Gamma background characterization on VESUVIO: Before and after the moderator upgrade

The VESUVIO spectrometer at the ISIS pulsed neutron and muon source is a unique instrument which makes use of eV neutrons and inverted geometry, allowing deep inelastic neutron scattering experiments with high values of energy and wavevector transfers. The neutron detection techniques on the VESUVIO forward-scattering detector banks is based on (n,gamma) conversion, therefore neutrons are indirectly detected and the signals produced by scattered neutrons, accordingly the photons, is recorded using gamma scintillators. The use of gamma-sensitive detectors make gamma-background one of the main limiting factors affecting the data quality and instrument sensitivity on VESUVIO. This work aims to assess how the sample-independent gamma background has changed after the recent upgrades to the water moderator viewed by the instrument, which resulted in a twofold increase of the thermal neutron flux. Here we show that the gamma background is mainly influenced by the thermal neutron flux and that the recent upgrade results in a fivefold increase in the gamma background in the photon energy range 300 keV-3 MeV. We point out the possibility of providing a thermal-neutron filter along the incident beam in order to suppress this background source.