Findings hint at a superfluid phase in ²⁹F and ²⁸O

Data collected by the SAMURAI spectrometer at RIKEN’s RI Beam Factory (RIBF) in Japan recently led to the detection of a rare fluorine (F) isotope, known as ³⁰F. This has opened interesting possibilities for the study of rare nuclear structures and corresponding phases, which could in turn help to test various physics theories.

The SAMURAI21-NeuLAND Collaboration, a large group of researchers that includes physicists at RIKEN, from GSI-FAIR and TU Darmstadt in Germany, and at other research facilities worldwide, set out to study the spectroscopy and neutron separation energy of the newly detected ³⁰F isotope.

Their findings, published in Physical Review Letters, hint at the presence of a superfluid state in the isotopes ²⁹F and ²⁸O.

“We are exploring the most neutron-rich nuclei on the chart of nuclides, pushing the boundaries of existence,” Julian Kahlbow, corresponding author for the paper, told Phys.org. “To date, we know the neutron-rich limits for the neon (Ne) and F isotopes, with the last fluorine isotope being ³¹F.

“Our initial goal was to study how nuclear structure behaves under extreme conditions, in particular determining if the nuclear ‘magic numbers’ hold.”

At a neutron number of N=20, nuclear structures typically display a large energy gap. As part of their study, Kahlbow and his colleagues explored the previously reported conflict between neutron-rich Ne and somewhat heavier nuclei, for which this energy gap breaks down, creating what is known as an “Island of Inversion,” against a ²⁸O nucleus that is supposedly twice as “magic.”

“In between these isotopes lie ²⁹F and ³⁰F,” explained Kahlbow. “Nothing is known about ³⁰F because it is unbound and exists for only about 10^-20 seconds, making any measurement very challenging.

“For the first time, my collaborators and I 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 extends also to the F isotopes.”

By measuring the mass of ³⁰F, the researchers were able to gather more information about this particular segment in the chart of nuclides (i.e., a graphical representation of all known isotopes that arranges them based on the number of protons and neutrons in their nuclei). This in turn led to more surprising results.

“³⁰F is an unbound nucleus, meaning it decays within 10^-20 seconds, making direct measurements impossible,” said Kahlbow. “By analyzing the decay products, however, we can reconstruct ³⁰F through the measurement of ²⁹F and a single neutron.”

First, Kahlbow and his colleagues produced an ion beam of ³¹Ne using the BigRIPS fragment separator at the RIBF/RIKEN facility in Japan. This beam, which traveled at about 60% the speed of light, was directed onto a liquid hydrogen target to knock out a single proton, resulting in the production of ³⁰F, which instantly decayed into ²⁹F and one neutron.

Measurements for both the neutron and the ²⁹F isotope were collected at the site where the SAMURAI experiment is taking place. To perform measurements on the neutron, however, the team used a 4-ton neutron detector called NeuLAND, which was shipped from GSI-FAIR research facility in Germany to Japan specifically for this research project.

“This study was a big team effort of 80+ people who collectively ran the experiment, combining expertise from all over the world working at the best accelerator facilities,” said Kahlbow. “In the data analysis, using the measured momentum information of ²⁹F and the neutron, the energy spectrum of ³⁰F is reconstructed in which we successfully identified a ground-state resonance and mass.”

This recent study by the SAMURAI21/NeuLAND collaboration could open new opportunities for research focusing on both the ³⁰F isotope and other interesting isotopes around ²⁸O. This oxygen isotope, 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 showed that the classical nuclear structure breaks down and the ‘magic number’ no longer holds at 20 neutrons (for Z=9, 8),” explained Kahlbow.

“We speculate that ²⁸O and ²⁹F 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 chart of nuclei. The excess neutrons are likely to form pairs and easily scatter between and occupy different energy levels.”

Notably, a pure superfluid regime is rarely encountered throughout isotopes in the chart of nuclides. This phase has previously been found in the heavier Tin (Sn) isotopic chain, in a Cooper-pair like regime, from neutron pairs with large distances between them.

“In our work, we propose superfluidity for the first time at the edge of stability in weakly bound systems,” said Kahlbow. “The possible implication of superfluidity in weakly bound or unbound systems is the change of regime, from that with neutrons at large distance to neutrons in pairs at shorter distance, close to 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 further experiments aimed at testing nuclear theories, potentially leading to unexpected discoveries.

“Our current results suggest the presence of a superfluid phase in ²⁹F and ²⁸O, which we aim to study in detail in the next step, for example by directly measuring the neutron correlations and size of neutron pairs,” said Kahlbow.

“In general, the evolution of pairing interactions towards weakly bound systems is also likely of importance for the equation of states used in the modeling of neutron stars.”

The calculations performed by the researchers also suggest that ²⁹F and ³¹F 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 about the surprising nuclear structure of neutron-rich nuclei along the fluorine isotopic chain,” added Kahlbow. “This entire region of the chart of nuclei at the edge of existence remains largely unexplored and has only recently become accessible due to advances in accelerator technology.

“Our work thus opens the opportunity to discover and study surprising behavior and properties of extremely neutron-rich nuclei.”

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