The population of metastable states as a probe of relativistic-energy fragmentation reactions

Abstract

Ac following the projectile fragmentation of a 1 A.GeV 238 U beam by a 9 Be target at GSI Helmholtzzentrum für Schwerionenforschung. The fragments were separated in the fragment separator (FRS) and identified by means of energy loss and time-of-flight techniques. They were brought to rest at the centre of the RISING gamma-ray detector array and intensities of gamma-rays emitted in the decay of isomeric states with half-lives between 100 ns and 40 µs and spin values up to 55/2 were used to obtain the corresponding isomeric ratios. The data are compared to theoretical isomeric ratios calculated in the framework of the abrasion-ablation model. Large experimental enhancements are obtained for high-spin isomers in comparison to expected values.The need for an understanding of relativistic heavyion collisions spans a number of scientific fields, from the safety of human space exploration [1] and cosmic-ray astrophysics [2], to the structure of the early Universe [3] and the generation of radioactive ion beams [4]. Nevertheless, the complexity of the nuclear-reaction processes and the large number of possible product isotopes have resulted in only limited tests of reaction models, which have focused mainly on hydrodynamic properties [5], isotope yields [6], and momentum distributions [7].In addition, the angular-momentum degree of freedom has the potential to reveal important aspects of the reaction dynamics. However, this requires special circumstances for study to be possible. Following a given collision, the excited nuclear products typically de-excite in less than 10 −8 s. Such a short time is insufficient to apply separation techniques that would enable the initial excitation energies and angular momenta to be determined, since the de-excitation radiations all occur in close proximity to the reaction target.A breakthrough came with the ability to separate the products of projectile-fragmentation reactions according to their mass and charge [8], combined with the detection of γ rays from nuclear isomeric states [9,10]. Excellent sensitivity was achieved for isomer half-lives in the 0.1-100 µs range, after the recoiling ions had been transported to a remote measurement station in less than 1 µs. In the present context, a key feature of nuclear isomers is that in many cases they carry high angular momentum, which is itself closely associated with their extended half-

Population of high-spin isomeric states following fragmentation of238U

“…The results of the current study and that of a parallel work [10] show a similar behavior of the isomeric ratios to that observed in Ref. [9] in heavy nuclei at high spins.…”

“…The largest difference observed between two experimental data points is for the (29/2 + ) state in 215 Ac [52] ( A = 23), where the isomeric ratio measured in Ref. [10] is just over a factor of 4 greater than the value of R exp presented in Ref. [9] (and neither of these measurements agree within uncertainty limits).…”

“…The highest-spin state for which an isomeric ratio has been measured in this mass region is the I π m = (55/2 + ) state in 213 Rn [42], obtained in the study by Bacelar et al [10]. Table II also shows the number of nuclei identified (from characteristic delayed γ rays) and the total number of isomeric ratios measured in each work.…”

“…In Ref. [10] the number of isomeric ratios presented is significantly greater than the number of identified nuclei. This is due to the yield of several isomeric states having been measured in the same nucleus, for example, Data from the literature summarized in Table II are plotted in Fig.…”

“…R exp is plotted against the angular momentum of the isomeric state of interest and includes measurements taken from the current work and from Refs. [1,9,10,48]. The mass loss A of the fragments compared with the beam is shown on the secondary x axis.…”