MACS—a new high intensity cold neutron spectrometer at NIST

Abstract

We describe a novel cold neutron spectrometer under development at NIST optimized for wave vector resolved spectroscopy with incident energies between 2.1 meV and 20 meV and energy resolution from 0.05 meV (Ei = 2.1 meV) to 3.0 meV (Ei = 20 meV). By using a 1428 cm2 double focusing PG (0 0 2) monochromator close to the National Institute of Standards and Technology (NIST) cold neutron source the instrument provides up to 5 × 108 neutrons cm−2 s−1 on a 8 cm2 sample area. The measured performance is consistent with Monte Carlo simulations. The monochromating system, which includes radial collimators, three filters and a variable beam aperture, offers considerable flexibility in optimizing Q-resolution, energy resolution and intensity. The detector system will consist of an array of 20 channels which combined will subtend a solid angle of 0.2 sr. This is approximately a factor of 40 more than a conventional triple axis spectrometer. Each detector channel contains a vertically focusing double crystal analyzer system (DXAL) actuated by a single stepping motor. We find identical integrated reflectivity at approximately 10% coarser energy resolution for the 130′ mosaic double bounce analyzer as compared to a conventional 25′ analyzer at the same energy. The vertical focusing of the DXAL allows for smaller detectors for enhanced signal to noise with 8° vertical acceptance. Options for post sample collimators and filters provide flexibility in the choice of scattered beam energy and wavevector resolution.