Visiting the Japanese Super-Kamiokande detector (part 1)
The Neutrino Trap made of Pure Water
Hardly any elementary particle occurs more frequently in the universe than the elusive neutrino. The investigation of the almost massless tiny particle is a focus of current elementary particle physics. Perhaps the most important contribution to the understanding of neutrino has been made over twenty years by the Japanese Super-Kamiokande detector, in which several Swiss research groups are involved. A visit to the Japanese mountains.
The village of Kamioka lies about 250 km northwest of Tokyo, embedded in the mountains of the northern Japanese Alps. Zinc and other ores were mined in the area until a few years ago. Although mining has meanwhile stopped, the tunnels are still very busy. To learn more, we board the bus in Kamioka and follow a winding road. In the valley bottom the Takahara river foams, the mountain flanks are covered by dense forest. After half an hour, the bus driver, wearing a cap and white gloves as usual in Japan, stops at the Mozumi bus stop. Next to a small temple there is one of the most renowned research laboratories in particle physics: the Kamioka Observatory.
A Life for Neutrino Research
On the stairs to the entrance a man with friendly eyes waits for the visitor. In the entrance we exchange our street shoes for green slippers. It is only a few steps to the spacious office of Masayuki Nakahata. Nakahata is director of the Kamioka Observatory and professor at the 'Institute for Cosmic Ray Research' of the University of Tokyo. The 60-year-old scientist has every reason to be proud of his work. He has been connected to the Kamioka Observatory since a neutrino detector was built here in the mid-1980s. His biography is closely linked to the scientific success of this research facility.
The first of these successes was seen on February 23, 1987. The detector, which had just been built, was used to measure neutrinos caused by the explosion of a star (supernova) 160,000 light years away. No one before had detected such 'cosmic' neutrinos - a scientific sensation. "Since then, our detector is known worldwide," says Nakahata, at the time mainly responsible for the data analysis of the observation that earned the Nobel Prize in Physics to the physicist in charge, Masatoshi Koshiba, in 2002.
Swiss research in Japan
This was only the first of a series of scientific highlights that the Kamioka Observatory was to be able to offer in the next few years: neutrinos originating in the sun were observed here for the first time in 1989. In 1998, the Japanese physicist Takaaki Kajita succeeded in proving that the three types of neutrinos can transform into each other ('oscillation') with 'atmospheric' neutrinos created by the impact of cosmic radiation in the Earth's atmosphere. For this he was awarded the Nobel Prize in 2015. Since then it has been clear that neutrinos have a mass (which, however, is hardly more than one millionth of the mass of an electron), although all is not yet known today about each of the three neutrino masses.
In 2009, the T2K experiment went into operation: neutrinos generated at Japan's south coast are shot 295 km across Japan to Kamioka and captured with the detector. With this experiment, in which researchers from the Universities of Bern and Geneva as well as ETH Zurich are also involved, the oscillation from muon to electron neutrinos could be shown for the first time in 2011, which means that the last of the three neutrino oscillations known today was experimentally proven.
A very special water tank
Masayuki Nakahata leads his visitor from his office to the parking lot. After a ten minute drive we arrive at the south flank of the 1369 meter high Ikenoyama mountain. A 1.7 km long tunnel leads into the mountain. Equipped with helmet and crocks we enter a rock cavern. The temperature here is 13 degrees Celsius. Above our heads is a 1000 meter thick rock layer, shielding this place from cosmic rays by a factor of 1/100’000. "We are now above the tank," says particle physicist Nakahata. The "tank" of which the researcher speaks is the Super-Kamiokanda detector. Super-KamiokaNDE' stands for 'Super-Kamioka Neutrino Detection Experiment'. It is the experiment that has had a decisive influence on Japanese neutrino research in the more than 20 years since its commissioning in 1996.
The detector is able to detect a – very small ! - part of the neutrinos that pass by here. A 41-meter-high steel cylinder with a diameter of 39 meters and filled with 50 million liters of water serves this purpose. If a neutrino hits the nucleus or electron of a hydrogen or oxygen atom of a H2O water molecule, an electron or muon is "knocked out" of the atom. When this electron or muon is fast enough – which must be faster than the speed of light in water! – weak blue light is emitted (Cherenkov radiation), which is recorded by a total of 13,000 photosensors within the water tank. Professor Nakahata takes the visitor to the adjacent control room. On one screen, the current measurements of the photosensors are represented by coloured dots. From the pattern of the pixels the physicists can deduce what kind of particles they observed and from which direction they came: Muon neutrinos have a clear ring shaped contour, whereas electron neutrinos have a fuzzy contour. Neutrinos of solar origin have an energy 100 times lower than atmospheric neutrinos; recognizable by a small radius of the ring shaped contour.
Fascinating images for the public
These are fascinating patterns which are conjured up by the neutrino events on the screen of the Super-Kamiokande. Images that tell of elementary particles that are otherwise inaccessible to human sensory perception. It was therefore a great idea to make these images accessible to people outside the research community. In March 2019, a small but fine science museum was opened in Kamioka, 15 km away. The images of this otherwise hidden world are transmitted there in real time. In addition, the museum uses interactive exhibits to tell the story of the elusive neutrinos. In the first six months, 100,000 visitors have been infected by the fascination of neutrino research.
Author: Benedikt Vogel
Information about the neutrino detector Super-Kamiokande: http://www-sk.icrr.u-tokyo.ac.jp/sk/index-e.html
Information about the T2K experiment: https://t2k-experiment.org