The Fermi chopper spectrometer 4SEASONS at J-PARC

Kajimoto, Ryoichi  ; Nakamura, Mitsutaka   ; Inamura, Yasuhiro  ; Mizuno, Fumio; Nakajima, Kenji  ; Kawamura, Seiko   ; Yokoo, Tetsuya*; Nakatani, Takeshi ; Maruyama, Ryuji   ; Soyama, Kazuhiko  ; Shibata, Kaoru  ; Suzuya, Kentaro; Sato, Setsuo*; Aizawa, Kazuya  ; Arai, Masatoshi; Wakimoto, Shuichi  ; Ishikado, Motoyuki; Shamoto, Shinichi  ; Fujita, Masaki*; Hiraka, Haruhiro*; Oyama, Kenji*; Yamada, Kazuyoshi*; Lee, C.-H.*

4SEASONS is a Fermi chopper spectrometer, which is in operation in Materials and Life Science Experimental Facility (MLF), J-PARC. It is intended to provide high-efficiency measurement of weak inelastic signals on novel spin and lattice dynamics with thermal neutrons. For this purpose, the spectrometer utilizes the coupled moderator and advanced instrumental design such as an elliptic-shaped converging neutron guide with high-critical angle supermirrors inside, long (2.5 m) position sensitive detectors, and a Fermi chopper feasible for multiple-incident-energy (multi-) measurement with the repetition rate multiplication technique. Here we show detailed design of the spectrometer, evaluation of its performance, and some examples of measurements.