SAMURAI experimental installation at RIBF/RIKEN, where 31Ne + p → 30F → 29The F+ value was not measured. Credit: SAMURAI RIBF/RIKEN collaboration.
Data collected by the SAMURAI spectrometer at RIKEN’s RI Beam Facility (RIBF) in Japan recently led to the detection of a rare isotope of fluorine (F), known as 30F. This opened up interesting possibilities for the study of rare nuclear structures and corresponding phases, which in turn could help test various physical theories.
The SAMURAI21-NeuLAND collaboration, a large group of researchers including physicists from RIKEN, GSI-FAIR and TU Darmstadt in Germany, as well as other research facilities around the world, set out to study the spectroscopy and neutron separation energy of the new 30Isotope F.
Their conclusions, published in Physical Exam Letterssuggests the presence of a superfluid state in the isotopes 29F and 28O.
“We are exploring the most neutron-rich nuclei in the nuclide list, pushing the boundaries of existence,” Julian Kahlbow, corresponding author of the paper, told Phys.org. “To date, we know the neutron-richness limits for the isotopes neon (Ne) and F, with the last fluorine isotope being sodium fluoride. 31F.
“Our initial goal was to study the behavior of nuclear structure under extreme conditions, in particular to determine whether nuclear ‘magic numbers’ are valid.”
At a neutron number of N = 20, nuclear structures typically exhibit a large energy gap. In their study, Kahlbow and colleagues explored the previously reported conflict between neutron-rich Ne and somewhat heavier nuclei, where this energy gap breaks down, creating a so-called “inversion island,” versus a 28Oh core that is supposedly twice as “magical”.
“Between these isotopes are 29F and 30“F,” Kahlbow explained. “We don’t know anything about that 30F because it is unrelated and only exists for about 10-20 seconds, which makes any measurement very difficult.
“For the first time, my colleagues and I have measured the mass of 30F, a fundamental quantity of any nucleus. By measuring the mass of 30F, (i.e. its neutron separation energy), we conclude that the region in which “magicity” is lost also extends to F isotopes.
By measuring the mass of 30F, the researchers were able to gather more information about this particular segment of the nuclide table (i.e., a graphical representation of all known isotopes that ranks them according to the number of protons and neutrons in their nuclei). This led to even more surprising results.
“30F is an unbound nucleus, meaning it decays into 10-20 seconds, making direct measurements impossible,” Kahlbow said. “By analyzing the decay products, however, we can reconstruct 30F by the measure of 29F and a single neutron.”
First, Kahlbow and his colleagues produced a beam of ions from 31Using the BigRIPS fragment separator at the RIBF/RIKEN facility in Japan. This beam, traveling at about 60% of the speed of light, was directed at a liquid hydrogen target to eliminate a single proton, resulting in the production of 30F, which instantly decomposed into 29F and a neutron.
Measurements for neutron and 29The F isotope was collected at the site where the SAMURAI experiment is taking place. To make neutron measurements, the team used a 4-ton neutron detector called NeuLAND, which was shipped from the GSI-FAIR research center in Germany to Japan especially for this research project.
Schematic of the nuclide graph, showing the trend of neutron separation energy (red line) in fluorine isotopes with the new result for 30F. The absence of a sharp drop proves the breakdown of the magic number of neutrons at N = 20. 28O and 29F, a superfluid phase forms with neutron pairs. Credit: Julian Kahlbow
“This study was the result of a large team effort of over 80 people who collectively conducted the experiment, combining expertise from around the world working in the best accelerator facilities,” Kahlbow said. “In analyzing the data, using the measured moment information from 29F and the neutron, the energy spectrum of 30F is reconstructed in which we successfully identified a resonance and a ground state mass.
This recent study from the SAMURAI21/NeuLAND collaboration could open new research opportunities focused on both 30Isotope F and other interesting isotopes around 28O. This isotope of oxygen, which was also recently detected and measured at RIKEN, is characterized by a nucleus that decays into four neutrons and 24O.
“Based on our results, we have shown that the classical nuclear structure breaks down and that the ‘magic number’ no longer holds at 20 neutrons (for Z=9.8),” explains Kahlbow.
“We speculate that 28O and 29“F neutrons exist in a superfluid state of nuclear matter. With the help of my French colleague Olivier Sorlin and theorists, we were able to identify this surprising state of matter in this region of the nuclear map. The excess neutrons are likely to form pairs and easily disperse between different energy levels and occupy different levels there.”
It is noteworthy that a pure superfluid regime is rarely encountered among the isotopes of the nuclide map. This phase has already been found in the heavier isotope chain of tin (Sn), in a Cooper pair-like regime, from neutron pairs with large distances between them.
“In our work, we propose for the first time superfluidity at the limit of stability in weakly bound systems,” Kahlbow said. “The possible implication of superfluidity in weakly bound or unbound systems is the change of regime, from one with long-range neutrons to neutrons in shorter-range pairs, close to the characteristics of Bose-Einstein condensates.”
The new measurements collected by the SAMURAI21/NeuLAND collaboration could have important implications for the study of exotic isotopes and their underlying phases. In the future, they could pave the way for new experiments to test nuclear theories, and potentially lead to unexpected discoveries.
“Our current results suggest the presence of a superfluid phase in 29F and 28“O, which we want to study in detail in the next step, for example by directly measuring neutron correlations and the size of neutron pairs,” Kahlbow said.
“In general, the evolution of pairing interactions towards weakly bound systems is also likely to be important for the equation of state used in modeling neutron stars.”
Calculations made by the researchers also suggest that 29F and 31F nuclei could be halo nuclei (i.e. nuclei in which one or two neutrons orbit far from the nuclear core). In their next studies, they would like to investigate this possibility in an experimental setting.
“Such studies would allow us to learn more about the surprising nuclear structure of neutron-rich nuclei along the fluorine isotope chain,” Kahlbow added. “This entire region of the nuclear map at the edge of existence remains largely unexplored and has only recently become accessible thanks to advances in accelerator technology.”
“Our work thus opens up the possibility of discovering and studying the behavior and surprising properties of extremely neutron-rich nuclei.”
More information:
J. Kahlbow et al, Magicality versus superfluidity around 28O seen from the study of 30F, Physical Exam Letters (2024). DOI: 10.1103/PhysRevLett.133.082501
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