Th. Paradellis

Description of the positive parity states in odd Xe isotopes in the framework of the intermediate coupling approach of the unified model

Abstract The nuclear structure and the properties of the lower lying levels in odd-mass Xe isotopes are investigated in the framework of the intermediate coupling approach of the unified model. It is assumed that the last odd neutron has available the 3s 1 2 and 2d 3 2 states and is coupled to the collective surface vibrations of the doubly even core of the nucleus. The Hamiltonian of the coupled system including all the states up to three phonons of quadrupole vibrations is diagonalized. In calculating the off-diagonal matrix elements, a reduction factor based on the occupation number of the state under consideration is used. The positive parity states in 127 Xe, 129 Xe, 131 Xe and 133 Xe are well reproduced by the model. The wave functions derived from the best fit were used to calculate nuclear moments and E.M. transition probabilities. The calculated magnetic moments compare favorably with the data. The calculated quadrupole moment for 129 Xe is in disagreement with the data, suggesting large collective interactions which the model does not account for. The model gives an overall description of 131 Xe which is satisfactory with regard to the known half-lives and multipolarities of various transitions. Finally the possible reasons for some of the discrepancies between the theoretical and experimental data are discussed.

Gamma decay of 75Se

Abstract A study was made of the γ-ray spectrum of 75Se using a variety of Ge(Li) detectors. The energies and intensities of 14 lines have been measured: 24.4 keV (0.044±0.006), 66.0 keV (1.72±0.04), 96.7 keV (5.12±0.10), 121.1 keV (27.7±0.50), 136.0 keV (95.00±1.80), 198.6 keV (2.38±0.07), 264.6 keV (100.0), 279.5 keV (42.0±0.80), 303.8 keV (2.19±0.07), 400.5 keV (20.40±0.50), 419.3 keV (0.023±0.002), 468.6 keV (0.001±0.0005), 572.6 keV (0.063±0.002), and 617.7 keV (0.0075±0.0002). Of these a 24.4 keV and 468.6 keV transition were substantiated for the first time in the γ-ray spectrum. The 80.8 keV transition deducedd from electron conversion spectra and a 373 keV transition previously reported in the γ-ray spectrum of 75Se were not observed. K X-ray intensities and branching ratios have been determined. The results of this work support the conclusions of previous studies in the decay of 75Ge and in the Coulomb excitations of 75As for the existence of a level at 468.6 keV. A modified γ-decay scheme of 75Se to include the 468.6 level is proposed.

Unified-model calculations in some Zr isotopes

Abstract The excited states of the Zr isotopes with 91 ≦ A ≦ 95 are investigated in the framework of the intermediate coupling approach of the unified model. The odd Zr isotopes are assumed to consist of a doubly even Zr core to which one or two extra core neutrons are coupled. The assumption is also made that the extra core neutrons may “feel” the vibrations of the doubly even core. The effect is not observed in the experimental spectra of the 90 Zr and 96 Zr nuclei, due to their closed shell structure. A good description of the observed energy levels, and their (d, p) spectroscopic factors is obtained. In addition it is possible to identify states which are tentatively assigned to the coupling of the extra core neutron to two proton excitations from the core.

Gamma decay of 78Se following the β-decay of 78As

Abstract The γ-ray spectrum of 78 Se following the β-decay of 78 As is investigated. Thirty γ-rays are reported, most of them for the first time. Accurate energy measurements of the observed γ-rays and γ-γ coincidence data support a decay scheme for 78 Se which includes levels at 614.1 keV, 1308.8 keV, 1503.0 keV, 1854.6 keV, 1996.0 keV, 2328.6 keV, 2453.5 keV, 2508.0 keV, 2535.0 keV, 2681.5 keV, 2798.0 keV, 2838.3 keV, 3148.0 keV, 3296.0 keV, 3528.0 keV and 3718.0 keV. The log ∫ t values for the branching to the excited levels of 78 Se support a spin 2 − assignment for the 78 As ground state, in agreement with the As systematics.

Capture reactions in the helium burning of stars

Abstract In the helium burning stage of stars there is a main sequence of reactions which is responsible for energy production and nucleosynthesis of the light elements 12C and 16O. Moreover some other reaction sequences may occur as for instance 14N(α,γ)18F(β)18O 18O(α,n)21Ne(α,n)24Mg and 18O(α,γ)22Ne(α,n)25Mg and 22Ne(α,γ)26Mg, 15N(α,γ)19F, 13C(α,n)16O[1]. Some of the more recent results on (α,n) reactions are given in [2,3].

The key reactions in Stellar helium burning: 12C(α,γ)16O and 22Ne(α,n)25Mg

Abstract The excitation functions of 12C(α,γ)16O and 22Ne(α,n)25Mg have been determined with a yet unequaled experimental sensitivity of 10−11 b. From γ-angular distributions the SE1-and SE2-factors for 12C(α,γ)16O have been deduced and extrapolated into the range of helium burning temperatures using the R-matrix method. An improved reaction rate has been calculated: NA〈σν〉=(7.9±2.5)x10−15cm3(mols)−1 at T9 = 0.2. For a more precise extrapolation the excitation function has to be measured with better statistics. Therefore a new experiment on 12C(α,γ)16O with a better suited setup has been started recently. The accuracy of the 22Ne(α,n)25Mg reaction rate could be improved considerably compared to previous compilations: NA〈σν〉 = (0.88−0.3+2.6) x 10−16cm3(mols)−1 at T9 = 0.2.