LEVEL  DENSITIES

Level densities represent one of key ingredients of nuclear reaction cross section calculations. Present directory, responding to practical formalisms of nuclear level densities and their parameterization, is subdivided into three parts.

• Total level densities, needed for statistical model calculations
• Fission level densities, including fission barriers and needed to calculate fission reaction channels
• Partial level densities, needed for preequilibrium model calculation

1.  Total Level Densities

      For  any applications of the statistical theory of nuclear reactions it is very important to obtain the parameters  of  the level   density description from the reliable experimental  data. The  cumulative   numbers  of low-lying levels  and  the  average spacings   between neutron resonances are usually  used  as  such data.   The  level  density parameters fitted to  such  data  are compiled  in  the  RIPL Starter File for the  tree  models   most frequently used in practical calculations:

1. For  the Gilbert-Cameron model the parameters  of  the Beijing  group,  based on a rather recent   compilations  of  the neutron  resonance  and low-lying level densities  and  included into  the beijing_gc.dat file, are chosen  as  recommended.   As alternative  versions the parameters provided  by  other groups  are  given into the files: jaeri_gc.dat, bombay_gc.dat, obninsk_gc.dat.  Additionally the iljinov_gc.dat and mengoni_gc.dat files include sets of the level density parameters that  take  into   account the damping of shell effects  at  high energies.  The iljinov_gc.dat also contains parameters taking into account collective enhancement of level densities.  

2. For   the   backed-shifted  Fermi  gas   model   the beijing_bs.dat   file  is  selected  as  the  recommended   one.   Alternative parameters of the Obninsk group are given  in  the obninsk_bs.dat file and those of Bombay in bombay_bs.dat.

3. For the generalized superfluid model the Obninsk group parameters included into the obninsk_bcs.dat file are  chosen   as recommended ones and the  beijing_bcs.dat file is included   as  an alternative set of parameters.

4. For the microscopic approach to the level densities the files are:  obninsk_micro.for  --- FORTRAN 77 source of the microscopical statistical level density code developed in Obninsk by Ignatyuk and coworkers, moller_levels.gz --- Mller single-particle level and ground state deformation data base, moller_levels.for --- retrieval code for Mller single-particle level scheme.

The directory is comprised of the subdirectories RECOMMENDED and OTHER_FILES:

RECOMMENDED

OTHER_FILES

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2.  Fission Level Densities

Fission level densities (or fissioning nucleus level densities at fission saddle deformations) are required for statistical model calculations of actinide fission cross sections. Back-shifted Fermi-Gas Model, Constant Temperature Model,  and Generalized Superfluid Model (GSM) are widely used for the description of level densities at ground state deformations. These models provide approximately identical level density description at excitations close to the neutron binding energy. It is low excitation energies where they are discrepant, while this energy region is crucial for fission cross section calculations. A drawback of  the back-shifted Fermi gas model and traditional constant temperature model approaches is that it is difficult to include in a consistent way pair correlations, collective effects, and shell effects. The pairing, shell and collective properties of nucleus do not reduce just to the renormalization of level density parameter a, but influence the energy dependence of level densities. These effects turn out to be important because they depend upon deformation of either equilibrium or saddle-point. These effects are easily introduced within the GSM approach. Fission barriers are another key ingredients involved in the fission cross section calculations. Fission level density and barrier parameters are strongly interdependent. This is the reason for including fission barrier parameters along with the fission level densities in the Starter File.

The recommended file is  maslov.dat --- fission barrier parameters.   Recent version of actinide fission barrier data obtained in Obninsk obninsk.dat should only be considered as a guide for selection of initial parameters.  These data are included in the Starter File, together with the fission barrier parameters recommended by CNDC (beijing.dat), for completeness.

The directory is comprised of the subdirectories RECOMMENDED and OTHER_FILES:

RECOMMENDED

OTHER_FILES

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3.  Partial Level Densities

The RIPL Starter File includes a Fortran code avrigeanu.for by M. Avrigeanu for using various equidistant and Fermi-gas single-particle models, including models that incorporate pairing and shell effects within the closed-form treatments and a Fortran code capote_micro.for by R. Capote, which performs combinatorial counting of p-h configurations in a realisitic shell-model space including pairing interaction.

The directory is comprised of the subdirectories RECOMMENDED and  OTHER_FILES:

RECOMMENDED

OTHER_FILES

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