B. J. Varley

The role of triaxiality in the ground states of even-even neutron-rich Ru isotopes

Abstract Partial decay schemes for the very neutron-rich nuclei 108,110,112 Ru and 114 Ru, about which nothing was previously known, have been determined from the study of prompt, triple-γ coincidences in 248 Cm fission fragments. Levels with probable spin up to 10ħ have been observed and γ-ray branching ratios obtained. Ratios of reduced electric quadrupole transition probabilities obtained from the branching ratios are in good agreement with predictions of a simple model of a rigid triaxial rotor.

Band crossings and near-rigid rotation in 76Kr and 78Kr

Abstract High-spin states in 78 Kr and 76 Kr were studied via the reactions 58 Ni( 24 Mg, 4p) 78 Kr and 58 Ni( 24 Mg, α2p) 76 Kr at 110 MeV beam energy. An array of fifteen Compton suppressed Ge detectors was used in the POLYTESSA framework to collect γγ-coincidence spectra. Rotational bands were observed up to probable spins of 24 + and 22 − in 76 Kr, and 24 + and 21 in 78 Kr. While the positive-parity yrast bands show strong variations in the moments of inertia caused by the alignments of pairs of ( g 9 2 ) proton and neutron quasiparticles, the negative-parity bands display a constant moment of inertia throughout most of their frequency range. The band crossings and alignments in both nuclei are discussed in the context of the cranked shell model. From an examination of the systematics of crossing frequencies along the Kr isotopes, the previous suggestion that protons align first is confirmed. The intrinsic structures of the bands are also analyzed with the Woods-Saxon-Strutinsky cranking model. The interplay between neutron and proton excitations, the shape changes induced by quasiparticle alignment, and the possible reduction of the static proton pairing in the negative-parity bands are discussed in detail. Three lifetimes in the 76 Kr ground band were remeasured by using the Doppler shift attenuation method, indicating a constant deformation of β 2 = 0.33 up to spin 10 + .

New results on

The very neutron-deficient isotope was produced in the reaction at 195 MeV and was identified using the Daresbury recoil separator. A pure -ray spectrum belonging to the de-excitation of the excited states of was obtained in-beam. Twenty -ray lines were identified, ten of them for the first time. The intensity balance concerning some of these -rays together with the results obtained from the coincident measurements show the existence of a new set of levels which may be interpreted as being a side band confirming the coupling of and configurations.

The observation of 84Mo

Abstract The 28 Si( 58 Ni, 2n) 84 Mo reaction has been studied at 195 MeV bombarding energy. The prompt gamma rays were detected in an array of twenty, escape-suppressed Ge detectors in coincidence with the recoiling nuclei, which were identified by A and Z with a recoil separator. A 443.8±0.3 keV gamma ray was observed from 84 Mo which is interpreted as the 2 + −0 + transition in this nucleus. This would indicate a quadrupole deformation e 2 ∼0.30. This result confirms that 76 Sr lie at the centre of the region of deformed nuclei with A ∼80.

Evidence for shape coexistence in the N=Z nucleus 7236Kr36

Abstract Gamma rays associated with the decay of states in the N = Z nucleus 72 Kr have been identified following the 16 O( 58 Ni, 2n) 72 Kr reaction at a mean beam energy of 170 MeV. Identification was made using the Daresbury Recoil Separator. The first excited state was found to be at 709.1 ± 0.3 keV and to be populated with a cross section of 60 ± 25 υ b. The pattern of gamma rays associated with 72 Kr indicates the co-existence of nuclear shapes.

Medium-spin structure of 96,97Sr and 98,99Zr nuclei and the onset of deformation in the A∼100 region

Abstract The N=58 , 59 nuclei 96,97Sr and 98,99Zr produced in the spontaneous fission of 248Cm were studied using the EUROGAM2 array. Regular rotational bands have been found in 97Sr and 99Zr, firmly establishing shape coexistence in both nuclei. Contrary to earlier suggestions, no rotational bands were found on top of 0 + 3 states in the N=58 nuclei 96Sr and 98Zr. It is likely, however, that the N=58 nuclei undergo a shape change along the yrast line from spherical ground state to weakly deformed medium-spin states. Numerous single-particle excitations of negative parity, observed in 97Sr and 99[loc=pre]Zr, suggest the important role of the h 11/2 shell in creating deformation in these nuclei. Quadrupole moments were determined for rotational bands in the N=58 , 59, 60, 62 and 64 Sr and Zr nuclei. Deformation parameters, which increase gradually from β 2 ∼0.1 at N=56 , through β 2 ∼0.2 at N=58 to β 2 ∼0.4 at N=64 , suggest that in strontium and zirconium isotopes the shape change happens gradually between N=56 and N=62 , and is most likely due to an occupancy of three or more deformation-driving orbitals of h 11/2 parentage.

Octupole correlations in neutron-rich, even-even barium isotopes

Abstract A = 140–148 even-even barium nuclei produced in the spontaneous fission of 248Cm were studied using the EUROGAM2 array. Spins and parities of excited levels have been firmly established from triple-γ angular correlation and direction-polarization correlation measurements. Known band structures have been extended and new bands were found. Band-crossing phenomena have been studied, which suggest termination of an alternating-parity band by particle alignment in 146Ba. The new data for 148Ba confirm predictions of a local decrease of electric dipole moment in 146Ba.

Suppression of Pairing Correlations in 77Rb

77Rb has been studied using the 40Ca(40Ca, 3p) reaction at (122 ÷ 132) MeV and has been demonstrated to be a very good rotor up to spin Iπ = 37/2±. Transitional quadrupole moments, extracted from extensive lifetime measurements, are large and show surprising constancy for this mass region. These moments, when taken together with signature splittings, indicate a charge distribution with β2 = 0.37(2) and γ -25°. Moments of inertia deduced from level spacings are close to that of a rigid body with the same shape as the charge distribution, a feature which indicates pairing correlations are strongly suppressed.

Triaxiality and isospin-forbidden E1 decays in the N = Z nucleus 64Ge

Abstract The neutron-deficient isotope 32 64 Ge 32 was produced in the reaction 12 C( 54 Fe, 2nγ) 64 Ge at a beam energy of 165 MeV. The production cross section for 64 Ge was measured to be 640 ± 70 μ b, which represents only ∼0.1% of the total fusion cross section. The Daresbury recoil separator was operated in conjunction with a large γ-array to study recoil-γγ correlations and measure recoil-γ angular distributions. A level scheme for 64 Ge was constructed containing 19 levels. The nucleus appears to have a structure consistent with a γ-soft shape. Shape changes induced by aligned g 9 2 nucleons are discussed. An overview of octupole correlations in the A ⋍ 60−80 mass region is given. Evidence for forbidden E1 transitions was found which is indicative of considerable isospin mixing.

High spin states, lifetimes and gamma-softness in 80Sr

Abstract High spin states in 80 Sr have been studied using the techniques of in-beam γ-ray spectroscpy. The ground state band has been investigated up to (26 + ) and three new side-bands have been discovered. The deduced level scheme is compared with cranking model calculations; the predicted transformation to mostly non-collective excitations of an oblate shape is not observed. The behaviour of the J (1) and J (2) moments of inertia is discussed and presented as evidence for γ-softness in the light Sr isotopes.