Gamma-ray spectroscopy of neutron-rich products of heavy-ion collisions

Thick-target γγ coincidence techniques are being used to explore the spectroscopy of otherwise hard-to-reach neutron-rich products of deep-inelastic heavy ion reactions. Extensive γγ coincidence measurements were performed at ATLAS using pulsed beams of 80Se, 136Xe, and 238U on lead-backed 122,124Sn targets with energies 10-15% above the Coulomb barrier. Gamma-ray coincidence intensities were used to map out yield distributions with A and Z for even-even product nuclei around the target and around the projectile. The main features of the yield patterns are understandable in terms of N/Z equilibration. We had the most success in studying the decays of yrast isomers. Thus far, more than thirty new μs isomers in the Z = 50 region were found and characterized. Making isotopic assignments for previously unknown γ-ray cascades proves to be one of the biggest problems. Our assignments were based (a) on rare overlaps with radioactivity data, (b) on the relative yields with different beams, and (c) on observed cross-coincidences between γ rays from light and heavy reaction partners. However, the primary products of deep inelastic collisions often are sufficiently excited for subsequent neutron evaporation, so γγ cross-coincidence results require careful interpretation