Researchers Determine Difference in Nuclear Charge Radius Between Two Boron Isotopes

Researchers have performed a study combining theoretical calculations and experimental work using supercomputers and found out the nuclear geometry of two isotopes of boron. The outcome could pave a new way to precisely calculate the structure of other nuclei that researchers could validate experimentally.

Scientists from the U.S. Department of Energy’s (DOE) Argonne National Laboratory collaborated with researchers in Germany and Poland to establish the difference in a quantity called the nuclear charge radius between boron-10 and boron-11. The nuclear charge radius signifies the size of an atomic nucleus, which usually has comparatively unclear edges.

It is challenging to calculate nuclear charge radii with high precision for atoms considerably larger than boron due to the sheer number of protons and neutrons whose properties and interactions must be deduced from quantum mechanics.

Nuclear theory builds from the physical rules of quantum chromodynamics (QCD), which are applicable to gluons and quarks that constitute the neutrons and protons inside the nucleus. However, attempts to solve the nuclear dynamics with the help of QCD alone would be a nearly impracticable task because of its complexity, and scientists have to depend on at least a few simplifying assumptions.

Since boron is comparatively light—with just a handful of neutrons and five protons—the researchers could successfully model the two boron isotopes using the Mira supercomputer and analyze them experimentally with the help of laser spectroscopy. Mira is part of the Argonne Leadership Computing Facility (ALCF), a DOE Office of Science User Facility.

This is one of the most complicated atomic nuclei for which it is possible to arrive at these precise measurements experimentally and derive them theoretically.

Peter Mueller, Nuclear Physicist, Argonne National Laboratory

Mueller helped lead the research.

In order to examine how the nuclear configurations of boron-11 (11B) and boron-10 (10B) varied, determinations were made at exceptionally small length scales: less than a femtometer, or one-quadrillionth of 1 m. In a contradictory discovery, the scientists found out that the 11 nucleons in boron-11, in fact, occupy a smaller volume when compared to the 10 nucleons in boron-10.

The researchers at the University of Darmstadt looked experimentally at the boron isotopes by carrying out laser spectroscopy on samples of the isotopes, which fluoresce at varied frequencies. Although a majority of the difference in the fluorescence patterns is brought about by the variation in the mass between the isotopes, there exists a component in the measurement that reflects the size of the nucleus, elucidated Argonne physicist Robert Wiringa.

Collaborators from the University of Warsaw and Adam Mickiewicz University in Poznan separated these components by performing most advanced atomic theory calculations that exactly depict the intricate dance of the five electrons around the nucleus in the boron atom.

Earlier electron scattering experiments couldn’t really say for sure which was bigger. By using this laser spectroscopy technique, we’re able to see for certain how the extra neutron binds boron-11 more closely.

Robert Wiringa, Physicist, Argonne National Laboratory

The better agreement between theory and experiment for the dimensions of the nucleus enables scientists to establish other properties of an isotope, for example, its beta decay rate, with improved confidence. “The ability to perform calculations and do experiments go hand-in-hand to validate and reinforce our findings,” stated Mueller.

The study’s next phase will probably involve the analysis of boron-8, an unstable isotope with a half-life of only about a second before it decays. With fewer neutrons in the nucleus, it is much less tightly bound compared to its stable neighbors and is considered to have an extended charge radius, stated Mueller. “There is a prediction, but only experiment will tell us how well it actually models this loosely bound system,” he explained.

An article based on the study, titled “Nuclear Charge Radii of 10,11B,” has been published in the May 10^th issue of Physical Review Letters. Apart from Argonne’s Alessandro Lovato, scientists from two German and two Polish universities also contributed to the study.