High-spin structure of N\simeq Z nuclei around the A = 72 region

High-spin states have been studied in ⁷²Kr and ⁷²Br using the ⁴⁰Ca + ⁴⁰Ca and ³⁶Ar + ⁴⁰Ca reactions at 164 and 145 MeV, respectively. The properties and configurations of the high-spin bands observed have been interpreted using unpaired cranked Nilsson-Strutinsky (CNS), and for ⁷²Kr, paired cranked relativistic Hartree-Bogoliubov (CRHB) calculations. In ⁷²Kr a new band has been identified that has the properties expected for the doubly aligned S-band configuration. In ⁷²Br the previously known bands have been extended to higher spin. This has lead to a re-interpretation of the configurations.Received: 31 October 2002, Published online: 24 February 2004PACS: 21.10.Re Collective levels - 23.20.Lv transitions and level energies - 27.50. + e      

58Ni: an unpaired band crossing at new heights of angular momentum for rotating nuclei

High-spin states in 58Ni have been investigated by means of the fusion-evaporation reaction 28Si(32S, 2p)58Ni at 130 MeV beam energy. Discrete-energy levels are observed in 58Ni at record-breaking 42 MeV excitation energy and angular momenta in excess of 30h. The states form regular rotational bands with unprecedented high rotational frequencies. A comparison with configuration dependent cranked Nilsson-Strutinsky calculations reveals an exceptional two-band crossing scenario, the interaction strength of which is strongly shape dependent.     

The A = 51 mirror nuclei 51Fe and 51Mn

Eighteen previously unknown γ-ray transitions were identified in the T _z = - 1/2 nucleus ⁵¹Fe following the fusion-evaporation reaction ³²S(²⁸Si,2α1n)⁵¹Fe. The level scheme reaches the fully aligned I ^π = 27/2^- terminating state of the five holes in the f _7/2 shell. The 17/2^- state was found to be isomeric, and the lifetime was measured to be 2.87 ns. The mirror symmetry of ⁵¹Fe and ⁵¹Mn is discussed, and the level scheme of ⁵¹Fe is compared to shell-model calculations. Received: 20 July 2000 / Accepted: 1 August 2000     

High-K multi-quasiparticle states in No

We report results from an experiment on the decay of the high-K isomers in ²⁵⁴No. We have been able to establish the decay from the known high-lying four-quasiparticle isomer, which we assign as a Kπ=¹⁶⁺${\mathrm{K}}^{\pi }={16}^{+}$ state at an excitation energy of Ex=2.928(3) MeV${\mathrm{E}}_x=2.928(3)\text{MeV}$. The decay of this state passes through a rotational band based on a previously unobserved state at Ex=2.012(2) MeV${\mathrm{E}}_x=2.012(2)\text{MeV}$, which we suggest is based on a two-quasineutron configuration with Kπ=¹⁰⁺${\mathrm{K}}^{\pi }={10}^{+}$. This state in turn decays to a rotational band based on the known Kπ=⁸⁻${\mathrm{K}}^{\pi }=8^{-}$ isomer, which we infer must also have a two quasineutron configuration. We are able to assign many new gamma-rays associated with the decay of the Kπ=⁸⁻${\mathrm{K}}^{\pi }=8^{-}$ isomer, including the identification of a highly K-forbidden ΔK=8$\mathrm{\Delta }\mathrm{K}=8$ E1 transition to the ground-state band. These results provide valuable new information on the orbitals close to the Fermi surface, pairing correlations, deformation and rotational response, and K-conservation in nuclei of the deformed trans-fermium region.     

Order-to-chaos transition in rotational nuclei

We have developed a new method to study the order-to-chaos transition in rotational nuclei. Correlations between successive gamma rays are used to determine the average complexity of the intermediate levels and thereby the ratio of the interaction potential between levels to the level spacing. The measured ratios, 0.15 to 1.5, span the range from nearly fully ordered to nearly fully chaotic.     

Superdeformation in the N = Z nucleus 36Ar: experimental, deformed mean field, and spherical shell model descriptions

A superdeformed rotational band has been identified in 36Ar, linked to known low-spin states, and observed to its high-spin termination at Ipi = 16(+). Cranked Nilsson-Strutinsky and spherical shell model calculations assign the band to a configuration in which four pf-shell orbitals are occupied, leading to a low-spin deformation beta(2) approximately 0.45. Two major shells are active for both protons and neutrons, yet the valence space remains small enough to be confronted with the shell model. This band thus provides an ideal case to study the microscopic structure of collective rotational motion.     

Rotational damping in ytterbium nuclei

We have made the first clear measurements of rotational damping widths in nuclei. In a mixture of three Yb nuclei, these widths are 300 +/- 60 keV between 1.2 and 1.5 MeV gamma-ray energy [approximately (37-57)Planck's constant]. Compound damping and motional narrowing are discussed in connection with these results.     

Lifetime measurement of the 2(1)+ state in 20C

Establishing how and when large N/Z values require modified or new theoretical tools is a major quest in nuclear physics. Here we report the first measurement of the lifetime of the 2(1)+ state in the near-dripline nucleus 20C. The deduced value of τ(2(1)+)=9.8±2.8(stat)(-1.1)(+0.5)(syst) ps gives a reduced transition probability of B(E2; 2(1)+→0(g.s.)+)=7.5(-1.7)(+3.0)(stat)(-0.4)(+1.0)(syst) e2 fm4 in good agreement with a shell model calculation using isospin-dependent effective charges.     

Lifetime measurement of the first excited 2+ state in 16C

The lifetime of the 2_+(1) state in 16C has been measured with the recoil distance method using the 9Be(9Be,2p) fusion-evaporation reaction at a beam energy of 40 MeV. The mean lifetime was measured to be 11.7(20) ps corresponding to a B(E2;2_+(1)-->0+) value of 4.15(73)e_2 fm_4 [1.73(30) W.u.], consistent with other even-even closed shell nuclei. Our result does not support an interpretation for "decoupled" valence neutrons.