Neutrons and protons in a nucleus can collectively oscillate against one another in what are called giant multipole resonances, a bit like the vibrational modes of a bell struck by a hammer. Researchers need to gather detailed and precise information on each of the multipole resonances to formulate a reliable equation of state for nuclear matter. The first two modes in the series, the isovector (when neutrons and protons oscillate out-of-phase) dipole resonance and the isoscalar (when neutrons and protons oscillate in phase) quadrupole resonance have already been systematically studied for a range of nuclear masses. The isovector giant quadrupole resonance has been much harder to tackle owing to the higher energies and low cross sections for exciting this mode.
Writing in Physical Review Letters, Seth Henshaw at Duke University, North Carolina, and colleagues report their measurements of the isovector giant quadrupole resonance in bismuth-209. The team developed a novel technique to measure Compton scattering of linearly-polarized monoenergetic gamma rays from the High Intensity Gamma Source at Duke. Measuring the asymmetries between horizontally and vertically scattered gamma rays allowed for precise determination of the resonance parameters (energy, width, and strength). Future efforts will involve measurement of other nuclei over a range of masses to compile a comprehensive database. The information thus gained should permit a better understanding of nuclear matter in extreme environments such as neutron stars. – David Voss