A Tin Flag for Nuclear Physics

Scientists have planted a flag far from the valley of stable nuclei that will serve as a launching point for future expeditions into the origins of the heavy elements. That flag is data from a new study on tin-132 (132Sn); an unstable isotope that holds together just long enough for nuclear physicists to smash it in the name of science.

The experiment took a beam of the rare isotope and collided it with a target of heavy hydrogen atoms in order to create 133Sn. By studying the properties of the single additional neutron, scientists hope to better understand nuclear structure and theories such as the r-process, which is responsible for creating elements heavier than iron in astrophysical phenomena like supernova.

Nature Cover

The research led by Kate Jones, assistant professor of experimental nuclear physics at the University of Tennessee, used the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory to conduct the study appearing in this week’s edition of Nature.

“The technology required to conduct this experiment has been in development for a long time,” said Filomena Nunes, nuclear reaction theorist at the National Superconducting Cyclotron Laboratory at Michigan State University, and co-author of the paper. “The nuclear physics community has been anxiously waiting for these results.”

132Sn is a doubly magic nucleus, which means that there is exactly the right number of protons and neutrons to completely fill all of the energy levels of a given shell in the nucleus. Just like filled electron shells give stability to the noble gases, fully occupied shells make these isotopes more stable than its neighbors.

data chart

Discontinuities in the energy states just beyond shell closure are compared between N=126 just above 208Pb and N=82 just above 132Sn. Also, the spectroscopic factors for both are shown to the right.

Populating the next levels outside of the shell with individual nucleons is essential to calibrating models and calculations of larger, unstudied isotopes. 132Sn plays a unique role because it is the only doubly magic nucleus in a region of neutron-rich nuclei that is critical to the creation of heavy elements.

In order to gain this insight, scientists used a technique called inverse kinematics. Because 132Sn decays too quickly to fabricate a solid target, the team created a beam of the rare isotope and smashed it into a target made of deuterium – a nucleus containing one proton and one neutron. This caused the neutron from the deuterium to fuse with the 132Sn nucleus creating 133Sn while the proton was ejected.

The paper presented in Nature gives detailed information about the properties of the added neutron and how it begins to fill the energy levels outside of the doubly magic shell.

“That’s the big fuss about 132Sn; the chance to actually make a milestone in that region of the nuclear chart,” said Nunes. “Now, at least there is a starting point for shell models to build on.”