The decay-out of superdeformed bands in theA=190 region: What have we learned?

One-step decay transitions linking the superdeformed (SD) bands 1 and 3 in¹⁹⁴Hg to yrast levels are discussed. Inter-band transitions between bands 1 and 3 have also been identified. For the first time, the spin, parity and excitation energy have been determined for two SD bands in the same nucleus. The low excitation energy of the excited band supports the view that it is based on an octupole excitation. It is believed that Porter-Thomas fluctuations play a major role in determining the strength of the one-step transitions as suggested by the fact that only one other SD band has been linked in theA=190 mass region (¹⁹⁴Pb) at the present time. When Porter-Thomas fluctuations prevent the observation of one-step or two-step linking transitions, as e.g. in the case of¹⁹²Hg, the analysis of the quasi-continuous part of the decay-out spectrum provides an alternative method for the determination of the excitation energy and spin of an SD band. This method is discussed in detail.     

High spin yrast states in even platinum isotopes

The high spin yrast states up toJ=20⁺ in^{184, 186}Pt and^{190, 192, 194}Pt are studied in a microscopic approach of variation with number-conserved projected states. The energy spectra, quadrupole moments andB(E2) values are calculated by employing the Hamiltonian with quadrupole plus pairing interactions. The results of the calculations are in fair agreement with the available experimental data.     

Observation of the single step links of the yrast superdeformed band in 194Pb

The EUROGAMarray has been used to investigate the decay out of the yrast superdeformed (SD) band in ¹⁹⁴Pb. Eigth single step decays from the lowest observed SD states to low-lying states at normal deformation (ND) have been identified. From this observation, the excitation energy (4877 ± 1.5 keV) and the spin (6⁺) of the lowest observed SD state in ¹⁹⁴Pb are established.     

Quadrupole moments of superdeformed bands in 193Tl

Lifetimes of states in the two strongest superdeformed (SD) bands in ¹⁹³Tl were measured using the Doppler-shift attenuation method. The reaction ¹⁷⁶Yb(²³Na,6n)¹⁹³Tl at a beam energy of 129 MeV was used and γ-rays were detected by the Gammasphere array. Quadrupole moments of 18.3(10) eb and 17.4(10) eb were extracted for SD bands 1 and 2, respectively, using the fractional Doppler-shifts of the SD transitions. The previously reported linking transitions of these SD bands to normal deformed near yrast levels could not be confirmed. No other candidates for linking transitions could be established. Received: 27 April 1999     

Study for Superdeformed Bands in A～190 Region with Quantum Group Uqp(u2) Model

superdeformed bands in the A～190 region are systematically analyzed by using the twoparameter expansion of quantum group U_qp(u₂) Model. The calculated results of E2 transition γ ray energies are in agreement with experimental data. According to three different methods for the spin assignment of rotational bands, the spin values of the band head I₀ of the yrast SD bands in ¹⁹⁴Hg(1) and ¹⁹⁴Pb(1) are assigned. Furthermore, the physical meaning of the nuclear softness is discussed, and these nuclear softness values of a signature partner are almost equal.     

Superdeformed bands in194Tl

Two superdeformed (SD) bands have been found in¹⁹⁴Tl using the reactions¹⁷⁶Yb(²³Na,5n) and¹⁸¹Ta(¹⁸O,5n). The mass assignments were made from excitation function data. These are the first SD bands found in the mass-190 region that are not in Hg nuclei. The energies of many of the transitions of one of these new SD bands are similar in energy to those in the SD band of¹⁹²Hg.     

Lifetimes of triaxial superdeformed states in 163Lu and 164Lu

Lifetimes of states in the yrast superdeformed bands of ¹⁶³Lu and ¹⁶⁴Lu were determined in a Doppler-shift attenuation-method experiment. From fractional Doppler shifts and line shapes, average transition quadrupole moments, Q _t = 8.2_-0.6 ^+1.0 b and 7.1_-0.6 ^+0.5 b, were deduced for one of the bands in ¹⁶³Lu and ¹⁶⁴Lu, respectively. These values are much larger than the quadrupole moment of the normal-deformed yrast band in ¹⁶³Yb, Q _t = 4.9_-0.4 ^+1.3 b, that was also determined in this experiment. Comparison to cranking calculations indicates that both superdeformed bands correspond to a local potential energy minimum with a pronounced triaxiality, γ∼ 20^°. Received: 8 November 2001 / Accepted: 9 January 2002     

Shape changes at high spin in 78Kr

High-spin states in ⁷⁸Kr have been studied via the ⁶³Cu (¹⁹F, 2p2n)⁷⁸Kr reaction at a beam energy of 60 MeV using the Indian National Gamma Array (INGA). In this nucleus, lifetimes have been measured upto the I^π=22⁺ level in the yrast positive-parity band and upto the I^π=15^- level in the negative-parity band using the Doppler Shift Attenuation Method (DSAM). The deduced transition quadrupole moments Q_t's are found to decrease with rotational frequency for both the bands.     

Investigation of the Type of Quadrupole Pairing Force in Superdeformed Nuclei

The variations of moments of inertia of superdeformed (SD) bands ¹⁹⁴Hg(1)and ¹⁹²Hg(1) with angular momentum (rotational frequency) are investigated using the particlenumber conserving (PNC) method for treating the cranked shell model. Calculations show that if both the monopole and Y₂₀ quadrupole pairing forces are taken into account, the observed co variation of the moments of inertia of ¹⁹⁴Hg(1) and ¹⁹²Hg(1) can be reproduced very well both in the low-and high- ranges, in particular the J⁽²⁾ downturn of ¹⁹⁴Hg(1) and the flattening in J⁽²⁾ of ¹⁹²Hg(1) at>0.40MeV. On the contrast, the calculated J⁽²⁾'s with the Y_2±1 or Y_2±2 quadrupole pairing forces are in disagreement with the experiments. The microscopic mechanism of the co variation of J⁽²⁾ is clearly exhibited in the PNC calculation (the contributions to J⁽²⁾ from various major Shells and individual cranked Nilsson orbitals, the occupancy of each cranked Nilsson Orbital etc). The variation of moment of inertia is the result of the competition among the shell effect (single particle motion in a deformed potential), pairing correlation, Pauli blocking effect and Coriolis (anti-pairing) interaction.     

Back-bending in182,184,186osmium

The high spin yrast states uptoJ=22⁺ in^182,184,186Os are studied in a microscopic variational approach with number-conserved projected states. The energy spectra, quadrupole, moments andB (E2) values are calculated by employing the Hamiltonian with quadrupole plus pairing interactions. The results of the calculations are in fair agreement with the available experimental data.     

Comparative quadrupole moments of triaxial superdeformed states in 163, 164, 165Lu

Average transition quadrupole moments in the yrast triaxial superdeformed bands of ¹⁶³Lu, ¹⁶⁴Lu and ¹⁶⁵Lu were determined in a Doppler-shift attenuation-method experiment. Fractional Doppler shifts were determined in γ-ray coincidence spectra measured with the Gammasphere array. The transition quadrupole moments derived from these data show a decrease from ¹⁶³Lu to ¹⁶⁵Lu which is not predicted by total-energy surface calculations. Received: 12 August 2002 / Accepted: 23 October 2002 / Published online: 10 December 2002 RID="a" ID="a"e-mail: schoenwa@iskp.uni-bonn.de Communicated by D. Schwalm     

Observation of superdeformed bands in194Hg

Two rotational bands, with energy spacings characteristic of superdeformed shapes, have been observed following bombardment of¹⁵⁰Nd with⁴⁸Ca. The more intensively populated band consists of 18 transitions and is assigned to¹⁹⁴Hg. The depopulation of this band occurs around spin 10. The second band, consisting of at least 16 transitions, was populated less strongly and is tentatively assigned to¹⁹⁴Hg also. The lowest level in this band is assigned spin 8. The energy differences between transitions for both bands decrease from ～40 keV at low rotational frequencies to ～30 keV at the highest observed frequencies. The moments of inertia of the bands are similar to those of the two previously observed superdeformed bands in^191,192Hg. The similarities and differences of the four known bands in the mercury region are discussed.     

超变形核中四极对力形式的探讨

用粒子数守恒方法分析了对力对于超变形(SD)带的转动惯量随角动量(角频率)变化的规律.计算中如包含了单极对力和Y₂₀四极对力,晕SD带¹⁹⁴Hg(1)和¹⁹²Hg(1)的转动惯量在观测的全部角动量范围中的变化规律,都可得到极好的重现,特别是ω≥0.40MeV时,¹⁹⁴Hg(1)的J⁽²⁾下弯和¹⁹²Hg(1)J⁽²⁾的变平.而考虑Y_2±1和Y_2±2四极对力的计算结果,都与实验不符.粒子数守恒计算中极清楚地展现了J⁽²⁾随ω变化的微观机制(可分别给出各大壳、各推转Nilsson能级上的粒子填布几率和对J⁽²⁾的贡献).J⁽²⁾随ω变化是粒子壳效应(变形场中单粒子运动)、对关联、Pauli堵塞效应以及Chriolis(反配对)作用相互竞争的结果.     

New results on the superdeformed 196Pb nucleus: decay of the excited bands to the yrast band

The study of the superdeformed ¹⁹⁶Pb nucleus has been revisited using the EUROGAMphase 2 array. In addition to the known yrast and two excited SD bands, a third excited SD band has been found. All of the three excited bands were found to decay to the yrast SD band through presumably E1 transitions. Comparisons with calculations using RPAapproximation indicate that the excited bands can be interpreted as octupole-vibrational structures.     

Energies and electromagnetic moments of deformed nuclei in the rare-earth region

Strutinsky-type cranking calculations with inclusion of pairing correlations have been performed for the rare-earth nuclei ^{156, 158, 164}Dy and ¹⁶⁴Er. The pairing effects contribute significantly and with their inclusion the calculated yrast spectra agree very well with experiments. Using Hartree-Fock-Bogouliubov cranking wave functions we have calculated the magnetic moments and quadrupole moments for states up to spin $\text{I = 20}\text{h}\text{?}$. The quadrupole moments are found to be constant over the whole spin range. The gyromagnetic factors g(I) show a strong I-dependence for ^{156, l58}Dy, a weaker one for ¹⁶⁴Er and none for ¹⁶⁴Dy. The sensitivity of this spin dependence on the single-particle occupation and the pairing degrees of freedom is studied. It is found that the spin variation of the gyrofactors is a rotational alignment effect.     

Projected shell model study of neutron-deficient 122Ce

The observed excited states of ¹²²Ce nucleus have been studied in the framework of projected shell model (PSM). The yrast band has been studied up to spin 26ħ. The first band crossing has been predicted above a rotational frequency of 0.4 MeV/ħ that corresponds to first backbending. The calculation reproduces the experimentally observed ground state band up to spin 14 ħ. The electromagnetic quantities, transition quadrupole moments and g-factors are predicted and there is a need to measure these quantities experimentally.     

Time-decay history of normal-deformation high-spin states in 193,194Hg

The time evolution of the decay of high-spin, normal deformed structures in ^193,194Hg was studied by measuring, in the Gammasphere array, the Doppler shift attenuation of selected transitions depopulating various bands. The data suggest that a fraction of states feeding selective yrast levels is non-collective in nature and show resemblance to A = 150 nuclei where shape polarizing effects have been linked to particles occupying high-j orbitals.     

Collectivity in neutron-deficient Pb and Po nuclei

Lifetimes of low-lying yrast states in ¹⁸⁶Pb and ¹⁹⁴Po have been determined by employing, for the first time, the recoil-decay tagging technique in recoil distance Doppler-shift lifetime measurements. In addition, lifetime measurements of prolate states in ¹⁸⁸Pb up to the 8⁺ state were carried out using the recoil gating method. The deformation parameters and for the prolate (^186,188Pb) and the oblate (¹⁹⁴Po) bands, respectively, have been extracted from the measured lifetimes. 27.70.+q–     

Rotational and single-particle structures in83Zr

The band structure of⁸³Zr has been investigated using the Daresbury recoil separator and TESSA array with 20 Compton suppressed Ge detectors and the reaction²⁸Si (⁵⁸Ni, 2pn) at 195 MeV. DCO ratios have been used to make many new spin assignments. The three known rotational bands have been extended up to the onset of the five quasiparticle bands. On the basis of the 21/2⁺, 23/2⁺ and 25/2⁺ yrast and yrare states, a (3qp) structure similar to those found in the isotones⁷⁹Kr and⁸¹Sr has been identified. Additional bands have been observed for the first time, one of which appears to be non-collective. Strong mixing between the various negative parity bands is evident. A multiplet of states feeding the three quasiparticle region of the negative parity yrast band has been found. The data is interpreted in the framework of the cranked shell model.     

Dipole and quadrupole cascades in the yrast region of 143Gd

The high spin states of ¹⁴³Gd have been studied via the ¹¹¹Cd(³⁵Cl,1p2n) reaction at 170 MeV. One dipole cascade has been newly found and two dipole cascades have been extended to higher spin states. A relatively long E2 cascade consisting of irregular transition energies has been found, which has only weak connections to the other dipole and quadrupole cascades. This resembles the ones recently found in ^142,143,144Eu and ¹⁴⁴Gd. The dipole and quadrupole cascades have been found to appear by turns in an energy increment of about 0.2 MeV above the yrast line. Received: 1 November 1997