Super-Kamiokande to Be Upgraded

The Guardian has reported that some upgrades are coming for Super-K, the water Cherenkov neutrino detector in the Kamioka mine in the mountains of Japan. In particular, a small amount of gadolinium will be added to the water. Gadolinium is notable in particle and nuclear physics for its huge thermal neutron capture cross section (Gd-157 has a capture cross section of 2.5×105 barns for thermal neutrons according to ENDF tabulated data). Adding gadolinium to a detector allows it to tag neutron captures on gadolinium using gamma rays emitted in the capture & de-excitation process.

In antineutrino charged current interactions on nuclei, one of the signals of interest to Super-K measurements, protons are converted into neutrons while the antineutrino is converted to a charged lepton. If only the lepton is seen, then Super-K will typically not be able to distinguish neutrinos from antineutrinos since it can’t reconstruct the sign of a charged particle. Neutron tagging with gadolinium will allow Super-K to select an antineutrino-enhanced sample of events. Neutrons can still be found in neutrino events so a¬†pure sample is probably not really possible, but I also haven’t read all the papers to see exactly how much this will enhance the analysis.