arXiv subjects

Frederic Nowacki

Publications and source records attributed to Frederic Nowacki.

4 recordsLinked to original sources

arXiv2020

Limits on assigning a shape to a nucleus

The interpretation of nuclear observables in the laboratory frame in terms of the intrinsic deformation parameters beta and gamma is a classical theme in nuclear structure. Here we use the quadrupole invariants (Kumar), calculated within the framework of the configuration-interaction shell model, to clarify the meaning and limitations of nuclear shapes. We introduce a novel method that enables us to calculate accurately higher-order invariants and, therefore, the fluctuations in both beta and gamma. We find that the shape parameter beta often has a non-negligible degree of softness, and that the angle gamma is usually characterized by large fluctuations, rendering its effective value not meaningful. Contrary to common belief, we conclude that doubly magic nuclei are not spherical because the notion of a well-defined shape does not apply to them.

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arXiv2014

The merging of the islands of inversion at N=20 and N=28

The N=20 and N=28 "islands of inversion" are described by large scale shell model calculations with an extension of the interaction sdpf-u that makes it possible to mix configurations with different Nhw or equivalently with different number of particles promoted from the sd-shell to the pf-shell. It allows to connect the classical sd-shell calculations below N=18 with the sd(protons)-pf(neutrons) calculations beyond N=24-26, for all the isotopes from Oxygen to Calcium, using the same interaction. For some isotopes this range contains all the nuclei between the proton and the neutron drip lines and includes the N=20 and N=28 islands of inversion. We shall pay particular attention to the properties of the states at fix Nhw which turn out to be the real protagonists of the physics at N=20. The existence of islands of inversion/deformation will be explained as the result of the competition between the spherical mean field which favors the 0hw configurations and the nuclear correlations which favor the deformed Nhw configurations. The Magnesium chain is exceptional because in it, the N=20 and N=28 "islands of inversion" merge, enclosing all the isotopes between N=19 and N=30. Indeed, this would be also the case for the Neon and Sodium chains if their drip lines would reach N=28.

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arXiv2012

Shell Model description of the beta beta decay of 136Xe

We study in this letter the double beta decay of 136Xe with emission of two neutrinos which has been recently measured by the EXO-200 collaboration. We use the same shell model framework, valence space, and effective interaction that we have already employed in our calculation of the nuclear matrix element (NME) of its neutrinoless double beta decay. Using the quenching factor of the Gamow-Teller operator which is needed to reproduce the very recent high resolution 136Xe (3He, t)136Cs data, we obtain a nuclear matrix element M(2nu)=0.025 MeV^(-1) compared with the experimental value M(2nu)=0.019(2) MeV^(-1).

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arXiv2004

The N=28 shell closure; from N=Z to the neutron drip line

The evolution of the N=28 neutron shell closure is studied in the N=28 isotones from neutron drip line's $^{40}$Mg to N=Z doubly magic $^{56}$Ni. It is found that the N=28 closure vanishes at Z=12 in favor of a deformed ground state. For Z=14 and Z=16 closed shell and intruder configurations are almost degenerate, leading to highly mixed ground states and to very low lying excited 0$^+$ states. For Z$>$20 the intruder states are always above the closed shell ground states. We examine their structure and their possible existence as collective yrare bands of well defined particle-hole character.

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