VESUVIO was initially designed only to measure nuclear momentum distributions (NMDs) for the lightest elements (H, He) in materials using epithermal neutrons. The spectrometer, in its initial configuration, used ⁶Li-doped neutron detectors both at forward and backscattering angles [4]. In February 2008, the ⁶Li-doped neutron forward scattering detectors were replaced by yttrium aluminium perovskite (YAP)-doped gamma-ray detectors [5-8], resulting in the instrument setup depicted in Figure 1. Although the YAP detectors do not detect neutrons directly, they are made to function both as neutron detectors and energy selectors by placing a gold foil on the YAP detector surface [7].

Prior to installing the YAP detectors, energy analysis was done on VESUVIO at forward scattering angles using the 'filter difference' (FD) method [9]. As in the 'foil cycling' (FC) technique currently employed with the YAP detectors, two measurements were taken: one with a gold foil placed between the sample and detectors (foil-in) and one with the foil removed (foil-out). The YAP detectors with the FC technique give a number of clear advantages over the 6Li detectors with the FD technique [10]. With the YAP detectors, count rates are reduced to ∼10⁴ s⁻¹, and saturation effects are not significant. The much larger difference between the foil-out and foil-in counts gives much greater stability. Small drifts in detector efficiency with time (due, for example, to temperature changes) are much less significant with the FC technique. The better resolution and stability of the YAP detectors allows more accurate parametric measurements of momentum distributions. Perhaps just as important is that a much better separation between proton peaks and the peaks from heavier atoms can be achieved. 

Uniquely, on VESUVIO, neutrons of all energies reach the sample and can be scattered or adsorbed. Namely, VESUVIO, unlike many other direct or indirect neutron spectrometers, employs a noninvasive final-neutron-energy filter. This means that scattered neutrons of all energies and other particles, such as gamma rays, are produced within the sample and then reach the detectors and can be counted. This is in opposition to direct geometry instruments, where incident neutron energies are selected using choppers, and inverted geometry instruments,  where intermediate reflections select the scattered neutrons on the filter. In these cases, the secondary path is not a straight line connecting the sample and the detector, with the latter actually shielded against direct radiation from the sample. This unique VESUVIO detection setup makes it possible to concurrently perform NCS, neutron diffraction (ND), neutron transmission (NT), and gamma spectroscopy based on resonant neutron absorption followed by gamma emission [through techniques such as Neutron Resonance Capture Analysis (NRCA) and Prompt Gamma Activation Analysis (PGAA)][3, 12].

[1]          Andreani C, Colognesi D, Mayers J, Reiter G and Senesi R 2005 Measurement of momentum distribution of light atoms and molecules in condensed matter systems using inelastic neutron scattering Advances in Physics 54 377-469

[2]          Andreani C, Krzystyniak M, Romanelli G, Senesi R and Fernandez-Alonso F 2017 Electron-volt neutron spectroscopy: beyond fundamental systems Advances in Physics 66 1-73

[3]          Andreani C, Senesi R, Krzystyniak M, Romanelli G and Fernandez-Alonso F 2017 Experimental Methods in the Physical Sciences, ed F Fernandez-Alonso and D L Price: Academic Press) pp 403-57

[4]          Senesi R, Andreani C, Bowden Z, Colognesi D, Degiorgi E, Fielding A L, Mayers J, Nardone M, Norris J, Praitano M, Rhodes N J, Stirling W G, Tomkinson J and Uden C 2000 VESUVIO: a novel instrument for performing spectroscopic studies in condensed matter with eV neutrons at the ISIS facility Physica B 276 200-1

[5]          Andreani C, Pietropaolo A, Senesi R, Gorini G, Perelli-Cippo E, Tardocchi M, Rhodes N and Schooneveld E M 2004 A resonant detector for high-energy inelastic neutron scattering experiments Appl Phys Lett 85 5454-6

[6]          Perelli Cippo E, Gorini G, Tardocchi M, Andreani C, Pietropaolo A, Senesi R, Rhodes N J and Schoonveld E M 2008 Advances on detectors for low-angle scattering of epithermal neutrons Measurement Science and Technology 19 047001

[7]          Schooneveld E M, Mayers J, Rhodes N J, Pietropaolo A, Andreani C, Senesi R, Gorini G, Perelli-Cippo E and Tardocchi M 2006 Foil cycling technique for the VESUVIO spectrometer operating in the resonance detector configuration Review of Scientific Instruments 77 095103

[8]          Tardocchi M, Gorini G, Pietropaolo A, Andreani C, Senesi R, Rhodes N and Schooneveld E M 2004 YAP scintillators for resonant detection of epithermal neutrons at pulsed neutron sources Review of Scientific Instruments 75 4880-90

[9]          Seeger P A, Taylor A D and Brugger R M 1985 Double-difference method to improve the resolution of an eV neutron spectrometer Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 240 98-114

[10]        Mayers J 2011 Calculation of background effects on the VESUVIO eV neutron spectrometer Measurement Science and Technology 22 015903

[11]        Mayers J and Reiter G 2012 The VESUVIO electron volt neutron spectrometer Measurement Science and Technology 23

[12]        Romanelli G, Krzystyniak M, Senesi R, Raspino D, Boxall J, Pooley D, Moorby S, Schooneveld E, Rhodes N and Andreani C 2017 Characterisation of the incident beam and current diffraction capabilities on the VESUVIO spectrometer Measurement Science and Technology 28 095501​