Scientists use new method to study rare isotopes with 'halo' structures

Scientists from institutions including Texas A&M University, Brookhaven National Laboratory, Johannes Gutenberg University Mainz in Germany, the University of Surrey in the United Kingdom, and FRIB measured reactions resulting from the collision of the radioactive isotope beryllium-11 and the stable nucleus carbon-12. The goal of that experiment is to test the ratio method, a new technique for extracting properties of halo nuclei. The team published its results (“Experimental test of the ratio method for nuclear-reaction analysis”) in Physical Review Letters.

The ratio method was first proposed back in 2011 by three theorists (Pierre Capel, professor of theoretical physics at the Institute for Nuclear Physics of Johannes Gutenberg University Mainz; Ronald Johnson, emeritus professor of physics at the University of Surrey in the United Kingdom; and Filomena Nunes, professor of physics at FRIB and in MSU's Department of Physics and Astronomy) to study halo nuclei. A halo nucleus occurs when a core nucleus is surrounded by an orbiting “halo” of  one or two loosely bound neutrons (or protons), making it easy for it to break up. The idea of the ratio method is to determine the structure of halo nuclei from the ratio of their scattering and breakup angular cross sections. The main advantage of this approach is the improved accuracy in the properties extracted from the measurement because the ratio cancels model dependence in the reaction analysis. The challenge is to simultaneously measure the scattering and the breakup. After over a decade of work, the experimental team verified the validity of the ratio method for the well-known beryllium-11 halo. 

Next, the ratio method will be applied to a less well-known case, the carbon-19 halo. An approved experiment at FRIB will measure elastic-scattering and breakup angular cross sections simultaneously for the halo carbon-19 impinging on a carbon-12 target. Researchers will then take the ratio of the quantities over a wide range of angles. It is expected that this measurement will pin down the separation energy of carbon-19 with higher precision than before and provide crucial information on carbon-19’s radial wavefunction. With the help of gamma-ray spectroscopy, the data obtained from the upcoming FRIB experiment may also resolve conflicting structure information about carbon-18, carbon-19, and boron-18. 

The work was supported in part by funding from the U.S. Department of Energy (DOE), the DOE Office of Science Office of Nuclear Physics, and the U.S. National Nuclear Security Administration. 

Michigan State University (MSU) operates the Facility for Rare Isotope Beams (FRIB) as a user facility for the U.S. Department of Energy Office of Science (DOE-SC), with financial support from and furthering the mission of the DOE-SC Office of Nuclear Physics. Hosting the most powerful heavy-ion accelerator, FRIB enables scientists to make discoveries about the properties of rare isotopes in order to better understand the physics of nuclei, nuclear astrophysics, fundamental interactions, and applications for society, including in medicine, homeland security, and industry. User facility operation is supported by the DOE-SC Office of Nuclear Physics as one of 28 DOE-SC user facilities.

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