The KamLAND reactor antineutrino observatory: from imagination to discovery
Glenn Horton-Smith, October 11, 2004

Poem by John Updike: Cosmic Gall The neutrino is the smallest, hardest to detect particle that has ever been proven to exist. Neutrinos are emitted in the decay of radioactive elements and in the decay of some unstable elementary particles. Many neutrinos were also made in the big bang. Neutrinos might make up as much of the mass of the universe as all of the atoms heavier than hydrogen put together, but you don't normally notice them, because once created, neutrinos hardly ever get absorbed again. They hardly interact with matter at all.

By building very large detectors deep underground, scientists have observed neutrinos. By the late 1990's, we had observed neutrinos from the sun, from nuclear power stations, from cosmic rays, and from accelerators. But there was something strange about the number of neutrinos detected from the sun. There weren't enough of them. Something interesting was happening. Either the sun worked very differently than we thought (and by the late 1990's, there were many different kinds of measurements of the sun that said the sun was well understood) or else the neutrinos themselves were changing, "disappearing" by transforming themselves into some kind of neutrino we couldn't detect. This could happen by a process called "neutrino oscillation."

But why did this happen only for solar neutrinos, and not to the neutrinos from reactors? Theorists calculated the properties of the oscillating neutrinos, and found that all of the observations made at reactors were made too close to the reactor. Their calculation is shown at the right. Neutrino scientists wanted to make measurements of the neutrinos from reactors more than 100 kilometers away, but even though the most powerful nuclear power station reactor cores make over 500 trillion neutrinos in every millionth of a second, the neutrinos interact so rarely that no reactor was powerful enough to make enough neutrinos to be seen that far away. It would take almost 100 reactor cores to generate even one neutrino interaction event a day in a 1000 ton detector.

One day, a professor at Tohoku University named Atsuto Suzuki had a great idea: "There are almost one hundred reactor cores in Japan," he thought, "and a 1000 ton detector would fit in the cavern in the Kamioka mine where the first Kamiokande solar neutrino detector was built." He checked, and most the reactors in Japan were about 180 kilometers away. He decided to call the new experiment KamLAND: Kamioka Liquid-scintilaltor Anti-Neutrino Detector. His idea for the detector is shown on the left; a computer-generated visualization of the detector inside the mine is shown below; below that is a map of Japan showing the neutrinos travelling from each reactor to KamLAND.

Professor Suzuki got money from the Japanese government to start building his new neutrino detector.
The old Kamiokande detector was removed and the floor lowered a bit to make room for the new detector.

Then the assembly of the new detector started. American researchers joined the Japanese group, with funding from the U.S. Department of Energy.

After the main tank was finished...

...we began installing photomultiplier tubes, which work like biggest, most sensitive eyes imaginable, inside the detector.

Then, we installed a large balloon, 13 meters in diameter, inside the main tank.
We filled the inside of the balloon with liquid scintillator, brought into the mine by the truck full. Scintillator makes a flash of light when a certain kind of radiation deposits energy in it, including the kind created when a neutrino from a reactor, very rarely, interacts with a proton in the scintillator.

There were a lot of wires to hook up to the readout electronics.

Finally, we began reading out the data.

And when we looked at the number of neutrinos seen compared to the number of neutrinos emitted by the reactors, we saw just the deficit that was predicted. Hooray! We understand the sun and neutrinos too.

But wait: the theorists predict that a different effect should apply to lower energy solar neutrinos. We can't test that with the reactor neutrinos, and solar neutrinos have not yet been observed at that energy. Do we really understand the solar neutrinos? Let's find out...

G. Horton-Smith, 2004/10/11.