Abstract
The relative population of high-spin states in 211Po, 211-213At and 214Rn with angular momenta ranging from 39/2 to 25 h populated following projectile fragmentation of relativistic 238U ions are reported for the first time. Delayed γ decays from isomeric (metastable) states have been observed in 24 atomic nuclei from Pt to Rn. Focusing on the highest-spin state identified in each nucleus, the isomeric ratio has been determined experimentally for a total of 23 different isomeric states by calculating the fraction of nuclei produced in a particular isomeric state relative to the total number of identified nuclei. In addition new spectroscopic information regarding isomeric states populated in 195Au, 201T1 and 215Rn was obtained. Experimental isomeric ratios are compared to theoretical models assuming the angular momentum of the final fragment originates purely from holes in the ‘fermi sea’ of the projectile nucleus created following the removal of nucleons in the abrasion phase of the reaction. Both analytical and statistical models are presented, using the sharp-cutoff approximation to describe the population of isomeric states in the projectile-like fragments. We observe that the population of isomeric states with I ≳ 18 h is significantly underestimated by current theory while the population of lower-spin states is generally overestimated. While the latter may be explained by the limitations of the sharp-cutoff approximation for low-spin and/or non-yrast states, the former highlights the requirement for a more detailed theoretical approach to the generation of angular momentum in the final fragments than is currently available. The potential additional sources of angular momentum are discussed. The greater-than-expected population of high-spin states discovered here is surely an advantage to future studies involving the next generation of fragmentation radioactive ion beam facilities such as FAIR (Germany), FRIB (MSU, USA) and RIKEN-RIBF (Japan).