Population of high-spin isomeric states following fragmentation of238U

Abstract

Isomeric ratios have been determined for 23 metastable states identified in A ≈ 200 nuclei from Pt to Rn near the valley of stability following fragmentation of 238 U. This includes high-spin states with angular momenta ranging from (39/2)h to 25h. The experimental results are discussed together with those of similar experiments performed in this mass region. Isomeric ratios are compared with theoretical predictions where the angular momentum of the fragment arises purely due to the angular momentum of nucleons removed from the projectile. The theoretical yield of low-spin states is generally overestimated. In these cases the assumption of 100% feeding of the isomer may require modification. However, the yield of high-spin isomeric states [I m (39/2)h] is significantly underestimated and highlights the requirement for a more complete theoretical framework in relation to the generation of fragment angular momentum. The enhanced population of high-spin states reported here is advantageous to future studies involving isomeric beams at fragmentation facilities such as the Rikagaku Kenkyusho RI Beam Factory (Japan) and next-generation facilities at the Facility for Antiproton and Ion Research (Germany) and Facility for Rare Isotope Beams (USA).

Review of metastable states in heavy nuclei

“…Figure 23 is an example of the reach of these reactions for populating neutron-rich nuclei, resulting in the discovery of numerous isomers [416]. Studies of the high-spin population in terms of isomeric ratios indicate more favouring of higher-spins than expected [412,420], reaching spins of ≤ 55/2 .…”

Half-life measurements of isomeric states populated in projectile fragmentation

“…The gradient of each function, corresponding to the decay constant λ (s −1 ), was obtained using the χ 2 minimisation method. Previously reported isomeric states were identified in 21 additional nuclei from Pt to Rn and experimental isomeric ratios (the fraction of nuclei populated in an isomeric state relative to the total number of nuclei) were determined in the majority of cases [6]. 195 Au.…”

Application of isochronous mass spectrometry for the study of angular momentum population in projectile fragmentation reactions

“…Recently, isomeric yield ratios have been obtained from γray spectroscopy investigations of angular momentum population of projectile fragments [9][10][11][12][13][14][15][16][17][18]. γ-ray spectroscopy is a powerful technique, which provides valuable contributions to nuclear structure investigations.…”

“…In a storage ring the isomeric states can be stored as fully-stripped ions, thus providing an advantage that the internal conversion channel is disabled. The latter is helpful for the cases where the internal conversion coefficients are experimentally unknown [15]. Owing to dedicated time-of-flight detectors, IMS has a very high detection efficiency and is sensitive to single stored ions [26].…”

“…The question of how the isomeric yield ratio depends on the mass loss is not only related to the understanding of the origin of angular momentum, but plays also an important role in the production and application of isomeric beams [31] as well as spin-aligned beams [32]. In previous works [9][10][11][12][13][14][15][16][17][18], nuclides with the same spin had different structure and different isomer excitation energies, which masked a possible dependence of the isomeric ratios versus the mass loss [11,33]. Thus, in order to remove the nuclear structure effects, we use the isomeric ratio values of the high-spin isomeric 19/2 state in 53 Fe produced using different projectiles: 58 Ni (Ref.…”