To bring neutron stars to earth

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

A quick overview of the numbers suggests that this should be impossible: it is too many things and not enough space. But this strange density is a defining feature of celestial objects, known as neutron stars. These stars are only about 20 kilometers wide, but hold more mass than our sun, thanks to extreme physics.

Led by researchers from Michigan State University, an international collaboration has now tracked a neutron star’s cosmic conditions on Earth to better investigate extreme science. The team shared its results in the journal Physical overview letters.

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

The researchers sent the barrel vessel through a thin tin target, or foil, to strike tin cans together. The nuclei break and in just a moment – a billionth of a trillionth of a second – the wreck exists as a super-dense region 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’ us’ – the ‘pi’ comes from the Greek letter π).

By creating and locating these peonies, the team enables scientists to better answer ongoing questions about nuclear science and neutron stars. This work can help scientists, for example, better characterize the internal pressure that prevents neutron stars from collapsing under their own gravity and becoming black holes.

“The experiment we performed could not be done elsewhere except inside neutron stars,” Betty Tsang, a professor of nuclear science and researcher at the National Superconducting Cyclotron Laboratory, or NSCL, told MSU.

Unfortunately, scientists can not set up shops in neutron stars. Apart from blowing temperatures and crushing gravity, the nearest neutron star is about 400 light-years away.

However, there is another place in the universe where scientists can see matter in such incredible density. It is in laboratories for particle accelerators, where scientists can crush the nucleus of atoms or nuclei to compress large amounts of nuclear material into very small amounts.

Of course, this is also no cake walk.

“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 the Department of Physics and Astronomy at MSU, lead the Spartan contingent of researchers in the international team.

To achieve their common goals in this study, the collaborating institutes each played their strengths.

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

MSU, home to the United States’ top-rated nuclear physics program, has taken the lead in building the pawn detector. The instrument, called the SπRIT Time Projection Chamber, was built with collaborators from Texas A&M University and RIKEN.

RIKEN’s particle accelerator offered the power and scarce neutron – rich tin cores 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 the stringent requirements. Students, staff, and faculties from other institutions in Asia and Europe helped build the experiment and analyze data.

This experiment on RIKEN’s accelerator has helped to raise awareness to new heights in terms of energy and density, but there are many more challenges.

When the Rare Isotope Radiation Facility, or FRIB, is operational in 2022, it also promises to be a hub of international collaboration in nuclear science. And the facility will be uniquely equipped to continue to investigate how nuclear systems behave at extreme energy and density.

“If FRIB comes online, it will give us more choice of beams and allow us to make much more accurate measurements,” Tsang said. “And so we can better understand the inside of the neutron stars and discover things that are even more intriguing, more surprising.”