TARGET 080723 What's so small that it takes 11,000 eyes to see it? A neutrino

3281 feet under the earth in a mine near Kamioka, Japan, a huge tank of ultra-pure water, 138 feet high and 128 feet in diameter, has "eyes" which watch for tiny, ultra dim flashes of light caused by neutrinos hitting the molecules of water. The flashes are so dim that it takes over 11,000 "eyes", or photomultiplier tubes to see them. A neutrino is the smallest atomic particle known to nuclear physics. It is so small, in fact, that it tends to pass through the empty space of atoms and molecules without stopping. It does this so well that it can pass through the entire earth without slowing down or hitting anything. But, now and then, on very rare occasions, one of them will hit something, and when it does, it makes a tiny spark of light. So, nuclear physicists decided to put some light sensors into water so pure that it has perfect clarity, and to put this water in total darkness so far under the earth that no other types of radiation can possibly reach it. Then, if there is a tiny spark of light, they know that they have just witnessed the collision of a neutrino with an atom.

The eyes are not small, by any means. Each is approximately 20 inches across.

The walls, ceiling and floor of the tank - which is the major part of a detector called Super-Kamiokande - are covered at regular intervals by 11,146 light-sensitive phototubes, each about 20 inches in diameter. These pick up the light, which is called "Cherenkov radiation" as it is emitted by the tiny collisions. All construction and adjustments must be made before the tank is loaded with the water. Any intrusion after that could pollute it enough to ruin the whole tank. So, after construction is completed, the place is treated like one of the world's cleanest clean-rooms, and men in clean suits ride highly sanitized rafts, making final checks and adnjstments as the water rises (shown in the following photos).

And, when it is all done, a camera on the bottom looks up through 138 feet of pure water as though it isn't even there.

The Super-Kamiokande detector is located 3281 feet below Mt. Ikenoyama inside the Kamioka Mining and Smelting Company's zinc mine. The detector is reached by driving Toyota Landcruisers about a kilometer through a level drift in Japans fastest mine road (approximately 12.5 mph). (see diagram.)

The detector itself contains 50,000 metric tons (12.5 million gallons) of ultra pure water, some of the purest in the world. It consists of three optically separated and concetric cylinders. From innermost to outermost (see figure above) they are: The inner detector (ID) is 118 ft. high and 115 ft in diameter and is the main sensitive region. It is viewed by 11,146 20 in. inward facing Hamamatsu photomultiplier tubes (PMTs) and surrounds 32,500 tons of water. The dead space is a 20 in. thick cylindrical shell which contains the stainless steel support structure as well as more water. It is optically separated from the ID by opaque black plastic and from the outer shell by black polyethylene bonded to reflective DuPont Tyvek. The outer detector (OD) is a 6.5 ft. to 7.2 ft. thick cylindrical shell which surrounds the entire inner detector. It is viewed by another 1885 8 in wide outward facing PMTs with 60 cm by 60 cm wavelength shifter plates and is lined with reflective Tyvec to increase the number of photons detected.

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