Living at the Edge of Stability

Contact: Ken Kingery, NSCL, Office: 517-908-7482, Kingery@nscl.msu.edu
Published April 30, 2012

For Immediate Release

EAST LANSING, Mich. – Theoretical physicists from an international collaboration have successfully described the physics underpinning the structure of the heaviest oxygen isotope ever seen in the laboratory, oxygen-26. The theoretical results hold great promise for the understanding of nuclear forces in systems with many interacting protons and neutrons.

Experimental observations can reveal new and exciting phenomena at work in the formation of the elements in the universe. However, only a working model of the underlying forces involved may give a proper understanding of the underlying physics. Thanks to sophisticated mathematical methods and algorithms, the power of supercomputers, and a solid collaboration between theorists and experimentalists, nuclear physicists recently unveiled some of the fundamental interactions of the strange and rare isotopes not found on Earth.

The collaboration included theoretical and experimental physicists from the University of Idaho, Oak Ridge National Laboratory, Michigan State University, the University of Oslo, and the University of Tennessee, and made use of the power of the supercomputing facilities Jaguar and Kraken, housed at Oak Ridge, and the Titan supercomputing cluster at the University of Oslo.

“One of the overarching aims and intellectual challenges of basic research in nuclear physics is to understand why some nuclei are stable and others not, and to relate this to the underlying fundamental forces and particles of nature,” said Morten Hjorth-Jensen, theoretical physicist at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University. “This successful collaboration resulting in the theoretical description of these nuclei takes us closer to making that challenge a reality. It definitely helps to have theorists and experimentalists working on both sides of the same coin.”

Of the approximately 3,100 nuclei that are known to us today, less than 300 are stable and make up the world we see around us. As more and more neutrons or protons are added, the nuclei become less and less stable. These unstable isotopes go asunder by emitting various types of radiation and particles, or by breaking up in smaller parts by spontaneous fission. Seeing as how these nuclei break down so quickly, they only exist within the nuclear reactions of supernova, other astronomical phenomena, or specialized nuclear physics laboratories such as NSCL, which produces them for study.

As more protons or neutrons are added to a nucleus, there comes a point at which there is not enough energy to bind more protons or neutrons into a nucleus, not even for times smaller than fractions of microseconds. This point is called the drip-line. Neutron-rich isotopes are especially interesting to the study of atomic nuclei because their neutral charges allow many more of them to be bound together. They thus serve as an important testing ground for the strong force and its capability to bind large systems of nucleons.

In a recent paper on the experimental side of the coin in Physical Review Letters, physicists describe the production of neutron-rich oxygen-26 at NSCL. The paper confirms that oxygen-24 holds the most neutrons an oxygen nucleus can bind before hitting this drip-line, and goes on to describe the near-immediate decays when more neutrons are added to create oxygen-25 and oxygen-26.

The new paper – on the theoretical side and also appearing in Physical Review Letters – goes one step further by successfully describing the behavior of these oxygen isotopes near and beyond the drip-line. Using several supercomputing facilities and many-body methods like coupled-cluster theory, the team found that oxygen-24 is the heaviest bound oxygen isotope, in good agreement with the experimental results from NSCL and other laboratories. They predict several long-living resonances as excited states of oxygen-24, and reproduce the ground-state energies of oxygen-25 and oxygen-26 observed at the NSCL.

To reproduce these ground state properties, three-body forces are essential. Performing the calculations with a Hamiltonian, which contains only two-body forces, results in these two isotopes being bound with respect to oxygen-24. These calculations provide important information on many-body correlations in complicated and strongly interacting many-body systems like the atomic nucleus.

The results show the importance of having theorists and experimentalists working together. While theorists help make sense of experimental results and help steer the course of future experiments, those theories and models are substantiated and refined thanks to the physical data gathered by experimentalists.

“Combined with forthcoming experimental plans at NSCL and the future Facility for Rare Isotope Beams (FRIB )for studies of isotopic chains like neutron rich calcium, nickel and zirconium isotopes,” said Hjorth-Jensen, “these results hold great promise for our basic understanding of nuclear stability.”

The earlier article describing the production of oxygen-26 can be found by referencing Lunderberg et al, Phys. Rev. Lett. 108, 142503 (2012). The current study in the May 4 issue of Physical Review Letters is entitled “Continuum Effects and Three-nucleon Forces in Neutron-rich Oxygen Isotopes” and authored by Hagen et al.

New Experiments Confirm Old Predictions

Recent experiments performed by the MoNA Collaboration at NSCL [1], unveil the structure of heavy oxygen isotopes. These elements play a critical role in the theoretical understanding of nuclear many-body physics on the verge of stability. The new observations are essential in guiding theoretical developments and in validating our methods and techniques. The theory group at NSCL is particularly enthusiastic in seeing their old predictions [2] being confirmed.


26O Chart

Fig. 1. Binding energies of the oxygen isotopes are shown as a function of mass number. The experimental data is in black; the CSM theory from [2] is shown with dashed-blue and the recent calculations of Ref. [3] in red. The results above A=23 are predictions. Inset shows the theory-experiment discrepancy.

The plot in Fig. 1 shows the superior quality of those predictions. In the unified framework hundreds of states and resonances as well as neutron scattering observables have been predicted, see Fig. 2, many of them are still waiting to be tested. Our method of choice was the continuum shell model (CSM) that combines the rich experience of classical shell model with the correct account of continuum by means of the effective non-Hermitian Hamiltonian.

Currently, other techniques [3,4] are on the rise. These alternative approaches pursue an ambitious goal of solving the nuclear many-body problem ab-initio, i.e. starting from the free-space nucleons and their free-space effective interactions. The QCD nucleonic in-medium excitations in the ab-initio approaches are included by means of effective many-body forces. The CSM uses effective in-medium nucleon degrees of freedom and effective interactions, which implicitly include renormalizations from the QCD degrees of freedom. As seen from the plots below, the CSM with its accurate predictions of bound states, resonances and scattering cross sections remains a reliable practical tool.


26O Chart

Fig. 2. States and resonances (color coded decay width) in heavy oxygen isotopes, predicted in Ref [2] are shown on an absolute binding energy scale.

[1] C. R. Hoffman et al., Phys. Lett. B 672, 17 (2009); Phys.Rev.Lett.102,152501(2009); Phys.Rev.C 83,031303(R)(2011); E. Lunderberg et al., Phys. Rev. Lett. 108, 142503 (2012).
[2] A. Volya and V. Zelevinsky, Phys. Rev. Lett. 94, 052501 (2005); Phys. Rev. C 67, 054322 (2003); 74, 064314 (2006).
[3] G. Hagen et.al., http://arxiv.org/abs/1202.2839.
[4] T. Otsuka et al., Phys. Rev. Lett. 105, 032501(2010).

###

Michigan State University has been working to advance the common good in uncommon ways for more than 150 years. One of the top research universities in the world, MSU focuses its vast resources on creating solutions to some of the world’s most pressing challenges, while providing life-changing opportunities to a diverse and inclusive academic community through more than 200 programs of study in 17 degree-granting colleges.