Uranium, thorium neutrino research could determine Earth's age, energy production

BY WENDY PITLICK, Black Hills Pioneer
No Source

LEAD - Neutrino detectors at the 4,850-foot level of the Homestake Mine could help scientists understand the total energy produced in the earth's interior as well as the history of the earth.

Nikolai Tolich, of Lawrence Berkeley National Laboratory, along with scientists Yuen-Dat Chan, Charles Currat, Patrick Decowski, Brian Fujikawa, Reyco Henning, Kevin Lesko, Alan Poon, Kazumi Tolich, and Joseph Wang have sent in proposals to construct a 1-kiloton (1,000 tons) neutrino detector that will study electron antineutrinos which come from the uranium and thorium decay within the earth core.

"The energy coming from uranium and thorium decay is thought to be the most significant energy source inside the earth," Tolich said. "So this is the driving engine for things such as tectonic plate movements, volcanoes and earthquake. We are looking for neutrinos, particularly electron antineutrinos ... coming from uranium and thorium decay inside the earth. The uranium and thorium is distributed all through the earth in the mantle. One part in 10 million of the earth's mass is thought to be made of uranium and thorium. This should produce enough neutrinos that we should be able to see hundreds of events per year."

According to Tolich's proposal a significant fraction of the heat dissipation from the earth's interior is believed to originate from uranium and thorium minerals. However, the only estimates of this heat comes from Earth models that are based on meteorites, which have a large margin of error. The detection of electron antineutrinos produced by uranium and thorium decay will allow a more direct measure of the total uranium and thorium content in the earth, thus allowing a more direct measure of the Earth's heat production.

One hundred years ago, Tolich said Lord Kelvin used earth temperature changes to estimate that the planet is approximately 100 million years old. However, Tolich said Kelvin's findings were inaccurate because they failed to consider the uranium and thorium that makes up a significant mass of the earth. Taking the two minerals into consideration, Tolich said scientists now estimate that the earth is actually more than 10 times older than the original estimation.

"So these questions we're asking is really to try and determine the history of the earth," Tolich explained. "(We also) want to understand the total energy in the earth and that will sort of explain why the earth is still causing earthquakes and whether it will cause earthquakes in a billion years." In addition to developing a further understanding of the earth's energy production and age, Tolich said this experiment will also test a hypothesis that there is a naturally occurring nuclear reactor in the center of the planet.

In order to capture minute fractions of the billions of neutrinos that pass through the earth's core undetected, Tolich said he proposes to construct a detector that is mostly comprised of purified oil. The oil includes hydrogen atoms that will interact with the neutrino to create positrons and neutrons. These particles will then produce two flashes of light inside the detector that will be observed with light detecting objects around the detector. The detector's infrastructure, Tolich said, will be surrounded by water that will help reduce background particles that make neutrinos difficult to detect.

While this experiment can be done anywhere that he can construct his detector deep underground, Tolich, who works closely with Homestake Principal Investigator Dr. Kevin Lesko, said the Homestake site would be ideal for his project.

"One of the advantages of Homestake is they already have space in the mine," he said. "Obviously it would be nice to do this experiment as soon as possible. (If another site is selected for the deep underground science and engineering laboratory) it would just be a matter of waiting for a site to become available to put the detector in."

But while the prospect of building his detector at Homestake's interim 4,850-foot level is appealing to Tolich, funding is another reason he is holding out for the National Science Foundation's DUSEL site selection announcement. With an estimated price tag that is tens of millions of dollars for the detector, Tolich said he hopes the NSF will fund his project. That means setting up the experiment wherever the DUSEL site is located, as along as it is deep underground.

But that doesn't mean that Tolich isn't supportive of the DUSEL at Homestake. In fact, the Homestake mine is his preferred choice.

"The reason we have to go underground is because one of the largest backgrounds comes from the neutrons produced by particles coming from the atmosphere," Tolich said. "The flux is very large of these atmospheric particles so the further down you go the lower the flux is. Once you get to the levels of the Homestake proposed lab the flux becomes an insignificant background."

While determining the total energy in the earth's core and the actual age of the planet will satisfy general curiosity that relates to geophysics, Tolich said the practical applications for his experiment will not be immediate. However, the technology that is used to construct his neutrino detector can be used in many other ways around the world. One practical application for the neutrino detector, he said, is as a monitor for nuclear power plants, which are the second biggest source of neutrinos.