Opening the terahertz window on the OSIRIS spectrometer

Abstract

Abstract.A cooled and mechanically retractable beryllium filter has been installed and commissioned on the low-energy OSIRIS spectrometer at ISIS. This instrument development extends the energy-transfer range of the spectrometer up to ca. 20 meV (∼ 5 THz), leading to an excellent resolution at THz frequencies and substantial gains in detected flux relative to existing capabilities on the neighbouring IRIS spectrometer. Herein, we provide a concise account of this new capability for highresolution neutron spectroscopy in the THz domain, as well as outline a number of ongoing and potential scientific opportunities in condensed-matter physics, chemistry, and materials science. The reasons whyOSIRIS is an indirect-geometry neutron spectrometer with long-wavelength diffraction capabilities [1]. The secondary spectrometer sits at ca. 34 m from a 25 K liquid-hydrogen (H 2 ) moderator on ISIS Target Station I. Optimised for low-energy, high-resolution inelastic neutron-scattering studies, OSIRIS uses a broad band of incident wavelengths which are Bragg-scattered from a crystal-analyser array following interaction with the sample under investigation. Owing to the pulsed nature of the source, time-of-flight methods are used to determine energy transfer at a particular scattering angle.As the last of first-generation instruments built at ISIS, OSIRIS has been operational since 1997. A schematic diagram of the instrument is shown in Fig. 1. In the early days, its primary use was as a long-dspacing diffractometer, exploiting its dedicated, 960-pixel backscattering detector bank with a resolution as high as d/d = 2.5 × 10 −3 and a dynamic range extending up to 20Å. In 2003, the instrument was upgraded with a cooled pyrolytic graphite (PG) analyser bank, thereby paving the way for low-energy inelastic and quasielastic neutron-scattering studies [3]. Its primary spectrometer profited from early advances in neutronguide technology in the 1990s, with an m = 2 guide extending across the majority of its primary length, and terminated by an m = 3.5 focusing section immediately prior to the sample. In spectroscopic mode, OSIRIS uses PG crystals cooled to 10 K so as to define two primary final energies using PG002 (E f = 1.84 meV) and PG004 (E f = 7.38 meV) Bragg reflections. The spectral resolution a Corresponding author: F. Demmel (franz.demmel@stfc. ac.uk) b and F. Fernandwz-Alonso(felix.fernandez-alonso@stfc. ac.uk) of these two alternative settings amounts to 25 and 99 µeV for PG002 and PG004, respectively. Recent Monte Carlo simulations of the instrument have also provided important insights into its neutronic performance, including the various contributions to the resolution function and line shape [2]. As on similar neutron spectrometers like IRIS at ISIS [4] or, more recently, BASIS at SNS [5] or DNA at JPARC [6], a tight spectral resolution in the µeV domain comes as a result of several factors, including: a relatively long flight path (>30 m) at a short-pulse spallation source; tight incident neutron pulses from a cold neut...

Electron-volt neutron spectroscopy: beyond fundamental systems

“…Such capabilities have already been used extensively on the IRIS spectrometer [197] to study H 2 uptake in the same GIC [193]. Other neutron spectrometers where a similar experimental approach is also available include OSIRIS [198][199][200] and TOSCA [201,202], also located at the ISIS Facility.…”

Sodium ion self-diffusion in molten NaBr probed over different length scales

“…With an end energy of E f = 1.845 meV the energy resolution deduced from a vanadium measurement was FWHM = 0.025 meV, and the covered wave vector range is 0.25 Å −1 < Q < 1.8 Å −1 . In contrast to the previous measurement [33] a beryllium filter was installed this time on the spectrometer [47] to remove potential contaminations from second-order reflections of the pyrolytic graphite analyser. About 36 hr of beam time were used for the sample and 20 hr for the empty cell measurement to obtain data of high statistical quality [48].…”

A silicon analyser for the OSIRIS spectrometer: An analytical and Monte Carlo simulation study

“…Calculating the integral under the spectral curves we obtain a factor 1.8 higher intensity for the SA compared to the PG analyser, which is due to the increased solid angle. That factor does not include the losses due to the transmission of the Beryllium filter on the PG side, about 60% [23]. Hence, it can be expected that, considering the same sample size, the SA setup will outperform the PG analyser not only in the view point of resolution but also intensity.…”

“…The cold TOF spectrometer OSIRIS has been described in detail in references [9,10,21,22] and hence we will only provide a short overview. OSIRIS has been recently upgraded with a moveable Beryllium filter to remove the second order reflection of the pyrolytic graphite analyser [23], and its energy resolution has been modelled with Monte Carlo simulations [24]. It is mainly used to study low-energy excitations, e.g.…”