Kamioka ObservatoryICRR, The University of Tokyo

In July and August of 2020, Super-Kamiokande [SK] started a new phase of operations by introducing the rare earth element gadolinium [Gd] into the 50,000 tons of ultrapure water in the SK tank, with the aim of increasing SK’s sensitivity to make the world’s first observation of “supernova relic neutrinos”.  These are the neutrinos emitted since the onset of stellar formation by ancient supernova explosions, and they produce neutrons when they interact inside SK.  These neutrons are then made visible by the de-excitation light generated whenever one is captured by a gadolinium nucleus, announcing the presence of a relic neutrino.

Two years ago, gadolinium was loaded into SK until a concentration of 0.01% by mass of Gd³⁺ ions had been achieved, and data have been acquired continuously since then. After the introduction of the Gd the transparency of the water in the tank was unchanged, maintaining its pre-loading attenuation length of nearly 90 meters, and data have been collected as expected.

On June 1, 2022, the second Gd loading process began.

The latest loading is scheduled to dissolve 26,000 kilograms of gadolinium sulfate octahydrate, twice the amount used in 2020. The gadolinium sulfate to be used this time has been refined to an even higher purity and is more homogeneous than the previous batch. Furthermore, the dissolution equipment has been improved, making it possible to double the loading rate, and so this larger batch is expected to be completely dissolved in approximately 1 month, just as the previous 13,200 kg were in 2020.

With a 0.01% concentration of gadolinium by mass, the efficiency of neutron capture on gadolinium was 50%, but with the Gd concentration raised to 0.03% the neutron capture efficiency will reach 75%. This means that it will be possible to collect supernova relic neutrinos 1.5 times more quickly than before.

Link

• August 21, 2020
  Introduction of Gadolinium into Super-Kamiokande and the Start of New Observations
• August 21, 2020
  [Details] Introduction of Gadolinium into Super-Kamiokande and the Start of New Observations