Development of Ce:LiCAF scintillator system for high precision nuclear data measurement using short pulsed X-Band electron linac based neutron source

We are developing a new neutron source at Tokai campus of University of Tokyo. In order to measure high precision nuclear data using this system, the neutron detector should have a large neutron cross section and fast decaying time. An inorganic scintillator Ce:LiCAF has a great characteristic in time resolution. However, an electron Linac emits intense gamma-rays, and this is an important issue for precise neutron measurement. There are two methods of discriminating between neutrons and gamma-rays. The first is the method using a difference of the spatial range of secondary particles. The ranges of the secondary particles of neutron are shorter than that of gamma-rays. The calculated results of Geant4 show the crystal thickness must be less than 5 mm for discrimination. However, the crystal size should be larger than 1 mm x 1 mm to keep reasonable signal-to-noise ratio. The improvement of discrimination due to the change of the size is confirmed by experiment. The other is the method which uses a difference of the light decaying time. The rising time of the signal of gamma-rays is shorter than that of neutrons. By using two shaping amplifiers having different time constants, the signal of gamma-rays can be eliminated. (author)