Nuclear energy density optimization: Large deformations

A new Skyrme-like energy density suitable for studies of strongly elongated nuclei has been determined in the framework of the Hartree-Fock-Bogoliubov theory using the recently developed model-based, derivative-free optimization algorithm POUNDerS. A sensitivity analysis at the optimal solution has revealed the importance of states at large deformations in driving the parameterization of the functional. The good agreement with experimental data on masses and separation energies, achieved with the previous parameterization UNEDF0, is largely preserved. In addition, the new energy density UNEDF1 gives a much improved description of the fission barriers in 240Pu and neighboring nuclei.Comment: 16 pages, 11 figures, accepted for publication in Phys. Rev.

Nuclear energy density optimization: Shell structure

Nuclear density functional theory is the only microscopical theory that can be applied throughout the entire nuclear landscape. Its key ingredient is the energy density functional. In this work, we propose a new parameterization UNEDF2 of the Skyrme energy density functional. The functional optimization is carried out using the POUNDerS optimization algorithm within the framework of the Skyrme Hartree-Fock-Bogoliubov theory. Compared to the previous parameterization UNEDF1, restrictions on the tensor term of the energy density have been lifted, yielding a very general form of the energy density functional up to second order in derivatives of the one-body density matrix. In order to impose constraints on all the parameters of the functional, selected data on single-particle splittings in spherical doubly-magic nuclei have been included into the experimental dataset. The agreement with both bulk and spectroscopic nuclear properties achieved by the resulting UNEDF2 parameterization is comparable with UNEDF1. While there is a small improvement on single-particle spectra and binding energies of closed shell nuclei, the reproduction of fission barriers and fission isomer excitation energies has degraded. As compared to previous UNEDF parameterizations, the parameter confidence interval for UNEDF2 is narrower. In particular, our results overlap well with those obtained in previous systematic studies of the spin-orbit and tensor terms. UNEDF2 can be viewed as an all-around Skyrme EDF that performs reasonably well for both global nuclear properties and shell structure. However, after adding new data aiming to better constrain the nuclear functional, its quality has improved only marginally. These results suggest that the standard Skyrme energy density has reached its limits and significant changes to the form of the functional are needed.Comment: 18 pages, 13 figures, 12 tables; resubmitted for publication to Phys. Rev. C after second review by refere

Overconstrained estimates of neutrinoless double beta decay within the QRPA

Estimates of nuclear matrix elements for neutrinoless double beta decay (0nu2beta) based on the quasiparticle random phase approximations (QRPA) are affected by theoretical uncertainties, which can be substantially reduced by fixing the unknown strength parameter g_pp of the residual particle-particle interaction through one experimental constraint - most notably through the two-neutrino double beta decay (2nu2beta) lifetime. However, it has been noted that the g_pp adjustment via 2\nu2\beta data may bring QRPA models in disagreement with independent data on electron capture (EC) and single beta decay (beta^-) lifetimes. Actually, in two nuclei of interest for 0nu2beta decay (Mo-100 and Cd-116), for which all such data are available, we show that the disagreement vanishes, provided that the axial vector coupling g_A is treated as a free parameter, with allowance for g_A<1 (``strong quenching''). Three independent lifetime data (2nu2beta, EC, \beta^-) are then accurately reproduced by means of two free parameters (g_pp, g_A), resulting in an overconstrained parameter space. In addition, the sign of the 2nu2beta matrix element M^2nu is unambiguously selected (M^2nu>0) by the combination of all data. We discuss quantitatively, in each of the two nuclei, these phenomenological constraints and their consequences for QRPA estimates of the 0nu2beta matrix elements and of their uncertainties.Comment: Revised version (27 pages, including 10 figures), focussed on Mo-100 and Cd-116. To appear in J. Phys. G: Nucl. Phys. (2008

Multipole strength function of deformed superfluid nuclei made easy

We present an efficient method for calculating strength functions using the finite amplitude method (FAM) for deformed superfluid heavy nuclei within the framework of the nuclear density functional theory. We demonstrate that FAM reproduces strength functions obtained with the fully self-consistent quasi-particle random-phase approximation (QRPA) at a fraction of computational cost. As a demonstration, we compute the isoscalar and isovector monopole strength for strongly deformed configurations in $^{240}$Pu by considering huge quasi-particle QRPA spaces. Our approach to FAM, based on Broyden's iterative procedure, opens the possibility for large-scale calculations of strength distributions in well-bound and weakly bound nuclei across the nuclear landscape.Comment: 5 pages, 3 figure

Methane dynamics in different boreal lake types

This study explores the variability in concentrations of dissolved CH&lt;sub&gt;4&lt;/sub&gt; and annual flux estimates in the pelagic zone in a statistically defined sample of 207 lakes in Finland. The lakes were situated in the boreal zone, in an area where the mean annual air temperature ranges from &amp;minus;2.8 to 5.9&amp;deg;C. We examined how lake CH&lt;sub&gt;4&lt;/sub&gt; dynamics related to regional lake types assessed according to the EU water framework directive. Ten lake types were defined on the basis of water chemistry, color, and size. Lakes were sampled for dissolved CH&lt;sub&gt;4&lt;/sub&gt; concentrations four times per year, at four different depths at the deepest point of each lake. We found that CH&lt;sub&gt;4&lt;/sub&gt; concentrations and fluxes to the atmosphere tended to be high in nutrient rich calcareous lakes, and that the shallow lakes had the greatest surface water concentrations. Methane concentration in the hypolimnion was related to oxygen and nutrient concentrations, and to lake depth or lake area. The surface water CH&lt;sub&gt;4&lt;/sub&gt; concentration was related to the depth or area of lake. Methane concentration close to the bottom can be viewed as proxy of lake status in terms of frequency of anoxia and nutrient levels. The mean pelagic CH&lt;sub&gt;4&lt;/sub&gt; release from randomly selected lakes was 49 mmol m&lt;sup&gt;&amp;minus;2&lt;/sup&gt; a&lt;sup&gt;&amp;minus;1&lt;/sup&gt;. The sum CH&lt;sub&gt;4&lt;/sub&gt; flux (storage and diffusion) correlated with lake depth, area and nutrient content, and CH&lt;sub&gt;4&lt;/sub&gt; release was greatest from the shallow nutrient rich and humic lakes. Our results support earlier lake studies regarding the regulating factors and also the magnitude of global emission estimate. These results propose that in boreal region small lakes have higher CH&lt;sub&gt;4&lt;/sub&gt; fluxes per unit area than larger lakes, and that the small lakes have a disproportionate significance regarding to the CH&lt;sub&gt;4&lt;/sub&gt; release

Fitting (NLO)-L-3 pseudo-potentials through central plus tensor Landau parameters

Landau parameters determined from phenomenological finite-range interactions are used to get an estimation of next-to-next-to-next-to-leading order ((NLO)-L-3) pseudo-potentials parameters. The parameter sets obtained in this way are shown to lead to consistent results concerning saturation properties. The uniqueness of this procedure is discussed, and an estimate of the error induced by the truncation at (NLO)-L-3 is given

Testing the density matrix expansion against ab initio calculations of trapped neutron drops

Microscopic input to a universal nuclear energy density functional can be provided through the density matrix expansion (DME), which has recently been revived and improved. Several DME implementation strategies are tested for neutron drop systems in harmonic traps by comparing to Hartree-Fock (HF) and ab initio no-core full configuration (NCFC) calculations with a model interaction (Minnesota potential). The new DME with exact treatment of Hartree contributions is found to best reproduce HF results and supplementing the functional with fit Skyrme-like contact terms shows systematic improvement toward the full NCFC results.Comment: 10 pages, 5 figure

Nuclear matter response function with a central plus tensor Landau interaction

We present a method to obtain response functions in the random phase approximation (RPA) based on a residual interaction described in terms of Landau parameters with central plus tensor contributions. The response functions keep the explicit momentum dependence of the RPA, in contrast with the traditional Landau approximation. Results for symmetric nuclear matter and pure neutron matter are presented using Landau parameters derived from finite-range interactions, both phenomenological and microscopic. We study the convergence of response functions as the number of Landau parameters is increased