Exploring neutrinos

From Kamiokande and Super-Kamiokande to the Hyper-Kamiokande Exploring neutrinos and proton decay

On February 23, 1987, Kamiokande made the colossal achievement of history's first neutrino observation from a supernova explosion. This once-in-a-lifetime chance, triggered by a supernova 160,000 light-years away, was captured by Hamamatsu Photonics' PMTs. Our technology is always evolving, and it is being passed on, first to the current upgraded Super-Kamiokande, next to the Hyper-Kamiokande.

New 20-inch sensor for Hyper-Kamiokande

Keyman Interview

Advancing into the High-Energy Physics Field
Toshikazu Hakamata
Responsible for sales at the time of development, Corporate Adviser of Hamamatsu Photonics

What is a neutrino?

The smallest particles that make up matter are called elementary particles. As far as is currently known, the elementary particles include six quarks, which make up protons and neutrons, and six leptons, which include electrons.

Three of the leptons have no charge and are called neutrinos. Neutrinos are the second-most common thing to fly around space after light. They arise from nuclear fusion reactions at the centers of stars like our sun, from supernovas explosion at the deaths of the heavy stars, from Earth's atmosphere as it is bombarded with cosmic rays, from nuclear reactors and Earth's interior. They can go through any type of matter. At this very moment, they are pouring through our bodies.

The atom and elementary particles

Quarks and leptons

Detecting 11 neutrinos in 13 seconds

In the space of about 13 seconds starting 07:35:35, Feb 23, 1987 (UTC), Kamiokande detected 11 out of the 1058 neutrinos released by a supernova explosion in the Large Magellanic Cloud. These neutrinos were released in a supernova explosion 160,000 years before. Their probability of going through the earth was:

$$\frac{13\ sec.}{5\cdot10^{12}sec.}=2.6\cdot\ 10^{-12}=\frac{2.6}{1\ trillion}$$

Such a remote chance was seized by Kamiokande. Masatoshi Koshiba, Professor of the University of Tokyo, who planned out Kamiokande and directed the experiment, was awarded the Nobel Prize in Physics in 2002 for his accomplishment in observing cosmic neutrinos.

Kamiokande detector, photographed with a fisheye lens from the bottom
(Image courtesy of the Kamioka Observatory, Institute for Cosmic Ray Research, University of Tokyo)

Mass of ultrapure water filling the tank at Super-Kamiokande: 50,000 tons

Super-Kamiokande conceptual diagram
(image courtesy of the Kamioka Observatory, Institute for Cosmic Ray Research, University of Tokyo)

Super-Kamiokande, a project to upgrade the abilities of Kamiokande by 20 times, was built 1000 m underground in the Kamioka mine about 300 m from Kamiokande. Its giant tank, 39.3 m in diameter and 41.4 m in height, holds 50,000 tons of water, about 16 times the capacity of Kamiokande. The goals of the experiment are to clarify everything about neutrinos, by observing solar neutrinos, atmospheric neutrinos, artificial neutrinos, etc., and to illuminate the lifespan of protons.

Number of 20-inch-diameter PMTs at Super-Kamiokande: 11,200

In Super-Kamiokande's water tank, facing inside, there are about 11,200 photosensors called photomultiplier tubes (PMTs). These sensors capture the Cherenkov light emitted by charged particles in the water. From the amount of Cherenkov light and the time light is captured, the energy, direction, etc. of charged particles are determined. The photomultiplier tube used has a photoelectric surface of approximately 50 cm (20 inches) in diameter, the largest in the world.

Related content:
Development of 20-inch PMT

20-inch-diameter PMT

Capacity of the water Cherenkov detector tank at Hyper-Kamiokande: 260,000 tons

Illustration of Hyper-Kamiokande detector

Scematic image of Hyper-Kamiokande detector

It is the aim of Hyper-Kamiokande to elucidate the Grand Unified Theory and the history of the evolution of the Universe through an investigation of proton decay and CP violation (the difference between neutrinos and antineutrinos), together with the observation of neutrinos from supernova explosions. The huge tank of Hyper-Kamiokande will be used in order to obtain an amount of data corresponding to 100 years of data collection time using Super-Kamiokande, in only 10 years. Therefore, this allows the observation of previously unrevealed rare phenomena and small CP violations.

Photosensors in water Cherenkov detectors

Each photosensor in a water Cherenkov detector measures “when” and “how much” light hits the sensor. The capability of a water Cherenkov detector depends largely on the performance of its photosensors. For example, by using photosensors that measure the arrival time of light more precisely, one can estimate the point of neutrino interactions or proton decays in the detector with greater precision. An improvement in the timing performance of photosensors would also provide a better discrimination between signal events and the fake events referred to as backgrounds. Through the use of photosensors that measure the amount of light more precisely, one can estimate the energy of an elementary particle produced by neutrino interactions. For Hyper-Kamiokande, a new 20-inch photomultiplier tube with higher performance for Super-Kamiokande was developed.