Bringing neutron stars to Earth

Imagine taking all the water in Lake Michigan – more than a trillion gallons – and squeezing it into a 4-gallon bucket, the kind you would find at a hardware store.

A quick review of the numbers suggests this should be impossible: that’s too much stuff and not enough space. But this bizarre density is a defining feature of celestial bodies known as neutron stars. These stars are only about 25 miles across, but they are more massive than our sun due to extreme physics.

Led by researchers at Michigan State University, an international collaboration has now simulated the cosmic conditions of a neutron star on Earth to better investigate that extreme science. The team shared the results in the journal Physical Review Letters.

For the experiment, the team chose tin to help create a dense nuclear soup rich in neutrons, allowing it to better mimic the environment of neutron stars. The team accelerated a beam made from tin cores to nearly two-thirds the speed of light at Japan’s RIKEN Nishina Center for Accelerator-Based Science. The research was funded by the US Department of Energy Office of Science’s Office of Nuclear Physics, or DOE-SC, and the Department of Education, Culture, Sports, Science, and Technology – Japan, or MEXT, Japan.

The researchers sent that beam through a thin tin target, or foil, to hit tin cores against each other. The cores shatter and for a moment – a billionth of a trillionth of a second – the wreckage exists as a super-dense area of ​​nuclear building blocks called protons and neutrons. Although this environment is volatile, it lives long enough to create rare particles called pions (pronounced “pie-ons” – the “pi” comes from the Greek letter π).

By creating and detecting these pions, the team enables scientists to better answer lingering questions about nuclear science and neutron stars. For example, this work could help scientists better characterize the internal pressures that keep neutron stars from collapsing under their own gravity and becoming black holes.

“The experiment we conducted cannot be done anywhere else except in neutron stars,” said Betty Tsang, a professor of nuclear science and researcher at the National Superconducting Cyclotron Laboratory, or NSCL, at MSU.

Unfortunately, scientists cannot set up stores in neutron stars. Sweltering temperatures and crushing gravities aside, the closest neutron star is about 400 light-years away.

However, there is another place in the universe where scientists can observe matter packed to such incredible density. That’s in particle accelerator labs, where scientists can hit the nuclei of atoms, or nuclei, together to squeeze large amounts of nuclear matter into very small volumes.

This is of course not easy either.

“The experiment is very difficult,” said Tsang. “That’s why the team is so enthusiastic about this.” Tsang and William Lynch, professor of nuclear physics in MSU’s Department of Physics and Astronomy, lead the Spartan contingent of researchers on the international team.

To realize their common goals in this research, the collaborating institutes each played their strengths.

“That’s why we collect employees,” Tsang said. “We solve problems by expanding the group and inviting people who really know what they are doing.”

Home of the top-ranked nuclear physics graduate program in the United States, MSU took the lead in building the pawn detector. The instrument, called the SπRIT Time Projection Chamber, was built with employees from Texas A&M University and RIKEN.

RIKEN’s particle accelerator provided the power and rare neutron-rich tin cores needed to create an environment reminiscent of a neutron star. Researchers from the Technical University, Darmstadt, in Germany contributed the tin targets that had to meet high specifications. Students, staff, and faculty from other institutions in Asia and Europe helped build the experiment and analyze data.

This experiment with RIKEN’s accelerator helped take that understanding to new heights in terms of both energy and density, but there are many more challenges.

When the Rare Isotope Beams Facility, or FRIB, is operational in 2022, it too promises to become a center of international nuclear science cooperation. And the facility will be uniquely equipped to continue to investigate how nuclear systems behave at extreme energies and densities.

“When FRIB comes online, it gives us more choice of beams and allows us to take much more accurate measurements,” said Tsang. “And that’s how we can better understand the insides of the neutron stars and discover things that are even more intriguing and surprising.”