The Brueckner-Goldstone method and its application in light nuclei

A new method for solving the reaction matrix equation is described. In this method the Pauli operator is treated exactly and the reaction matrix is found straightforwardly from systems of differential and algebraic equations. The three-body cluster contribution is discussed. The low truncation used in computation of it in other papers is shown to be unsatisfactory. A dependence of computed binding energy on the formally introduced parameter λ, corresponding to the shift of the single-particle spectrum of unoccupied states, is studied. In principle, the exact results should not depend on λ and therefore in an approximate calculation one must achieve a weak dependence on this parameter. It is shown that in this case the consideration of the third order in the Goldstone series is necessary. Various conditions are suggested for choosing the parameter λ. As an example of light nuclei, the ⁴He nucleus is chosen. Its binding energy is found to be about 20 MeV, both for the Reid potential and the RHEL potential.     

Beta-decay properties of strongly neutron-rich nuclei

Experimental β-strength functions for about 50 neutron-rich mass-separated fission products with half-lives between 0.8 s and 30 s have been obtained at the OSIRIS facility. The β-strength to excited states depopulating by delayed-neutron emission is evaluated separately. The β^? strength functions are found to increase strongly with excitation energy, which is in contrast to the approximately energy-independent β-strength found for EC decay. By using semiempirical values for the level density, the average transition rate per energy level was evaluated under the assumption that only allowed transitions contribute to the decay. This alternative way of analyzing the data gives a more uniform picture of the β-decay to highly excited states since the transition rates are found to be roughly independent of excitation energy (above the pairing energy). A model of constant transition rate to each final level is introduced and its systematic behaviour is studied. Its use for estimating half-lives of unmeasured nuclides is of value for calculations on nucleosynthesis by the “r-process”. (A listing of β-feed and β-strength functions is available on request.)     

Transition rates between negative-parity states in odd-mass gold nuclei

The half-lives of low-lying negative-parity states in ¹⁸⁵Au and ¹⁸⁷Au have been measured using mass-separated sources from the ISOCELE separator. The results,

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, are discussed within the generalized particle-plus-asymmetric-rotor model, giving information on the structure of the states and on the nuclear-shape parameters.     

Band crossing in184Pt

The levels in the ground band of¹⁸⁴Pt are identified up to the member with spin 20 in an in-beam spectroscopy experiment on theγ-rays following the¹⁷⁷Hf(¹²C, 5n)¹⁸⁴Pt reaction. In contrast to the situation in the heavier Pt isotopes, where the 10⁺ to 12⁺ level spacing is very small, the transition energies in¹⁸⁴Pt show a monotonie increase with spin. However, between spin 10⁺ and 14⁺ the moment of inertia grows quite rapidly and this appears to be reminiscent of the much more pronounced irregularity encountered in the isotopes of larger mass. This property of the moment of inertia is discussed within a model where twoi _13/2 neutron quasiparticles or twoh _9/2 orh _11/2 proton quasiparticles may be excited and coupled to the rotation of the core.     

Band crossing and surface states in solids

A symmetry criterion for the existence of surface states in model calculations in solids is derived. The criterion applies to surfaces that coincide with symmetry planes of the ideal crystal. A detailed analysis is carried out for one-dimensional model calculations and it is shown how band crossing can be interpreted in the framework of the crystal symmetry.     

Band crossing in the particle-rotor model

The sharpness of the backbending associated with a two-band crossing is shown to be an oscillating function of the degree of shell filling in a particle-rotor model that includes an exact treatment of the two-body recoil term. Both the even-and the odd-nucleon cases are considered. A detailed comparison is made with the cranking model, and also the physical implications of the results are discussed.     

Band crossing phenomena inN=Z nuclei a probe toT=0 pairing correlations?

The structure of theN=Z nuclei⁶⁰Zn,⁶²Ga,⁶⁴Ge, and⁷²Kr has been investigated at GASP. Total Routhian surface (TRS) calculations have been performed for theN=36, 38 and 40 Kr isotones. In the case of⁷²Kr the four quasi-particleg _9/2 alignment is observed to be significantly delayed in rotational frequency with respect to the heavier Kr isotopes. Such a delay contradicts the predictions of mean field calculations and may be viewed as the first sign of additional correlations in theT=0 pairing channel.     

Renormalization of the axial-vector coupling constant in β-decay

Relationships are established between different theoretical approaches which attempt to account for the quenching of the axial-vector β-decay coupling constant g_A. It is concluded that it is probably not meaningful to attribute the quenching observed in light nuclei to any one particular mechanism.     

Proton spin flip at 12 MeV for nuclei near the neutron shell closure, N = 28

Spin-flip angular distributions for the nuclear pairs ^{48, 50}Ti and ^{54, 56}Fe have been measured using the (p, p'γ) correlation method. These data are compared to collective model DWBA calculations using a distorted spin-orbit potential of the full Thomas form. The spinflip angular distribution data for ⁵⁴Fe exhibit two peaks of nearly equal magnitude while the data for ⁵⁶Fe and ^{48, 50}Ti more closely resemble the usual single large peak at back angles predicted by DWBA calculations.     

Vibronic coupling and intersystem crossing in aromatic amines

The fluorescence and phosphorescence spectra of the aromatic amines acridan, iminobibenzyl, and carbazole have been measured in Shpolskii matrices at 10 K. Under these conditions the emission exhibits a detailed vibrational structure which has been analyzed. The change of the polarization degree observed within the fluorescence spectra at 77 K, particularly pronounced in acridan and iminobibenzyl, is attributed to vibronic interaction between the closely lying S₁(¹A₁) and S₂(¹B₁) excited states. This process activates a b₁ vibration with a frequency of 1200 cm⁻¹ in the ground state. The appearance of a long-axis (b₁) polarized vibration (700 cm⁻¹) following the out-of-plane polarized 0-0 band of the phosphorescence of these amines at 77 K is suggested to arise from vibronic interactions in the triplet manifold. This second-order spin-orbit coupling (soc) process is superimposed upon the dominant first-order electronic soc mechanism, which couples the lowest π, π^* triplet with high-energy (σ, π)^* singlet states.     

High-resolution studies of the gamma rays from isomeric states with half-lives of 10 μs - 30ms in nuclei with Z = 63−83

Eight new isomeric states with half-lives in the range 10 μs-30 ms have been identified following bombardment with 17.5 MeV protons of the natural elements and separated isotopes of the elements Eu to Pb. Improved data on 14, previously reported, isomeric states have also been obtained. The energies, half-lives and multipolarities of the gamma rays from these states have been measured using an 8 cm³ Ge(Li) detector. In most cases, level schemes have been constructed, and the spin and parity of the isomeric level and intermediate levels populated in the decay have been determined. A group of hindered E2 transitions have been observed, all of which are retarded by approximately the same degree e.g. 3 × 10⁻³ Weisskopf units. The results are discussed in terms of the Nilsson model and in terms of the selection rules for transitions between Nilsson states.     

The GT matrix elements in the decay of the mirror nuclei 47Cr, 51Fe and 55Ni

Three new mirror nuclei, ⁴⁷Cr, ⁵¹Fe and ⁵⁵Ni, have been identified in (³He, 2n) reactions. The measured half-lives were as follows: ${}^{47}\text{Cr}\text{, 452 ± 18}\text{ms}\text{;}{}^{55}\text{Fe}\text{, 245 ± 7}\text{ms}\text{;}{}^{55}\text{Ni}\text{, 189 ± 5}\text{ms}$. Known mass data were utilized to deduce log $\text{?t}$ values and Gamow-Teller matrix elements for the three mirror β-decays. The experimental GT matrix elements are compared to the results of shell-model calculations.     

Particle-vibration coupling and many-particle correlation in nuclei

The formation of many-particle clusters within atomic nuclei as a result of the coupling between particle and vibrational modes has been investigated. Expressions for the formation probabilities have been obtained for different excitation energies of the nuclei, characterized by the nuclear temperature. The methods used in the analysis are those of the double-time temperature Green function and the Fredholm method of solving integral equations.     

Nucleon channel coupling in electrodisintegration of nuclei

The process of nuclear excitation above threshold for nucleon decay in high energy electron scattering is considered. On the basis of the particle-hole shell model a formalism is proposed which allows one to describe electroexcitation of nuclei in a unified manner both in the resonance and quasielastic scattering regions. Numerical calculations were made for the ¹²C nucleus taking into account the mixing of proton and neutron configurations (channels) of the particle-hole type in the continuum.     

Deformed even-even nuclei in a quark-meson coupling model with tensor coupling

Neutron total cross sections of ¹⁹⁷Au and ^natTa have been measured at the nELBE photoneutron source in the energy range 0.1–10MeV with a statistical uncertainty of up to 2% and a total systematic uncertainty of 1%. This facility is optimized for the fast neutron energy range and combines an excellent time structure of the neutron pulses (electron bunch width 5ps) with a short flight path of 7m. Because of the low instantaneous neutron flux transmission measurements of neutron total cross sections are possible, that exhibit very different beam and background conditions than found at other neutron sources.     

Phantom divide crossing with general non-minimal kinetic coupling

We propose a model of dark energy consists of a single scalar field with a general non-minimal kinetic couplings to itself and to the curvature. We study the cosmological dynamics of the equation of state in this setup. The coupling terms have the form \({\xi_{1} Rf(\phi)\partial_{\mu}\phi\partial^{\mu}\phi}\) and \({\xi_{2}R_{\mu\nu}f(\phi)\partial^{\mu}\phi\partial^{\nu}\phi}\) where ξ ₁ and ξ ₂ are coupling parameters and their dimensions depend on the type of function \({f(\phi)}\). We obtain the conditions required for phantom divide crossing and show numerically that a cosmological model with general non-minimal derivative coupling to the scalar and Ricci curvatures can realize such a crossing.