arXiv subjects

D. S. Delion

Publications and source records attributed to D. S. Delion.

30 records · Page 2Linked to original sources

arXiv2013

Self-Consistent RPA from a Coupled Cluster Wave Function Perspective

Self-Consistent RPA is rederived in a consistent way with the help of the Coupled Cluster ground state wave function truncated at the two body level. An exact killing operator for this wave function is introduced allowing for a detailed discussion of the approximation scheme. Several exactly solvable models are reanalyzed under this new perspective giving raise to a quantitative evaluation of the performances of this many body method.

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arXiv2010

Coexistence of 'alpha+ 208Pb' cluster structures and single-particle excitations in 212Po

Excited states in 212Po have been populated by alpha transfer using the 208Pb(18O,14C) reaction at 85MeV beam energy and studied with the EUROBALL IV gamma multidetector array. The level scheme has been extended up to ~ 3.2 MeV excitation energy from the triple gamma coincidence data. Spin and parity values of most of the observed states have been assigned from the gamma angular distributions and gamma -gamma angular correlations. Several gamma lines with E(gamma) < 1 MeV have been found to be shifted by the Doppler effect, allowing for the measurements of the associated lifetimes by the DSAM method. The values, found in the range [0.1-0.6] ps, lead to very enhanced E1 transitions. All the emitting states, which have non-natural parity values, are discussed in terms of alpha-208Pb structure. They are in the same excitation-energy range as the states issued from shell-model configurations.

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arXiv2010

Systematics of 2+ states in semi-magic nuclei

We propose a simple systematics of low lying 2+ energy levels and electromagnetic transitions in semi-magic isotopic chains Z=28,50,82 and isotonic chains N=28,50,82,126. To this purpose we use a two-level pairing plus quadrupole Hamiltonian, within the spherical Quasiparticle Random Phase Approximation (QRPA). We derive a simple relation connecting the 2+ energy with the pairing gap and quadrupole-quadupole (QQ) interaction strength. It turns out that the systematics of energy levels and B(E2) values predicted by this simple model is fulfilled with a reasonable accuracy by all available experimental data. Both systematics suggest that not only active nucleons but also those filling closed shells play an important role.

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arXiv2010

Magic radioactivity of 252Cf

We show that the sharp maximum corresponding to 107Mo in the fragment distribution of the 252Cf cold fission is actually a Sn-like radioactivity, similar to other decay processes in which magic nuclei are involved, namely alpha-decay and heavy cluster emission, also called Pb-like radioactivity. It turns out that the mass asymmetry degree of freedom has a key role in connecting initial Sn with the final Mo isotopes along the fission path. We suppose the cold rearrangement of nucleons within the framework of the two center shell model, in order to compute the cold valleys in the charge equilibrated fragmentation potential. The fission yields are estimated by using the semiclassical penetration approach. We consider five degrees of freedom, namely the inter-fragment distance, the shapes of fragments, the neck parameter and mass asymmetry. We found an isomeric minimum between the internal and external barriers. It turns out that the inner cold valley of the total potential energy is connected to the double magic isotope 132Sn

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arXiv2009

A novel manifestation of $α$-clustering: new '$α$ + $^{208}$Pb' states in $^{212}$Po revealed by their enhanced E1 decays

Excited states in $^{212}$Po were populated by $α$ transfer using the $^{208}$Pb($^{18}$O, $^{14}$C) reaction and their deexcitation $γ$-rays were studied with the Euroball array. Several levels were found to decay by a unique E1 transition (E$_γ<$ 1 MeV) populating the yrast state with the same spin value. Their lifetimes were measured by the DSAM method. The values, found in the range [0.1-1.4] ps, lead to very enhanced transitions, B(E1) = 2$\times10^{-2}$ - 1$\times10^{-3}$ W.u.. These results are discussed in terms of an $α$-cluster structure which gives rise to states with non-natural parity values, provided that the composite system cannot rotate collectively, as expected in the '$α$ + $^{208}$Pb' case. Such states due to the oscillatory motion of the $α$-core distance are observed for the first time.

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arXiv2009

Microscopic description of Cf-252 cold fission yields

We investigate the cold fission of 252Cf within the two center shell model to compute the potential energy surface. The fission yields are estimated by using the semiclassical penetration approach. It turns out that the inner cold valley of the total potential energy is strongly connected with Z=50 magic number. The agreement with experimental values is very much improved only by considering mass and charge asymmetry degrees of freedom. Thus, indeed cold fission of 252Cf is a Sn-like radioactivity, related the other two "magic radioactivities", namely alpha-decay and heavy-cluster decay, called also Pb-like radioactivity. This calculation provides the necessary theoretical confidence to estimate the penetration cross section in producing superheavy nuclei, by using the inverse fusion process.

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arXiv2009

Universal decay rule for reduced widths

Emission processes including $α$-decay, heavy cluster decays, proton and di-proton emission are analyzed in terms of the well known factorisation between the penetrability and reduced width. By using a shifted harmonic oscilator plus Coulomb cluster-daughter interaction it is possible to derive a linear relation between the logarithm of the reduced width squared and the fragmentation potential, defined as the difference between the Coulomb barrier and Q-value. This relation is fulfilled with a good accuracy for transitions between ground states, as well as for most $α$-decays to low lying $2^+$ excited states. The well known Viola-Seaborg rule, connecting half lives with the Coulomb parameter and the product between fragment charge numbers, as well as the Blendowke scalling rule connecting the spectroscopic factor with the mass number of the emitted cluster, can be easily understood in terms of the fragmentation potential. It is shown that the recently evidenced two regions in the dependence of reduced proton half-lives versus the Coulomb parameter are directly connected with the corresponding regions of the fragmentation potential.

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arXiv2008

Synthesis of superheavy elements beyond Z=118

We investigate the cold fission/fusion paths of superheavy nuclei within the two center shell model, in order to find the best projectile-target combinations of their production. The fission/fusion yields are estimated by using the semiclassical approach. We predict several asymmetric combinations of relative long living fragments, which can be used in fusion experiments of superheavy nuclei with Z>118.

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arXiv2006

Systematics of proton emission

A very simple formula is presented that relates the logarithm of the half-life, corrected by the centrifugal barrier, with the Coulomb parameter in proton decay processes. The corresponding experimental data lie on two straight lines which appear as a result of a sudden change in the nuclear shape marking two regions of deformation independently of the angular momentum of the outgoing proton. This feature provides a powerful tool to assign experimentally quantum numbers in proton emitters.

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arXiv2004

Self Consistent Random Phase Approximation and the restoration of symmetries within the three-level Lipkin model

We show that it is possible to restore the symmetry associated with the Goldstone mode within the Self Consistent Random Phase Approximation (SCRPA) applied to the three-level Lipkin model. We determine one and two-body densities as very convergent expansions in terms of the generators of the RPA basis. We show that SCRPA excitations correspond to the heads of some rotational bands in the exact spectrum. It turns out that the SCRPA eigenmodes for N=2 coincide with exact solutions, given by the diagonalisation procedure.

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arXiv2002

High Spin Proton Emitters in Odd-Odd Nuclei and Shape Changes

We present a formalism to describe proton emission from odd-odd nuclei based on a scattering like approach. Special emphasis is given to the case of transitions between states with different deformations. As an example we estimate the proton half-life of the odd-odd nucleus $^{58}$Cu. Our calculations show that the change of deformation in the decay process has a significant influence on the half-life. In addition, the angular momentum coupling of proton and neutron orbitals can result in an important $K$-hindrance of the decay. To fully account for the observed half-life in $^{58}$Cu, one has to consider a shape mixing in the final state.

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arXiv1998

Self consistent random phase approximation within the O(5) model and Fermi transitions

Self Consistent Quasiparticle Random Phase Approximation (SCQRPA) is considered in application to the Fermi transitions within the O(5) model. It is demonstrated that SCQRPA improves on renormalized QRPA (RQRPA), a method that has recently become rather popular in this context. The analytical form of the SCQRPA vacuum is used to evaluate all the matrix elements. The SCQRPA results show a general trend similar to the exact solutions. The necessity to change the single particle basis beyond the transition point, and to include the proton-proton and neutron-neutron channels in the QRPA operator, in addition to the proton-neutron one, is pointed out.

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