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F. Nowacki

Publications and source records attributed to F. Nowacki.

90 records · Page 5Linked to original sources
arXiv2002

The superdeformed excited band of Ca-40

The superdeformed band, recently discovered in Ca-40 is analysed in an spherical shell model context. Two major oscillator shells, sd and pf are necessary to describe it. The yrast band of the fixed 8p-8h configuration fits extremely well with the experimental energies and transition rates of the superdeformed band. The 4p-4h configuration generates a normally deformed band plus a gamma-band pattern, both are also present in the experimental data.

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arXiv2001

First-forbidden mirror beta-decays in A=17 mass region

The first-forbidden beta-decay of 17Ne into the 'halo' state J=1/2+ of 17F presents one of the largest measured asymmetries for mirror beta-decay feeding bound final states. This asymmetry is studied in the framework of the Shell Model Embedded in the Continuum (SMEC). The spatial extent of single particle orbits is constrained by the proton capture cross-section 16O(p,gamma)17F calculated in SMEC . This allows to estimate the mirror symmetry breaking in 17F/17O and 17Ne/17N nuclei.

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arXiv2001

Canonical form of Hamiltonian matrices

On the basis of shell model simulations, it is conjectured that the Lanczos construction at fixed quantum numbers defines---within fluctuations and behaviour very near the origin---smooth canonical matrices whose forms depend on the rank of the Hamiltonian, dimensionality of the vector space, and second and third moments. A framework emerges that amounts to a general Anderson model capable of dealing with ground state properties and strength functions. The smooth forms imply binomial level densities. A simplified approach to canonical thermodynamics is proposed.

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arXiv2001

Fast Rotation of the N=Z Nucleus 36Ar

A highly-deformed rotational band has been identified in the N=Z nucleus 36Ar. At high spin the band is observed to its presumed termination at I=16+, while at low spin it has been firmly linked to previously known states in 36Ar. Spins, parities, and absolute excitation energies have thus been determined throughout the band. Lifetime measurements establish a large low-spin quadrupole deformation (beta_2=0.46+-0.03) and indicate a decreasing collectivity as the band termination is approached. With effectively complete spectroscopic information and a valence space large enough for significant collectivity to develop, yet small enough to be meaningfully approached from the shell model perspective, this rotational band in 36Ar provides many exciting opportunities to test and compare complementary models of collective motion in nuclei.

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arXiv2001

Shell Model Description of Isotope Shifts in Calcium

Isotope shifts in the nuclear charge radius of even and odd calcium isotopes are calculated within the nuclear shell model. The model space includes all configurations of nucleons in the $2s, 1d_{3/2}, 1f_{7/2}, {\rm and} ~2p_{3/2}$ orbits. The shell model describes well the energies of the intruder states in Sc and Ca, as well as the energies of the low-lying $2^+$ and $3^-$ states in the even Ca isotopes. The characteristic features of the isotope shifts, the parabolic dependence on $A$ and the prominent odd-even staggering, are well reproduced by the model. These features are related to the partial breakdown of the $Z = 20$ shell closure caused by promotion, due to the neutron-proton interaction, of the $ds$ shell protons into the $fp$ shell.

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arXiv2000

Shell model study of the isobaric chains A=50, A=51 and A=52

Shell model calculations in the full pf-shell are carried out for the A=50, 51 and 52 isobars. The most frequently used effective interactions for the pf-shell, KB3 and FPD6 are revisited and their behaviour at the N=28 and Z=28 closures examined. Cures to their -relatively minor- defaults are proposed, and a new mass dependent version called KB3G is released. Energy spectra, electromagnetic transitions and moments as well as beta decay properties are computed and compared with the experiment and with the results of the earlier interactions. A high quality description is achieved. Other miscellaneous topics are addressed; the Coulomb energy differences of the yrast states of the mirror pair 51Mn-51Fe and the systematics of the magnetic moments of the N=28 isotones.

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arXiv2000

Shell model studies of neutron rich nuclei

We discuss the present status of the description of the structure of the very neutron rich nuclei, in the framework of modern large scale shell model calculations. Particular attention is paid to the interaction related issues, as well as to the problems of the shell model approach at the neutron drip line. We present detailed results for nuclei around N=20 and, more briefly, we discuss some salient features of the regions close to N=8, 28 and 40. We show that most experimental features can be understood in a shell model context.

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arXiv2000

Shell model description of 16O(p,gamma)17F and 16O(p,p)16O reactions

We present shell model calculations of both the structure of 17F and the reactions 16O(p,gamma)17F, 16O(p,p)16O. We use the ZBM interaction which provides a good description of the properties of 16O and neighbouring nuclei and, in particular it takes account for the complicated correlations in coexisting low-lying states of 16O.

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arXiv1999

Lifetimes of 26Al and 34Cl in an astrophysical plasma

We study here the onset of thermal equilibrium affecting the lifetimes of 26Al and 34Cl nuclei within a hot astrophysical photon gas. The 26Al isotope is of prime interest for gamma ray astronomy with the observation of its delayed (t_1\2=0.74 My) 1.809MeV gamma-ray line. Its nucleosynthesis is complicated by the presence of a short lived (t_1\2=6.34s) spin isomer. A similar configuration is found in 34Cl where the decay of its isomer (34mCl, t_1\2=32m) is followed by delayed gamma-ray emission with characteristic energies. The lifetimes of such nuclei are reduced at high temperature by the thermal population of shorter lived levels. However, thermal equilibrium within 26Al and 34Cl levels is delayed by the presence of the isomer. We study here the transition to thermal equilibrium where branching ratios for radiative transitions are needed in order to calculate lifetimes. Since some of these very small branching ratios are not known experimentally, we use results of shell model calculations.

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arXiv1999

Analysis of the 16O(p,gamma)17F capture reaction using the shell model embedded in the continuum

We apply the realistic shell model which includes the coupling between many-particle (quasi-)bound states and the continuum of one-particle scattering states, called the Shell Model Embedded in the Continuum, to the spectroscopy of mirror nuclei: 17F and 17O, as well as to the description of low energy cross section (the astrophysical S-factor) for E1, E2 and M1 components in the capture reaction: 16O(p,gamma)17F. With the same microscopic input we calculate the phase shifts and differential cross-section for the elastic scattering of protons on 16O.

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arXiv1999

Shell model calculations of stellar weak interaction rates: I. Gamow-Teller distributions and spectra of nuclei in the mass range A=45-65

Electron capture and beta-decay rates on nuclei in the mass range A=45-65 play an important role in many astrophysical environments. The determination of these rates by large-scale shell model calculations is desirable, but it requires to reproduce the Gamow-Teller strength distributions and spectra of the pf shell nuclei. We show in this paper that large-scale shell model calculations, employing a slightly monopole-corrected version of the wellknown KB3 interaction, fulfill these necessary requirements. In particular, our calculations reproduce the experimentally available GT+ and GT- strength distributions and the nuclear halflives, and describe the nuclear spectra appropriately.

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arXiv1998

Full $0\hbarω$ shell model calculation of the binding energies of the $1f_{7/2}$ nuclei

Binding energies and other global properties of nuclei in the middle of the $pf$ shell, such as M1, E2 and Gamow-Teller sum rules, have been obtained using a new Shell Model code (NATHAN) written in quasi-spin formalism and using a $j-j$-coupled basis. An extensive comparison is made with the recently available Shell Model Monte Carlo results using the effective interaction KB3. The binding energies for -nearly- all the $1f_{7/2}$ nuclei are compared with the measured (and extrapolated) results.

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arXiv1998

A study of nuclei of astrophysical interest in the continuum shell model

We present here the first application of realistic shell model (SM) including coupling between many-particle (quasi-)bound states and the continuum of one-particle scattering states to the spectroscopy of 8B and to the calculation of astrophysical factors in the reaction 7Be(p,gamma)8B.

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arXiv1997

$β^+$ and electron capture of fp shell nuclei with $Tz=-2$

Allowed $β^+$ branches of very proton-rich $fp$ shell $Tz=-2$ nuclei at the proton drip-line are calculated in the full fp valence space. The $β^+$ decay half-lives calculated with the standard quenching factor ($g^{eff}_{A}/g_{A}$)=0.74 are in good agreement with existing experimental data. Detailed branching Gamow-Teller strength are predicted but comparison with experiment is still difficult since, in most cases, spectroscopic information is not yet available.

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arXiv1996

Shell Model Study of the Neutron-Rich Nuclei around N=28

We describe the properties of the neutron rich nuclei around N=28 in the shell mode framework. The valence space comprises the $sd$ shell for protons an the $pf$ shell for neutrons without any restriction. Good agreement is found with the available experimental data. The N=28 shell closure, even if eroded due to the large neutron excess, persists. The calculations predict that $^{40}$S and $^{42}$S are deformed with $β=0.29 $ and $β=0.32$ respectively.

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arXiv1996

Backbending in 50Cr

The collective yrast band and the high spin states of the nucleus 50Cr are studied using the spherical shell model and the HFB method. The two descriptions lead to nearly the same values for the relevant observables. A first backbending is predicted at I=10\hbar corresponding to a collective to non-collective transition. At I=16\hbar a second backbending occurs, associated to a configuration change that can also be interpreted as an spherical to triaxial transition.

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arXiv1996

Shell Model Study of the Double Beta Decays of $^{76}$Ge, $^{82}$Se and $^{136}$Xe

The lifetimes for the double beta decays of $^{76}$Ge, $^{82}$Se and $^{136}$Xe are calculated using very large shell model spaces. The two neutrino matrix elements obtained are in good agreement with the present experimental data. For $ <1$ eV we predict the following upper bounds to the half-lives for the neutrinoless mode: $T^{(0ν)}_{1/2}(Ge) > 1.85\,10^{25} yr.$, $T^{(0ν)}_{1/2}(Se) > 2.36\,10^{24} yr.$ and $T^{(0ν)}_{1/2}(Xe) > 1.21\,10^{25} yr$. These results are the first from a new generation of Shell Model calculations reaching O(10$^{8}$) dimensions.

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