C. J. Gross

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 + .

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.

First identification of excited states in 84–86Nb

We report on the observation of excited states in the niobium isotope with masses 84, 85 and 86. These nuclei lie in the shape transition region known to occur in nuclei with 35 less-than-or-equal-to Z less-than-or-equal-to 40 and N = 45. Unambiguous identification was achieved by detecting gamma-rays in coincidence with mass-separated recoils produced in heavy-ion fusion reactions. Rotational bands were observed in 84Nb and 85Nb which show decoupling schemes similar to those in the neighbouring nuclei. A rotational model was used to interpret these bands and the quasiparticle configurations involved. Two sequences of transitions were observed in 86Nb, at least one of which appears to be a rotational band.

First Observation of the Nuclei 87Tc and 88Tc

The gamma-decay of excited states in the neutron deficient isotopes Tc-87 and Tc-88 has been observed for the first time. The fusion-evaporation reactions Ni-58(S-32, p2n)Tc-87 and Ni-58(S-32, pn)Tc-88 at 110 MeV, Ca-40(Cr-50, p2n)Tc-87 and Ca-40(Cr-50, pn)Tc-88 at 170 MeV and Ni-58(Ar-36, alpha-pn)Tc-88 at 145 MeV beam energy were used. Unambiguous identification of these nuclei which are populated with less than 1 mb cross section was achieved with coincidences between the gamma-radiation and the evaporation residues detected in a recoil mass separator. From gamma-gamma-coincidence data, a decay scheme of 11 transitions was constructed for Tc-88, the ground state of which is suggested to have I-pi = 7- or 8+. Two transitions identified in Tc-87 follow the pattern of a g9/2 one-quasiparticle band. (Less)

Proton and Neutron g9/2 Alignments in the N=46 Isotones 86Zr and 87Nb

The magnetic moments of the 8(+) yrast state in Zr-86 and the 17/2(-) and 21/2(+) yrast states in Nb-87 have been measured via the IMPAD technique, The nuclei were produced in the Ni-58(S-32,Xp) reaction; the time-integral Larmor precessions were measured after recoil implantation into a polarised Fe foil. The g-factors are used to deduce the character of the g(9/2) Shell alignments within the states and their structures are interpreted with the shell model in the (g(9/2),p(1/2)) single-particle space, using the Gross-Frenkel two-body matrix elements. A survey on the various alignment schemes in these neutron-deficient isotopes is given. We find that neutron-pair alignment is favoured for even-parity states in the near-spherical nuclei with N greater than or equal to 46 but a major component of the odd-parity 17/2(1)(-) state arises from the alignment of a neutron-proton pair in the g(9/2) shell coupled to an unpaired neutron in the p(1/2) shell. (Less)

High-spin States in the Transitional Nucleus 88Mo

The reaction58Ni(36Ar,α qρ)88Mo has been studied at 145 MeV beam energy. A detector array consisting of the OSIRIS spectrometer, four charged-particleΔE detectors and seven NE213 neutron detectors has been used to meaure the gamma radiation inγγ- and particle-γγ-coincidence mode. The level scheme of88Mo has been extended up to 11.6 MeV excitation energy and probable spin 23ħ; some 70 transitions and 40 levels have been identified. Spin assignments have been proposed on the basis of measured DCO ratios. Hartree Fock cranking calculations of the Total Routhians and shell model calculations of the high spin states are presented which imply near-sphericity of the yrast line up to the highest spins found. A classification of the high spin states according to their leading seniority is proposed.

Seniority v=5 States in 89Nb

Yrast and yrare states in the nucleus [sup 89]Nb have been studied in the reaction [sup 58]Ni([sup 36]Ar,5[ital p]) by measuring [gamma][gamma] coincidences and directional correlation orientation with the OSIRIS array. In addition, [gamma]-ray singles angular distributions have been taken with the reaction [sup 58]Ni([sup 35]Cl,4[ital p]). Up to the highest spins observed (37/2[sup +],33/2[sup [minus]]), the states identified line up with the ones predicted by the spherical shell model, taking into account three proton particles and two neutron holes in the [ital g][sub 9/2] and [ital p][sub 1/2] single-particle orbits. The shell model also reproduces most of the measured branching and [ital E]2/[ital M]1 mixing ratios. The mean lifetime of the 2152 keV 17/2[sup [minus]] yrast state was determined as 0.74(7) ns.

The abrupt onset of collectivity in 87Mo

Abstract Excited states in the transitional N=45 nucleus 4287Mo were identified for the first time using the 58Ni(32S,2pn)87Mo reaction Low-lying states were found to be non-collective, but above 2 MeV an abrupt change in structure was observed. Two regularly spaced bands of states were found and the measurement of transitional quadrupole moments indicates a mean deformation of β2=0.17(3).

Heavy-ion In-beam Studies of the Nucleus 87Nb

High spin states in the transitional nucleus87Nb up to 14 MeV excitation have been established for the first time via the reactions40Ca(50Cr, 3p)87Nb and58Ni (32S, 3p)87Nb. The87Nbγ-radiations have been identified throughγ-ray spectra taken in coincidence with the evaporation residues detected in the Daresbury recoil separator or with multiple proton emission. Gamma-gamma coincidences, DCO ratios,γ-ray angular distributions and lifetimes have been measured. A total of some 100 transitions have been placed into a level scheme comprising of sixty states. The one-quasiparticle (1qp) bands of either parity and several other band-like structures have been identified, some containing alignedg9/2 nucleons. Moderately enhancedE2 in-band transitions of 13–48 W.u. as well as several weakE2 yrast transitions connecting bands with different quasiparticle numbers have been found. Similarities with respect to theN=46 isotones83Rb,84Sr,85Y,86Zr and88Mo are discussed.

Multiparticle-hole States of High Spin in N

High spin states in the nucleus Tc-89 have been studied via the fusion evaporation reaction Ni-58(Ca-40,2 alpha p)Tc-89 at 180 MeV beam energy. The NORDBALL gamma-ray spectrometer equipped with auxiliary detectors for light particle selection was used to measure gamma gamma- and particle-gamma gamma coincidences. Some 60 transitions were placed into a level scheme comprising 38 levels reaching up to 9.2 MeV excitation energy and a possible spin of I = 45/2HBAR The level scheme is compared to those of neighbouring nuclei and interpreted in terms of the spherical shell model. The calculations were performed with different sets of parameters within a restricted pi(p(1/2)), pi(g(9/2)), nu(p(1/2)) and nu(g(9/2)) configuration space. States above 2.3 MeV excitation energy are well reproduced by shell model calculations based on an empirical residual interaction, whereas collective excitations are suggested to contribute to the wave functions of lower lying states. (Less)