Relative quadrupole moments of exotic shapes at ultrahigh spin in 154Er: Calibrating the TSD/SD puzzle

Transition quadrupole moments, Qt, of two ultrahigh-spin, collective structures in 154Er have been measured for the first time using the Doppler Shift Attenuation Method (DSAM). Data were acquired at the ATLAS accelerator facility of Argonne National Laboratory, using the Gammasphere detector array. A thick, gold-backed 110Pd foil was bombarded by a beam of 48Ti ions at 215 MeV. The Qt for each band was determined from the Doppler shift of gamma rays emitted by the resulting recoil nuclei. The extracted transition quadrupole moments are significantly different in magnitude, suggesting the two structures in 154Er represent distinct exotic nuclear shapes, namely axial superdeformed (SD) with Q t 20 eb, and triaxial strongly deformed (TSD) with Qt ≈ 11 eb. Indeed, the results calibrate the quadrupole moments of TSD bands recently measured in light erbium nuclei, 157,158Er

Quadrupole moments of coexisting collective shapes at high spin in 154Er

Four high-spin collective bands have been populated in 68154Er86 via the 110Pd(48Ti,4nγ)154Er reaction. Average transition quadrupole moments Qt have been measured for three of the bands by using the Doppler-shift attenuation method. The strongest band has a value of Q t=11.0±1.0eb, similar to values found recently for four triaxial strongly deformed (TSD) bands in 157,158Er. The second band has a value of Qt=19.5±3.2eb, consistent with a predicted axially symmetric superdeformed (SD) shape, similar in deformation to the 152Dy isotone, and is used as a calibration point. The third, new band has a value of Qt=9.9±2.2eb. The results confirm the unexpectedly large Qt moments for the favored TSD bands in light erbium isotopes

High-spin yrast structure of 159Ho

An investigation of the yrast structure of the odd-Z 159Ho nucleus to high spin has been performed. The 159Ho nucleus was populated by the reaction 116Cd(48Ca,p4nγ) at a beam energy of 215 MeV, and resulting γ decays were detected by the Gammasphere spectrometer. The h11/2 yrast band has been significantly extended up to Iπ=75/2- (tentatively 79/2-). A lower frequency limit for the second (h11/ 2)2 proton alignment was extracted consistent with the systematics of this alignment frequency, indicating an increased deformation with neutron number in the Ho isotopes. The energy-level splitting between the signature partners in the h11/2 structures of the Ho isotopes and the neighboring N=92 isotones is discussed

High-spin proton alignments and evidence for a second band with enhanced deformation in 171Hf

High-spin properties of the nucleus 171Hf were studied through the 128Te(48Ca,5n) reaction. Previously known bands have been extended to significantly higher spins and four new bands have been extracted from these data. The results are discussed within the framework of the cranked shell model aided by a comparison with level structures in the neighboring nuclei. The band crossings at rotational frequencies ∼500 keV are interpreted as caused by the alignments of h11/2 and h9/2 proton orbitals. Band ED2 exhibits an alignment pattern similar to that of band ED1 which was reported in a recent paper and proposed to be built on a second potential energy minimum involving the deformation-driving proton i13/2 - h9/2 configuration. It is likely that band ED2 is also associated with a deformation enhanced with respect to that of the normal deformed structures. Further experimental investigation is needed to ascertain the nature of this band

Rotational alignments in Np235 and the possible role of j15/2 neutrons

The role j15/2 neutron orbitals play in the transuranic region of actinides has been studied by exploring γ-ray transitions between yrast states in Np235, populated utilizing the nucleon-transfer reaction Np237(Sn116,Sn118). Two rotational sequences, presumably the two signatures of the ground-state band, have been delineated to high spin for the first time, with the α=+1/2 and α=-1/2 signature partners reaching 49/2 (tentatively 53/2+) and 47/2+ (tentatively 51/2+), respectively. Definite isotopic assignments for these in-band transitions were established through γ-ray cross correlations between Np235 and Sn118 and events where at least three γ rays corresponding to neptunium-like particles were detected. These transitions reveal clear upbends in the aligned angular momentum and kinematic moment of inertia plots; such a phenomenon could indicate a strong interaction between an aligned νj15/2 configuration crossing the ground-state band in Np235, which is based on a πi13/2 orbital. However, the lack of any signature splitting over the observed frequency range of the Np235 rotational sequences cannot remove the possibility of a πh9/2 assignment for the observed band. The role of the νj15/2 and πi13/2 alignment mechanisms in the deformed U-Pu region is discussed in light of the current spectroscopic data and in the context of the cranked-shell model

Structure changes in Er160 from low to ultrahigh spin

A spectroscopic investigation of the γ decays from excited states in Er160 has been performed in order to study the changing structural properties exhibited from low spin up toward ultrahigh spin (I~60). The nucleus Er160 was populated by the reaction Cd116(Ca48,4nγ) at a beam energy of 215 MeV, and resulting γ decays were studied using the Gammasphere spectrometer. New rotational structures and extensions to existing bands were observed, revealing a diverse range of quasiparticle configurations, which are discussed in terms of the cranked shell model. At spins around 50 there is evidence for oblate states close to the yrast line. Three rotational bands that have the characteristics of strongly deformed triaxial structures are observed, marking a return to collectivity at even higher spin. The high-spin data are interpreted within the framework of cranked Nilsson-Strutinsky calculations

New structures in 178Hf and coulomb excitation of isomers

A 985 MeV 178Hf beam was Coulomb excited by a 208Pb target at the ATLAS accelerator of Argonne National Laboratory. Gammasphere and the CHICO particle detector recorded particle-γ coincidence data. The aim was to populate and determine the mechanism of previously observed Coulomb excitation of the Kπ = 6+ (t1/2 = 77 ns), 8- (4 s) and 16+ (31 y) isomer bands. New rotational bands were identified including an aligned band which appears to mix with the ground-state band (GSB) and the γ-vibrational band above ∼ 12 ℏ of angular momentum. Newly observed γ-decay transitions into the three isomer bands may elucidate the K-mixing which allows Coulomb excitation of these isomer bands, but direct decays from the GSB into the 16+ isomer band have not yet been confirmed

Nuclear shapes of highly deformed bands in Hf171,172 and neighboring Hf isotopes

A Gammasphere experiment was carried out to search for triaxial strongly deformed (TSD) structures in Hf171,172 and the wobbling mode, a unique signature of nuclei with stable triaxiality. Three strongly deformed bands in Hf172 and one in Hf171 were identified through Ca48(Te128, xn) reactions. Linking transitions were established for the band in Hf171 and, consequently, its excitation energies and spins (up to 111/2) were firmly established. However, none of the Hf172 sequences were linked to known structures. Experimental evidence of triaxiality was not observed in these bands. The new bands are compared with other known strongly deformed bands in neighboring Hf isotopes. Theoretical investigations within various models have been performed. Cranking calculations with the Ultimate Cranker code suggest that the band in Hf171 and two previously proposed TSD candidates in Hf170 and Hf175 are built on proton (i13/2h9/2) configurations, associated with near-prolate shapes and deformations enhanced with respect to the normal deformed bands. Cranked relativistic mean-field calculations suggest that band 2 in Hf175 has most likely a near-prolate superdeformed shape involving the πi13/2νj15/2 high-j intruder orbitals. It is quite likely that the bands in Hf172 are similar in character to this band

Quadrupole moments of collective structures up to spin ̃65h in 157Er and 158Er: A challenge for understanding triaxiality in nuclei

The transition quadrupole moments, Qt, of four weakly populated collective bands up to spin ̃65h in 157,158Er have been measured to be ̃11 eb demonstrating that these sequences are associated with large deformations. However, the data are inconsistent with calculated values from cranked Nilsson-Strutinsky calculations that predict the lowest energy triaxial shape to be associated with rotation about the short principal axis. The data appear to favor either a stable triaxial shape rotating about the intermediate axis or, alternatively, a triaxial shape with larger deformation rotating about the short axis. These new results challenge the present understanding of triaxiality in nuclei

Non-yrast positive-parity structures in the γ-soft nucleus Er156

Weakly populated band structures have been established in Er156 at low to medium spins, following the Cd114(Ca48,6nγ) reaction at 215 MeV. High-fold γ-ray coincidence data were recorded in a high-statistics experiment with the Gammasphere spectrometer. Bands built on the second 0+ and 2+ (γ-vibrational) states have been established. A large energy staggering between the even- and odd-spin members of the γ-vibrational band suggests a γ-soft nature of this nucleus. An additional band is discussed as being based on a rotationally aligned (νh9/2,f 7/2)2 structure, coexisting with the systematically observed, more favorable (νi13/2)2 aligned structure seen in this mass region