EAST LANSING, Michigan — Michigan state researchers have helped peer inside a nova — a type of astrophysical nuclear explosion — without leaving Earth.
These stellar events help forge the chemical elements of the universe, and the Spartans have helped explore their nature with an intense isotope beam and a custom experimental device with record-breaking sensitivity at the National Superconducting Cyclotron Laboratory, or NSCL. The team published their work on May 3 in the journal Physical examination letters.
“We’ve been working on this project for about five years, so it’s really exciting to see this document come out,” said Christopher WeredeProfessor of physics at the Rare Isotope Beam Facility, or FRIB, and in the Department of Physics and Astronomy at MSU. Wrede, an MSU/FRIB faculty member, led the international research project.
NSCL was a National Science Foundation facility that served the scientific community for decades. FRIB, a user installation of the United States Department of Energy Office of Science, was officially launched on May 2nd. Now, FRIB will usher in a new era of experiments that will allow researchers like Wrede to better test and verify scientific theories explaining the cosmos.
For example, with their experiments at NSCL, the researchers provided better calibration for so-called “nuclear thermometers.” The experimental results have improved the accuracy of calculations used by scientists to determine the interior temperature of novae – the plural of nova. With their results, the team confirmed that the interior of a nova named V838 Herculis was about 50,000 times hotter than the surface of the sun.
“Ultimately, the information we extracted from our experiments reduced the uncertainties in this calculation by a factor of two to four,” Wrede said. “We were actually surprised how close it was to the temperature we expected.”
This agreement makes it possible to solidify the theories underlying the nuclear physics of novae, which is saying something. Our understanding of novae has come a long way since people first observed them hundreds of years ago – a fact illustrated by the name nova itself, which means “new”.
“A long time ago, if something in the sky popped up out of nowhere, you can imagine people going, ‘Wait a minute. What the hell is that?’” Wrede said. It must be a star that wasn’t there before.'”
Scientists have since learned that novae are not new stars, but distant existing stars that become visible on Earth when they explode or trigger explosions. Perhaps the best-known example of a “new star” is a supernova, which is when an entire star explodes. In our galaxy, the Milky Way, this is relatively rare, occurring once every hundred years or so.
The nuclear reactions that Wrede and his team study, however, are found in so-called classical novae, which are more common in our cosmic neighborhood. Scientists observe about a dozen in a typical year, often aided by amateur astronomers. And, because a star doesn’t fully explode in a classic nova, the same one can appear more than once (although the typical time between appearances is around 10,000 years, Wrede said).
A classic nova is created by two stars orbiting close enough that one star can siphon off nuclear fuel from the other. When the siphoning star borrows enough fuel, it can set off an energetic series of nuclear explosions.
Understanding the nuclear processes of all stars helps researchers understand where elements in the universe come from, and those involving two stars are particularly important in the Milky Way, Wrede said.
“About half of the stars we see in the sky are actually two-star systems, or binary star systems,” he said. “If we really want to understand how our galaxy works to produce chemical elements, there’s no way to ignore them.”
Wrede studied a specific nuclear reaction within novae that, in nature, involves versions, or isotopes, of phosphorus. The phosphorus inside a nova can gobble up an extra proton to create sulfur isotopes, but unfortunately scientists cannot recreate this reaction under stellar conditions on Earth. So Wrede and the team did the next best thing.
Instead, they started with chlorine isotopes that decay into sulfur isotopes. They then watched these sulfur isotopes spit out protons to become phosphorus. It’s the reverse interest reaction, which essentially allows researchers to synthesize an instant replay of the action that they can rewind to better understand nature’s playbook.
But there was another wrinkle. To achieve their goal, the team needed to take record-breaking measurements of the lower-energy protons emerging from sulfur. To do this, the researchers built an instrument they called the Germanium Tagged Gas Detector, or GADGET.
“These protons have very low energy, and using conventional techniques the signal would be swamped by background noise,” Wrede said. GADGET took an unconventional approach – using a gaseous detector component instead of solid silicon – to achieve the sensitivity needed to see protons.
“In terms of sensitivity, it’s a world record,” Wrede said.
Of course, tools and techniques are only part of the equation. The team also needed talent to build the instrument, conduct the experiments and interpret the data. Wrede, in particular, praised Spartan graduate student researcher Tamas Budner, the paper’s first author who was involved in every phase of the project.
Budner will get his doctorate this summer from MSU ranked at the top graduate program in nuclear physics thanks in large part to this project, which he described as fortuitous. When he started his graduate program in 2016, he didn’t know which lab he would work in or what project he would embark on.
“When I came to MSU, I didn’t really know what I wanted to work on. But it seemed like an exciting environment where people were working on a lot of different things with a lot of cool cutting-edge technology,” Budner said.
“I emailed Chris about this project, and he ticked a lot of boxes for me. I would see every step of the process: build a new detector, do a new experiment, and analyze the data,” he said “It had everything I wanted to try.”
Researchers from around the world have also joined the Spartans on this project. Team members came from institutions in France, Spain, China, Israel, Canada and South Korea. There was also a national cohort of collaborators from the University of Notre Dame in Indiana and the Oak Ridge National Laboratory in Tennessee.
MSU, however, was the epicenter of the experiments as the headquarters for NSCL, which provided the required high-intensity chlorine isotope beam. Now, FRIB will continue the tradition of NSCL, continuing to attract the best researchers from around the world to answer some of the biggest scientific questions with experiments that are not possible anywhere else.
And Wrede’s team will be part of it. He already has permission to conduct a new experiment at FRIB, with a new GADGET system to boot.
“We have already updated GADGET. We call it GADGET 2,” Wrede said. “It’s a much more complex system and it can measure protons even more sensitively.”
Michigan State University operates the Rare Isotope Beam Facility as a user facility for the U.S. Department of Energy’s Office of Science, or DOE-SC, supporting the Physics Office’s mission Nuclear from the DOE-SC. The establishment of FRIB was funded by the DOE-SC, MSU, and the State of Michigan, with operation of user facilities supported by the DOE-SC Office of Nuclear Physics.
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