No need to worry about the positron annihilation going on at Union College. It’s just scientists trying to improve the disposal of nuclear waste and more accurately date Earth materials billions of years old.
A new research project headed by Professor Heather Watson, a geophysicist at Union, could help scientists better measure the impact of microscopic defects found in crystals and other natural materials.
Watson, working with her co-investigator, professor Daniele Cherniak of Rensselaer Polytechnic Institute and Union Physics professor Mike Vineyard, hopes a novel method of characterizing the atomic-scale properties of different Earth materials can be used to improve disposal of nuclear waste and increase the precision with which scientists can age natural materials.
The research team last year won a one-year National Science Foundation grant worth roughly $73,000 that supports early concept and high-risk projects; about $50,000 of the grant will be dedicated to Union. Over the year of the grant-funded project, the researchers hope to build a proof-of-concept system that can lay the groundwork for multiyear funding and further research, Watson said.
The researchers’ approach focuses on what microscopic imperfections caused by radioactive decay can tell scientists about how different materials would handle nuclear waste disposal. The same approach to studying the defects caused by decay can also help researchers better pinpoint the age of materials billions of years old.
PETER R. BARBER/STAFF PHOTOGRAPHER
Union College professor Heather Watson looks over computer screens with graduate student Elise Liebon Wednesday, February 12, 2020.
“There’s one engineering application related to nuclear waste and another that’s more scientific,” Watson said of the project.
Watson said the research aims to help better understand how radioactivity creates defects that would negatively impact the ability of different materials to contain nuclear waste; that understanding could help engineers and scientists determine the best materials for disposing of the waste or, at least, know how long materials might last before breaking down from radioactive decay.
The process can also be used in measuring the age of planetary materials like minerals and meteorites, some of which formed close to the birth of the solar system, Watson said. The study of defects in materials underway at Union can potentially help scientists understand how the radioactive decay signatures found in materials may have been disrupted over time and correct for those disruptions to more accurately predict the age of materials.
The team started under the federal grant in December, ordering a new set of detectors expected to arrive later in the spring that will help in taking measurements down to fractions of fractions of seconds – down to a trillionth of a second.
The researchers have designed an experiment, called positron annihilation spectroscopy, that allows a source of radiation (decaying sodium, for instance) to shoot positrons into a crystalline sample. When the positron collides with an electron, an annihilation event occurs, sending off two gamma rays in opposite directions. Those gamma rays can then be measured and serve as a signature of the positron annihilation.
In a theoretical perfect crystal, which doesn’t exist in nature or in a lab, Watson said, a positron would travel unabated and easily find an electron. In crystals riddled with microscopic defects, like what would be found in natural materials, the positrons are obstructed from finding that electron, delaying the time it takes for the annihilation to occur and be measured thanks to the release of gamma rays.
Using lab-produced crystals, the researchers can control the defects and pinpoint how long it takes for positron annihilation to occur under different conditions. Those measurements, the researchers hope, can then be applied to imperfect natural materials, where an understanding of how the defects impact radioactive decay within materials can be used to better triangulate the age of materials.
“It’s like carbon dating, but for things that have existed for a really, really long time,” she said. “We are trying to understand events that happened billions of years ago based on indirect evidence we can get from tiny little materials, and piece together this whole history.”
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Watson acknowledged that at least the name of her approach, positron annihilation, might sound like something a Bond villain is plotting.
“It sounds like you are going to blow up the world,” she said. Watson, who earned her doctorate at RPI and has worked as a researcher in labs around the country, joined Union about five years ago. She teaches introductory courses in physics and planetary sciences, as well as mechanics, heat and light, and electricity and magnetism.
In Union’s research labs, now housed at the college’s new Integrated Science and Engineering Complex, Watson works with students and faculty conducting experiments at extreme temperatures – from 30 degree below Celsius to 1,700 degrees Celsius – and at pressures the equivalent of over 100 kilometers of rock sitting on your head.
She said she likes Union’s small community of scientists and researchers, noting the environment helps break down the barriers that have traditionally made it difficult for scientists in different disciplines to work together. As today’s students becomes tomorrow’s scientists, Watson said those lines will only continue to blur.
“The way we compartmentalize a lot of classes and majors is a bit artificial, and in real life all these things are bleeding into each other,” she said.