Gadolinium Doped Water Detectors555

Neutron Detection in Water Detectors

One of the big advantages of neutrino detectors made with organic liquid scintillator is that they can detect the capture of neutrons on hydrogen, which produces a 2.2 MeV gamma ray. The enhaced light yield of water Cherenkov detectors (about a factor of 40) makes these visible with detection efficiencies that can approach 100%.

Water Cherenkov detectors have a difficult time with detecting this 2.2 MeV gamma due to the facts that: (1) the light yield is much lower per MeV deposited energy, and (2) the gamma will typically spread the energy over several Compton recoil electrons, some of which may be below the Cherenkov Threshold. For example, Super-Kamiokande has reported an efficiency of about 17% for this capture.

Addition of dissolved gadolinium can solve this problem due to the fact that the resulant gamma cascade has an energy of about 8 MeV instead of 2.2 MeV. With a cross-section of around 40,000 barns (compared to 0.2 barns for hydrogen) only about 0.1% of the water solution need be gadolinium by weight to capture 90% of the thermal neutrons.

While at LSU, Professor Svoboda worked with postdoc Mark Vagins (now a professor the IPMU in Tokyo) to initiate a material compatibility study for detector materials exposed to dissolved gadolinium. At UC Davis  we are continuing these on material compatibility studies for ANNIE and for WATCHMAN.