic- itly stated otherwise, the beam power when the experimental results were obtained was 100–120 kW. 2. Design of 4SEASONS Figure 1 shows schematic views of 4SEASONS, and Ta- ble I summarizes its specifications. The moderator is a coupled moderator that provides a high neutron flux and wide pulse width. The moderator–sample (L 1 ) and sample– detector (L 2 ) distances are 18 m and 2.5 m, respectively. The monochromating chopper is located 1.7 m (≡ L 3 ) upstream of the sample. 2.1 Neutron guide A supermirror-coated neutron guide is installed between the moderator and the sample. Figure 2 shows the shape of the guide and the kinds of supermirrors used. Here, m indicates the critical angle of the supermirror divided by that of the Ni J. Phys. Soc. Jpn. Downloaded from journals.jps.jp by 3.239.82.167 on 06/11/22