Search Results (5 total)

The yrast J^π=8⁺ states in neutron-rich ^70,72,74,76Ni nuclei are predicted to be isomeric. The present paper describes two GANIL experiments. In the first of them a search was made for the 8⁺ isomeric states in ^72,74Ni nuclei via fragmentation of ⁷⁶Ge using the ion γ-decay correlation technique. Although these states were not observed, limits for their lifetimes were determined. In the second experiment the decay spectroscopy of ^70,72Co nuclei was performed using fragmentation of a ⁸⁶Kr³⁶⁺ beam and the new LISE2000 spectrometer. The β delayed γ rays from the decay of ^70,72Co to ^70,72Ni were observed using the EXOGAM germanium detectors. The half life of ⁷²Co was measured to be 62(3) ms and the level sequence of the lowest excited states in ⁷²Ni was suggested, with the 2⁺ state at 1096  keV. An attempt to reproduce the level scheme in terms of shell-model calculations was undertaken. The reasons for the disappearance of the 8⁺ isomer in ⁷²Ni are discussed.

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 nucleus ⁶⁹As was studied using the ⁴⁰Ca(³²S,3p)⁶⁹As reaction at a beam energy of 105 MeV. An extension of the band built on the g_9/2 orbital was observed to exhibit a band crossing at a rotational frequency of 0.511 MeV with an associated alignment of 7ħ. This alignment is interpreted as being due to a pair of g_9/2 neutrons. Total Routhian surface calculations have been carried out which confirm that the shape of this nucleus changes from oblate at low spin to a triaxial prolate shape at intermediate spin.

Two weakly populated rotational bands with energy spacings characteristic of superdeformation (SD) have been found in the ¹⁸⁶W(¹⁸O,xn) reaction at a beam energy of E=113 MeV. One of the bands corresponds to the known sequence in ¹⁹⁶Pb, the other to an extremely weak cascade previously assigned to ¹⁹⁸Pb. Both bands have been extended to higher frequency. The behavior of the dynamic moments of inertia for the known SD bands in the Pb nuclei are discussed. Deformation and pairing self-consistent cranked Woods-Saxon calculations of the scrI⁽²⁾ moments of inertia for Z=80 gap resulting in a slightly larger quadrupole deformation of the Pb nuclei as compared to the Hg nuclei. Thus, there seems to be a slight cancellation between increased deformation and reduced alignment, both effects being rather subtle. In very recent calculations by Satul/a and Wyss [35], it is shown that the low frequency discrepancy in scrI⁽²⁾ is removed, when quadrupole pairing calculations are taken into account, the main effect originating from the Y₂₁ component of the quadrupole pairing interaction.

Lifetimes of states in four of the oblate dipole bands in ^197,198Pb have been measured with the recoil distance technique. Using all the available data on lifetimes and branching ratios, and assuming pure magnetic dipole transitions, we deduce transition probabilities. The results are consistent with weakly oblate collective structures involving high-K proton configurations coupled to rotationally aligned neutrons. Comparsions are made to the theoretical estimates of the Dönau and Frauendorf semi-classical model and the tilted axis cranking (TAC) model.