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Manuscript Title: Code SPINDIS: nuclear shell-model level densities at high energies with a simplified pairing interaction.
Authors: D.K. Sunko
Program title: SPINDIS
Catalogue identifier: ADEU_v1_0
Distribution format: tar.gz
Journal reference: Comput. Phys. Commun. 101(1997)171
Programming language: C.
Computer: DEC Ultrix.
Operating system: UNIX.
RAM: 1.1M words
Keywords: Nuclear physics, Shell model, Level density, Spin distributions, Reactions nuclear, Neutron resonances.
Classification: 17.19.

Nature of problem:
The estimation of the final-state level density is an important ingredient in all calculations of reaction rates. Typically, the level density can be measured directly only in narrow windows of energy and spin. The present program efficiently calculates the exact level densities predicted by the shell model up to very high excitation, even in heavy nuclei, and with a simplified pairing interaction taken into account. When fitted to known neutron resonance data, the program provides a benchmark estimate of the level densities and spin distributions for a given nucleus.

Solution method:
The generating functions of spin distributions of a shell-model configuration with given seniority are used, along with an efficient method for the calculation of their coefficients. The distribution calculator is provided with configurations by the configuration generator, which uses auxiliary tables and C's pointer mechanism to avoid searching through configurations which are predictably too high in energy.

Restrictions:
Spherical shell-model single-particle levels for both protons and neutrons are input to the program. Only the short-range part of the residual interaction is considered, and this in a simplified form, diagonal in seniority. There is no restriction in principle on the input itself, since the basic combinatorial algorithms are guaranteed to terminate. However, the running time can become impractical.

Running time:
Depends heavily on the number of single-particle levels involved. On a DEC Alpha AXP running at 233 MHz, from one second for 59Ni up to 17 MeV excitation, to twent-six hours for 244Am up to 30 MeV excitation.