Double beta decay to the first 2+2^+ state within a boson expansion formalism with a projected spherical single particle basis

The Gamow-Teller transition operator is written as a polynomial in the dipole proton-neutron and quadrupole charge conserving QRPA boson operators, using the prescription of the boson expansion technique of Belyaev-Zelevinski type. Then, the $2\nu\beta\beta$ process ending on the first $2^+$ state in the daughter nucleus is allowed via one, two and three boson states describing the odd-odd intermediate nucleus. The approach uses a single particle basis which is obtained by projecting out the good angular momentum from an orthogonal set of deformed functions. The basis for mother and daughter nuclei have different deformations. The GT transition amplitude as well as the half lives were calculated for ten transitions. Results are compared with the available data as well as with some predictions obtained with other methods.Comment: 12 page

New results for the fully renormalized proton-neutron quasiparticle random phase approximation

A many-body Hamiltonian describing a system of Z protons and N neutrons moving in spherical shell model mean field and interacting among themselves through proton-proton and neutron-neutron pairing and a dipole-dipole proton-neutron interaction of both particle-hole and particle-particle type, is treated within a fully renormalized (FR) pnQRPA approach. Two decoupling schemes are formulated. One of them decouples the equations of motion of particle total number conserving and non-conserving operators. One ends up with two very simple dispersion equations for phonon operators which are formally of Tamm-Dancoff types. For excitations in the (N-1,Z+1) system, Ikeda sum rule is fully satisfied provided the BCS equations are renormalized as well and therefore solved at a time with the FRpnQRPA equations. Next, one constructs two operators ${\cal R}^{\dagger}_{1\mu}$, ${\cal R}_{1,-\mu}(-)^{1-\mu}$ which commutes with the particle total number conserving operators, ${\cal A}^{\dagger}_{1\mu}$ and ${\cal A}_{1,-\mu}(-)^{1-\mu}$, and moreover could be renormalized so that they become bosons. Then, a phonon operator is built up as a linear combination of these four operators. The FRpnQRPA equations are written down for this complex phonon operator and the ISR is calculated analytically. This formalism allows for an unified description of the dipole excitations in four neighboring nuclei (N-1,Z+1),(N+1,Z-1),(N-1,Z-1),(N+1,Z+1). The phonon vacuum describes the (N,Z) system ground state.Comment: 24 page

New features of some proton-neutron collective states

Using a schematic solvable many-body Hamiltonian, one studies a new type of proton-neutron excitations within a time dependent variational approach. Classical equations of motion are linearized and subsequently solved analytically. The harmonic state energy is compared with the energy of the first excited state provided by diagonalization as well as with the energies obtained by a renormalized RPA and a boson expansion procedure. The new collective mode describes a wobbling motion, in the space of isospin, and collapses for a particle-particle interaction strength which is much larger than the physical value. A suggestion for the description of the system in the second nuclear phase is made. We identified the transition operators which might excite the new mode from the ground state.Comment: 28 pages and 3 figure

Effects of the secondary decays on the isotopic thermometers

The sharp microcanonical multifragmentation model from [Al. H. Raduta and Ad. R. Raduta, Phys. Rev. C 55, 1344 (1997); Phys. Rev. C, in press] is employed for evaluating the nuclear caloric curve predictions of nine isotopic thermometers for three representative nuclei. Evaluations are performed for both primary decay and asymptotic stages. Effects of the secondary decays on the primary decay caloric curves are evidenced and discussed. In both cases a dispersive character of the isotopic caloric curves with increasing the source excitation energy is observed. A procedure of calibrating the isotopic thermometers on the microcanonical predictions for both primary decay and asymptotic stages is proposed. The resulting set of calibrating parameters for each thermometer is independent on the source size, on its excitation energy and, in the case of the primary decay, on the freeze-out radius assumption.Comment: 13 pages, 5 figures, Nuclear Physics A, in pres

Echos of the liquid-gas phase transition in multifragmentation

A general discussion is made concerning the ways in which one can get signatures about a possible liquid-gas phase transition in nuclear matter. Microcanonical temperature, heat capacity and second order derivative of the entropy versus energy formulas have been deduced in a general case. These formulas are {\em exact}, simply applicable and do not depend on any model assumption. Therefore, they are suitable to be applied on experimental data. The formulas are tested in various situations. It is evidenced that when the freeze-out constraint is of fluctuating volume type the deduced (heat capacity and second order derivative of the entropy versus energy) formulas will prompt the spinodal region through specific signals. Finally, the same microcanonical formulas are deduced for the case when an incomplete number of fragments per event are available. These formulas could overcome the freeze-out backtracking deficiencies.Comment: accepted to Nuclear Physics

Homogeneity and Size Effects on the Liquid-Gas Coexistence Curve

The effects of (in)homogeneity and size on the phase diagram of Lennard-Jones fluids are investigated. It is shown that standard multifragmentation scenarios (finite equilibrated systems with conserved center of mass position and momentum) are implying a strong radial inhomogeneity of the system strongly affecting the phase diagram. The homogeneity constraint is therefore necessary for finite systems in order to align to the ``meaning'' of infinite systems phase diagrams. In this respect, a method which deduces the equation of state of homogeneous finite systems from the one corresponding to bulk matter is designed. The resultant phase diagrams show a strong dependence on the system's size.Comment: 4 pages, 4 figure