A DOUBLE CHOPPER SYSTEM NEUTRON SCATTERING TIME-OF-FLIGHT SPECTROMETER

A double chopper system neutron scattering time-of-flight spectrometer is built and installed at the Heavy Water Research Reactor (HWRR) of China Institute of Atomic Energy in Beijing with the purpose of phonon measurement using neutron inelastic scattering technique. The two choppers are rotated in a synchro way (the highest speed is 15000 r/min ). Scattering neutrons are detected by 54 ~3He counters. distributing in an angle region of 90°. The total number of the ~3He detector will be extended to 108 in future. Neutron signals from ~3He detectors are coded by a time-of-flight coding system and than transferred to the data acquisition system of DG-10/SP micro-computer. An universal intelligent interface was used in the data acquisition system, which has the function of displaying the real time spectrum separately for each counter. Boron containing resin is filled in the chopper body, thus increasing anti fast neutron backqround ability of this system by a factor of 10, and enabling the spectrometer to be used on the radial experimental channels of the HWRR. In addition, Gd foil of 0. 025 mm is used as the material for the collimation system within the core of the Fermi chopper, which also strengthens the collimation efficiency and reduces the background duo to the large neutron absorption cross section of Gd. The technique specifications of the spectrometer are as following: energy region of monochromatic incident neutrons : 5—200 meV resolution : \E/E～(3—8) % monochromatic neutron flux at sample position: ～10~3 s~(-1)·cm~(-2); signal to noise ratio: better than 20: 1; phase and speed shift of the chopper system for a 10 day non-stop operation at 13000 r/min less than 0. 04%.