Expertise in demand: Substantial GSI/FAIR participation in international review article on the origin of the heaviest elements
Where and how does nature produce noble metals such as gold and platinum? This is one of the most exciting questions in physics. Astrophysical observations were able to unveil this mystery only a few years ago. The worldwide attention was immense; the interest in the topic has grown strongly since then. A group of top-class experts now evaluated and summarized the current state of knowledge and presented a review article in the renowned scientific journal "Reviews of Modern Physics". More than half of the authors are researchers at GSI/FAIR or have close connections here.
The place of origin of the heavy and heaviest elements, which include gold and platinum, has long occupied the scientific community. The U.S. National Research Council had listed this question as one of the eleven biggest unsolved problems in physics in the 21st century. A breakthrough came in August 2017, when a previously never observed astrophysical phenomenon was detected by both gravitational waves and a burst of light (known as a kilonova). Analysis of the gravitational waves showed that the observed event could be identified as the merger of two neutron stars, while the light curve gave evidence for the production of heavy elements in the so-called astrophysical r-process.
The r-process, a sequence of neutron capture reactions and beta decays by extremely neutron-rich nuclei, has been postulated as the origin of the heavy elements for a long time, but now a location where this occurs in the universe is finally known. The identification of neutron star mergers as an astrophysical site of the r-process has thus opened the door to a fascinating new field of scientific research that is attracting considerable global attention. This is one of the reasons why the prestigious scientific journal "Reviews of Modern Physics", published by the American Physical Society, invited a group of experts to comprehensively summarize and evaluate the latest knowledge on the formation of the heavy elements. Among the eight authors are three researchers working at GSI and two others closely associated with GSI/FAIR.
"It was, of course, a great honor to provide a review of this rapidly evolving field of research for ‘Reviews of Modern Physics’. Especially, it was a challenge to present in a balanced way the wide spectrum from astrophysical observations to nuclear and atomic physics laboratory measurements and simulations of such events. I am glad that competent colleagues from the different disciplines supported me with their expertise," says Professor Friedrich-Karl Thielemann, who also conducts research at GSI since his retirement from the University of Basel and who was recently awarded the Karl Schwarzschild Medal of the German Astronomical Society not least for his pioneering work on the r-process.
However, Thielemann also emphasizes that there are still many unresolved questions about the r-process, which the review also addresses. This particularly concerns the nuclear processes that are essential in the fusion of neutron stars as well as in r-process nucleosynthesis. Exciting findings can be expected here once new large-scale accelerator facilities have begun operation. At FAIR, the Facility for Antiproton and Ion Research, which is currently being built at GSI as an international accelerator project, matter can be compressed to extreme densities and temperatures in ultrarelativistic heavy ion collisions and studied under conditions that exist in neutron star mergers just before the transition to a black hole.
"At FAIR, we will also produce many of the exotic nuclei for the first time and measure their properties at the storage rings and detectors available there," says co-author Gabriel Martinez-Pinedo, head of the GSI theory department and professor at TU Darmstadt. Professor Martinez-Pinedo had led with Brian Metzger of Columbia University the team that predicted the kilonova signal as a fingerprint of the r-process several years before it was observed.
Until now, the properties of the short-lived nuclei, which are important in the r-process, had to be modeled theoretically, which is always associated with a certain degree of uncertainty. Another co-author, Professor Michael Wiescher from Notre Dame University, who is connected to GSI through a prestigious Humboldt Research Award, is working on changing this in the future. Together with other researchers, primarily from Goethe University Frankfurt and GSI, Wiescher is developing plans to use the unique storage rings at FAIR to generate important experimental data for the r-process. “I am fascinated by the idea of my colleague Professor René Reifarth from Frankfurt, that the FAIR rings will make it possible to measure neutron captures at short-lived nuclei," Wiescher points to a long-held dream of nuclear astrophysics that could come true at FAIR. The FAIR storage rings also promise first-time access to measuring atomic physics data from heavy element ions, as needed to model the kilonova light curve.
The review article appears in the new volume 93 (February 1, 2021) of “Reviews of Modern Physics". Because of the actuality and complexity of the subject, the editors have allowed the page limit to be significantly exceeded. The text summarizes in 85 pages what is currently known about the formation of the heavy elements by the astrophysical r-process. However, it also shows which questions are still unsolved and what progress can be expected from improved astronomical observations, from computer simulations, and above all from the unique possibilities opened up by the next generation of accelerator facilities in Europe, America, and Asia.
The scientists involved are looking to the future: "When another review article on the r-process will appear in the ‘Reviews of Modern Physics’ in a decade or two, it will probably answer many of today's unanswered questions on the basis of the knowledge now described. But surely, as is typical and fruitful for science, it also will identify new open questions." (BP)