The quest for improved neutron capture cross sections for advanced reactor concepts, transmutation of radioactive wastes as well as for astrophysical scenarios of neutron capture nucleosynthesis has motivated new experimental efforts based on modern techniques. Recent measurements in the keV region have shown that a 4π BaF 2 detector represents an accurate and versatile instrument for such studies. The present work deals with the potential of such a 4π BaF 2 detector in combination with spallation neutron sources, which offer large neutron fluxes over a wide energy range. Detailed Monte Carlo simulations with the GEANT package have been performed to investigate the critical backgrounds at a spallation facility, to optimize the detector design, and to discuss alternative solutions.
High-spin states in 174 W (Z = 74) have been populated using the reaction 128 Te( 50 Ti, 4n) 174 W at beam energies of 215 and 225 MeV. The Gammasphere array was used to detect the γ rays emitted by the evaporation residues. Four previously known collective band structures have been extended, and 16 new rotational sequences observed. Two are built upon isomeric states, one corresponding to a two-quasiparticle K = 8 isomer, the other to a four-quasiparticle K = 12 isomer, with the latter exhibiting strong K-violating K = 12 decays to the ground state band. Nucleonic configurations for the two-and four-quasiparticle excitations are proposed, and Woods-Saxon cranking calculations are presented to understand the rotational structures. Decay mechanisms of multi-quasiparticle K isomers are discussed in terms of the prevalent phenomenological models, with special emphasis on γ -tunneling calculations. Surprisingly, the latter underpredict the decay hindrance for the K = 12 isomer by three orders of magnitude, unlike all other isomer decays in this mass region.
Pulsed238 U and 208 Pb beams have been used to populate multi-quasiparticle high-K isomers in neutron-rich hafnium isotopes at and beyond the line of -stability, via inelastic excitation and transfer. Spectroscopic properties and configuration assignments of several new high-K isomers are compared with earlier theoretical predictions. A striking example of the robustness of the K quantum number is demonstrated by the observed competition between E1 and E3 decay modes in 180 Hf, the heaviest stable isotope of the element. ͓S0556-2813͑99͒50503-7͔PACS number͑s͒: 21.10. Tg, 23.20.Lv, 27.70.ϩq, 29.30.Kv The conditions under which K, the projection of the aligned nucleonic spins on the symmetry axis in deformed nuclei ͓1͔, is a good quantum number remain a topic of much current interest. On the one hand, there are striking examples of K conservation at high spins, such as the K ϭ16 ϩ isomer in 178 Hf, with t 1/2 ϭ31 yr ͓2͔. On the other, experiments with the new generation of detector arrays have isolated multiquasiparticle ͑multi-qp͒ K isomers in the AϷ180 region with unexpectedly fast decay rates, suggesting a severe breakdown of K-selection rules ͓3,4͔. Clearly, a systematic study of the properties of K isomers as a function of different parameters, such as the number of quasiparticles in the isomeric configuration, the spin and excitation energy of the isomers, and the magnitude of the K quantum number are needed, preferably over a wide range of N/Z ratios, from the most neutron-deficient to the most neutron-rich nuclei.The Hf(Zϭ72) isotopic chain is rich in K isomers, due to a robust axial symmetry and the availability of multiple high-⍀ orbitals near the Fermi surface. High-K isomers have long been predicted in isotopes heavier than 180 Hf ͓5͔, the heaviest stable Hf isotope. Experimental information to date on neutron-rich Hf nuclei, however, has been limited, primarily because fusion reactions with stable beam-target combinations are unable to populate neutron-rich nuclei beyond the line of  stability. Recent advances with inelastic and transfer reactions in populating high spins in target-and projectile-like fragments ͓6-10͔ have motivated our current investigation of high-K isomers in the AϷ180 region using these techniques. Our pilot experiment, with pulsed 238 U beams on the heaviest stable isotopes of Lu(Zϭ71), Ta(Z ϭ73), and W(Zϭ74), was very successful in populating new multi-qp isomers in the target nuclei ͓10͔. This article presents the results of our follow-up experiments with Hf(Zϭ72) targets, where inelastic excitation and transfer mechanisms were used to populate several new 4-qp K isomers in a 180 Hf target, as well as other new K isomers in neutron-rich Hf nuclei. Prior to our present study, only very long-lived 2-qp 8 Ϫ isomers were known in the even-even 180,182,184 Hf nuclei, the first two from -decay studies ͓2͔, and the last one from a recent experiment using a transfer reaction ͓11͔.In our primary experiment, thick targets of isotopically enriched
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