arXiv subjects

Eckart Grosse

Publications and source records attributed to Eckart Grosse.

15 recordsLinked to original sources

arXiv2022

Broken axial symmetry as essential feature for a consistent modelling of various observables in heavy nuclei

Although most nuclear spectroscopy as well as atomic hyperfine structure data do not deliver accurate information on nuclear axiality the ad-hoc assumption of symmetry about one axis found widespread use in nuclear model calculations. In the theoretical interpretation of nuclear properties as well as in the analysis of experimental data triaxiality was considered - if at all - only for some, often exotic, nuclides. A breaking of axial symmetry combined to a spin-independent moment of inertia results in a surprisingly simple heuristic triaxial parametrization of the yrast sequence in all heavy nuclei, including well deformed ones. No additional fit parameters are needed in detailed studies of the mass and charge dependence of the electric dipole strength in the range of and outside of giant dipole resonances. Allowing triaxiality also avoids the introduction of an arbitrary level density parameter ā to fit the accurate values observed in n-capture experiments and ā can be taken from nuclear matter studies. A combination of this value to the yrast energies no longer based on axiality and the related I(I+1) rule results in agreement to data independent of spin. And predictions for radiative neutron capture as derived on the basis of non-axiality are improved as well. The experimentally favoured broken axial symmetry is in accord to HFB and MC-shell model calculations already for nuclei in the valley of stability.

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arXiv2019

Consequences of broken axial symmetry in heavy nuclei -- an overview of the situation in the valley of stability

An overview on the various effects of axial symmetry breaking is presented for medium heavy and heavy nuclei covering the mass number range 70 < A <240. The discussion includes various observations for nuclei: level densities, spectroscopic features as energies and transition rates, ground state masses and finally the splitting of giant dipole resonances. Quadrupole moments and rates can be derived from models of triaxial rigid rotation or cranking for a given triaxiality parameters γ, but microscopic considerations are needed to predict these for each nucleus investigated. Respective predictions were made by recently made Hartree- Fock-Bogolyubov (HFB) calculations extended to arbitrary triaxiality by a generator coordinate method. In accord to these, various observations as reported in this overview demonstrate the importance of allowing a breaking of axial symmetry for heavy nuclei already in the valley of stability. Considering this breaking as indicated from the HFB approach surprisingly many experimental data are well described globally without the need for local fit parameters. In addition to these comparisons it will be shown that it is advantageous to consider cγ=cos(3γ) an indicator of axiality for heavy nuclei independent of their quadrupole moment.

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arXiv2019

Novel data and a new parametrization of the electric dipole strength in nuclei with 88 < A < 116

A hitherto unexplored method for the experimental determination of the photon strength function up to the neutron separation energy was developed at the Radiation Source ELBE in Dresden. It was applied to various heavy nuclei, preferentially to nuclides with increasing distance to the N=50 neutron shell, and it covers the high level density excitation energy range above 4 MeV. The observed quasi-continuous spectra of scattered photons can be -- after a proper correction for multi-step processes -- directly combined to nuclear photo effect data from literature. A remarkably good match of the photon strengths as measured below and above the neutron emission threshold is observed. The wide energy coverage of the combined data forms an excellent basis to derive a parameterization for the dipole strength function fully covering the range across the nucleon separation energies. In addition to the parameters defining the deformation of the nuclear ground states only one additional constant is needed to describe the dipole strength in the nuclei with 88<A<116. The new parameterization differs significantly from the prescriptions generally used in network calculations, e.g. those of interest for the cosmic nucleo-synthesis.

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arXiv2017

Experimental signals for broken axial symmetry in excited heavy nuclei from the valley of stability

An increasing number of experimental data indicates the breaking of axial symmetry in many heavy nuclei already in the valley of stability: Multiple Coulomb excitation analysed in a rotation invariant way, gamma transition rates and energies in odd nuclei, mass predictions, the splitting of Giant Resonances (GR), the collective enhancement of nuclear level densities and Maxwellian averaged neutron capture cross sections. For the interpretation of these experimental observations the axial symmetry breaking shows up in nearly all heavy nuclei as predicted by Hartree-Fock-Bogoliubov (HFB) calculations [1] ; this indicates a nuclear Jahn-Teller effect. We show that nearly no parameters remain free to be adjusted by separate fitting to level density or giant resonance data, if advance information on nuclear deformations, radii etc. are taken from such calculations with the force parameters already fixed. The data analysis and interpretation have to include the quantum mechanical requirement of zero point oscillations and the distinction between static vs. dynamic symmetry breaking has to be regarded.

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arXiv2017

Breaking of axial symmetry in excited heavy nuclei as identified in Giant Dipole Resonance data

A recent theoretical prediction of a breaking of axial symmetry in quasi all heavy nuclei is confronted to a new critical analysis of photon strength functions of nuclei in the valley of stability. For the photon strength in the isovector giant dipole resonance (IVGDR) regime a parameterization of GDR shapes by the sum of three Lorentzians (TLO) is extrapolated to energies below and above the IVGDR. The impact of non-GDR modes adding to the low energy slope of photon strength is discussed including recent data on photon scattering and other radiative processes. These are shown to be concentrated in energy regions where various model calculations predict intermediate collective strength; thus they are obviously separate from the IVGDR tail. The triple Lorentzian (TLO) ansatz for giant dipole resonances is normalized in accordance to the dipole sum rule. The nuclear droplet model with surface dissipation accounts well for positions and widths without local, nuclide specific, parameters. Very few and only global parameters are needed when a breaking of axial symmetry already in the valley of stability is admitted and hence a reliable prediction for electric dipole strength functions also outside of it is expected.

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arXiv2017

Effect of broken axial symmetry on the electric dipole strength and the collective enhancement of level densities in heavy nuclei

The basic parameters for calculations of radiative neutron capture , photon strength functions and nuclear level densities near the neutron separation energy are determined based on experimental data without an ad-hoc assumption about axial symmetry - at variance to previous analysis. Surprisingly few global fit parameters are needed in addition to information on nuclear deformation, taken from Hartree Fock Bogolyubov (HFB) calculations with the Gogny force, and the generator coordinator method (GCM) assures properly defined angular momentum. For a large number of nuclei the GDR shapes and the photon strength are described by the sum of three Lorentzians (TLO), extrapolated to low energies and normalized in accordance to the dipole sum rule. Level densities are influenced strongly by the significant collective enhancement based on the breaking of shape symmetry. The replacement of axial symmetry by the less stringent requirement of invariance against rotation by 180 degree leads to a novel prediction for radiative neutron capture. It compares well to recent compilations of average radiative widths and Maxwellian average cross sections for neutron capture by even target nuclei. An extension to higher spin promises a reliable prediction for various compound nuclear reactions also outside the valley of stability. Such predictions are of high importance for future nuclear energy systems and waste transmutation as well as for the understanding of the cosmic synthesis of heavy elements.

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arXiv2015

Broken axial symmetry as essential feature to predict radiative capture in heavy nuclei

Cross sections for neutron capture in the range of unresolved resonances are predicted for more than 140 spin-0 target nuclei with A > 50. Allowing the breaking of spherical and axial symmetry in nearly all these nuclei a combined parameterization for both, level density and photon strength is obtained which employs a surprisingly small number of parameters only. The strength functions used are based on a global fit to IVGDR shapes by the sum of three Lorentzians. They are based on theoretical predictions for the A-dependence of pole energies and spreading widths and add up to the TRK sum rule. For the small spins reached by capture resonance spacings are well described by a level density parameter close to the nuclear matter value; a significant collective enhancement is apparent due to the deviation from axial symmetry. Reliable predictions for compound nuclear reactions also outside the valley of stability (as important for nuclear astrophysics and for the transmutation of nuclear waste) are expected to result from the global parameterization presented.

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arXiv2014

Importance of nuclear triaxiality for electromagnetic strength, level density and neutron capture cross sections in heavy nuclei

Cross sections for neutron capture in the range of unresolved resonances are predicted simultaneously to level distances at the neutron threshold for more than 100 spin-0 target nuclei with A >70. Assuming triaxiality in nearly all these nuclei a combined parameterization for both, level density and photon strength is presented. The strength functions used are based on a global fit to IVGDR shapes by the sum of three Lorentzians adding up to the TRK sum rule and theory-based predictions for the A-dependence of pole energies and spreading widths. For the small spins reached by capture level densities are well described by only one free global parameter; a significant collective enhancement due to the deviation from axial symmetry is observed. Reliable predictions for compound nuclear reactions also outside the valley of stability as expected from the derived global parameterization are important for nuclear astrophysics and for the transmutation of nuclear waste.

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arXiv2014

Impact of Triaxiality on the Emission and Absorption of Neutrons and Gamma Rays in Heavy Nuclei

For many spin-0 target nuclei neutron capture measurements yield information on level densities at the neutron separation energy. Also the average photon width has been determined from capture data as well as Maxwellian average cross sections for the energy range of unresolved resonances. Thus it is challenging to use this data set for a test of phenomenological prescriptions for the prediction of radiative processes. An important ingredient for respective calculations is the photon strength function for which a parameterization was proposed using a fit to giant dipole resonance shapes on the basis of theoretically determined ground state deformations including triaxiality. Deviations from spherical and axial symmetry also influence level densities and it is suggested to use a combined parameterization for both, level density and photon strength. The formulae presented give a good description of the data for low spin capture into 124 nuclei with 72<A<244 and only very few global parameters have to be adjusted when the predetermined information on ground state shapes of the nuclei involved is accounted for.

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arXiv2013

Neutron total cross section measurements of gold and tantalum at the nELBE photoneutron source

Neutron total cross sections of $^{197}$Au and $^\text{nat}$Ta have been measured at the nELBE photoneutron source in the energy range from 0.1 - 10 MeV with a statistical uncertainty of up to 2 % and a total systematic uncertainty of 1 %. This facility is optimized for the fast neutron energy range and combines an excellent time structure of the neutron pulses (electron bunch width 5 ps) with a short flight path of 7 m. Because of the low instantaneous neutron flux transmission measurements of neutron total cross sections are possible, that exhibit very different beam and background conditions than found at other neutron sources.

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arXiv2011

Description of dipole strength in heavy nuclei in conformity with their quadrupole degrees of freedom

In conformity to new findings about the widespread occurrence of triaxiality arguments are given in favor of a description of the giant dipole resonance in heavy nuclei by the sum of three Lorentzians. This TLO parameterization allows a strict use of resonance widths Γ in accordance to the theoretically founded power law relation to the resonance energy. No additional variation of Γ with the photon energy and no violation of the sum rule are necessary to obtain a good agreement to nuclear photo-effect, photon scattering and radiative capture data. Photon strength other than E1 has a small effect, but the influence of the level density on photon emission probabilities needs further investigation.

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arXiv2010

Resonance strengths in the 14N(p, γ)15O and 15N(p, αγ)12C reactions

The 14N(p, γ)15O reaction is the slowest reaction of the carbon-nitrogen-oxygen cycle of hydrogen burning in stars. As a consequence, it determines the rate of the cycle. The 15N(p, αγ)12C reaction is frequently used in inverse kinematics for hydrogen depth profiling in materials. The 14N(p, γ)15O and 15N(p, αγ)12C reactions have been studied simultaneously, using titanium nitride targets of natural isotopic composition and a proton beam. The strengths of the resonances at Ep = 1058 keV in 14N(p, γ)15O and at Ep = 897 and 430 keV in 15N(p, αγ)12C have been determined with improved precision, relative to the well-known resonance at Ep = 278 keV in 14N(p, γ)15O. The new recommended values are ωγ= 0.353$\pm$0.018, 362$\pm$20, and 21.9$\pm$1.0 eV for their respective strengths. In addition, the branching ratios for the decay of the Ep = 1058 keV resonance in 14N(p, γ)15O have been redetermined. The data reported here should facilitate future studies of off-resonant capture in the 14N(p, γ)15O reaction that are needed for an improved R-matrix extrapolation of the cross section. In addition, the data on the 430 keV resonance in 15N(p, αγ)12C may be useful for hydrogen depth profiling.

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arXiv2010

Dipole strength in 144Sm studied via (gamma,n), (gamma,p) and (gamma,alpha) reactions

Photoactivation measurements on 144Sm have been performed with bremsstrahlung endpoint energies from 10.0 to 15.5 MeV at the bremsstrahlung facility of the superconducting electron accelerator ELBE of Forschungszentrum Dresden-Rossendorf. The measured activation yield for the 144Sm(gamma,n) reaction is compared with the calculated yield using cross sections from previous photoneutron experiments. The activation yields measured for all disintegration channels 144Sm(gamma,n), (gamma,p) and (gamma,alpha) are compared to the yield calculated by using Hauser-Feshbach statistical models. A new parametrization of the photon strength function is presented and the yield simulated by using the modified photon strength parameters are compared to the experimental data.

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arXiv2008

The energy dependence of the electric dipole strength in heavy nuclei

On the basis of new photon scattering measurements and a reevaluation of average neutron resonance capture data we investigate how well Lorentzians adjusted to photo-neutron data in the giant dipole resonances give a good description of the photon strength also below the neutron threshold. If deformation effects are properly taken into account this is verified down to about 5 MeV for various nuclei with A>80 such that the previously employed differentiation between deformed and non-deformed nuclei is no longer necessary.

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arXiv2005

Pygmy dipole strength close to particle-separation energies - the case of the Mo isotopes

The distribution of electromagnetic dipole strength in 92, 98, 100 Mo has been investigated by photon scattering using bremsstrahlung from the new ELBE facility. The experimental data for well separated nuclear resonances indicate a transition from a regular to a chaotic behaviour above 4 MeV of excitation energy. As the strength distributions follow a Porter-Thomas distribution much of the dipole strength is found in weak and in unresolved resonances appearing as fluctuating cross section. An analysis of this quasi-continuum - here applied to nuclear resonance fluorescence in a novel way - delivers dipole strength functions, which are combining smoothly to those obtained from (g,n)-data. Enhancements at 6.5 MeV and at ~9 MeV are linked to the pygmy dipole resonances postulated to occur in heavy nuclei.

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