A rotational band of nineteen transitions with a moment of inertia 3$ nA of 84£ 2 MeV" 1 has been observed in 152 Dy. The band feeds into the oblate yrast states between 19" and 25" and it is proposed that the lowest member of the band has a spin of 22 + and thus the band extends up to 6Qfr. It is identified as the yrast superdeformed band and its intensity accounts for the whole of the ridge structure seen previously in continuum E y -E r correlations.PACS numbers: 21.10. Re, 23.20.Lv, 27.70.+q The nucleusDy has been extensively studied and three different structures have been identified. The low-spin yrast levels have a pseudovibrational structure 1 which develops into a low-deformation (fi = 0.15) prolate rotational band 2 extending up to 40fr. This band, in the spin region between SR and 38£, lies between 0.5 and 1.5 MeV above the yrast states which have a weak oblate structure formed by particles in equatorial orbits. 3 " 5 At higher spins the y-ray continuum is dominated by a collective E2 bump. 6 Part of this bump has been shown to arise from superdeformed (/J^O^) bands from the existence of ridges with a moment of inertia3 (2) = (85 ±2)H 2 MeV" 1 in E y 'Ey correlation spectra. 7,8 In this Letter we present data showing a discrete-line rotational band extending over nineteen transitions from 602 to 1449 keV with an almost constant energy separation of 47 keV which corresponds to the superdeformed moment of inertia. The major -y-ray decay deexciting the band feeds into the yrast oblate structure between the 19"" and 25"" states and then proceeds via the 60-ns 17 + isomer. Additionally 25% of the decay intensity bypasses this isomer. We propose that the decay process from the bottom of the band is essentially statistical, involving several transitions, and we assign the spin at the bottom of the band to be 22£, thus establishing the spin at the top of the band to be 60fr. This is the first observation of a discrete-line superdeformed band and it extends the spin at which discrete states have been seen from about 46* (e.g., 158 Er, Tj0m etaL 9 ) to 60T.The experiment was carried out on the tandem accelerator at the Daresbury Laboratory using the TES-SAS spectrometer, which consists of a 50-element bismuth germanate (BGO) crystal ball similar to that used in TESSA2 10 with twelve escape-suppressed germanium detectors. 11 The states in 152 Dy were populated by the reaction 108 Pd( 48 Ca,4>7) at 205 MeV with a target consisting of two 500-/ig-cm~2 self-supporting foils isotopically enriched at 95% in 108 Pd. A 15-mgcm" 2 gold catcher foil was positioned 5 cm downstream of the targets such that it was outside the focus of the germanium detectors but within the full detection efficiency of the BGO ball. A total of over 150 million double (Ge-Ge) coincidences were recorded together with the sum energy and number of hits (fold) in the BGO ball. The time difference between the BGO ball and the second-coincidence germanium detector was recorded and enabled most of the neutron-induced events in the germanium detec...
Shape coexistence in the light krypton isotopes was studied in two low-energy Coulomb excitation experiments using radioactive 74 Kr and 76 Kr beams from the SPIRAL facility at GANIL. The ground-state bands in both isotopes were populated up to the 8 + state via multi-step Coulomb excitation, and several non-yrast states were observed. Large sets of matrix elements were extracted for both nuclei from the observed γ -ray yields. Diagonal matrix elements were determined by utilizing the reorientation effect. In both isotopes the spectroscopic quadrupole moments for the ground-state bands and the bands based on excited 0 + 2 states are found to have opposite signs. The experimental data are interpreted within a phenomenological two-band mixing model and model-independent quadrupole invariants are deduced for the relevant 0 + states using the complete sets of matrix elements and the formalism of quadrupole sum rules. Configuration mixing calculations based on triaxial Hartree-Fock-Bogolyubov calculations with the Gogny D1S effective interaction have been performed and are compared both with the experimental results and with recent calculations using the Skyrme SLy6 effective interaction and the full generator-coordinate method restricted to axial shapes.
The neutron-rich (66,68)Ni have been produced at GANIL via interactions of a 65.9A MeV 70Zn beam with a 58Ni target. Their reduced transition probability B(E2;0(+)(1)-->2+) has been measured for the first time by Coulomb excitation in a (208)Pb target at intermediate energy. The B(E2) value for (68)Ni(40) 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 reduced transition probabilities, B E2; 0 gs ! 2 1 , have been measured in the radioactive isotopes 108;106 Sn using subbarrier Coulomb excitation at the REX-ISOLDE facility at CERN. Deexcitation rays were detected by the highly segmented MINIBALL Ge-detector array. The results, B E2; 0 gs ! 2 1 0:222 19 e 2 b 2 for 108 Sn and B E2; 0 gs ! 2 1 0:195 39 e 2 b 2 for 106 Sn were determined relative to a stable 58 Ni target. The resulting B E2 values are 30% larger than shell-model predictions and deviate from the generalized seniority model. This experimental result may point towards a weakening of the N Z 50 shell closure. DOI: 10.1103/PhysRevLett.101.012502 PACS numbers: 23.20.Js, 21.60.Cs, 25.70.De, 27.60.+j Precision measurements in unstable nuclei together with recently developed models of the nucleon-nucleon interaction, stemming from many-body techniques and QCD, show promise to improve our understanding of the finer aspects of the dynamics of the atomic nucleus. One approach to this question is to measure reduced transition probabilities -B E2; 0 gs ! 2 1 -for specific nuclei in the vicinity of a shell closure and to compare these results with calculations based on such models. In particular, one of the pressing questions in nuclear physics today is whether the shell closures, that are well established close to stability, remain so also for isotopes with a more extreme proton-toneutron ratio. Intuitive models, such as the generalized seniority scheme [1], predict that these B E2 values follow a parabolic trend, that peaks at midshell, for a sequence of isotopes between two shell closures. In the following we address the 100 Sn shell closure and consequently present results from measurements in the sequence of neutron-deficient even-mass Sn isotopes. This approach has been made possible by newly developed facilities that produce high-quality radioactive ion beams. Recent measurements in 110;108 Sn [2 -4] consistently deviate from the broken-pair model as given by the generalized seniority scheme and from current large-scale shell-model calculations [2]. Parallel work [4], using intermediate energy Coulomb excitation, suggests a constant trend of the reduced transition probabilities extending to 106 Sn. In this Letter we report results from the first measurements of 108;106 Sn using subbarrier Coulomb excitation. This is the only experiment so far for 106 Sn that has permitted for complete control of the scattering process and thus explicitly fulfills the conditions for safe Coulomb excitation. Our result still deviates significantly from theoretical predictions but indicates a decreasing trend of the B E2 with a decreasing number of valence particles outside of the 100 Sn core. Note that with this Letter three different isotopes have been used for normalization as 112 Sn [2] and 197 Au [4] have been used previously. All three experiments yield similar PRL 101, 012502 (2008)
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