Neutronic developments on TOSCA and VESPA: Progress to date

Abstract

A B S T R A C TWe report recent developments regarding the TOSCA (ISIS, UK) and VESPA (ESS, Sweden) neutron broadband chemical spectrometers, both joint ventures between CNR (IT) and ISIS (UK). TOSCA has seen the first major upgrade since it first became operational over fifteen years ago. The new design of the primary spectrometer, which exploits a state-of-the-art, high-m neutron guide and associated chopper system, is boosting the useful neutron flux by over an order of magnitude. Feasibility studies for an upgrade of the secondary spectrometer have been performed, outlining an additional order-of-magnitude gain in performance. In the case of VESPA, the novel characteristics and challenges arising from a long-pulse spallation source such as ESS are part of the drivers of the instrument design. For both the primary and secondary spectrometers, a detailed analysis of expected performance, supported by both simulations and analytical models, is being carried out, also capitalizing from experience on TOSCA. Indeed, for instrument design and optimization, extensive neutron-transport simulations and baseline studies of neutronic response have become a must, along with extensive benchmarking against much-needed experimental data. All these combined efforts represent the first opportunity to benchmark a broadband, high-resolution chemical spectrometer in terms of measured vs. simulated response. High-resolution broadband chemical spectrometersInelastic Neutron Scattering (INS) as a vibrational spectroscopy technique [1] is widely used across chemistry, materials science, biology, and beyond. The workhorses for this kind of studies can be called "High-resolution Broadband Chemical Spectrometers". Here, we focus on time-of-flight instruments at pulsed spallation sources in socalled indirect geometry, and whereby final-energy selection is attained with a crystal after scattering by the sample. With them, good-quality spectra (i.e. high count rate and resolution) can be obtained with reasonably small quantities of specimen and easy operation. This type of instruments is generally divided into a Primary Spectrometer (PS), dedicated to neutron delivery from a moderator to the sample, and a Secondary Spectrometer (SS) which processes neutrons scattered from the sample and delivers them to the detectors.In the PS the polychromatic beam, coming from the moderator, is generally worked on by a chopper system (i.e. shaping or cleaning it) and transported onto the sample by a neutron delivery device (e.g. an evacuated tube or a super-mirror guide). The SS is arranged in such a way that the final energy of neutrons that can be detected after scattering by the sample is fixed. This is achieved via the use of an analyser crystal, typically HOPG (Highly Oriented Pyrolytic Graphite), which selects the neutrons through Bragg scattering. In order to eliminate higher-order crystal reflections, a low-pass energy filter is used. The filter is typically made with a thick block of beryllium, selecting the bandwidth by exploiting the Bragg cut...

A combined deep inelastic neutron scattering and ab initio lattice dynamics study of the hydride anion dynamics and bonding in La2LiHO3 oxyhydride

“…The experimental high-resolution vibrational neutron spectroscopy, INS, was carried out at the ISIS Pulsed Neutron and Muon source on the inverted-geometry TOSCA instrument at ≈10 K in a closed-cycle helium refrigerator [18][19][20][21]. A 5 g powder sample was measured in a flat aluminium cell with 4×4.8 cm 2 cross-section area.…”

Crystal Analyzers for Indirect-Geometry Broadband Neutron Spectrometers: Adding Reality to Idealized Design

“…The contributions to the resolution can be described with analytical formulae [10,11] or estimated independently via Monte Carlo simulations, and then summed in quadrature as in Eq.(1). In particular, in our analyses, we used an exact analytic estimation of the primary flight path terms and simulated the ones due to the analysers [8], with the McStas [12] package. This approach gives results which are in very good agreement with the ones obtained with a full simulation of a "δ-sample", as described previously [8].…”

“…In principle, the adoption of a curved analyser does not guarantee time focusing, however, this may be obtained to a certain extent (i.e. limiting the uncertainty to a certain threshold) by tuning the position and the orientation of each of the HOPG tiles [8]. Energy and, most importantly, time focussing need to be provided by the analysers.…”

“…Here, we provide insights into the design of this type of analyser, starting from the most ideal conditions and then evaluating the effects of the uncertainties in their physical realization. These analyses have been performed as part of the design of two instruments: the VESPA [8] spectrometer, jointly developed by the Italian "Consiglio Nazionale delle Ricerche" (CNR) and the "ISIS Neutron and Muon Source" (UK) for the "European Spallation Source" (Sweden); and the upgrade of the TOSCA spectrometer (ISIS, UK). In particular, the results here reported are based on the VESPA layout [8,9], in its High-Resolution mode, but the same conclusions are applicable to TOSCA.…”

Differentiating the Role of Organic Additives to Assemble Open Framework Aluminosilicate Zeolites Using INS Spectroscopy

“…We have employed inelastic neutron scattering (INS) spectroscopy 32,33 in order to probe the dynamics of the occluded 18C6 species and hence how the organic-framework interactions differ between the zeolites. This was achieved by comparison of the spectra of 18C6 in addition to the 18C6 containing (filled) zeolites and the calcined analogues where the additive has been removed (empty).…”