Finding an Elusive Giant Resonance
Although nuclei are complex systems of strongly interacting neutrons and protons, they can undergo relatively simple collective motions, in which all nucleons move in unison. In a nuclear reaction, vibrational modes can be excited in nuclei. Experimenters observe such excitations as broad structures at high excitation energy, referred to as “giant resonances”. By studying them, one can learn about the properties of bulk nucleonic matter. Such knowledge is important to understand the properties of neutron stars, for example. Neutron stars, the remnants of supernovae, are very compact objects, only about 10 km in diameter, but very massive (~1.4 times the mass of our sun).
A well-known giant resonance is a mode in which the nucleus as a whole undergoes a breathing-mode type of oscillation, as shown on the left-hand side of the figure. By studying this resonance in variety of nuclei, researchers have learned about the compressibility of nuclear matter, i.e., its ability to be squeezed. In a slightly more complex resonance, the protons and neutrons in the nucleus move out of phase: when the neutrons move inward, the protons move outward, and vice versa, as shown on the right-hand side of the figure. Researchers are very interested to studying the properties of this giant resonance as well, because it provides complementary information. Unfortunately, it has proven difficult to disentangle the signatures of this mode from the background present in experiments. The availability of rare-isotope beams at NSCL has provided experimentalists with a number of new nuclear reactions that can be used to isolate specific types of resonances.
In a work, recently published in PRL 118, 172501 (2017), experimenters at NSCL impinged a beam of unstable berrylium-10 nuclei on a thin foil of silicon-28 nuclei and investigated reactions in which boron-10 nuclei were produced, leaving behind aluminum-28, which, with a small but measurable probability, was excited to a giant resonance. The boron-10 nuclei were characterized with the S800 magnetic spectrograph and used to reconstruct the excitation energy spectrum that contained the signatures of the giant resonance. Gamma rays emitted by the boron-10 following the reaction were used to cleanly select the reaction path that excites the giant resonance of interest. This was enabled by the high resolution of the GRETINA detector array. The successful experiment constitutes a breakthrough for the giant resonance studies. In the future, researchers plan to use this new method to study heavier nuclei, such as lead-208, from which it is easier to extract information about the bulk properties of nuclear matter. For this, higher berrylium-10 beam intensities are required. At FRIB, beryllium-10 will be available at orders of magnitude more intensity, transforming studies of collective motions in atomic nuclei. This effort was part of the Ph.D. thesis work of Mike Scott.
