Search Results (23 total)

A detailed study of known and new SD bands in Tb isotopes has been performed with the use of the EUROBALL IV γ-ray array. The high-statistics data set has allowed for the extension of known SD bands at low and high spins by new γ-ray transitions. These transitions, as it turns out, correspond to the rotational frequencies where the principal superdeformed gaps (Z=66,N=86) close giving rise to up- or down-bending mechanisms. This enables to attribute the underlying theoretical configurations with much higher confidence as compared to the previous identifications. Five new SD bands have been discovered, three of them assigned to the ¹⁵²Tb and the two others to the ¹⁵¹Tb nuclei. Nuclear mean-field calculations have been used to interpret the structure of known SD bands as well as of the new ones in terms of nucleonic configurations.

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.

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.

The excitation energy of the lowest-energy superdeformed band in ¹⁹⁶Pb is established using the techniques of time-correlated γ-ray spectroscopy. Together with previous measurements on ¹⁹²Pb and ¹⁹⁴Pb, this result allows superdeformed excitation energies, binding energies, and two-proton and two-neutron separation energies to be studied systematically, providing stringent tests for current nuclear models. The results are examined for evidence of a “superdeformed shell gap.”

The ^92-96Zr nuclei were produced as fission fragments following the fusion reactions ²⁸Si+¹⁷⁶Yb and ³¹P+¹⁷⁶Yb at 145 and 152 MeV bombarding energy, respectively. Prompt γ rays emitted in the two reactions were detected with the EUROGAM II and EUROBALL IV arrays. Sequences of γ-ray transitions observed in coincidence were newly assigned to ^93-96Zr. The previously known level schemes have been extended to higher excitation energies and higher spins. The experimental results are discussed in the framework of shell-model calculations with ⁸⁸Sr assumed to be an inert core and the valence protons and neutrons filling the π(2p_1/2,1g_9/2) and ν(2d_5/2,3s_1/2) orbitals.

The three superdeformed (SD) bands in ¹³²Ce and the two SD bands in ¹³¹Ce have been extended to higher spin, following experiments with the Euroball IV spectrometer. The two SD bands in ¹³¹Ce have been linked together. However, despite the relatively high population intensity of the bands (up to 5% of the respective channel), it has not been possible to unambiguously link any of the five SD bands into the low-spin, normally deformed structures of ^131,132Ce. At the highest spins (>60ℏ), some of the bands exhibit the characteristic behavior of smooth band termination, i.e., energetically costly angular-momentum generation near the finite mesoscopic limit. Configuration-dependent cranked Nilsson-Strutinsky calculations have been performed to suggest single-particle configurations for the SD bands and to investigate the possibility of termination in these bands.

Lifetimes have been measured for states in four magnetic-rotational bands in ¹⁹⁶Pd using the Doppler-shift attenuation method. Line shapes of several transitions in each band have been analyzed in γ-ray coincidence spectra measured with the EUROBALL spectrometer array. The B(M1) values deduced from the lifetimes show a decrease with increasing spin within the bands as expected for the shears mechanism. The B(M1) values are compared to calculations within the framework of the tilted-axis cranking model.

Lifetimes have been measured for states in the yrast (band 1) and an excited superdeformed band (band 2) in ¹⁹⁶Pb. The excited band is thought to be built on an octupole vibration, rather than a quasiparticle excitation. Centroid shifts and line shapes of transitions in γ-ray coincidence spectra measured with the EUROBALL spectrometer have been analyzed. The transition quadrupole moments deduced from these data for the yrast and the excited superdeformed band are identical within the experimental uncertainties. This result shows that the excited band is not built on different deformation-driving orbitals compared to band 1. It is compatible with the previous suggestion of an octupole-vibrational excitation for the signature partner bands 2 and 3.

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.

A study of the ¹³⁶Nd nucleus has been performed with the EUROBALL III multidetector to establish the decay-out of the yrast superdeformed (SD) band. Three discrete linking transitions (754, 1456, and 1493 keV) have been discovered, establishing the position of the SD band at 7.03 MeV excitation energy with proposed spin and parity 17^(-) for the lowest observed SD state. Neutron pairing gap parameters for SD shapes have been extracted in Nd isotopes, using the strong-coupling model and odd-even mass difference formulas. The major conclusion of our phenomenological analysis is that the pairing correlations do subsist in the SD configurations of nuclei in the A=130 mass region.

High spin states in ¹⁹³Tl have been populated by the ¹⁸¹Ta(¹⁶O, 6n) reaction at 110 MeV and γ-ray spectroscopy was performed using the EUROGAM II array. In addition to the two known superdeformed (SD) bands, three new superdeformed bands have been identified. The dynamic moments of inertia of two of these new bands display a sudden and opposite change of slope and two of them have identical transition energies within 2 keV. The behavior of their dynamic moments of inertia and their γ transitions allowed us to propose configurations for the proton orbitals involved around the SD shell closure Z=80. The intrinsic proton states identified are the two signature partners of the [411]1/2 and the negative signature of the [651]1/2 orbitals. Their properties are compared to the predictions of two theoretical calculations.

The high-spin normally deformed and superdeformed states of ¹⁹⁴Hg have been populated in an exit channel of the ¹⁶O+¹⁸⁶W reaction at 110 MeV bombarding energy. The Doppler shift analysis of the energies of the γ transitions de-exciting both normally deformed and superdeformed states in ¹⁹⁴Hg indicates a lower recoil velocity of the ¹⁹⁴Hg nucleus compared to that of the ¹⁹⁶Pb nucleus obtained from the 6n exit channel of the complete-fusion compound nucleus. This suggests that incomplete fusion plays the main role in the population of normally deformed and superdeformed states of ¹⁹⁴Hg in this reaction.

The decay-out spectrum of the superdeformed yrast band in ¹⁹²Hg comprises a quasicontinuum, from which about 50 weak discrete transitions of energy between 1 and 3.2 MeV are resolved. The fluctuations of the one-dimensional quasicontinuum spectrum are studied with the fluctuation analysis method, which shows that of the order of a few thousand different transitions are available in the first step of the decay-out cascades. The experimental effective number of transitions is compared to schematic theoretical calculations.

Superdeformed (SD) states of ¹⁹⁰Hg have been studied with the Eurogam Phase 2 γ-ray spectrometer using the ¹⁶⁰Gd(³⁴S,4n) reaction. Two new excited SD bands have been found and identified as belonging to this nucleus, bringing the total number of SD bands in ¹⁹⁰Hg to 4. One of the new bands has a dynamic moment of inertia that is very similar to that of the yrast SD band of ¹⁹⁰Hg and most other SD bands in the A∼190 region. In contrast, the other band has a dynamic moment of inertia which is mainly constant as a function of rotational frequency and exhibits a dramatic increase at the lowest frequencies. The observed dynamic moments of inertia are compared with the results of random phase approximation calculations based on the cranked shell model. Finally, the known excited SD band has been extended towards lower frequencies and new transitions have been found linking this band to the yrast SD band. The extracted B(E1) values of the new linking transitions give further support for the possible octupole vibrational character of this band. © 1996 The American Physical Society.

The six superdeformed bands in ¹⁹³Pb have been studied with the EUROGAM 2 γ-ray spectrometer using the ¹⁶⁸Er(³⁰Si,5n)¹⁹³Pb reaction. The results are discussed in terms of cranked-Hartree-Fock-Bogolioubov-Lipkin-Nogami calculations. From the @FF ⁽²⁾ moment of inertia behavior as function of the rotational frequency and the M1 and E2 decay competition of the superdeformed states, the bands are interpreted as three pairs of signature partners based on quasineutron excitations. Dipole transitions linking two signature partner superdeformed bands have been observed and, for the first time in lead isotopes, the branching ratio B(M1)/B(E2)=0.15±0.04 μ_N²/e² b² has been extracted. © 1996 The American Physical Society.

From the observed competition between M1 and E2 γ transitions deexciting the states of the two signature partner yrast superdeformed bands in ¹⁹³Tl, the i_{13 / 2} (Ω=5 / 2) intruder proton orbital is found to have (g_K-g_R)K / Q₀=0.138±0.008. A model dependent value of g_s^eff=(0.7±0.2)g_s^free is deduced. A saturation of the T² moments of inertia, at rotational frequency ℏω>0.32 MeV, is observed for the two bands. This feature is discussed in terms of exhausted quasineutron alignment in the presence of substantially reduced quasiproton alignment due to Pauli blocking.

An experiment using the Eurogam phase II γ-ray spectrometer confirms the existence of an excited superdeformed (SD) band in ¹⁹⁰Hg and its very unusual decay into the lowest SD band over 3–4 transitions. The energies of the transitions linking the two SD bands have been firmly established, and their angular distributions are consistent with a dipole character. Comparisons with calculations using random-phase approximation indicate that the excited SD band can be interpreted as an octupole-vibrational structure.

The complete spectrum of γ rays following the decay of superdeformed (SD) states in ¹⁹²Hg has been extracted. The spectrum, which comprises a quasicontinuous component and sharp lines, reveals the decay mechanism and defines the excitation energies of the SD band. The decay of SD states results from the coupling between a cold, ordered SD system with a hot, chaotic one with normal deformation.

Dipole transitions linking signature partner superdeformed bands in ¹⁹³Hg have been observed. Measurements of the photon decay branching ratios, taken together with the average superdeformed quadrupole moment measured in neighboring nuclei, enable the absolute M1 strengths to be determined. From these data, using the strong coupling model, we find (g_K-g_R)K/Q₀=-0.14±0.01 (e b)^-1, g_K=-0.65±0.14 with Ω=2.8±0.8. These data are consistent with the superdeformed neutron orbital being [512]5/2^- as predicted by cranked Woods-Saxon calculations.

Self-consistent axial Hartree-Fock plus BCS calculations have been performed in order to analyze the superdeformed single-particle orbitals near the chemical potentials in Hg and Pb nuclei. The influence of an approximate restoration of the particle-number symmetry is investigated. A discussion of available data is made, taking stock of our results, in the framework of the strong coupling model.

An approximate restoration of the particle number symmetry, in the manner of Lipkin-Nogami, is numerically investigated in the context of constrained Hartree-Fock plus BCS calculations. Its effect is assessed in a variety of physical situations like potential energy landscapes in transitional nuclei, shape isomerism at low spin, and fission barriers of actinide nuclei.

Self-consistent calculations of the potential-energy surfaces for triaxial quadrupole deformations have been performed for 15 proton-rich cadmium and samarium isotopes. The light Cd isotopes show the expected transition towards spherical nuclei near the doubly magic ¹⁰⁰Sn nucleus. Proton-rich Sm isotopes are strongly deformed whereas a region of γ instability is found around ¹³⁸Sm and explained by a shell structure at N=76.