The design and performance of QENS, a medium resolution, inverted geometry, tof quasielastic and inelastic spectrometer at IPNS

Abstract

A multidetector, medium resolution, quasielastic and inelastic neutron scattering spectrometer has been designed and built at the Intense Pulsed Neutron Source of Argonne National Laboratory. The main objective in the design is to combine a moderate resolution around zero energy transfer with relatively uniform and high resolution at finite energy transfer up to about 1000 wave numbers. By using the principles of time and energy focusing together with pyrolytic graphite monochrometers and small detectors, we achieve a best resolution at the quasielastic peak of 56 μeV at the final energy of E f = 3.63 meV. The relative energy resolution at finite energy transfer, Δ E/ E, is fairly constant varying between 2% and 4%. The spectrometer has three arms which rotate around the sample covering the angular range 20°–155°. Two of the arms have 13 detectors and the third has 9 detectors. The q range at the elastic position varies from 0.4 to 2.6 Å −1. The incident neutron intensity on the sample area of 1 cm×10 cm is about 10 6 neutrons/s. Results of preliminary measurements on n-butane in the plastic phase, on crystallized Kaolinite, and on tetramethylammonium template cations occluded within zeolites are outlined.