Japan’s next neutrino detector to have huge tanks, bright beam

Construction on Hyper-Kamiokande, the largest-ever water Cherenkov neutrino detector, is to begin in April 2020, the University of Tokyo announced on 19 September.

The power of the new underground detector will rest largely on its size: With an order of magnitude larger volume than that of its predecessor (Super-Kamiokande), sheer statistics dictate that there will be more detections of neutrino-related events. Hyper-Kamiokande will also gain in sensitivity by using state-of-the-art photo sensors. The tremendously successful Super-Kamiokande is particularly known for its observation of neutrino oscillations (see Physics Today, August 1998, page 17 and December 2015, page 16 ).

With Hyper-Kamiokande, physicists plan to search for proton decay, measure neutrino CP violation, search for clues about dark matter, determine the neutrino mass hierarchy, and more. The physics goals and timetable are similar to those of the US-led Deep Underground Neutrino Experiment, a liquid-argon detector under construction in South Dakota.

Hyper-Kamiokande will cover a wide energy range, says project leader Masato Shiozawa of the University of Tokyo, “from a few MeV to observe solar neutrinos and supernova neutrinos, to 1 GeV for proton decays, and up to TeV for atmospheric neutrinos.” High-intensity neutrinos and antineutrinos will be beamed to Hyper-Kamiokande from the accelerator nearly 300 km away at the Japan Proton Accelerator Research Complex. An upgrade will nearly triple the J-PARC beam’s current neutrino intensity.

In the case of proton decay, Hyper-Kamiokande will surpass in two years the data collected over two decades by Super-Kamiokande. “If we see proton decay, it would signify that the strong force is unified with the weak and electromagnetic forces,” says Boston University’s Ed Kearns, a member of the Hyper-Kamiokande steering committee. If the experiment does spot proton decay, the next step would be to measure the different decay modes to learn about how the forces are unified, he says. “It’s a beautiful goal, a huge goal.”

The Hyper-Kamiokande design consists of two cylindrical tanks, each 60 m in diameter and 74 m tall. The tanks will be blanketed with 40 000 photo sensors each. The experiment requires digging a new access tunnel and cavern in the Kamioka mine; the experiment will be located about 8 km south of Super-Kamiokande.

The cavern size “is pushing the limits of the size of an underground cavern, at least with the rock structure” at the site, notes Kearns, which is why the full-volume experiment is divided into two tanks. Another technical challenge is to make sure the photo sensors can withstand the greater pressure due to the increased depth of the tanks. “There must be no chance of a chain reaction if one implodes,” says Kearns, referencing a 2001 accident that destroyed half of Super-Kamiokande’s photo sensors (see Physics Today, January 2002, page 22 ).

Meanwhile, Super-Kamiokande is undergoing an upgrade to boost its game in observing supernovae. Gadolinium sulfate is being added to the tank to make the detector sensitive to neutrons, which are emitted along with neutrinos in supernova explosions; the neutrons can be detected when excited Gd nuclei emit gamma rays upon decay.

The Japanese Ministry of Education, Culture, Sports, Science, and Technology included ¥50 million ($440 000) in seed funding for Hyper-Kamiokande in its 2019 budget. In Japan that pledge is seen as a signal that the project will go ahead. The total cost for excavation and the first tank is estimated at ¥55 billion.

The Hyper-Kamiokande collaboration has researchers from 75 institutions in 15 countries. Contributions from outside Japan have yet to be worked out. Data collection is to start in 2027.