Super-Kamiokande at Sheffield
Super-Kamiokande is the far detector for T2K, but it also has a long and distinguished history as a stand-alone detector of atmospheric and solar neutrinos
Members of T2K only have access to Super-K data from the times corresponding to the expected arrival at Super-K of neutrinos from the T2K beam, and therefore do not share in the rich non-accelerator physics of Super-K.
For this reason, a small group of UK physicists from Imperial College, Liverpool, Oxford, QMUL and Sheffield have recently joined Super-K. This is particularly important in preparation for Hyper-Kamiokande, which will have a similar non-accelerator physics programme in addition to its role as a long-baseline neutrino oscillation experiment.
Super-Kamiokande is a water Cherenkov experiment consisting of 50 kilotons total (22.5 kilotons fiducial) of water located about 1 km underground in the Mozumi mine in western Japan.
The water tank is 39 m in diameter and 42 m high, and is instrumented by 11129 20" photomultiplier tubes in the inner detector (as shown on the right), with a further 1885 8" PMTs instrumenting the outer detector (a "skin" outside the main tank, designed to veto incoming charged particles).
The Super-Kamiokande physics programme includes:
- neutrino oscillations using accelerator neutrinos (acting as the T2K far detector);
- neutrino oscillation measurements using atmospheric neutrinos;
- solar neutrinos;
- supernova neutrinos (in the event of a core-collapse supernova in our Galaxy or one of its satellites);
- searches for supernova relic neutrinos;
- searches for signatures of proton decay;
- indirect dark matter searches using neutrinos from neutralino annihilation in the Sun or the Galactic centre.
An exciting recent development is the decision, in June 2015, to approve the addition of 0.2% gadolinium sulphate to the water in Super-K.
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.
Therefore, adding Gd to the water in Super-K will greatly improve the efficiency with which neutrons produced in neutrino interactions are tagged.
This has particularly significant implications for:
- 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 (ν̅e + p → e+ + n), does;
- 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;
- proton decay searches—a proton decay event should not contain a neutron, but some neutrino-induced backgrounds do.
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% Gd2(SO4)3 would be counterproductive if we also added uranium and thorium salts as accidental contaminants.
Sheffield's access to the Boulby Underground Laboratory provides a site where radiopurity assays can be carried out in very low background conditions.
Boulby Mine is a working salt and potash mine located on the north-east coast of the North York Moors. It is 1.1 km deep, or 2805 mwe (metres of water equivalent—the standard method of describing depth in particle physics contexts, as it corrects for the fact that the overburden of different mines, road tunnels, etc., will have different mean densities).
As a result of the shielding provided by this, the cosmic ray flux in the mine is reduced by about 5 orders of magnitude compared to a detector on the surface. In addition, rock salt has inherently low levels of background radioactivity, leading to a very low overall background for radiation measurements. Boulby is also the proposed location for the WATCHMAN experiment.
The Boulby Underground Germanium Suite (BUGS) contains four HPGe detectors, which are capable of measuring γ-ray spectral lines with very good energy resolution, allowing one to identify the radioactive decay responsible and measure the activity.
Using BUGS, radioactivity from the uranium and thorium decay chains can be detected down to levels of ~100 parts per trillion (i.e. 10–10).
In addition to the Gd assay work, Sheffield's contribution to the development of Hyper-Kamiokande calibration systems includes a plan to deploy prototype Hyper-K calibration instrumentation in Super-Kamiokande. Although this is, of course, primarily R&D for Hyper-K, the systems will be operated in Super-K alongside the existing Super-K calibration systems.