About Kamioka Underground Observatory Purpose of Research System Used for Detection Achievements
About Kamioka Underground Observatory
Construction of Kamioka underground observatory, the predecessor of the present Kamioka Observatory, Institute for Cosmic Ray Research, University of Tokyo began in 1982 and was completed in April, 1983. The purpose of the observatory was to investigate the stability of matter, one of the most fundamental questions of elementary particle physics.
Kamioka Underground Observatory was located 1,000 m underground of Mozumi Mine of the Kamioka Mining and Smelting Co. located in Kamioka-cho, Gifu, Japan. The detector (KAMIOKANDE: Kamioka Nucleon Decay Experiment) was a tank which contained 3,000 tons of pure water and had about 1,000 photomultiplier tubes (PMTs) attached to the inner surface. The size of the tank was 16.0 m in height and 15.6 m in diameter. The PMTs collect the pale blue light called cerenkov light which is emitted by high-velocity charged particles travelling as fast as light in the water.
An upgrade of the detector was started in 1985 to observe elementary particles called neutrino of cosmic origin. As a result, the detector had become hightly sensitive and had succeeded in detecting neutrinos from a supernova explosion which occured in the Large Magellanic Cloud in Febrary 1987. Solar neutrinos were observed in 1988 adding to the advancements in neutrino astronomy and neutrino astrophysics.
Purpose of Research
Study of Cosmic Neutrino
Kamioka Underground Observatory had been researching neutrinos from supernova explosions and solar neutrinos. Research of Neutrinos enable us to probe stellar core which is inaccessible with optical or radio observations and all other regions of the electromagnetic spectrum. Observations of cosmic neutrinos would help determine the intrinsic properties of this elusive particle, neutrino itself. Neutrino observations are of great importance to both astronomy and paricle physics.
Verification of the Grand Unified Theories
Experimental verification of the Grand Unified Theories is another purpose of this experiment at Kamioka.
Among various ways to detect charged particles such as electrons and ??-mesons, we detect blue Cherenkov light emitted from a charged particle propagating in water at a high velocity.
Light sensitive detectors, in particular high sensitive photomultiplier tubes, are arrayed for the purpose of detecting a doughnut-like Cherenkov light ring, as shown in the figure above??. The energy of the charged particle that emits Cherenkov light is determined by the light intensity. The shape of the light pattern and the arrival time of the light give the direction and position of the particle.
The Kamiokande detector employs a large water tank surrounded by photomultiplier tubes on the walls of the detector. The detector is located deep underground to avoid contamination of proton decay and neutrino events by other abundant ground level cosmic-ray events.
a. Detection of supernova neutrinos
On February 23,1987, a supernova explosion occured in the Large Megallanic Cloud which produced the first neutrino detection ever in the world, made by Kamiokande with 11 neutrino events detected. This initiated the dawn of a new era in neutrino astronomy.
This supernova explosion was the result of the collapse of a progenitor star 20 times more massive than our Sun, occurring at the very end of its evolution. An enormous amount of energy had been released in the form of neutrinos in just 20 seconds. The amount is one-thousand times as much energy than the energy emitted by our own Sun in 4.5 billion years! The huge energy release, however, is in agreement and verifies astrophysical predictions of long standing. The duration of the burst of neutrinos is also important, because from it we can infer the neutrino mass is ?? 20 eV.
The Sun keeps on shining via nuclear fusion reactions. During these reactions, neutrinos are emitted which reach the Earth. Only one observation of solar neutrinos has been made so far (except for Kamiokande). Surprisingly, the number of neutrinos obtained by this observation was only 1/4 of a theoretical prediction. This so-called "Solar Neutrino Puzzle" has troubled researchers a long time.
Kamiokande succeeded in the observation of solar neutrinos by analyzing data from the beginning of 1987 to the spring of 1990.?? The observed neutrino flux was 45% of the theoretical prediction. Thus, we confirmed that the neutrino flux was actually less than expected.
The purpose of elementary particle physics is to understand the mechanisms for mass and force. The current theory is that matter is made up of quarks (three quarks compose a proton or a neutron) and leptons (electrons, and neutrinos). We find four forces in nature, gravitational, electromagnetic, weak and strong forces. two of these forces, the electromagnetic and the weak, were recentry?? unified in one theory. The Grand Unified Theories were proposed to unify all but gravity, and some Grand Unified Theories include gravity.
Protons are predicted to decay with a lifetime of 10?? years in the simplest Grand Unified Theories. Kamiokande ruled out this posibility by extending the limit of detectable lifetime to 10?? years.