In an exciting development in neutron science, the Spallation Neutron Source (SNS) project is a partnership of six US DOE National Laboratories to design and construct the most powerful spallation source in the world for neutron-scattering R&D. Instruments being built at the SNS include neutron spectrometers and diffractometers, used to determine the positions, or arrangements, of atoms in crystals, ceramics, superconductors and proteins. Novel neutron diffractometers and detectors at the Oak Ridge National Laboratory will be using neutron scintillators, developed in conjunction with Applied Scintillation Technologies (AST).
AST has received two major orders for this project. The first is the supply of glass scintillator plates for use on an Anger camera. The scintillators are 158 mm square and 2 mm thick and are used in a 4 x 4 array. Each plate is coupled to a photomultiplier tube to give area position-sensitive neutron detection. The scintillator material has a fast decay time (80-90 ns) and offers excellent discrimination between neutrons and gamma rays. The plates are robust and extremely stable over time.
The second order is for novel neutron detection screens for use in the crossed-fibre neutron detectors for both the VULCAN and POWGEN3 diffractometers. POWGEN3 will be an extremely flexible and versatile general purpose diffractometer for a wide range of structural studies. VULCAN is an engineering diffractometer which will be used for deformation and residual stress related studies. Other uses include spatial mapping of chemistry, microstructure and texture.
The crossed-fibre detector combines the AST neutron detection screen with a network of overlaid, wavelength shifting optical fibres, which are coupled to photomultiplier tubes. The screen itself has a blend of 6Li compounds and phosphors with an increased 6Li loading per unit area, which gives up to 20% increase in neutron capture compared to previous formulations. Each detector module utilizes 32 of these screens, tiled together in a 4 x 8 array.
The screen design enables optimum contact area with the optical fibres. Neutron interaction with the screen produces scintillation at 450 nm. The 450 nm photons are absorbed by the wavelength-shifting fibres, and converted to 520 nm photons, which propagate out of the ends of the fibres. The photomultiplier tubes provide co-incidence counting and position coding to allow 2-D detection of neutron events.