Successful experiment with FAIR detector in Japan – First measurement of nucleus oxygen-28

Researchers of the GSI Helmholtzzentrum für Schwerionenforschung and the Technical University in Darmstadt, together with an international team, succeeded in producing and detecting the long-sought oxygen atomic nucleus ²⁸O for the first time. The experiment was conducted at the Japanese research center RIKEN. A decisive factor was the first-time use of the meter-high neutron detector NeuLAND, which weighs several tons and was developed for the future accelerator center FAIR (Facility for Antiproton and Ion Research) in Darmstadt. At FAIR, it will be an important component of one of the first experiments to go into operation, starting in 2028. The current results are published in the journal Nature.

The experiment was conducted at the Radioactive Ion Beam Factory (RIBF) at the RIKEN research center in Japan. The ²⁸O nuclei were produced in collisions of accelerated ions of the radioactive fluorine isotope ²⁹F with a hydrogen target, in which a proton was shot out of the fluorine. Subsequently, the decay of the ²⁸O into ²⁴O and four neutrons had to be measured. Thanks to the utilization of the NeuLAND neutron detector setup, four neutrons could be observed in coincidence with the charged remnant nucleus for the first time.

“NeuLAND is being developed at GSI/FAIR and built with the participation of German university groups for the R3B experiment at the FAIR facility. For the current experiment, we flew the detector to RIKEN in Japan and recommissioned it on site,” explains Professor Thomas Aumann, who heads the Research department Nuclear Reactions at GSI/FAIR and holds a professorship for experimental nuclear physics with exotic ion beams at TU Darmstadt. “The realization required an extraordinary effort, in which the Darmstadt groups at GSI/FAIR and the TU Darmstadt made a central contribution.”

The most stable oxygen isotope is composed of eight protons and eight neutrons, while ²⁸O has eight protons and 20 neutrons. Understanding the properties of such extremely neutron-rich nuclei is of great importance for the further development and for tests of modern nuclear theories. These, in turn, form the basis for predicting and understanding properties of neutron-rich nuclei and neutron-rich nuclear matter, which play a major role in our universe, for example in the synthesis of the heavy elements. They are for example produced in collisions of neutron stars, which have recently been detected by multi-messenger astronomy using the measurement of gravitational waves.

“The result impressively highlights the relevance and contribution of the detector setups developed for FAIR, such as in this case the NeuLAND detector, which was essential to conduct the experiment,” says Professor Paolo Giubellino, Scientific Managing Director of GSI and FAIR. “Together with our Japanese colleagues, with whom we have a long-standing successful collaboration, and in an international team of top researchers, we were able to achieve this outstanding result, of which all involved can be very proud.”

The participation of German universities in the development and construction of the R3B NeuLAND detector was substantially supported through the BMBF's collaborative research program. The experiment was funded by the DFG through the collaborative research center SFB 1245 “Atomic nuclei: From Fundamental Interactions to Structure and Stars” at the TU Darmstadt.