How big is a nucleus?
Where do atomic nuclei come from? How are they organized? Why is matter stable? What are their practical and scientific uses?
Conceptual art connects the atomic underpinnings of the neutron-rich calcium-48 nucleus with the Crab Nebula, which has a neutron star at its heart. Zeros and ones depict the computational power needed to explore objects that differ in size by 18 orders of magnitude. Image credit: Oak Ridge National Laboratory, U.S. Dept. of Energy; conceptual art by LeJean Hardin and Andy Sproles
The answers to these and many other questions are central overarching aims and intellectual challenges of basic research in nuclear physics. To relate the existence and properties of nuclei to the underlying fundamental forces and laws of motion, is central to present and planned facilities like the Facility for Rare Ion Beams (FRIB) at Michigan State University in the USA.
From nuclei to stars
Atomic nuclei are made of protons and neutrons. Due to their electric charge, the distribution of protons in a nucleus can be accurately measured. Neutron densities, that is how neutrons organize themselves in dense matter, are difficult to assess experimentally and are presently not well understood.
An accurate knowledge of neutron distributions is crucial for our understanding of a variety of physical systems ranging from tiny objects such as neutron-rich nuclei up to macroscopically large and massive objects such as neutron stars.
In an article published in Nature Physics, entitled Neutron and weak-charge distributions of the 48Ca nucleus, authored by Dr. Gaute Hagen of Oak Ridge National Laboratory in Tennessee and his collaborators, amongst these Morten Hjorth-Jensen from the University of Oslo, advanced computational tools and sophisticated theoretical models have been used to compute the neutron density and related observables of the doubly-magic nucleus calcium-48 (a nucleus with 20 protons and 28 neutrons).
The authors obtain an excellent agreement with experiment for the radius of the charge distribution and predict that the neutron radius of calcium-48 is smaller than previously thought.
The authors provide predictions for the electric dipole polarizability and the weak form factor, quantities that are currently targeted by precision measurements worldwide.
The authors point also to how the predicted neutron distribution can be used to constrain the size of neutron stars, opening up for a better understanding of the link between the physics of minute objects like nuclei and massive stellar objects like neutron stars.
The progress achieved enables a quantitative description of nuclei based on first principles. The obtained results offer new insights into neutron-rich matter and illuminate how features of nuclear forces determine the basic properties of atomic nuclei.
This work has significant consequences for our basic understanding of nuclei. Combined with the exciting experimental program at present and planned experimental facilities worldwide, there is great hope that within the next two decades, researchers will be able to gain deeper insights about our fascinating microscopic world, impacting intellectual challenges like why matter is stable and how the elements are formed.
About the authors
Several of the authors have their education and training from the University of Oslo and the Computational Physics group at the Department of Physics.
Dr Gaute Hagen was a PhD student at the University of Bergen and Oslo with Morten Hjorth-Jensen and Jan Vaagen as supervisors.
Dr Hagen obtained his PhD in 2005 and has since then been employed at Oak Ridge National Laboratory in the USA. In 2013 he received the Department of Energy's prestigious Young Investigator award.
Gustav Jansen obtained his PhD at the University of Oslo in 2012 with Morten Hjorth-Jensen as supervisor. He is employed as a researcher at Oak Ridge National Laboratory.
Andreas Ekstrøm has his PhD from Lund University in Sweden and worked as a post-doctoral fellow at the University of Oslo from 2010 to 2014. He is now a post-doctoral fellow at Oak Ridge National Laboratory.