arXiv subjects

B. G. Carlsson

Publications and source records attributed to B. G. Carlsson.

At least 19 recordsLinked to original sources

arXiv2026

Microscopic optical potential framework applied to neutron scattering on deformed $^{48,50}$Cr

We formulate and implement a microscopic framework to derive an optical potential from the solution to an effective Hamiltonian and use it to calculate neutron scattering cross sections for the deformed nuclei $^{24}$Mg, $^{48}$Cr and $^{50}$Cr. This approach is based on a symmetry-restored multi-excitation generator coordinate method (GCM), enabling the consistent treatment of both nuclear structure and reaction observables. Through this method, non-local optical potentials corresponding to a Hamiltonian can potentially be constructed for any nucleus in the whole nuclide chart. We use this to perform reaction calculations employing quadrupole deformed triaxial configurations, obtaining results for $A\approx 50$ chromium isotopes, and study the properties of the calculated non-local optical potentials. This work further advances the unified treatment of structure and reaction, within a framework that exploits the intrinsic symmetries of nuclei.

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arXiv2025

Nuclear cross sections from low-energy interactions

We present a method to calculate neutron scattering cross sections for deformed nuclei using many--body wavefunctions described with multiple reference states. Nuclear states are calculated with the generator coordinate method using a low energy effective Hamiltonian. Using these states, a non--local and energy dependent optical potential is consistently constructed, allowing to directly investigate the role of nuclear structure properties in nuclear scattering. The case of neutron scattering on $^{24}$Mg is presented. The results are compared to experiment and to phenomenological optical potentials at energies below 13 MeV, demonstrating the importance of low--energy collectivity in elastic and non--elastic scattering.

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arXiv2024

Formation of transfermium elements in reactions with $^{208}$Pb

Within the Langevin framework, we investigate the dynamics of the fusion process for production of transfermium elements in reactions of $^{48}$Ca, $^{50}$Ti, $^{54}$Cr, and $^{58}$Fe with $^{208}$Pb. After the reacting nuclei have made contact, the early dynamical stage is dominated by the dissipation of the initial radial kinetic energy, while the subsequent shape evolution is diffusive. The probability for surmounting the inner barrier and forming a compound system is obtained by simulating the evolution as a Metropolis random walk in a five-dimensional potential-energy landscape. Good agreement with the available data is obtained, especially for the maximal formation probability.

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arXiv2022

Beyond-Mean-Field with an Effective Hamiltonian Mapped from an Energy Density Functional

A method for beyond-mean-field calculations based on an energy density functional is described. The main idea is to map the energy surface for the nuclear quadrupole deformation, obtained from an energy density functional at the mean-field level, into an effective Hamiltonian expressed as a many-body operator. The advantage of this procedure is that one avoids the problems with density dependence which can arise in beyond-mean-field methods. The effective Hamiltonian is then used in a straightforward way in the generator-coordinate-method with the inclusion of projections onto good particle numbers and angular momentum. In the end, both spectra and wave functions are obtained. As an example of the method, calculations for the nucleus $ ^{62} $Zn is performed with three different parametrizations of the Skyrme functional. The results are compared with experiment.

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arXiv2022

Nuclear spectra from low-energy interactions

A method to describe spectra starting from nuclear density functionals is explored. The idea is based on postulating an effective Hamiltonian that reproduces the stiffness associated with collective modes. The method defines a simple form of such an effective Hamiltonian and a mapping to go from a density functional to the corresponding Hamiltonian. In order to test the method, the Hamiltonian is constrained using a Skyrme functional and solved with the generator-coordinate method to describe low-lying levels and electromagnetic transitions in $^{48,49,50,52}$Cr and $^{24}$Mg.

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arXiv2021

A new and practical formulation for overlaps of Bogoliubov vacua

In this letter we present a new expression for the overlaps of wavefunctions in Hartree-Fock-Bogoliubov based theories. Starting from the Pfaffian formula by Bertsch et al (Phys. Rev. Lett. 108,042505 (2012)), an exact and computationally stable formula for overlaps is derived. We illustrate the convenience of this new formulation with a numerical application in the context of the particle-number projection method. This new formula allows for substantially increased precision and versatility in chemical, atomic, and nuclear physics applications, particularly for methods dealing with superfluidity, symmetry restoration and uses of non-orthogonal many-body basis states.

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arXiv2020

Correlation studies of fission fragment neutron multiplicities

We calculate neutron multiplicities from fission fragments with specified mass numbers for events having a specified total fragment kinetic energy. The shape evolution from the initial compound nucleus to the scission configurations is obtained with the Metropolis walk method on the five-dimensional potential-energy landscape, calculated with the macroscopic-microscopic method for the three-quadratic-surface shape family. Shape-dependent microscopic level densities are used to guide the random walk, to partition the intrinsic excitation energy between the two proto-fragments at scission, and to determine the spectrum of the neutrons evaporated from the fragments. The contributions to the total excitation energy of the resulting fragments from statistical excitation and shape distortion at scission is studied. Good agreement is obtained with available experimental data on neutron multiplicities in correlation with fission fragments from $^{235}$U(n$_{\rm th}$,f). At higher neutron energies a superlong fission mode appears which affects the dependence of the observables on the total fragment kinetic energy.

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arXiv2020

Calculated fission-fragment mass yields and average total kinetic energies of heavy and superheavy nuclei

Fission-fragment mass and total-kinetic-energy (TKE) distributions following fission of even-even nuclides in the region $74 \leq Z \leq 126$ and $92 \leq N \leq 230$, comprising 896 nuclides have been calculated using the Brownian shape-motion method. The emphasis is the region of superheavy nuclei. To show compatibility with earlier results the calculations are extended to include earlier studied regions. An island of asymmetric fission is obtained in the superheavy region, $106\leq Z\leq114$ and $162\leq N\leq 176$, where the heavy fragment is found to be close to $^{208}$Pb and the light fragment adjusts accordingly. Most experimentally observed $α$-decay chains of superheavy nuclei with $Z > 113 $ terminate by spontaneous fission in our predicted region of asymmetric fission. In these cases, the pronounced large asymmetry is accompanied by a low TKE value compatible with measurements.

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arXiv2019

Fission modes in fermium isotopes with Brownian shape-motion model

Fission-fragment mass and total-kinetic-energy distributions following fission of the fermium isotopes $^{256,258,260}\text{Fm}$ at low excitation energy have been calculated using the Brownian shape-motion model. A transition from asymmetric fission in $^{256}\text{Fm}$ to symmetric fission in $^{258}\text{Fm}$ is obtained. The total-kinetic-energy distributions for the three isotopes show radically different behaviour due to varying contributions from different fission modes, with a double-humped distribution for $^{258}\text{Fm}$.

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arXiv2018

Microscopic treatment of energy partition in fission

The transformation of an atomic nucleus into two excited fission fragments is modeled as a strongly damped evolution of the nuclear shape, until scission occurs at a small critical neck radius, at which point the mass, charge, and shape of each fragment are extracted. The available excitation energy then is divided statistically on the basis of the microscopic level densities. This approach takes account of the important (and energy-dependent) finite-size effects. After the fragments have been fully accelerated and their shapes have relaxed to their equilibrium form, they undergo sequential neutron evaporation. The dependence of the resulting mean neutron multiplicity on the fragment mass, $\barν$(A), including the dependence on the initial excitation energy of the fissioning compound nucleus, is in good agreement with the observed behavior, as demonstrated here for $^{235}$U(n,f).

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arXiv2016

The effect of realistic nuclear charge distributions on isotope shifts and towards the extraction of higher order nuclear radial moments

Background: Atomic spectral lines from different isotopes display a small shift in energy, commonly referred to as the line isotope shift. One of the components of the isotope shift is the field shift, which depends on the extent and the shape of the nuclear charge density distribution. Purpose: To investigate how sensitive field shifts are with respect to variations in the nuclear size and shape and what information of nuclear charge distributions that can be extracted from measured field shifts. Methods: Nuclear properties are obtained from nuclear density functional theory calculations based on the Skyrme-Hartree-Fock-Bogoliubov approach. These results are combined with multiconfiguration Dirac-Hartree-Fock methods to obtain realistic field shifts. Results: Phenomena such as nuclear deformation and variations in the diffuseness of nuclear charge distributions give measurable contributions to the field shifts. Using a novel approach, we demonstrate the possibility to extract new information concerning the nuclear charge densities from the observed field shifts. Conclusions: Combining methods used in atomic and nuclear structure theory gives an improved description of field shifts. Extracting additional nuclear information from measured field shifts is possible in the near future with improved experimental methods.

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arXiv2015

Recoil-alpha-fission and recoil-alpha-alpha-fission events observed in the reaction Ca-48 + Am-243

Products of the fusion-evaporation reaction Ca-48 + Am-243 were studied with the TASISpec set-up at the gas-filled separator TASCA at the GSI Helmholtzzentrum für Schwerionenforschung. Amongst the detected thirty correlated alpha-decay chains associated with the production of element Z=115, two recoil-alpha-fission and five recoil-alpha-alpha-fission events were observed. The latter are similar to four such events reported from experiments performed at the Dubna gas-filled separator. Contrary to their interpretation, we propose an alternative view, namely to assign eight of these eleven decay chains of recoil-alpha(-alpha)-fission type to start from the 3n-evaporation channel 115-288. The other three decay chains remain viable candidates for the 2n-evaporation channel 115-289.

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arXiv2014

Spectroscopic properties of nuclear Skyrme energy density functionals

We address the question of how to improve the agreement between theoretical nuclear single-particle energies (SPEs) and experiment. Empirically, in doubly magic nuclei, the SPEs can be deduced from spectroscopic properties of odd nuclei that have one more, or one less neutron or proton. Theoretically, bare SPEs, before being confronted with experiment, must be corrected for the effects of the particle-vibration-coupling (PVC). In the present work, we determine the PVC corrections in a fully self-consistent way. Then, we adjust the SPEs, with PVC corrections included, to empirical data. In this way, the agreement with experiment, on average, improves; nevertheless, large discrepancies still remain. We conclude that the main source of disagreement is still in the underlying mean fields, and not in including or neglecting the PVC corrections.

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arXiv2014

Structure of Superheavy Nuclei Along Element 115 Decay Chains

A recent high-resolution $α$, $X$-ray, and $γ$-ray coincidence-spectroscopy experiment offered first glimpse of excitation schemes of isotopes along $α$-decay chains of $Z=115$. To understand these observations and to make predictions about shell structure of superheavy nuclei below $^{288}115$, we employ two complementary mean-field models: self-consistent Skyrme Energy Density Functional approach and the macroscopic-microscopic Nilsson model. We discuss the spectroscopic information carried by the new data. In particular, candidates for the experimentally observed $E1$ transitions in $^{276}$Mt are proposed. We find that the presence and nature of low-energy $E1$ transitions in well-deformed nuclei around $Z=110, N=168$ strongly depends on the strength of the spin-orbit coupling; hence, it provides an excellent constraint on theoretical models of superheavy nuclei. To clarify competing theoretical scenarios, an experimental search for $E1$ transitions in odd-$A$ systems $^{275,277}$Mt, $^{275}$Hs, and $^{277}$Ds is strongly recommended.

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arXiv2014

Low-lying states in near-magic odd-odd nuclei and the effective interaction

The iterative quasi-particle-random-phase approximation (QRPA) method we previously developed to accurately calculate properties of individual nuclear states is extended so that it can be applied for nuclei with odd numbers of neutrons and protons. The approach is based on the proton-neutron-QRPA (pnQRPA) and uses an iterative non-hermitian Arnoldi diagonalization method where the QRPA matrix does not have to be explicitly calculated and stored. The method is used to calculate excitation energies of proton-neutron multiplets for several nuclei. The influence of a pairing interaction in the $T=0$ channel is studied.

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arXiv2014

$α$-decay calculations of heavy nuclei using an effective Skyrme interaction

Background: For nuclei heavier than $^{208}$Pb $α$ decay is a dominating decay mode, and in the search of new superheavy elements one often observes chains of $α$ decays. Purpose: Explore and test microscopic descriptions of $α$ decay based on theories with effective nuclear interactions. Methods: The nuclear ground states are calculated with the Hartree-Fock-Bogoliubov (HFB) method using the Skyrme interaction. Microscopic $α$-decay formation amplitudes are calculated from the HFB wave functions, and the $R$-matrix formalism is utilized to obtain decay probabilities. Results: Using a large harmonic-oscillator basis we obtain converged $α$-decay widths. A comparison with experiment including all spherical even-even $α$ emitting nuclei shows that the model consistently predicts too small formation amplitudes while relative values are in good agreement with experiment. Conclusions: The method was found to be numerically practical even with a large basis size. The comparison of formation amplitudes suggests that the pairing type correlations included in the HFB approach cannot produce sufficient $α$-particle clustering.

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arXiv2014

Alpha-particle formation and decay rates from Skyrme-HFB wave functions

$α$ decay is treated microscopically, where the unstable mother nucleus and residual daughter nucleus are described using HFB wave functions, obtained with the Skyrme effective interaction. From these wave functions the amplitude for forming $α$ particles in the mother nucleus is computed. Two different Skyrme parametrizations with different pairing properties are compared, and we find good agreement with experiment for relative decay rates in both cases. The absolute values of the decay rates are under-estimated.

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arXiv2013

Polarization corrections to single-particle energies studied within the energy-density-functional and QRPA approaches

Background: Models based on using perturbative polarization corrections and mean-field blocking approximation give conflicting results for masses of odd nuclei. Purpose: Systematically investigate the polarization and mean-field models, implemented within self-consistent approaches that use identical interactions and model spaces, so as to find reasons for the conflicts between them. Methods: For density-dependent interactions and with pairing correlations included, we derive and study links between the mean-field and polarization results obtained for energies of odd nuclei. We also identify and discuss differences between the polarization-correction and full particle-vibration-coupling (PVC) models. Numerical calculations are performed for the mean-field ground-state properties of deformed odd nuclei and then compared to the polarization corrections determined by using the approach that conserves spherical symmetry. Results: We have identified and numerically evaluated self-interaction (SI) energies that are at the origin of different results obtained within the mean-field and polarization-correction approaches. Conclusions: Mean-field energies of odd nuclei are polluted by the SI energies, and this makes them different from those obtained by using polarization-correction methods. A comparison of both approaches allows for the identification and determination of the SI terms, which then can be calculated and removed from the mean-field results, giving the self-interaction-free energies. The simplest deformed mean-field approach that does not break parity symmetry is unable to reproduce full PVC effects.

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