The performance of the Super-Kamiokande neutrino detector in Japan could be greatly improved by dissolving 100, 000 kilograms of gadolinium trichloride, a rare-earth metal salt, in the massive tank of ultrapure water at the heart of the experiment. John Beacom of Ohio State University and Mark Vagins of the University of California at Irvine say that the gadolinium trichloride (GdCl3) would allow physicists to detect neutrinos from outside our galaxy for the first time (Phys. Rev. Lett. 93 171101).
Neutrinos are one of the fundamental constituents of matter, and come in three flavours — electron, muon and tau neutrinos. Large numbers of electron neutrinos are produced by the Sun, and all three flavours are produced in supernova explosions. However, all three types of neutrino are extremely difficult to detect because they are electrically neutral, have very little mass and only interact with other matter through the weak interaction.
The Super-Kamiokande experiment consists of 50, 000 tons of ultrapure water in a tank some 1000 metres below ground in central Japan. It can detect both electron neutrinos and muon neutrinos — but not tau neutrinos — from the flashes of Cerenkov radiation that are emitted when the neutrinos interact with electrons in the water molecules in the detector. Using the new method, Super-K will also be able to identify antineutrinos — the antiparticle equivalents of neutrinos — for the first time.
Electron antineutrinos currently interact with protons in the water molecules to produce a neutron and a positron for each interaction. However, the neutrons cannot be seen at present and the positrons cannot be distinguished from the background radiation of electrons and gamma rays.
Beacom and Vagins have proposed that these problems can be overcome by adding just 0.2% GdCl3 by mass — which is roughly 100 tons — to the water in the detector. This is because gadolinium is much more efficient at capturing neutrons than free protons. Indeed, gadolinium has a cross-section of 49,000 barns for thermal neutron capture, compared with just 0.3 barns on protons.
“Existing experiments can only detect supernova neutrinos produced within our own galactic neighborhood,” Beacom and Vagins told PhysicsWeb. “We have identified a method that will extend the reach of these experiments to about half the known universe. Instead of waiting for years or decades for a nearby explosion, we will record a continual stream of supernova neutrinos from distant galaxies.” The GdCl3 method would also make Super-Kamiokande 50 times more sensitive to the antineutrinos from nuclear reactors than the dedicated KamLAND detector that is located in the same mine.
Beacom and Vagins are currently performing R&D on handling and filtration techniques for the GdCl3 at Irvine, and hope to apply these techniques to a 1000 ton detector in Japan next year, and to the Super-Kamiokande detector itself in the summer of 2006.
Last year physicists at the Sudbury Neutrino Observatory (SNO) in Canada reported that they had increased the sensitivity of their detector by factor of three by adding 2000 kilograms of ultrapure table salt (sodium chloride). By using heavy water rather than ordinary water SNO is able to detect all three flavours of neutrino. However, it is much smaller than Super-Kamiokande, which means that it cannot detect as many events.
