New insights on cosmic explosions’ nuclear physics and extreme settings

An illustration of a classical nova shows a bright white dwarf star near the center accumulating fuel in an orange disc from its neighboring main sequence star (shown in bright orange). Credit: NASA/JPL-Caltech

Researchers from Michigan State have helped look inside a nova, which is a type of nuclear explosion in space, without leaving Earth.

These stellar events help make the chemical elements in the universe, and Spartans at the National Superconducting Cyclotron Laboratory, or NSCL, used an intense isotope beam and a custom experimental device with record-setting sensitivity to learn more about them. The group’s work was written up in the journal Physical Review Letters on May 3.

“We’ve been working on this project for about five years, so it’s exciting to see this paper come out,” said Christopher Wrede, a professor of physics at the Facility for Rare Isotope Beams, or FRIB, and in MSU’s Department of Physics and Astronomy. The international research project was led by Wrede, who taught at MSU/FRIB.

NSCL was a building run by the National Science Foundation that scientists used for many years. FRIB, which is run by the Office of Science in the U.S. Department of Energy, got its official start on May 2. Now, FRIB will start a new era of experiments that will help scientists like Wrede test and confirm scientific theories about how the universe works in a better way.

For example, with their experiments at NSCL, the researchers helped make “nuclear thermometers” more accurate. The results of the experiment improved the accuracy of the calculations scientists use to figure out the temperature inside novae, which is the plural of nova. With their results, the team confirmed that the inside of a nova called V838 Herculis was about 50,000 times hotter than the surface of the sun.

Wrede said, “In the end, the information we got from our experiments cut this calculation’s uncertainty by a factor of two to four.” “We were surprised by how close the temperature was to what we thought it would be.”

This agreement helps make nuclear physics theories about novae more solid, which is saying something. Since people first saw novae hundreds of years ago, we’ve learned a lot about them. The name “nova,” which means “new,” shows how far we’ve come.

“A long time ago, if something suddenly appeared in the sky, people might have thought, “Wait a minute. “What in the world is that?'” Wrede said. “‘It must be a new star,'”

Scientists have since found out that novae are not new stars, but rather old stars far away that can be seen from Earth when they explode or cause explosions. A supernova, which is when a whole star blows up, is probably the most well-known example of a “new star.” In our galaxy, the Milky Way, this doesn’t happen too often—about once every hundred years.

Wrede and his team, on the other hand, study nuclear reactions in what are called classical novae, which are more common in our part of the universe. In a typical year, scientists look at about a dozen comets with the help of amateur astronomers. And because a classical nova doesn’t completely destroy a star, the same one can show up more than once (although the typical time between appearances is about 10,000 years, Wrede said).

A classic nova happens when two stars are close enough to each other in their orbits that one star can take nuclear fuel from the other. When it has enough fuel, the syphoning star can set off a powerful series of nuclear explosions.

Understanding the nuclear processes of all stars helps scientists figure out where the elements of the universe come from. Wrede said that the nuclear processes that involve two stars are especially important in the Milky Way.

“About half of the stars we see in the sky are actually binary star systems, which means they are made up of two stars,” he said. “There’s no way we can ignore them if we really want to know how our galaxy makes chemical elements and how it works.”

Wrede has been looking at a specific nuclear reaction that happens in novae and involves different forms of phosphorus, called isotopes. Inside a nova, phosphorus can eat an extra proton to make sulphur isotopes, but scientists can’t make this happen on Earth under stellar conditions. So Wrede and his team did what they thought was best.

Instead, they started with isotopes of chlorine that break down into isotopes of sulphur. Then they watched as these sulphur isotopes spit out protons to make phosphorus. It’s the opposite of the reaction of interest, and it lets researchers make an instant replay of the action that they can watch over and over again to learn more about how nature works.

But there was one more twist. To reach its goal, the team had to measure the protons that came out of the sulphur with the least amount of energy. To do this, the researchers made a device they call GADGET, which stands for Gaseous Detector with Germanium Tagging.

Wrede said, “These protons have a very low amount of energy, and if we used traditional methods, the signal would get lost in the noise.” GADGET used a gaseous detector component instead of solid silicon to get the sensitivity needed to see the protons.

“It’s a world record for sensitivity,” Wrede said.

Tools and methods are important, but they aren’t the whole story. The team also needed people with the skills to make the instrument, run the experiments, and figure out what the results meant. Wrede praised graduate student researcher Tamas Budner, who was the first author of the paper and worked on every part of the project, in particular.

Budner will get his Ph.D. in nuclear physics from MSU’s top-ranked graduate programme this summer, in part because of this project, which he called a happy accident. When he started his graduate programme in 2016, he didn’t know in whose lab he’d work or what project he’d take on.

“When I first got to MSU, I wasn’t sure what I wanted to do. But it seemed like a fun place where people were working on many different projects with a lot of cool, new technology “Budner said.

“I sent Chris an email about this project because it seemed to meet a lot of my needs. I’d get to see the whole process, from making a new detector to doing a new experiment to analysing the results “he said. “It had everything I’d like to try.”

Researchers from all over the world worked on this project with the Spartans. The people on the team came from France, Spain, China, Israel, Canada, and South Korea. There were also people from the University of Notre Dame in Indiana and Oak Ridge National Laboratory in Tennessee who worked on the project.

MSU was the centre of the experiments, though, because it was home to NSCL, which made the high-intensity beam of chlorine isotopes that was needed. Now, FRIB will continue NSCL’s work by bringing together the best scientists from all over the world to do experiments that can’t be done anywhere else. These experiments will help answer some of science’s most important questions.

This will include Wrede’s team. It has already been given permission to do a new experiment at FRIB, along with a new GADGET system.

“We have already made changes to GADGET. It is called GADGET 2 “Wrede said. “It’s a much more complicated system that can measure protons with even greater accuracy.”

Further information: T. Budner et al, Constraining the ³⁰P(p, γ)³¹S Reaction Rate in ONe Novae via the Weak, Low-Energy, β -Delayed Proton Decay of ³¹Cl, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.128.182701

Journal information: Physical Review Letters

Source: Michigan State University