Our principal contribution to Super-Kamiokande at Sheffield is assaying commercial gadolinium samples for radiopurity. Super-K goes to great lengths to minimise the background radioactivity in the detector, and adding 0.2% Gd 2(SO 4) 3 would be counterproductive if we also added uranium and thorium salts as accidental contaminants. proton decay searches-a proton decay event should not contain a neutron, but some neutrino-induced backgrounds do.neutrino/antineutrino separation in quasi-elastic events-neutrino CCQE events, ν + n → ℓ – + p, do not produce neutrons, while antineutrino events, ν̅ + p → ℓ + + n, do.searches for supernova relic neutrinos-the main background, decay of a sub-Cherenkov-threshold "invisible" muon producing an apparently isolated electron, does not produce a neutron, whereas the signal, inverse beta decay (ν̅ e + p → e + + n), does. This has particularly significant implications for: Therefore, adding Gd to the water in Super-K will greatly improve the efficiency with which neutrons produced in neutrino interactions are tagged. Gadolinium has an enormous neutron capture cross section (49700 barns for the natural element, dominated by 157Gd (abundance 15.7%, neutron capture cross-section 259000 barns) and 155Gd (14.8%, 61100 barns)), and neutron capture on Gd produces γ rays which have higher energies, and are thus easier to detect, than the 2.2 MeV γ ray emitted when neutrons are captured on hydrogen.
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