Search Results (6 total)

Neutron-rich, radioactive Zn isotopes were investigated at the Radioactive Ion Beam facility REX-ISOLDE (CERN) using low-energy Coulomb excitation. The energy of the 2₁⁺ state in ⁷⁸Zn could be firmly established and for the first time the 2⁺→0₁⁺ transition in ⁸⁰Zn was observed at 1492(1) keV. B(E2,2₁⁺→0₁⁺) values were extracted for ^74,76,78,80Zn and compared to large scale shell model calculations. With only two protons outside the Z=28 proton core, ⁸⁰Zn is the lightest N=50 isotone for which spectroscopic information has been obtained to date. Two sets of advanced shell model calculations reproduce the observed B(E2) systematics. The results for N=50 isotones indicate a good N=50 shell closure and a strong Z=28 proton core polarization. The new results serve as benchmarks to establish theoretical models, predicting the nuclear properties of the doubly magic nucleus ⁷⁸Ni.

We report on the first radioactive beam experiment performed at the recently commissioned REX-ISOLDE facility at CERN in conjunction with the highly efficient γ spectrometer MINIBALL. Using ³⁰Mg ions accelerated to an energy of 2.25  MeV/u together with a thin ^natNi target, Coulomb excitation of the first excited 2⁺ states of the projectile and target nuclei well below the Coulomb barrier was observed. From the measured relative deexcitation γ-ray yields the B(E2;0_gs⁺→2₁⁺) value of ³⁰Mg was determined to be 241(31)e²   fm⁴. Our result is lower than values obtained at projectile fragmentation facilities using the intermediate-energy Coulomb excitation method, and confirms the theoretical conjecture that the neutron-rich magnesium isotope ³⁰Mg resides outside the “island of inversion.”

In-beam γ-ray spectroscopy using fragmentation reactions of both stable and radioactive beams has been performed in order to study the structure of excited states in neutron-rich oxygen isotopes with masses ranging from A=20 to 24. For the produced fragments, γ-ray energies, intensities, and γ-γ coincidences have been measured. Based on this information new level schemes are proposed for ^21,22O up to the neutron separation energy. The nonobservation of any γ-decay branch from ²³O and ²⁴O suggests that their excited states lie above the neutron decay thresholds. From this, as well as from the level schemes proposed for ²¹O and ²²O, the size of the N=14 and 16 shell gaps in oxygen isotopes is discussed in the light of shell-model calculations.

The yrast J^π=8⁺ states in neutron-rich ^70,72,74,76Ni nuclei are predicted to be isomeric. The present paper describes two GANIL experiments. In the first of them a search was made for the 8⁺ isomeric states in ^72,74Ni nuclei via fragmentation of ⁷⁶Ge using the ion γ-decay correlation technique. Although these states were not observed, limits for their lifetimes were determined. In the second experiment the decay spectroscopy of ^70,72Co nuclei was performed using fragmentation of a ⁸⁶Kr³⁶⁺ beam and the new LISE2000 spectrometer. The β delayed γ rays from the decay of ^70,72Co to ^70,72Ni were observed using the EXOGAM germanium detectors. The half life of ⁷²Co was measured to be 62(3) ms and the level sequence of the lowest excited states in ⁷²Ni was suggested, with the 2⁺ state at 1096  keV. An attempt to reproduce the level scheme in terms of shell-model calculations was undertaken. The reasons for the disappearance of the 8⁺ isomer in ⁷²Ni are discussed.

A systematic study of the population probabilities of microsecond isomers produced following the fragmentation of ²⁰⁸Pb projectiles at 1 GeV/nucleon has been undertaken at the SIS/FRS facility at GSI Darmstadt. Gamma decays from approximately 20 isomeric states, mainly in the rare-earth and transitional nuclei with A∼180, were identified and the corresponding isomeric ratios deduced. The results are compared with a model based on the statistical abrasion-ablation description of relativistic fragmentation and simple assumptions concerning gamma cascades in the final nucleus (sharp cutoff). The model is found to represent an upper limit for the population of isomeric states in relativistic projectile fragmentation.

The neutron-rich ^66,68Ni have been produced at GANIL via interactions of a 65.9A MeV ⁷⁰Zn beam with a ⁵⁸Ni target. Their reduced transition probability B(E2;0₁⁺→2⁺) has been measured for the first time by Coulomb excitation in a ²⁰⁸Pb target at intermediate energy. The B(E2) value for ⁶⁸Ni₄₀ 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.