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If you want to understand a part of Earth’s galactic history, such as whether it passed close to a supernova on its way around our galaxy, you may be able to find the answer in the crystalline structure of a rock, according to the University of Michigan. she studies.
The study outlines a method that uses paleo-detectors, an idea inspired by 1960s work, which used ancient minerals to search for new physics. The idea is this: the Earth is constantly flooded with cosmic rays. Cosmic rays are particles produced by an energetic universe, in which stars explode into supernovae, supermassive black holes in the center of galaxies accelerate particles to almost the speed of light, and neutron stars collide and produce bright flashes of gamma rays and other energetic particles.
“There are energetic particles produced in cosmic events all the time: mergers of neutron stars and black holes and active galactic nuclei,” said lead author Joshua Spitz, a particle physicist and Norman M. Leff Assistant Professor of Physics at UM. “Some of these energetic particles reach the Earth and interact with the atmosphere, producing showers of particles that rain down on us all the time.”
Some of these particles are neutrinos, which are fundamental particles that interact only with matter very weakly. As a result, these atmospheric neutrinos can pass through the Earth without interacting, allowing them to reach ancient minerals deep within the Earth.
Occasionally, one of these neutrinos will interact with an atomic nucleus in the ancient mineral, leaving a trace within the rock’s crystalline structure. By examining these traces in excavated rocks, scientists can study the flow of cosmic radiation on Earth over time. The researchers’ method is published in Physical Review Letters.
“About a hundred billion neutrinos from the sun pass through your fingertip every second, but hardly any of them interact. The same is true for atmospheric neutrinos, which rarely interact. However, every now and then, one of these atmospheric neutrinos will hit a core, ”said co-author and UM graduate student Johnathon Jordan. “And when they do, they kick the core.”
After being kicked, the nucleus shifts a small distance, from a few microns to a few hundred in length. A human hair is about 70 microns wide. As the core retracts into the mineral, it creates a tiny path of destruction through the rock’s crystal lattice.
Finding these structures in rocks and then determining the age of the rocks could help scientists pinpoint when an event occurred that may have increased cosmic ray exposure during a certain period of Earth’s history and answer broader questions about the speeds of cosmic rays and radiation that hit the Earth over time, Spitz says.
“It’s a big question: has the speed of cosmic rays changed as a function of time? Was it always the same rate or was it higher in the past? Was there a single event that caused it to increase for a short period of time, or did it slowly increase or decrease? “Spitz said.” These are questions we don’t really know the answers to.
No observations have yet been made about the damage of atmospheric neutrinos inside the crystals, but it would be similar to the damage caused by spontaneous fission of uranium-238, Jordan says. During this fission, the heavy uranium-238 core splits in two and each half shoots outward, away from each other, and creates tiny scars in the rock’s crystalline structure. Scientists use these traces to determine the age of the rocks.
The same idea could also be used to search for dark matter, the researchers say. Currently, one method of looking for dark matter is to monitor dark matter particles as they pass through argon- or xenon-filled detectors buried deep underground – a costly endeavor, Jordan says, because argon and xenon are expensive and because detectors have to be great.
“You want these big detectors and you want them to be able to work for a long time, because you want them to have the maximum exposure possible,” Jordan said. “What the paleo-detectors do is turn that script upside down. At most, the rocks are 100 grams or one kilogram. And instead of waiting 10 years, we are waiting a billion years. The novelty of paleo-detectors is that you win by not having a great detector, but by having a really long exposure time. “
The study stems from Spitz and Jordan’s hopes of using paleo-detectors to observe proton decay, a big issue facing particle physics. Typically, physicists monitor huge water reservoirs for flashes of light that could mean proton decay, but they realized you might be able to examine hundreds of millions of years of rocks for tiny incisions of damage that could mean the same thing. But when the researchers looked at this idea, they found they couldn’t discern the signature of the proton’s potential decay due to damage from atmospheric neutrinos.
“This article is actually the result of turning lemons into lemonade,” Spitz said. “These atmospheric neutrinos were the background of the research we were initially interested in.”
Instead, the team realized that this method could provide a window into Earth’s history in a different way.
“Our solar system revolves around the galaxy once every 250 million years,” he said. “An extremely interesting question is: what did the Earth encounter on its way around the galaxy?”
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