Abstract
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,γ) conversion, therefore neutrons are indirectly detected and the signals produced by scattered neutrons, accordingly the photons, is recorded using gamma scintillators. The use of γ-sensitive detectors make γ-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.
Highlights
The ISIS pulsed Neutron and Muon Source [1] has over 30 neutron and muon instruments distributed in two main buildings, Target Station 1 (TS1) and Target Station 2 (TS2)
By exploiting the gamma-ray line at 478 keV emitted by the decay of the excited 7Li to the ground state in the reaction n + 10B → 7Li* + α, clearly recognizable by its shape, the neighbouring gamma ray corresponding to 511 keV, due to electron-positron annihilation following pair production, and the 1292 keV line of 115In present in the detector contact
Both spectra are normalized to their own acquisition time, and the differences in the peak-to-background ratio are attributable to the logarithmic scale used to emphasize the increment in the baseline on the entire energy range, showing that there is a rigid shift of the background, as expected
Summary
The ISIS pulsed Neutron and Muon Source [1] has over 30 neutron and muon instruments distributed in two main buildings, Target Station 1 (TS1) and Target Station 2 (TS2). A water moderator, decoupled and poisoned, at 295 K serve several beamlines on TS1, providing neutrons with the energy useful for the various fields of application. The “poisoning”of the moderator with gadolinium foils is used to lower the Maxwellian component of the neutron velocity distribution and reduce the intensity of the over-thermalized neutrons in the reflectors system [4]. Several beamlines on TS1 like MAPS, SXD and, in particular VESUVIO, benefit from an increase factor of up to two in thermal neutron flux. The benefits of the extra flux for these instruments include the capability to measure smaller samples (SXD), improve the speed and statistical quality of low energy excitations measurements (MAPS) and improve diffraction and transmission data collection done in parallel with spectroscopy experiments (VESUVIO) [8]
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