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...
