Search Results (17 total)

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.

The linear polarization of γ rays between excited and yrast superdeformed (SD) states in ¹⁹⁰Hg was measured using the four-element CLOVER detectors of the EUROBALL IV γ-ray spectrometer. This measurement shows in a model-independent way that the interband transitions which compete with the highly collective in-band quadrupole transitions are largely enhanced electric dipoles. Not only do these results represent the first measurement of the multipolarity of transitions between different SD states, but they also provide strong evidence for the interpretation of the structures in the SD minimum of the A∼190 region in terms of octupole excitations.

High spin states in ^127,129,131La have been investigated using the ¹⁰⁰Mo(³²S,p4n), E=160 MeV, ¹⁰⁰Mo(³⁴S,p4n), E=155 MeV, and ¹⁰⁰Mo(³⁶S,p4n), E=160 MeV reactions, respectively. γ rays were detected using the EUROBALL IV (^127,131La) and EUROGAM II (¹²⁹La) arrays. The results have enabled the negative parity yrast band, built on the πh_11/2 orbital at low spin, and the lowest lying (π,α)=(+,±1 / 2) bands to be extended to higher spins. The positive parity, α=-1 / 2, bands in ^127,129La and the positive parity, α=1 / 2, band in ¹²⁷La can be interpreted, with the aid of cranked Nilsson-Strutinsky calculations, as examples of smoothly terminating bands in valence space based on both signatures of the π[(g_7/2d_5/2)⁵(h_11/2)²]⊗ν[(g_7/2d_5/2)^x(h_11/2)⁶(d_3/2s_1/2)²] configuration, where x=12,14 for ^127,129La, respectively. The favored α=-1 / 2 bands of these configurations are predicted to terminate at 95 / 2⁺ and 83 / 2⁺, respectively, and experimentally the bands are observed to within two transitions of these terminating states. The data for the positive parity, α=1 / 2, band in ¹²⁹La do not allow any firm conclusions to be made regarding the high spin structure of this band. The positive parity bands in ¹³¹La have a different structure at high spin compared with the lighter nuclei. In this case the bands may be interpreted in terms of a π[(g_7/2d_5/2)⁵(h_11/2)²]⊗ν[(g_7/2d_5/2)¹²(h_11/2)⁸(d_3/2s_1/2)⁴] configuration relative to a ¹⁰⁰Sn core.

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.

In experiments employing fast ion-guide-based on-line mass separation we have identified six new, rare nuclides of highly refractory elements produced in the fission of ²³⁸U by 20-MeV protons. The new isotopes ¹⁰⁵Zr, ¹⁰⁷Nb, ¹⁰⁹Mo, ¹¹⁰Mo, ¹¹³Tc, and ¹¹⁵Ru were identified through (β, K_α x-ray) coincidences and β-delayed γ decay. The observed β half-lives range from 130 ms to approximately 1 s. All our recent β half-life data are compared with predictions of the deformed quasirandom-phase-approximation model. The main uncertainty is found to be the inaccuracy of the decay energy predictions.