Showing papers by "Nils Paar published in 2013"

Electric dipole polarizability in 208 Pb: Insights from the droplet model

Abstract: We study the electric dipole polarizability αD in 208Pb based on the predictions of a large and representative set of relativistic and nonrelativistic nuclear mean-field models. We adopt the droplet model as a guide to better understand the correlations between αD and other isovector observables. Insights from the droplet model suggest that the product of αD and the nuclear symmetry energy at saturation density J is much better correlated with the neutron skin thickness Δrnp of 208Pb than the polarizability alone. Correlations of αDJ with Δrnp and with the symmetry energy slope parameter L suggest that αDJ is a strong isovector indicator. Hence, we explore the possibility of constraining the isovector sector of the nuclear energy density functional by comparing our theoretical predictions against measurements of both αD and the parity-violating asymmetry in 208Pb. We find that the recent experimental determination of αD in 208Pb in combination with the range for the symmetry energy at saturation density J=[31±(2)est] MeV suggests Δrnp(208Pb)=0.165±(0.009)expt±(0.013)theor±(0.021)est fm and L=43±(6)expt±(8)theor±(12)est MeV.

Giant Quadrupole Resonances in 208 Pb, the nuclear symmetry energy and the neutron skin thickness

Abstract: Recent improvements in the experimental determination of properties of the isovector giant quadrupole resonance (IVGQR), as demonstrated in the A=208 mass region, may be instrumental for characterizing the isovector channel of the effective nuclear interaction. We analyze properties of the IVGQR in 208Pb, using both macroscopic and microscopic approaches. The microscopic method is based on families of nonrelativistic and covariant energy density functionals (EDF), characterized by a systematic variation of isoscalar and isovector properties of the corresponding nuclear matter equations of state. The macroscopic approach yields an explicit dependence of the nuclear symmetry energy at some subsaturation density, for instance S(ρ=0.1 fm−3), or the neutron skin thickness Δrnp of a heavy nucleus, on the excitation energies of isoscalar and isovector GQRs. Using available data it is found that S(ρ=0.1 fm−3)=23.3±0.6 MeV. Results obtained with the microscopic framework confirm the correlation of the Δrnp to the isoscalar and isovector GQR energies, as predicted by the macroscopic model. By exploiting this correlation together with the experimental values for the isoscalar and isovector GQR energies, we estimate Δrnp=0.14±0.03 fm for 208Pb, and the slope parameter of the symmetry energy: L=37±18 MeV.

Information content of the weak-charge form factor

Abstract: Background: Parity-violating electron scattering provides a model-independent determination of the nuclear weak-charge form factor that has widespread implications across such diverse areas as fundamental symmetries, nuclear structure, heavy-ion collisions, and neutron-star structure. Purpose: We assess the impact of precise measurements of the weak-charge form factor of 48Ca and 208Pb on a variety of nuclear observables, such as the neutron skin and the electric-dipole polarizability. Methods: We use the nuclear density functional theory with several accurately calibrated nonrelativistic and relativistic energy density functionals. To assess the degree of correlation between nuclear observables and to explore systematic and statistical uncertainties on theoretical predictions, we employ the chi-square statistical covariance technique. Results: We find a strong correlation between the weak-charge form factor and the neutron radius, that allows for an accurate determination of the neutron skin of neutron-rich nuclei. We determine the optimal range of the momentum transfer q that maximizes the information content of the measured weak-charge form factor and quantify the uncertainties associated with the strange quark contribution. Moreover, we confirm the role of the electric-dipole polarizability as a strong isovector indicator. Conclusions: Accurate measurements of the weak-charge form factor of 48Ca and 208Pb will have a profound impact on many aspects of nuclear theory and hadronic measurements of neutron skins of exotic nuclei at radioactive-beam facilities.

Pairing transitions in finite-temperature relativistic Hartree-Bogoliubov theory

Abstract: We formulate the finite-temperature relativistic Hartree-Bogoliubov theory for spherical nuclei based on a point-coupling functional, with the Gogny or separable pairing force. Using the functional PC-PK1, the framework is applied to the study of pairing transitions in Ca, Ni, Sn, and Pb isotopic chains. The separable pairing force reproduces the gaps calculated with the Gogny force not only at zero temperature, but also at finite temperatures. By performing a systematic calculation of the even-even Ca, Ni, Sn, and Pb isotopes, it is found that the critical temperature for a pairing transition generally follows the rule Tc=0.6Δn(0), where Δn(0) is the neutron pairing gap at zero temperature. This rule is further verified by adjusting the pairing gap at zero temperature with a strength parameter.

Incompressibility of finite fermionic systems: stable and exotic atomic nuclei

Abstract: The incompressibility of finite fermionic systems is investigated using analytical approaches and microscopic models. The incompressibility of a system is directly linked to the zero-point kinetic energy of constituent fermions, and this is a universal feature of fermionic systems. In the case of atomic nuclei, this implies a constant value of the incompressibility in medium-heavy and heavy nuclei. The evolution of nuclear incompressibility along Sn and Pb isotopic chains is analyzed using global microscopic models, based on both nonrelativistic and relativistic energy functionals. The result is an almost constant incompressibility in stable nuclei and systems not far from stability and a steep decrease in nuclei with pronounced neutron excess, caused by the emergence of a soft monopole mode in neutron-rich nuclei.

Large-scale calculations of supernova neutrino-induced reactions in Z = 8 –82 target nuclei

Abstract: Background: In the environment of high neutrino fluxes provided in core-collapse supernovae or neutron star mergers, neutrino-induced reactions with nuclei contribute to the nucleosynthesis processes. A number of terrestrial neutrino detectors are based on inelastic neutrino-nucleus scattering and modeling of the respective cross sections allow predictions of the expected detector reaction rates. Purpose: To provide a self-consistent microscopic description of neutrino-nucleus cross sections involving a large pool of Z=8–82 nuclei for the implementation in models of nucleosynthesis and neutrino detector simulations. Methods: Self-consistent theory framework based on relativistic nuclear energy density functional is employed to determine the nuclear structure of the initial state and relevant transitions to excited states induced by neutrinos. The weak neutrino-nucleus interaction is employed in the current-current form and a complete set of transition operators is taken into account. Results: We perform large-scale calculations of charged-current neutrino-nucleus cross sections, including those averaged over supernova neutrino fluxes, for the set of even-even target nuclei from oxygen toward lead (Z=8–82), spanning N=8–182 (OPb pool). The model calculations include allowed and forbidden transitions up to J=5 multipoles. Conclusions: The present analysis shows that the self-consistent calculations result in considerable differences in comparison to previously reported cross sections, and for a large number of target nuclei the cross sections are enhanced. Revision in modeling r-process nucleosynthesis based on a self-consistent description of neutrino-induced reactions would allow an updated insight into the origin of elements in the Universe and it would provide the estimate of uncertainties in the calculated element abundance patterns.

Neutron-skin thickness of 208Pb from the energy of the anti-analogue giant dipole resonance

Abstract: The energy of the charge-exchange anti-analogue giant dipole resonance (AGDR) has been calculated for the Pb-208 isotope using the state-of-the-art fully self-consistent relativistic proton-neutron quasiparticle random-phase approximation based on the relativistic Hartree-Bogoliubov model. It is shown that the AGDR centroid energy is very sensitively related to the corresponding neutron-skin thickness. The neutron-skin thickness of Pb-208 has been determined very precisely by comparing the theoretical results with the available experimental data on E(AGDR). The result Delta R-pn = 0.161 +/- 0.042 agrees nicely with the previous experimental results.

Neutron-skin thickness of 208Pb from the energy of the anti-analog giant dipole resonance

Abstract: The energy of the charge-exchange Anti-analog Giant Dipole Resonance (AGDR) has been calculated for the 208Pb isotope using the state-of-the-art fully self-consistent relativistic proton-neutron quasiparticle random-phase approximation based on the Relativistic Hartree-Bogoliubov model. It is shown that the AGDR centroid energy is very sensitively related to the corresponding neutron-skin thickness. The neutron-skin thickness of 208Pb has been determined very precisely by comparing the theoretical results with the available experimental data on E(AGDR). The result DR(pn)= 0.161(42) agrees nicely with the previous experimental results.

Neutron-skin thickness of $^{208}$Pb, and symmetry-energy constraints from the study of the anti-analog giant dipole resonance

Abstract: The $^{208}$Pb($p$,$n\gamma\bar p$) $^{207}$Pb reaction at a beam energy of 30 MeV has been used to excite the anti-analog of the giant dipole resonance (AGDR) and to measure its $\gamma$-decay to the isobaric analog state in coincidence with proton decay of IAS. The energy of the transition has also been calculated with the self-consistent relativistic random-phase approximation (RRPA), and found to be linearly correlated to the predicted value of the neutron-skin thickness ($\Delta R_{pn}$). By comparing the theoretical results with the measured transition energy, the value of 0.190 $\pm$ 0.028 fm has been determined for $\Delta R_{pn}$ of $^{208}$Pb, in agreement with previous experimental results. The AGDR excitation energy has also been used to calculate the symmetry energy at saturation ($J=32.7 \pm 0.6$ MeV) and the slope of the symmetry energy ($L=49.7 \pm 4.4$ MeV), resulting in more stringent constraints than most of the previous studies.

Nuclear Symmetry Energy: constraints from Giant Quadrupole Resonances and Parity Violating Electron Scattering

Abstract: Experimental and theoretical efforts are being devoted to the study of observables that can shed light on the properties of the nuclear symmetry energy. We present our new results on the excitation energy [X. Roca-Maza et al., Phys. Rev. C 87, 034301 (2013)] and polarizability of the Isovector Giant Quadrupole Resonance (IVGQR), which has been the object of new experimental investigation [S. S. Henshaw et al., Phys. Rev. Lett. 107, 222501 (2011)]. We also present our theoretical analysis on the parity violating asymmetry at the kinematics of the Lead Radius Experiment [S. Abrahamyan et al. (PREx Collaboration), Phys. Rev. Lett. 108, 112502 (2012)] and highlight its relation with the density dependence of the symmetry energy [X. Roca-Maza et al., Phys. Rev. Lett. 106, 252501 (2011)].

A new method for measuring neutron-skin thickness in rare isotope beams

Abstract: A new method, based on the excitation of the anti-analog giant dipole resonance (AGDR) in (p, n) reaction, for measuring the neutron-skin thickness has been tested. The gamma-decay of the AGDR to the isobaric analog state (IAS) has been measured. The difference in excitation energy of the AGDR and IAS was calculated. By comparing the theoretical results with the measured one, the Delta R-pn value for Sn-124 was deduced to be 0.209 +/- 0.066 fm. The present method provides a new possibility for measuring neutron-skin thickness of very exotic nuclei. DOI:10.5506/APhysPolB.44.559

Measurement of the 92,93,94,100Mo(g,n) reactions by Coulomb Dissociation

Abstract: The Coulomb Dissociation (CD) cross sections of the stable isotopes 92,94,100Mo and of the unstable isotope 93Mo were measured at the LAND/R3B setup at GSI Helmholtzzentrum fur Schwerionenforschung in Darmstadt, Germany. Experimental data on these isotopes may help to explain the problem of the underproduction of 92,94Mo and 96,98Ru in the models of p-process nucleosynthesis. The CD cross sections obtained for the stable Mo isotopes are in good agreement with experiments performed with real photons, thus validating the method of Coulomb Dissociation. The result for the reaction 93Mo(g,n) is especially important since the corresponding cross section has not been measured before. A preliminary integral Coulomb Dissociation cross section of the 94Mo(g,n) reaction is presented. Further analysis will complete the experimental database for the (g,n) production chain of the p-isotopes of molybdenum.