Bringing neutron stars to Earth

Imagine that you took all the water from Lake Michigan – more than a billion gallons – and collected it in a 4-gallon bucket, the kind you would find at a hardware store.

A quick analysis of the numbers suggests that this should be impossible: there are too many things and not enough space. But this bizarre density is a defining feature of celestial objects known as neutron stars. These stars have a radius of only 15 miles, yet they have more mass than our sun due to extreme physics.

Led by researchers at Michigan State University, an international collaboration has now mimicked the cosmic conditions of a neutron star on Earth to better test that extreme science. The team shared their results in the diary Physical review letters.

For the experiment, the team selected tin to help create a dense, neutron-rich nuclear soup, helping it to more closely mimic the environment of neutron stars. The team accelerated a beam of tin cores at nearly two-thirds the speed of light at the Japanese RIKEN Nishina Center for Accelerator-based Science. The research was funded by the Office of Nuclear Physics in the US Department of Energy, Office of Science, or DOE-SC, and the Ministry of Education, Culture, Sports, Science and Technology – Japan or MEXT, Japan.

The researchers sent that beam that passes through a thin target of tin, or foil, to break the tin cores together. The nuclei break and for just a moment – a billionth of a trillionth of a second – the wreckage exists as a super-dense region of nuclear blocks called protons and neutrons. Although this environment is transient, it lives long enough to create rare particles called pawns (pronounced “pie-ons” – “pi” comes from the Greek letter π).

By creating and detecting these pawns, the team enables scientists to better answer persistent questions about nuclear science and neutron stars. For example, this paper can help scientists better characterize the internal pressure that prevents neutron stars from collapsing under their own gravity and becoming black holes.

“The experiment we performed can only be done inside neutron stars,” said Betty Tsang, a professor of nuclear science and a researcher at the National Laboratory of Superconducting Cyclotrons, or NSCL, at MSU.

Unfortunately, scientists cannot set up the store in neutron stars. Aside from boiling temperatures and the crushing of gravitational forces, the nearest neutron star is about 400 light-years away.

There is, however, another place in the universe where scientists can observe packaged matter at such incredible density. This happens in particle acceleration laboratories, where scientists can break up the cores of atoms or nuclei to gather large amounts of nuclear matter in very small volumes.

Of course, this is not a cake either.

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

In order to meet their collective objectives in this study, the collaborating institutes each played according to their strengths.

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

MSU, which hosts the top graduate program in nuclear physics in the United States, has taken over the construction of the pawn detector. The instrument, called the SπRIT Time Projection Chamber, was built with collaborators from Texas A&M University and RIKEN.

The RIKEN particle accelerator provided the power and rare-earth neutron nuclei needed to create an environment reminiscent of a neutron star. Researchers at the Technical University of Darmstadt, Germany, contributed to the target targets that had to meet the exact specifications. Students, staff and faculty from other institutions in Asia and Europe helped build the experiment and analyze the data.

This RIKEN accelerator experiment has helped push this agreement to new heights in terms of both energy and density, but there are many other challenges.

When the Rare Isotopic Radiation Facility, or FRIB, is operational in 2022, it also promises to be a center for international collaboration in nuclear science. And the facility will be uniquely equipped to continue to explore how nuclear systems behave at extreme energies and densities.

“When FRIB is online, it will give us more beam options and allow us to make more accurate measurements,” Tsang said. “And that will allow us to better understand the interior of neutron stars and discover things that are even more interesting, more surprising.”