What's a Neutrino? Super-Kamiokande Neutrino Oscillations What's It All Mean?

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The Detector

The Super-Kamiokande detector is a 50,000 ton tank of water, located approximately 1 km underground. The water in the tank acts as both the target for neutrinos, and the detecting medium for the by-products of neutrino interactions.sk_inside_small.jpg (12784 bytes)

The inside surface of the tank is lined with 11,146 50-cm diameter light collectors called "photo-multiplier tubes". In addition to the inner detector, which is used for physics studies, an additional layer of water called the outer detector is also instrumented light sensors to detect any charged particles entering the central volume, and to shield it by absorbing any neutrons produced in the nearby rock.

In addition to the light collectors and water, a forest of electronics, computers, calibration devices, and water purification equipment is installed in or near the detector cavity.  

Cherenkov Light

Above: A view from inside the Super-Kamiokande tank during filling

Below: Illustration of the conical geometry of Cherenkov radiation.

To detect the high-energy particles which result from neutrino interactions, Super-Kamiokande exploits a phenomenon known as Cherenkov radiation.cone.gif (21620 bytes)

Charged particles (and only charged particles) traversing the water with a velocity greater than 75% of the speed of light radiate light in a conical pattern around the direction of the track, as at left. Bluish Cherenkov light is transmitted through the highly-pure water of the tank, and eventually falls on the inner wall of the detector, which is covered with photo-multiplier tubes (PMT's). These PMT's are each sensitive to illumination by a single photon of light - a light level approximately the same as the light visible on Earth from a candle at the distance of the moon!

Each PMT measures the total amount of light reaching it, as well as the time of arrival. These measurements are used to reconstruct energy and starting position, respectively, of any particles passing through the water. Equally important, the array of over 11,000 PMTs samples the projection of the distinctive ring pattern, which can be used to determine the direction of a particle. Finally, the details of the ring pattern - most notably whether it has the sharp edges characteristic of a muon, or the fuzzy, blurred edges characteristic of an electron, can be used to reliably distinguish muon-neutrino and electron-neutrino interactions. 

Neutrino Interactions

Since neutrinos themselves cannot be directly detected, Super-Kamiokande detects the by-products of their interactions inside the water volume of the detector and the nearby rock outside. Two sources of neutrinos are available for our studies.

"Atmospheric" neutrinos are produced when cosmic ray particles from outer space collide with the Earth's atmosphere, producing a spray of secondary particles including electron- and muon-neutrinos. Neutrinos are produced in the atmosphere above Super-Kamiokande, and everyplace else on Earth. Hence neutrinos produced on the opposite side of the Earth actually pass all the way through the Earth, and arrive at the detector from below.

In addition to neutrinos produced in the Earth's atmosphere, the Sun is also a source of neutrinos. These are produced in the complex chain of reactions which generate the Sun's power. These "solar" neutrinos are all of the electron type, and are considerably lower in energy than atmospheric ones. As a result the solar neutrino analysis is inherently more difficult since radioactive decays of materials in and around the detector create charged particles of comparable energy.

Five distinct classes of data are analyzed, classified by whether the neutrinos come from the Sun or the Earth, and in the latter case, whether products of the neutrino interaction enter and/or exit the detector. Click on the links below to find out more about each type of neutrino data:


    Atmospheric Neutrino Interaction Occurs:
    Inside Detector Outside Detector
Exiting Particles? No Fully-Contained Stopping Muon
Yes Partially-Contained Through-going Muon

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Dave Casper