Scientists say developing a more precise understanding of the nuclear reactions in stars that creates some of earth's most basic elements is the central idea behind one of the many astrophysics experiments proposed for the Sanford Underground Science and Engineering Laboratory.
Paul Vetter, a staff scientist at the Lawrence Berkeley National Lab, is part of a collaboration of astrophysicists who are proposing constructing a particle accelerator at the 4,850-foot level of the former Homestake Mine. The accelerator would simulate the conditions in stars - as the massive balls of gas burn, forming various elements - to understand how the nuclear reactions happen. Then, scientists will seek to measure how often those nuclear reactions happen. This, Vetter said, will lead to a greater understanding of the fundamentals of the universe, and the engineering necessary for the experiment could be one of the many steps toward understanding how to re-create basic elements through nuclear fusion processes.
"We know in the universe when the Big Bang happened in the beginning pretty much everything that got made, all of the matter was in the form of hydrogen with a tiny bit of helium," Vetter explained. "That begs the question, where did we get the carbon that we are made of? Where did we get the oxygen that we breathe? Where did we get the nitrogen in the air and the silicon that makes up the rocks and the earth?
"The answer to that is that all of those elements come from the nuclear reactions inside stars. Stars fuse hydrogen together to make heavier nuclei and so on and over the course of many stars' lifetimes all of these elements get cooked and then eventually get flung back out into space and then eventually make us. The general idea is that all of the atoms inside your body have been recycled through about 40 stars before arriving at the earth and making you and everything we know about."
All of this, Vetter said, has been well known for about 40 to 50 years. However, stars don't burn quite as hot as some scientists originally believed, which is why astrophysicists have become intrigued with how the nuclear reactions work. In fact, Vetter said, stars are only warm enough to keep those nuclear reactions going, which keeps the stars burning. That burning activity is fueled only by the massive size of the stars themselves.
Deep in the Homestake Mine, Vetter said, scientists can simulate the nuclear activity on stars to study these processes while the earth shields the accelerators from normal cosmic rays. These cosmic rays - as in neutrino studies - make it difficult for the astrophysicists to detect a signal that shows how often the nuclear reactions occur.
"We want the energy in the accelerator to be not too high," Vetter explained. "We want it to be about the energy that particles have when they're inside the stars. Then you study the particular nuclear reaction that you care about and how often that happens.
"The challenge in doing these experiments is when you have very low energy the probability that the particles will actually stick together to make the nuclear reaction that you want to study is very, very low. So, if you are running at very low energies to try and mimic what happens inside stars, then you might have the appropriate nuclear reaction happen only once an hour. If you want to see a signal from the experiments that you are running you have to compete against all kinds of backgrounds coming from cosmic rays in the atmosphere, or environmental radiation coming from the background and the materials in the laboratory.
"That's where going underground helps, because you shield out all of the cosmic rays that ordinarily bathe all of us in a natural sea of radiation all the time."
On the other hand, Vetter said, scientists have to be very careful about the small amount of natural radiation coming from the rock underground as well - particularly when working in a hard rock mine.
"So you have to be very careful about which nuclear reactions could I study underground and not have problems from the environmental radioactivity deep underground," he said.
Though Vetter said he is still awaiting funding news from the National Science Foundation or the Department of Energy - two of the major funding sources for large-scale, modern science experiments - he estimates that his experiment will cost approximately $5 million to construct and maintain with about a half dozen scientists. If he gets his funding, Vetter hopes to have his accelerator constructed and begin doing work in Homestake within the next two to three years.
Overall, Vetter said his experiment could ultimately lead to a better understanding of the solar system's fate, as well as the fate of stars.
"Understanding exactly how the nuclear reactions of stars work allows us to answer questions about the cosmological time frames or the galactical evolution, how the galaxies evolve and how our solar system evolves and so on," he said.