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     

Favoured superdeformed states in 89Tc

A superdeformed band consisting of a cascade of ten γ-ray transitions has been identified and assigned to the nucleus ⁸⁹Tc, close to the proton dripline. The quadrupole moment of the band (Q _t= 6.7^+3.0 _−2.3 eb, as measured by the Residual Doppler Shift Method) as well as a large dynamic moment of inertia point to a highly elongated shape. With a relative population of approximately 15% of the γ-ray flux in the ⁸⁹Tc exit channel, the band is among the most intense superdeformed bands observed to date. Received: 6 July 1999     

Count fluctuation analysis in the triaxial strongly deformed potential well of 163Lu

High-spin properties of the triaxial strongly deformed potential well in ¹⁶³Lu at excitation energies above resolvable bands have been investigated. Gated E _γ-E _γ spectra display clear ridges with moments of inertia corresponding to those observed for the discrete strongly deformed bands. A fluctuation analysis of the ridges yields a number of two-step paths of ≈ 40 and ≈ 20, when gating on triaxial strongly or normally deformed bands, respectively. These results show that a potential well at large deformation coexists with the normally deformed well, and indicate a mixing between states in the two wells at higher excitation energy. Received: 4 January 2002 / Accepted: 6 May 2002     

Observation of superdeformed states in 88Mo

High-spin states in ⁸⁸Mo were studied using the Gammasphere germanium detector array in conjunction with the Microball CsI(Tl) charged-particle detector system. Three γ-ray cascades with dynamic moments of inertia showing similar characteristics to superdeformed rotational bands observed in the neighbouring A= 80 region have been identified and assigned to the nucleus ⁸⁸Mo. The quadrupole moment of the strongest band, deduced by the Residual Doppler Shift Method, corresponds to a quadrupole deformation of β₂≈ 0.6. This confirms the superdeformed nature of this band. The experimental data are interpreted in the framework of total routhian surface calculations. All three bands are assigned to two-quasi-particle proton configurations at superdeformed shape. Received: 20 May 1999 / Revised version: 25 August 1999     

Quadrupole moment measurement of the highly deformed πg 9/2⊗νh 11/2 band in 130Pr

The quadrupole moment for the πg _9/2⊗νh _11/2 band in the ^130>Pr nucleus has been measured using the Doppler-shift attenuation method. A centroid shift analysis was carried out and a value of Q₀=6.1±0.4 eb, corresponding to an axial prolate deformation of β₂=0.35(3), has been determined. This is the first direct experimental confirmation of the deformation-driving character of the πg _9/2 orbital in an odd-odd nucleus in the A≃135 superdeformed region. Received: 23 April 1998     

First evidence for triaxial superdeformation in 161Lu and 162Lu

High-spin states in ¹⁶¹Lu and ¹⁶²Lu have been investigated using the GASP γ-ray spectrometer array. Excited states in these nuclei have been populated through the ¹⁰⁰Mo(⁶⁵Cu, xn) reaction at a beam energy of 260 MeV. Four presumably triaxial superdeformed bands, three in ¹⁶²Lu and one in ¹⁶¹Lu, have been observed. This is the first evidence for triaxial superdeformation in the two isotopes. Received: 18 October 2002 / Accepted: 21 November 2002 / Published online: 4 February 2003 RID="a" ID="a"e-mail: bringel@iskp.uni-bonn.de RID="b" ID="b"Present address: Wright Nuclear Structure Laboratory, Yale University, New Haven, CT 06520, USA. RID="c" ID="c"Present address: Physik Department, Technische Universit?t München, James Franck Str., D-85748 Garching, Germany. Communicated by D. Schwalm     

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.     

High-spin states in 160Lu

High-spin states of ¹⁶⁰Lu have been studied through the ¹⁴⁴Sm( ¹⁹F, 3n) reaction. The previously known πh _11/2⊗υi _13/2 yrast band is extended from I ^π = 21^- to 25^- and a four quasiparticle band with configuration πh _11/2[523]7/2 ^-⊗υh _9/2[521]3/2 ^-⊗ (υi _13/2)² is reported. Received: 21 May 2001 / Accepted: 19 July 2001     

Lifetime measurement of the ((11/2)−) isomer in 125Cs

A new rotational band has been observed in ¹⁶⁷Lu by ¹⁵²Sm (¹⁹F,4n)reaction at the HI-13 tandem accelerator of CIAE in Beijing. The high spin transition energies of the new band are almost identical to the triaxial superdeformed bands recently discovered in ¹⁶³Lu and ¹⁶⁵Lu. This new band is predicted as a triaxial superdeformed band by total routhian surface calculations. Received: 6 October 1997     

Magnetic properties of the υj15/2 intruder orbital in the superdeformed minimum of 197Pb

This work has established eight cross-talk transitions between the two signature partner superdeformed (SD) bands in ¹⁹⁷Pb with the EUROBALL IV spectrometer. Directional correlations from oriented states measurements confirm the ΔI = 1 character of these transitions. The flat behaviour of the dynamical moment of inertia and the agreement between the experimental and microscopic HF+BCS values of (g _K - g _R)K/Q ₀ suggest that the configuration of the SD bands is based upon the υ[752]5/2^- neutron intruder orbital. The derived effective spin gyromagnetic factor g _s ^eff is found to be not quenched, and is close to the theoretical g _s ^free value. Received: 14 June 2000 / Accepted: 14 September 2000     

Search for hyperdeformed structures populated in the <Superscript>37</Superscript>Cl+<Superscript>120</Superscript>Sn reaction by using EUROBALL III

The ridge structure with ΔE_γ=±30 keV, observed in the past in coincidence with protons emitted in the reaction 187 MeV ³⁷Cl +¹²⁰Sn and attributed to an hyperdeformed nuclear shape in ¹⁵²Dy, has been studied in a new experiment performed with the EUROBALL III array. The ridge is now observed in coincidence with transitions in the yrast superdeformed band of ¹⁵²Dy but no discrete rotational bands have been identified. Received: 7 February 2000     

High-spin states in <Superscript>36</Superscript>Ar and their underlying shell model configurations

Eight high-spin states in ³⁶Ar below 10MeV excitation energy, among them a prospective J ^π = 8^- state at 9408keV and the J? 8 levels of the recently discovered superdeformed rotational band, have been observed by n-γ coincidence measurements with the ³³S(α, nγ) reaction at E _α = 14.4 and 13.4MeV. High-spin assignments of, respectively, J ^π = 6⁺ and 5^- were obtained for the E _p = 1209 and 1462keV (E _x = 9682 and 9927keV) resonances of the ³⁵Cl (p,γ) reaction by a measurement of γ-ray angular distributions. The spectrum of the high-spin and of the E _x? 7.4MeV levels is decomposed according to the underlying shell model configurations with n = 0, 1, 2, 4 particles excited from the N = 2 into the N = 3 major shell. The role of four-particle excitations, all connected with large prolate distortions, is elucidated for the entire A = 36-40 mass region. Received: 21 December 2001 / Accepted: 25 March 2002     

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     

High-spin states in odd-odd 168Lu

High-spin states of odd-odd ¹⁶⁸Lu were observed up to 18ħ for the first time using the ¹⁵⁹Tb(¹³C, 4nγ)¹⁶⁸Lu reaction at E _13C= 58.5 MeV. γ-γ coincidence, E _γ singles, excitation function, and the DCO ratios were measured. Two rotational bands with signature splitting were populated, among which the lower-energy band is identified as π 7/2⁺[404] ⊗ν 5/2⁺[642] in view of alignment and energy systematics. Received: 27 October 1997     

Rotational bands in the doubly odd 130Cs nucleus

The band structures built on the 5^- isomeric state ( T _1/2 = 3.46 m) in the doubly odd ¹³⁰Cs nucleus have been established up to I = 24? via the ¹²⁴Sn(¹¹B, 5n)¹³⁰Cs reaction. The previously observed bands based on the πh _11/2⊗νh _11/2, πg _7/2⊗νh _11/2 and πd _5/2⊗νh _11/2 configurations and a positive-parity side band with multiple connections to the α = 0 signature partner of the yrast πh _11/2⊗νh _11/2 band have been extended to higher spins. A new band based on the πh _11/2⊗νg _7/2 configuration is observed. The yrast πh _11/2⊗νh _11/2 band exhibits anomalous signature splitting whose magnitude decreases up to spin 15 and then increases without restoring the normal signature splitting. Received: 20 February 2001 / Accepted: 9 May 2001     

Systematics of high-spin isomers in N = 83 isotones and a high-spin isomer beam

Isomers in N = 83 isotones of Z = 60-66, were studied systematically. Their spins and parities are 49/2⁺ and 27⁺ for odd and odd-odd nuclei, respectively. Nearly constant excitation energies of these isomers indicated a decrease of a Z = 64 shell gap energy as Z decreases from 64 to 60 within the framework of a deformed independent-particle model (DIPM). Their configurations are [ν(f _7/2 h _9/2 i _13/2),π(h _11/2)²]_49/2+ and [ν(f _7/2 h _9/2 i _13/2),π(h _11/2)²(d _5/2)^-1]₂₇₊ for odd and odd-odd nuclei, respectively. The shape of the yrast states changes suddenly at spin 49/2(odd) and 27(odd-odd) from a near spherical to an oblate shape. Transitions from isomers are highly hindered because of the shape changes. They may be categorized to be shape isomers. The development of a secondary beam produced by using these high-spin isomers is also described. Received: 1 May 2001 / Accepted: 4 December 2001     

The collectivity in the yrast band of100Zr: the half-life of the 4 1 + state

The half-lives of the 2⁺ and 4⁺ levels of the yrast band in¹⁰⁰Zr have been determined to be 0.54(4) ns and 37(4) ps, respectively, at the fission-product separator JOSEF, in a β^?- γ- γ coincidence experiment which exploited the fast timing properties of plastic and BaF₂ detectors. The half- life of the 4₁ ⁺ state corresponds to an intrinsic quadrupole moment of ¦Q₀¦=3.14(17)b which confirms the strongly deformed nature, in a way independent of the configuration mixing in the ground state. The value of t_1/2(2₁ ⁺) corroborates a recently published value. There is evidence for a decrease of ¦Q₀¦ with increasing spin in the yrast bands of the N=60 isotones⁹⁸Sr and¹⁰⁰Zr.     

Rotational bands in the near-drip-line nucleus 128Nd

The even-even nucleus ¹²⁸Nd was studied via in-beam γ-ray spectroscopy using the ⁴⁰Ca + ⁸²Mo reaction at 190 MeV. Two new bands were observed besides the yrast one, that has been extended up to spin 34⁺. Configurations were assigned to the three bands by analysing their rotational properties and by comparison with the neighboring even-even nuclei. Received: 18 July 2001 / Accepted: 25 October 2001     

Accurate mass measurements of very short-lived nuclei

Mass measurements of ³⁴Ar, ^73-78Kr, and ^74,76Rb were performed with the Penning-trap mass spectrometer ISOLTRAP. Very accurate Q _EC-values are needed for the investigations of the t-value of 0⁺ → 0⁺ nuclear β-decays used to test the standard model predictions for weak interactions. The necessary accuracy on the Q _EC-value requires the mass of mother and daughter nuclei to be measured with δm/m ⩽ 3 ^. 10^-8. For most of the measured nuclides presented here this has been reached. The ³⁴Ar mass has been measured with a relative accuracy of 1.1 ^. 10^-8. The Q _EC-value of the ³⁴Ar 0⁺ → 0⁺ decay can now be determined with an uncertainty of about 0.01%. Furthermore, ⁷⁴Rb is the shortest-lived nuclide ever investigated in a Penning trap. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: frank.herfurth@cern.ch     

High spin states and evidence for octupole correlations in 117Xe

High-spin states of ¹¹⁷Xe were investigated by means of in-beam γ-ray spectroscopic techniques via the reaction of ²⁸Si bombarding a ⁹²Mo target at beam energies of 100-120 MeV. A positive-parity rotational band decaying into the yrast negative-parity band by a series of enhanced E1 transitions was observed for the first time, implying the existence of octupole correlations in ¹¹⁷Xe. The B(E1) values increase with spin. The νd_5/2 band was firmly established up to 27/2⁺ and the B(M1)/B(E2) ratios were extracted from the relative intensities of γ-rays in this band. The previously known νh_11/2 and νg_7/2 [413]5/2⁺α=−1/2 bands were confirmed and extended up to high spins and two bandcrossings are observed in the latter at ħω=0.33 and 0.44 MeV, respectively. The bandcrossings and configurations of these bands are discussed by TRS and CSM calculations. In a γ–γ–t measurement, the 11/2⁻ and 7/2⁻ levels were identified as two isomers with half-lives of 59.4±20 ns and 16.5±8.0 ns, respectively. Received: 17 June 1997 / Revised version: 29 September 1997