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Study Offers Insights into Processes that Create Exotic Nuclei

In the field of nuclear physics, there has been a long-time puzzle of why the universe is made of the particular materials that are observed at present. Put differently, why is it composed of “this” material and not other materials?

A look inside the ISOLDE Solenoid Spectrometer at CERN. Image Credit: Ben Kay, Argonne National Laboratory.

Of particular interest are the physical processes that underlie the formation of heavy elements—such as uranium, gold, and platinum—that are considered to occur when there are explosive stellar events and neutron star mergers.

At the U.S. Department of Energy’s (DOE) Argonne National Laboratory, researchers designed an international nuclear physics experiment performed at CERN, the European Organization for Nuclear Research, that employs innovative methods formulated at Argonne to analyze the nature and origin of heavy elements in the universe.

The study could offer a crucial understanding of the processes that function jointly to develop the exotic nuclei, and will also inform models of the early universe and stellar events.

The nuclear physicists who collaborated for this study are the first to visualize the neutron-shell structure of a nucleus including fewer protons than lead and over 126 neutrons—termed “magic numbers” in the field of nuclear physics. Nuclei with these magic numbers—of which 8, 20, 28, 50 and 126 are canonical values—exhibit improved stability, quite similar to the noble gases that have closed electron shells.

Nuclei that have neutrons more than the magic number of 126 are unexplored to a large extent as they are challenging to produce. Insights into their behavior are vital for perceiving the fast neutron-capture process, or r-process, which creates several of the heavy elements in the universe.

It is considered that the r-process occurs under extreme stellar conditions like supernovae or neutron-star mergers. Nuclei can grow fast in these neutron-rich environments, by capturing neutrons to form new, heavier elements before they can decay.

The focus of this study was on the mercury isotope 207Hg, the study of which could offer insights into the properties of its close neighbors—nuclei that directly take part in important aspects of the r-process.

One of the biggest questions of this century has been how the elements formed at the beginning of the universe. It’s difficult to research because we can’t just go dig up a supernova out of the earth, so we have to create these extreme environments and study the reactions that occur in them.

Ben Kay, Study Lead Scientist and Physicist, Argonne National Laboratory

The team first employed the HIE-ISOLDE facility at CERN in Geneva, Switzerland, to analyze the structure of 207Hg. The collisions resulting when a high-energy beam of protons was fired at a molten lead target produced hundreds of exotic and radioactive isotopes.

Then, the 206Hg nuclei were separated from the other fragments and CERN’s HIE-ISOLDE accelerator was used to develop a beam of the nuclei with the highest energy ever realized at that accelerator facility. Then, the beam was focused at a deuterium target within the new ISOLDE Solenoidal Spectrometer (ISS).

No other facility can make mercury beams of this mass and accelerate them to these energies. This, coupled with the outstanding resolving power of the ISS, allowed us to observe the spectrum of excited states in 207Hg for the first time.

Ben Kay, Study Lead Scientist and Physicist, Argonne National Laboratory

The ISS, a newly built magnetic spectrometer, was used by the nuclear physicists to identify instances when the 206Hg nuclei captured a neutron and transformed into 207Hg. The solenoidal magnet of the spectrometer is a recycled 4-Tesla superconducting MRI magnet obtained from an Australian hospital.

A UK-led collaboration between the University of Liverpool, the University of Manchester, Daresbury Laboratory, and collaborators from KU Leuven in Belgium enabled it to be moved to CERN and installed at ISOLDE.

Deuterium is a rare heavy isotope of hydrogen that includes a neutron and a proton. The proton tends to recoil when a neutron is captured by 206Hg from the deuterium target. The protons released as part of these reactions move to the detector in the ISS.

From the energy and position of the protons, researchers can obtain vital information related to the structure of the nucleus and how it is held together. These properties have a considerable effect on the r-process, and the outcomes could enable significant calculations in nuclear astrophysics models.

The ISS employs a pioneering concept proposed by John Schiffer, an Argonne distinguished fellow, which was developed as the lab’s helical orbital spectrometer, HELIOS—the instrument that led to the development of the ISS spectrometer.

HELIOS has enabled exploration of nuclear properties that were once challenging to investigate to be analyzed at Argonne from 2008. CERN’s ISOLDE facility is capable of producing beams of nuclei that complement those that can be formed at Argonne.

In the last century, nuclear physicists have been in a position to collect information on nuclei by investigating the collisions in which light ion beams hit heavy targets. But when light targets are hit by heavy beams, the physics of the collision turns distorted and more challenging to parse. The HELIOS concept proposed by Argonne was the solution to preventing this distortion.

When you’ve got a cannonball of a beam hitting a fragile target, the kinematics change, and the resulting spectra are compressed. But John Schiffer realized that when the collision occurs inside a magnet, the emitted protons travel in a spiral pattern towards the detector, and by a mathematical ‘trick’, this unfolds the kinematic compression, resulting in an uncompressed spectrum that reveals the underlying nuclear structure.

Ben Kay, Study Lead Scientist and Physicist, Argonne National Laboratory

The theoretical predictions of the existing nuclear models are confirmed by the preliminary analyses of the data from the CERN experiment, and the researchers intend to analyze other nuclei in the region of 207Hg with the help of these new potentials, which would offer a deeper understanding about the unknown regions of the r-process and nuclear physics.

Apart from performing the experiment at CERN, the Argonne scientists also contributed to the design of the ISS, by powering the spectrometer with data acquisition electronics and their HELIOS detectors.

Kay has also contributed to the development of SOLARIS—another solenoidal spectrometer at the DOE-sponsored Facility for Rare Isotope Beams (FRIB) at Michigan State University—offering his expertise in nuclear physics to another cross-institutional collaboration.

The study findings were reported in an article titled “First exploration of neutron shell structure below lead and beyond N = 126” published in the Physical Review Letters journal on February 13th, 2020. The study was funded by the DOE’s Office of Nuclear Physics, the UK Science and Technology Facilities Council, and the European Research Council.


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