One of the biggest puzzles of the 21st century: How were the elements formed after the Big Bang?

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A long-standing mystery in nuclear physics is: Why is the Universe made of the specific stuff we see around us? In other words, why is it made of "this" stuff and not something else? Of particular interest are the physical processes responsible for the creation of heavy elements such as gold, platinum and uranium, which are thought to occur during neutron star mergers and supernova explosions. Scientists from the U.S. Department of Energy's Argonne National Laboratory (DOE), leading an international nuclear physics experiment at CERN, have published their findings in the journal Physical Review Letters.

The experiment uses new technology developed at Argonne National Laboratory to study the properties and origins of heavy elements in the universe. The research may provide key insights into the processes that co-create "exotic" nuclei and will inform models of stellar events and the early universe. Nuclear physicists involved in the collaboration are the first to observe the neutron shell structure of nuclei with fewer protons than lead and more than 126 neutrons, a "magic number" in the field of nuclear physics. At these magic numbers (of which 8, 20, 28, 50 and 126 are canonical values), the stability of the nucleus is enhanced, just as noble gases do with closed electron shells.

Nuclei with neutrons above the magic number 126 have largely gone undetected because they are difficult to produce. Knowledge of their behavior is crucial to understanding the rapid neutron capture process, or r-process, that produces many of the heavier elements in the universe. The r-process is thought to occur under extreme stellar conditions, such as neutron star mergers or supernovae. These neutron-rich environments are where nuclei can grow quickly, capturing neutrons before they have a chance to decay to create new, heavier elements. This experiment focused on an isotope of mercury, Hg207.

Studies of Hg207 may help shed light on the properties of its closest neighbors, the atomic nuclei that are directly involved in key aspects of the r-process. "One of the biggest questions of this century is how the elements were formed at the beginning of the universe, after the Big Bang," said Argonne physicist Ben Kay, lead scientist on the study. "It's difficult to study because we can't just dig up a supernova on Earth, so we have to create these extreme environments and study the reactions that occur in them." To study the structure of Hg207, the researchers first used the HIE-ISOLDE facility at CERN in Geneva, Switzerland.

Pictured: Inside the Isolde solenoid spectrometer at CERN

A high-energy proton beam was fired at a molten lead target, and the resulting collisions created hundreds of exotic and radioactive isotopes. The nuclei of Hg206 were then separated from the other fragments and, using CERN's HIE-ISOLDE accelerator, a beam of the highest-energy nuclei ever achieved at this accelerator facility was created. The beam was then focused onto a deuterium target inside the new ISOLDE Solenoid Spectrometer (ISS). No other facility can create a beam of mercury atoms of this quality and accelerate them to such high energies.

This, combined with the excellent resolution of the ISS, has allowed us to observe the spectrum of excited states of Hg207 for the first time. The ISS has a newly developed magnetic spectrometer that nuclear physicists use to detect instances where Hg206 nuclei capture neutrons and become Hg207. The spectrometer solenoidal magnet is a recycled 4 Tesla superconducting magnetic resonance magnet from a hospital in Australia. It was transferred to CERN and installed at ISOLDE thanks to a UK-led collaboration between collaborators at the University of Liverpool, the University of Manchester, the Daresbury Laboratory and KU Leuven in Belgium.

Deuterium is a rare heavy isotope of hydrogen that consists of protons and neutrons, and when Hg206 captures neutrons on the deuterium target, the protons recoil. The protons emitted in these reactions are transmitted to detectors on the International Space Station, and their energy and position yield key information about the structure of the nucleus and how it is held together. These properties have a major impact on the r-process, and the results can guide important calculations in nuclear astrophysics models, which the International Space Station uses a pioneering concept developed by Argonne Distinguished Fellow John Schiffer.

The concept was built as the laboratory's Helical Orbiter Spectrometer HELIOS, an instrument that inspired the development of the International Space Station spectrometer. Explorations into nuclear properties that were once impossible to study, but thanks to HELIOS, have been underway at Argonne since 2008. CERN's ISOLDE facility can generate atomic beams that complement those that Argonne can make. For the past century, nuclear physicists have been able to glean information about atomic nuclei from collisions of light ion beams hitting heavy targets.

"However, when a heavy beam hits a lightweight target, the physics of the collision becomes distorted and harder to resolve. Argonne's HELIOS concept is a solution to eliminate this distortion. When the beam hits a fragile target, the kinematics are altered and the resulting spectrum is compressed. But when the collision occurs inside a magnet, the emitted protons move in a spiral pattern toward the detector. Through a mathematical 'trick,' this unfolds the kinematic compression, resulting in an uncompressed spectrum that reveals the underlying nuclear structure."

This first analysis of data from the CERN experiment confirms the theoretical predictions of current nuclear models, and the team plans to use these new capabilities to study other nuclei in the Hg207 region, gaining deeper insights into unknown regions of nuclear physics and the r-process.

Boco Park | Research/From: Argonne National Laboratory

Reference journal: Physical Review Letters

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