scholarly journals Fission dynamics with microscopic level densities

2018 ◽  
Vol 169 ◽  
pp. 00019
Author(s):  
Jørgen Randrup ◽  
Daniel Ward ◽  
Gillis Carlsson ◽  
Thomas Døssing ◽  
Peter Möller ◽  
...  
Keyword(s):  
Potential Energy ◽  
Energy Surface ◽  
Local Level ◽  
Level Density ◽  
Deformation Energy ◽  
Excitation Energies ◽  
Level Densities ◽  
Mass Distributions ◽  
Shell Effects ◽  
The Individual

Working within the Langevin framework of nuclear shape dynamics, we study the dependence of the evolution on the degree of excitation. As the excitation energy of the fissioning system is increased, the pairing correlations and the shell effects diminish and the effective potential-energy surface becomes ever more liquid-drop like. This feature can be included in the treatment in a formally well-founded manner by using the local level densities as a basis for the shape evolution. This is particularly easy to understand and implement in the Metropolis treatment where the evolution is simulated by means of a random walk on the five-dimensional lattice of shapes for which the potential energy has been tabulated. Because the individual steps between two neighboring lattice sites are decided on the basis of the ratio of the statistical weights, what is needed is the ratio of the local level densities for those shapes, evaluated at the associated local excitation energies. For this purpose, we adapt a recently developed combinatorial method for calculating level densities which employs the same single-particle levels as those that were used for the calculation of the pairing and shell contributions to the macroscopic-microscopic deformation-energy surface. For each nucleus under consideration, the level density (for a fixed total angular momentum) is calculated microscopically for each of the over five million shapes given in the three-quadratic-surface parametrization. This novel treatment, which introduces no new parameters, is illustrated for the fission fragment mass distributions for selected uranium and plutonium cases.

HEAVY PARTICLE RADIOACTIVITIES

2007 ◽  
Vol 16 (04) ◽  
pp. 995-1007
Author(s):  
D. N. POENARU ◽  
R. A. GHERGHESCU ◽  
I. H. PLONSKI ◽  
W. GREINER
Keyword(s):  
Potential Energy ◽  
Potential Energy Surface ◽  
Energy Surface ◽  
Energy Levels ◽  
Heavy Particle ◽  
Deformation Energy ◽  
Decay Modes ◽  
Shell Effects ◽  
Mass Asymmetry ◽  
First Time

The potential energy surface for binary decay modes of 228 Th versus the distance between the fragment centers and the mass asymmetry is calculated by using the macroscopic-microscopic model with energy levels obtained within the most advanced asymmetric two center shell model. The valley due to 20 O radioactivity is evidenced as a result of the very strong shell effects due to the doubly magic daughter 208 Pb . For the first time the potential barrier of this decay mode is produced by adding the shell and pairing corrections to the phenomenological deformation energy. The half-lives of cluster emitters predicted by the analytical superasymmetric fission model have been confirmed in all measurements performed until now.

scholarly journals Nuclear level densities away from line of β-stability

2021 ◽  
Author(s):  
Tanmoy Ghosh ◽  
Bhoomika Maheshwari ◽  
Sangeeta Arora ◽  
Gaurav Saxena ◽  
Bijay Agrawal
Keyword(s):  
Single Particle ◽  
Level Density ◽  
Mass Range ◽  
Density Parameter ◽  
Structural Effects ◽  
Mass Model ◽  
Nuclear Level ◽  
Level Densities ◽  
Shell Effects ◽  
Order Of Magnitude

Abstract The variation of total nuclear level densities (NLDs) and level density parameters with proton number Z are studied around the β-stable isotope, Z0, for a given mass number. We perform our analysis for a mass range A=40 to 180 using the NLDs from popularly used databases obtained with the single-particle energies from two different microsopic mass-models. These NLDs which include microscopic structural effects such as collective enhancement, pairing and shell corrections, do not exhibit inverted parabolic trend with a strong peak at Z0 as predicted earlier. We also compute the NLDs using the single-particle energies from macroscopic-microscopic mass-model. Once the collective and pairing effects are ignored, the inverted parabolic trends of NLDs and the corresponding level density parameters become somewhat visible. Nevertheless, the factor that governs the (Z-Z0) dependence of the level density parameter, leading to the inverted parabolic trend, is found to be smaller by an order of magnitude. We further find that the (Z-Z0) dependence of NLDs is quite sensitive to the shell effects.

Analysis of pairing phase transition in Sn-isotopes within semiclassical approach

2020 ◽  
Vol 29 (09) ◽  
pp. 2050071
Author(s):  
Saniya Monga ◽  
Harjeet Kaur ◽  
Sudhir R. Jain
Keyword(s):  
Phase Transition ◽  
Shell Structure ◽  
Level Density ◽  
Critical Temperatures ◽  
Periodic Orbit Theory ◽  
Level Densities ◽  
Shell Effects ◽  
Particle Level ◽  
Pairing Correlations ◽  
Orbit Theory

We demonstrate that pairing phase transition (superfluid to normal) can be described quite generally in terms of the thermodynamical properties after verifying the obtained level densities with the available experimental data for [Formula: see text]- isotopes. Periodic-orbit theory conveniently connects the oscillatory part of level density to the underlying classical periodic orbits and hence, leads to the shell effects in the single-particle level density. Such methods incorporated with pairing effects can be used effectively to study the phase transitions in [Formula: see text]-isotopes. In addition to this, an interplay between pairing correlations and the shell effects has been understood by analyzing the results obtained for the critical temperatures and shell structure energies for [Formula: see text] isotopes. A relation between variation in critical temperatures caused by shell effects and the shell structure energies determined with and without pairing effects has been established. Furthermore, the systematics of the heat capacity (giving a clear signature of pairing phase transition) as function of temperature for these nuclei are investigated as well.

Large scale vibrational calculations on IVR in S0 thiophosgene

2016 ◽  
Vol 15 (01) ◽  
pp. 1650005 ◽  
Author(s):  
Svetoslav Rashev ◽  
David C. Moule
Keyword(s):  
Potential Energy ◽  
Potential Energy Surface ◽  
Large Scale ◽  
Energy Surface ◽  
Vibrational Excitation ◽  
Dissociation Limit ◽  
Excitation Energies ◽  
Local Coupling ◽  
Very High

In this work, using a recently derived refined potential energy surface for [Formula: see text] thiophosgene, we perform large scale vibrational calculations to explore the IVR characteristics and vibrational mixing at very high vibrational excitation energies, ranging up to the dissociation limit (at [Formula: see text]20,000[Formula: see text]cm[Formula: see text]). The results from our calculations have been compared to the conclusions based on the available experimentally measured dataset (obtained from SEP and LIF spectra) as well as to the conclusions from the analyses by other authors using local coupling models.

scholarly journals Theoretical Study on Pentiptycene Molecular Brake: Photoinduced Isomerization and Photoinduced Electron Transfer

2021 ◽  
Author(s):  
Hailong Wang ◽  
Qiuping Guan ◽  
Xueye Wang
Keyword(s):  
Electron Transfer ◽  
Double Bond ◽  
Potential Energy ◽  
Potential Energy Surface ◽  
Energy Surface ◽  
Basis Set ◽  
Isomerization Reaction ◽  
Excitation Energies ◽  
Dft Computations ◽  
Molecular Brake

Abstract The isomerization of the double bond plays an important role in the braking and de-braking of the light-driven molecular brake. Therefore, the Pp-type light-controlled molecular brake system containing the C=C double bond was theoretically studied. Combining the 6-31G(d) basis set, the ωB97XD functional with dispersion correction was applied to implement the (E)-configuration and (Z)-configuration initial optimization. Next, using the 6-311G(d,p) basis set, the relaxed potential energy surface scans of the rotation angle were operated, and then the optimization calculations of the transition states at the extremum high points. Analyzing the stagnation points and the rotational transition state on the MEPs, the rotation mechanism and basic energy parameters of the molecular brake were obtained. Then the DFT computations at ground states and the TD-DFT computations of vertical excitation energy was put into practice at the accuracy of the def-TZVP basis set for the two configurations, and using the natural transition orbital (NTOs) analyses combining the excitation energies and absorption spectrums of the molecules, the electronic transition characteristics and electron transfer properties of light-driven molecular brake were studied. Afterwards, in order to investigate the photo-induced isomerization reaction, the C=C double bond was scanned on the relaxed potential energy surface, and the intermediates of the isomerization reaction was searched and analyzed, thus, the braking mechanism of the light-driven molecular brake was proposed.

scholarly journals Shell-structure and asymmetry effects in level densities

2021 ◽  
Author(s):  
A. G. Magner ◽  
A. I. Sanzhur ◽  
S. N. Fedotkin ◽  
A. I. Levon ◽  
S. Shlomo
Keyword(s):  
Shell Structure ◽  
Level Density ◽  
Fermi Gas ◽  
Saddle Point Method ◽  
Density Parameter ◽  
Excitation Energies ◽  
Periodic Orbit Theory ◽  
Shell Effects ◽  
Opposite Case ◽  
Energy Formula

Level density [Formula: see text] is derived for a nuclear system with a given energy [Formula: see text], neutron [Formula: see text], and proton [Formula: see text] particle numbers, within the semiclassical extended Thomas–Fermi and periodic-orbit theory beyond the Fermi-gas saddle-point method. We obtain [Formula: see text], where [Formula: see text] is the modified Bessel function of the entropy [Formula: see text], and [Formula: see text] is related to the number of integrals of motion, except for the energy [Formula: see text]. For small shell structure contribution one obtains within the micro–macroscopic approximation (MMA) the value of [Formula: see text] for [Formula: see text]. In the opposite case of much larger shell structure contributions one finds a larger value of [Formula: see text]. The MMA level density [Formula: see text] reaches the well-known Fermi gas asymptote for large excitation energies, and the finite micro-canonical limit for low excitation energies. Fitting the MMA [Formula: see text] to experimental data on a long isotope chain for low excitation energies, due mainly to the shell effects, one obtains results for the inverse level density parameter [Formula: see text], which differs significantly from that of neutron resonances.

A COMPOSITE NUCLEAR-LEVEL DENSITY FORMULA WITH SHELL CORRECTIONS

1965 ◽  
Vol 43 (8) ◽  
pp. 1446-1496 ◽  
Author(s):  
A. Gilbert ◽  
A. G. W. Cameron
Keyword(s):  
Lower Energy ◽  
Level Density ◽  
Energy Levels ◽  
Fermi Gas ◽  
Experimental Information ◽  
Shell Correction ◽  
Nuclear Level Density ◽  
Excitation Energies ◽  
Nuclear Level ◽  
Level Densities

At low excitation energies a "constant nuclear temperature" representation of nuclear-level densities is used, and at high excitation energies the regular Fermi gas formula is adopted. A method is developed for determining the parameters of the Fermi gas formula by using both the pairing and the shell-correction energies found by Cameron and Elkin for their semiempirical atomic mass formula in its exponential form. This procedure determines level densities at neutron-binding-energy excitations subject to an average factor error of 1.8. Methods are also developed for determining the parameters for the lower-energy formula in such a way that it best fits the lower-energy levels and joins smoothly to the Fermi gas formula. Correlations of the resulting parameters with shell and pairing effects are found. A composite prescription is given for calculating level densities in nuclei for which no experimental information is known. Tables give level density parameters for a wide variety of nuclei for which some experimental information is known. Some of the derivations of the Fermi gas formula in the literature were found to be slightly incorrect, so new derivations are presented in Appendixes.

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