RIPL – Reference Input Parameter Library for Calculation of Nuclear Reactions and Nuclear Data Evaluations

The production of about half of the heavy elements found in nature is assigned to a specific astrophysical nucleosynthesis process: the rapid neutron capture process (r-process). Although this idea has been postulated more than six decades ago, the full understanding faces two types of uncertainties/open questions: (a) The nucleosynthesis path in the nuclear chart runs close to the neutron-drip line, where presently only limited experimental information is available, and one has to rely strongly on theoretical predictions for nuclear properties. (b) While for many years the occurrence of the r-process has been associated with supernovae, more recent studies have cast substantial doubts on this environment. Alternative scenarios include the mergers of neutron stars, neutron-star black hole mergers, but possibly also rare classes of supernovae as well as hypernovae/collapsars with polar jet ejecta and also accretion disk outflows related to the collapse of fast rotating massive stars with high magnetic fields. Stellar r-process abundance observations, have provided insights into, and constraints on the frequency of and conditions in the responsible stellar production sites. One of them, neutron star mergers, was just identified and related to the Gravitational Wave event GW170817. High resolution observations, increasingly more precise due to improved experimental atomic data, have been particularly important in defining the heavy element abundance patterns of the old halo stars, and thus determining the extent, and nature, of the earliest nucleosynthesis in our Galaxy. Combining new results and important breakthroughs in the related nuclear, atomic and astronomical fields of science, this review attempts to provide an answer to the question "How Were the Elements from Iron to Uranium Made?" (Abridged)

Origin of the heaviest elements: The rapid neutron-capture process

New developments have been brought to our energy-, spin-, and parity-dependent nuclear level densities based on the microscopic combinatorial model. As in our previous study, a detailed calculation of the intrinsic state density and of the rotational enhancement factor is included, but this time the vibrational contributions explicitly take the phonon excitations into account. The present model predicts the experimental $s$- and $p$-wave neutron resonance spacings with a degree of accuracy comparable to that of the best global models available. It is also shown that the model gives a reliable extrapolation at low energies where experimental data on the cumulative number of levels can be extracted. The predictions are also in good agreement with the experimental data extracted from the analysis of particle-$\ensuremath{\gamma}$ coincidence in the ($^{3}\mathrm{He}$, $\ensuremath{\alpha}\ensuremath{\gamma}$) and ($^{3}\mathrm{He}$, $^{3}\mathrm{He}$${}^{'}\ensuremath{\gamma}$) reactions. Total as well as partial level densities for more than 8500 nuclei are made available in a table format for practical applications. For the nuclei for which experimental $s$-wave spacings and enough low-lying states exist, renormalization factors are provided to reproduce simultaneously both observables. The same combinatorial method is used to estimate the nuclear level densities at the fission saddle points of actinides and at the shape isomer deformation. Finally, the new nuclear level densities are applied to the calculation of radiative neutron capture cross sections.

Improved microscopic nuclear level densities within the Hartree-Fock-Bogoliubov plus combinatorial method

General Description of Fission Observables: GEF Model Code

Microscopic HFB+QRPA predictions of dipole strength for astrophysics applications

We present a new iterative procedure to extract the level density and the gamma strength function from primary gamma spectra for energies close up to the neutron binding energy. The procedure is tested on simulated spectra and on data from the Yb-173(He-3,alpha)Yb-172 reaction.

https://arxiv.org/pdf/nucl-ex/9910009.pdf

Extraction of level density and gamma strength function from primary gamma spectra

The level densities and $\gamma$-strength functions of the weakly deformed $^{148}$Sm and $^{149}$Sm nuclei have been extracted. The temperature versus excitation energy curve, derived within the framework of the micro canonical ensemble, shows structures, which we associate with the break up of Cooper pairs. The nuclear heat capacity is deduced within the framework of both the micro canonical and the canonical ensemble. We observe negative heat capacity in the micro canonical ensemble whereas the canonical heat capacity exhibits an S-shape as function of temperature, both signals of a phase transition. The structures in the $\gamma$-strength functions are discussed in terms of the pygmy resonance and the scissors mode built on exited states. The samarium results are compared with data for the well deformed $^{161,162}$Dy, $^{166,167}$Er and $^{171,172}$Yb isotopes and with data from (n,$\gamma$)-experiments and giant dipole resonance studies.

https://arxiv.org/pdf/nucl-ex/0111012.pdf

Level densities and $γ$-strength functions in $^{148,149}$Sm

The density of accessible levels at low spin in the ${(}^{3}\mathrm{He},\ensuremath{\alpha}\ensuremath{\gamma})$ reaction has been extracted for the ${}^{161,162}\mathrm{Dy}$ and ${}^{171,172}\mathrm{Yb}$ nuclei The entropy of the even-odd and even-even nuclei has been deduced as a function of excitation energy The entropy of one quasiparticle outside an even-even core is found to be 170(15) ${k}_{B}$ This quasiparticle picture of hot nuclei is well accounted for within a simple pairing model The onset of two, four, and six quasiparticle excitations in the ${}^{162}\mathrm{Dy}$ and ${}^{172}\mathrm{Yb}$ nuclei is discussed and compared to theory The number of quasiparticles excited per excitation energy is a measure for the ratio of the level energy spacing and the pairing strength

Entropy in hot161,162Dyand171,172Ybnuclei

A method to extract simultaneously level densities and γ-ray transmission coefficients has for the first time been tested on light nuclei utilizing the 28Si(3He,αγ)27Si and 28Si(3He,3He'γ)28Si reactions. The extracted level densities for 27Si and 28Si are consistent with the level densities obtained by counting known levels in the respective nuclei. The extracted γ-ray strength in 28Si agrees well with the known γ-decay properties of this nucleus. Typical nuclear temperatures are found to be T ~ 2.4 MeV at around 7 MeV excitation energy. The entropy gap between nuclei with mass number A and A ± 1 is measured to be δS ~ 1.0 kB, which indicates an energy spacing between single-particle orbitals comparable with typical nuclear temperatures.

Level density and γ-ray strength in 27,28Si

A method to extract simultaneously level densities and $\gamma$-ray transmission coefficients has for the first time been tested on light nuclei utilizing the $^{28}$Si($^3$He,$\alpha\gamma)^{27}$Si and $^{28}$Si($^3$He,$^3$He'$\gamma)^{28}$Si reactions. The extracted level densities for $^{27}$Si and $^{28}$Si are consistent with the level densities obtained by counting known levels in the respective nuclei. The extracted $\gamma$-ray strength in $^{28}$Si agrees well with the known $\gamma$-decay properties of this nucleus. Typical nuclear temperatures are found to be $T\sim 2.4$ MeV at around 7 MeV excitation energy. The entropy gap between nuclei with mass number $A$ and $A\pm 1$ is measured to be $\delta S\sim 1.0 k_B$, which indicates an energy spacing between single-particle orbitals comparable with typical nuclear temperatures.

Level density and $\gamma$-ray strength in $^{27,28}$Si

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