Search Results (48 total)

A Gammasphere experiment was carried out to search for triaxial strongly deformed (TSD) structures in ^171,172Hf and the wobbling mode, a unique signature of nuclei with stable triaxiality. Three strongly deformed bands in ¹⁷²Hf and one in ¹⁷¹Hf were identified through ⁴⁸Ca(¹²⁸Te, xn) reactions. Linking transitions were established for the band in ¹⁷¹Hf and, consequently, its excitation energies and spins (up to 111/2ℏ) were firmly established. However, none of the ¹⁷²Hf 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 ¹⁷¹Hf and two previously proposed TSD candidates in ¹⁷⁰Hf and ¹⁷⁵Hf are built on proton (i_13/2h_9/2) 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 ¹⁷⁵Hf has most likely a near-prolate superdeformed shape involving the πi_13/2⊗νj_15/2 high-j intruder orbitals. It is quite likely that the bands in ¹⁷²Hf are similar in character to this band.

High-spin states in ¹²⁶Xe have been populated in the ⁸²Se(⁴⁸Ca,4n)¹²⁶Xe reaction in two experiments, one at the VIVITRON accelerator in Strasbourg using the Euroball detector array, and a subsequent one with ATLAS at Argonne using the Gammasphere Ge-detector array. Levels and assignments made previously for ¹²⁶Xe up to I=20 have been confirmed and extended. Four regular bands extending to a spin of almost I=60, which are interpreted as two pairs of signature partners with opposite parity, are identified for the first time. The α = 0 partner of each pair is connected to the lower-lying levels, whereas the two α = 1 partners remain floating. A fractional Doppler shift analysis of transitions in the strongest populated (π,α)=(-,0) band provides a value of 5.2_0.5^0.4 b for the transition quadrupole moment, which can be related to a minimum in the potential-energy surface calculated by the ULTIMATE CRANKER cranked shell-model code at ε≈0.35 and γ≈5^°. The four lowest bands calculated for this minimum compare well with the two signature pairs experimentally observed over a wide spin range. A sharp upbend at ℏω~1170 keV is interpreted as a crossing with a band involving the j_15/2 neutron orbital, for which pairing correlations are expected to be totally quenched. The four long bands extend to within ∼5 spin units of a crossing with an yrast line defined by calculated hyperdeformed transitions and will serve as important stepping stones into the spin region beyond 60ħ for future experiments.

High-spin states in ¹⁶⁴Lu were populated in the ¹²¹Sb(⁴⁸Ca,5n) reaction at 215 MeV and γ-ray coincidences were measured with the Gammasphere spectrometer. Through this experiment the eight known triaxial superdeformed bands in ¹⁶⁴Lu could be confirmed. Some of these bands were extended to higher as well as to lower spins. Evidence is reported for the first time for weak ΔI=1,E1 transitions linking TSD3 and TSD1. This observation may imply coupling to octupole vibrational degrees of freedom. The decay mechanism is different from the one observed in the neighboring even-N isotopes, which exhibit wobbling excitations built on the πi_13/2 structure with E2(M1),ΔI=1 interband decay. An additional sequence decaying at high spin into TSD1 was observed up to I^π=(50^-). This band has a constant dynamic moment of inertia of ∼70ℏ²MeV^-1 and an alignment that is ∼2ℏ larger than that found for TSD1. A revision of the assumed spin-parity-assignment of TSD2 is based on the observed decay-out to normal-deformed structures. The parity and signature quantum numbers of TSD2 are now firmly assigned as (π,α)=(+,0), in disagreement with the former assignment of (π,α)=(-,1), which was based on the assumption that TSD2 is the signature partner of TSD1. TSD1 and TSD2 show an alignment gain at ℏω∼0.67 and 0.60 MeV, respectively. In TSD1 the involvement of the j_15/2 neutron orbital is suggested to be responsible for the high-frequency crossing.

Rotational structures in the ¹⁶⁹Ta nucleus were studied via the ¹²⁴Sn(⁵¹V, 6n) reaction. These data were obtained as a side channel of an experiment focusing on ¹⁷¹Ta, but the sensitivity provided by the Gammasphere spectrometer proved sufficient for a significant extension of the level scheme of this rare-earth nucleus. Over 170 new transitions and four new band structures were placed in ¹⁶⁹Ta, including the intruder πi_13/2 structure. Linking transitions between all of the sequences were identified, and the relative excitation energies between the different configurations were determined for the first time. The rotational sequences were interpreted within the framework of the cranked shell model.

High-spin states in ¹²⁴Ba were investigated in two experiments using the ⁶⁴Ni(⁶⁴Ni, 4n)¹²⁴Ba reaction at three different beam energies. In-beam γ-ray coincidences were measured with the Euroball and Gammasphere detector arrays. In the experiment with Euroball, the CsI detector array Diamant was employed to discriminate against charged-particle channels. Six new rotational bands were observed in ¹²⁴Ba, and previously known bands were extended to higher spins. One of the bands shows a transition from collective to noncollective behavior at high spins. Configuration assignments are suggested on the basis of comparison with cranked shell model and cranked Nilsson-Strutinsky calculations.

High-spin states in ¹⁶¹Lu were populated in the ¹³⁹La(²⁸Si, 6n) reaction and γ-ray coincidences were measured with the EUROBALL spectrometer. On the basis of these data, the previously known level scheme is extended with new band structures and is partly revised. Configuration assignments are made to all bands based on comparison of experimental properties with cranked shell model calculations. A strongly populated band with parity and signature (π,α)=(+,-1/2) is found to be yrast above spin I≃33. This band shows characteristics resembling those of two triaxial superdeformed bands in this nucleus based on the occupation of the shape-driving i_13/2 proton orbital. This structure, unique to ¹⁶¹Lu within the chain of even-N Lu isotopes, is discussed in terms of a quasiparticle configuration in a local triaxial minimum with a larger triaxiality and a smaller quadrupole deformation than calculated for the i_13/2 proton excitation.

The experimental information on the observed nearly degenerate bands in the N=75 isotones, in particular ¹³⁴Pr⁡ and ¹³⁶Pm, which are often considered as the best candidates for chiral bands, is critically analyzed. Most properties of the bands, in particular, the recently measured branching ratios and lifetimes, are in clear disagreement with the interpretation of the two bands as chiral bands. For I=14–18 in ¹³⁴Pr⁡, where the observed energies are almost degenerate, we have obtained a value of 2.0(4) for the ratio of the transition quadrupole moments of the two bands, which implies a considerable difference in the nuclear shape associated with the two bands. The insufficiency of the near-degeneracy criterion to trace nuclear chirality is emphasized.

The high-spin structure of ¹⁷⁰Hf was investigated using the EUROBALL spectrometer. The previously known level scheme was extended in the low-spin region as well as to higher spins, and several new bands were discovered. In particular, two bands were identified which show the characteristics of triaxial superdeformation. One of these bands is strongly populated, and its excitation energy and spins are established. Configuration assignments are made to the normal-deformed bands based on comparisons of their properties with cranked shell model calculations. The results for the very high spin states provide important input for such calculations.

High-spin states in ¹⁷¹Ta were populated in the ¹²⁴Sn(⁵¹V,4n) reaction at 228 MeV to search for evidence of stable triaxial deformation. Identification of a wobbling sequence based on the previously known πi_13/2 structure would provide a unique signature for this rarely observed shape. No such sequence was identified in these data, which suggests that the island of triaxial strongly deformed bands may be smaller than once thought. However, over 200 new transitions and two new bands were placed in the level scheme and the sequence based on the πi_13/2 orbital could be observed up to spin and parity I^π=(101/2⁺). The relative excitations of all the sequences were determined and the ground state of ¹⁷¹Ta was found to have 5/2⁺ quantum numbers, contrary to previous reports. All of the previously known structures were extended to much higher spin and their high-frequency band crossings are interpreted within the framework of the cranked shell model.

The level structure of ₆₀¹⁴⁰Nd₈₀ has been established up to spin 48 by in-beam γ-ray spectroscopy by use of the ⁹⁶Zr(⁴⁸Ca, 4n) reaction. High-fold γ-ray coincidences were measured with the EUROBALL spectrometer. Twelve new rotational bands have been discovered at high spins. They are interpreted as being formed in a deep triaxial minimum at ɛ₂≈0.25 and γ≈35^°. Possible configurations are assigned to the observed bands on the basis of configuration-dependent cranked Nilsson-Strutinsky calculations.

The γ decay of the nuclei ^124,125Ba has been investigated by means of the EUROBALL spectrometer, coupled to the DIAMANT array of charged-particle detectors, using the reaction ⁶⁴Ni + ⁶⁴Ni at E_beam=255 and 261 MeV. In the nucleus ¹²⁵Ba six new E1 transitions have been found to link opposite-parity bands currently interpreted as νd_5/2(+g_7/2),νh_11/2 structures. The previously unknown J^π=3^- level in the nucleus ¹²⁴Ba has also been identified; its excitation energy is accurately reproduced by a microscopic calculation including octupole correlations. Both issues are bolstered by sizable B(E1)/B(E2) ratios.

Significant extensions to the high-spin excitation spectrum of the N=91, 92, 93 isotopes ^157,158,159Dy have been achieved using the high-efficiency γ-ray spectrometers Euroball and Gammasphere. These nuclei were populated via weak 3n or αxn exit channels in fusion evaporation reactions. In ¹⁵⁷Dy, the yrast band has been extended to I^π=101 / 2⁺ (tentatively to 105 / 2⁺) with four sideband structures (two of which are new) observed in the 35–50ℏ spin range. In ¹⁵⁸Dy, three bands have been extended to 42⁺(44⁺),40^-, and 41^-(43^-), whereas in ¹⁵⁹Dy the yrast band is observed to 81 / 2⁺(85 / 2⁺). The high-spin behavior and band crossing systematics of the Dy isotopes and of the neighboring N = 91, 92, and 93 isotones are discussed in terms of rotational alignments and shape transitions. Cranked Nilsson-Strutinsky calculations without pairing have been performed for detailed comparisons with the very high-spin states observed in ¹⁵⁷Dy.

The γ decay in the quasicontinuum is used to study the order-to-chaos transition in the thermally excited ¹⁶³Er nucleus. The experimental analysis is performed on high-statistics EUROBALL data, focusing on the spin region I≈(20–40)ℏ, and internal excitation energy up of ≈2.5  MeV. The results are compared to cranked shell model calculations for this nucleus, taking into account the dependence on the K quantum number. Two main topics are investigated. First, the validity of the selection rules associated with the K quantum number are studied as a function of the internal energy U above yrast. K-selection rules are found to be obeyed in the decay along discrete unresolved rotational bands up to U≈1.2  MeV, whereas in the interval U≈1.2–2.5  MeV, where the order-to-chaos transition is expected to take place, selection rules are found to be only partially valid. Second, the line-shape analysis of γ-γ coincidence spectra provides a direct experimental measurement of the rotational and compound damping widths (Γ_rot and Γ_μ), yielding values of 200 and 20 keV, respectively, in good agreement with theory.

An unexpected strong interaction between normal deformed and triaxial strongly deformed levels at high spin, ∼32ℏ, is observed in ¹⁶⁷Lu. This constitutes the first observation of accidental degeneracy, at such high spins, causing cross talk between levels in the normal deformed and in the second potential well in any known mass region. Furthermore, evidence of quasi-particle excitations highly competing with the wobbling excitation mode in a triaxial superdeformed well has been established in ¹⁶⁷Lu.

A new high-spin superdeformed band has been discovered in ₆₀¹⁴⁰Nd₈₀. It was populated in the ⁹⁶Zr(⁴⁸Ca,4n) reaction and investigated using the EUROBALL γ-ray spectrometer array. The band is observed in the approximate spin range of I=36 to 66. It is associated with shell gaps around Z=60 and at N=80 at large deformation. These gaps produce a pronounced minimum in the calculated total Routhian surfaces at a quadrupole deformation of ϵ₂=0.45. The new band which lies between the high-deformation bands in the A≈130 region and the superdeformed bands in A≈150 nuclei provides insight into the development of the deformation between these two regions. Two possible configurations are suggested involving four neutrons of i_13∕2 origin (ν6⁴) and either six protons of h_11∕2∕h_9∕2 origin (π5⁶) or five protons of h_11∕2∕h_9∕2 and one of i_13∕2 origin (π5⁵6¹).

Two high-spin regularly spaced rotational bands with large dynamical moments of inertia have been identified in ¹⁷⁵Hf with the Gammasphere spectrometer. These new bands are very similar to the previously identified triaxial superdeformed bands in the hafnium nuclei. However, the new bands in ¹⁷⁵Hf have been linked into the known level scheme and thereby provide the first firm spin assignments for these structures in this region. In order to understand the new bands, theoretical calculations have been performed based on the ULTIMATE CRANKER code. The new bands in ¹⁷⁵Hf are deduced to be built upon highly deformed structures. No experimental evidence for triaxiality was established and this work suggests that the structure of the so-called “triaxial” superdeformed bands in the Hf nuclei may be quite different from those identified in the lighter mass Lu nuclei. Since the two highly deformed bands in ¹⁷⁵Hf are associated with different deformations, this work also identifies the role of the intruder orbits in polarizing the nuclear shape.

High-spin states in ¹²³Cs were populated in the ⁶⁴Ni(⁶⁴Ni,p4n) reaction at a beam energy of 265  MeV. Gamma-ray coincidences were measured using the Gammasphere spectrometer. Two additional bands have been placed in the level scheme and the four previously known bands have been extended to higher spin. At the highest spins, two of the bands show irregular level sequences. These structures of noncollective excitations, which compete with collective rotation, are interpreted as band-terminating states. The results are compared to cranked Nilsson-Strutinsky calculations, and configuration assignments to the bands and to the terminating states are discussed.

The γ decay in the quasicontinuum from selected configurations of the rotational nucleus ¹⁶³Er has been measured with the EUROBALL array. A new analysis technique has allowed for the first time to directly measure the compound and rotational damping widths Γ_μ and Γ_rot. Values of Γ_μ≈20  keV and Γ_rot≈200  keV are obtained in the spin region I≈30–40ℏ︀, in good agreement with microscopic cranked shell model calculations. A dependence of Γ_μ and Γ_rot on the K-quantum number of the nuclear states is also presented.

Lifetimes of states in the triaxial strongly deformed bands of ¹⁶³Lu have been measured with the Gammasphere spectrometer using the Doppler-shift attenuation method. The bands have been interpreted as wobbling-phonon excitations from the characteristic electromagnetic properties of the transitions connecting the bands. Quadrupole moments are extracted for the zero-phonon yrast band and, for the first time, for the one-phonon wobbling band. The very similar results found for the two bands suggest a similar intrinsic structure and support the wobbling interpretation. While the in-band quadrupole moments for the bands show a decreasing trend towards higher spin, the ratio of the interband to the in-band transition strengths remains constant. Both features can be understood by a small increase in triaxiality towards higher spin. Such a change in triaxiality is also found in cranking calculations, to which the experimental results are compared.

The wobbling excitations in the presence of an appreciable amount of alignment are expected to appear more easily at lower angular momenta of the yrast spectra, compared with those in the textbook example. The large B(E2;I→I-1) value for Δn̂=1 transitions where n̂ expresses the number of wobbling phonons is shown to be a strongly increasing function of the triaxiality parameter γ, especially for γ≳+20°, while it is relatively independent of moments of inertia. On the other hand, the relation of the wobbling phonon energy to the total angular momentum may be used to extract quantitative information on nuclear moments of inertia. It is concluded that the γ value of the triaxial, strongly deformed bands in ¹⁶³Lu is about equal to +20° and may be slightly increasing as a function of I.

The nucleus ¹⁶³Lu has been populated through the reaction ¹³⁹La(²⁹Si,5n) with a beam energy of 157 MeV. Three triaxial, strongly deformed (TSD) bands have been observed with very similar rotational properties. The first excited TSD band has earlier been assigned as a one-phonon wobbling excitation built on the lowest-lying (yrast) TSD band. The large B(E2)_out/B(E2)_in value obtainable for one of four observed transitions between the second and first excited TSD bands is in good agreement with particle-rotor calculations for a two-phonon wobbling excitation.

The incomplete fusion reactions ⁷Li→^158,160Gd at beam energies of 8 MeV/nucleon have been used to study the first band crossing region in the heavy stable Dy isotopes ^160,162Dy. The γ rays were detected in the GASP spectrometer in coincidence with fast charged particles detected in the ISIS silicon ball. We succeeded to observe the first backbending in ¹⁶²Dy at a crossing frequency of ħω≈350 keV, a value much higher than expected from other nuclei in this mass region. Moreover, for the first time in a nucleus with a very large interaction strength, the yrare band in ¹⁶⁰Dy could be established up to rather high spin (I=20ħ) allowing for a precise determination of the interaction strength between the ground state and the Stockholm band, |V_g-S|=219(2) keV. Together with |V_g-S|=14(2) keV determined for the corresponding interaction in ¹⁶²Dy, a full oscillation of the strengths from one node to the next could be observed within an isotopic chain. In addition to the ground state and Stockholm bands, many other known bands in the two nuclei were considerably extended to higher spin and the experimental results are compared to calculations within the projected shell model.

The neutron-rich ^66,68Ni have been produced at GANIL via interactions of a 65.9A MeV ⁷⁰Zn beam with a ⁵⁸Ni target. Their reduced transition probability B(E2;0₁⁺→2⁺) has been measured for the first time by Coulomb excitation in a ²⁰⁸Pb target at intermediate energy. The B(E2) value for ⁶⁸Ni₄₀ is unexpectedly small. An analysis in terms of large scale shell model calculations stresses the importance of proton core excitations to reproduce the B(E2) values and indicates the erosion of the N  =  40 harmonic-oscillator subshell by neutron-pair scattering.