Unexpected high-energy γ emission from decaying exotic nuclei

The N = 52⁸³ Gaβ decay was studied at ALTO. The radioactive ⁸³Ga beam was produced through the ISOL photofission technique and collected on a movable tape for the measurement of γ-ray emission following beta decay. While β-delayed neutron emission has been measured to be 56-85% of the decay path, in this experiment an unexpected high-energy 5-9 MeV γ-ray yield of 16(4)% was observed, coming from states several MeVs above the neutron separation threshold. This result is compared with cutting-edge QRPA calculations, which show that when neutrons deeply bound in the core of the nucleus decay into protons via a Gamow-Teller transition, they give rise to a dipolar oscillation of nuclear matter in the nucleus. This leads to large electromagnetic transition probabilities which can compete with neutron emission, thus affecting the beta-decay path. This process is enhanced by an excess of neutrons on the nuclear surface and may thus be a common feature for very neutron-rich isotopes, challenging the present understanding of decay properties of exotic nuclei. (authors)