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Manuscript Title: A program for calculation of spectra and cross sections within the combined pre-equilibrium compound nucleus model of nuclear reactions.
Authors: M. Herman, A. Marcinkowski, K. Stankiewicz
Program title: EMPIRE
Catalogue identifier: ACCQ_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 33(1984)373
Programming language: Fortran.
Computer: CYBER 73.
Operating system: SCOPE 3.4.4.
RAM: 48K words
Word size: 60
Peripherals: disc.
Keywords: Nuclear physics, Preequilibrium decay, Gamma cascade, Energy spectra, Angular momentum, Scattering, Compound, Cross section, Width fluctuations.
Classification: 17.12.

Nature of problem:
The program calculates spectra and cross sections for capture and/or multistep nuclear reactions in the frame of a combined preequilibrium and compound nucleus mechanisms model. Angular momentum conservation is observed throughout the whole calculation. To this end angular momentum considerations have been incorporated into the hybrid model describing the nucleon emission prior to equilibration of the excited nucleus. The decay of the compound nucleus is treated either in terms of the HRTW theory, to account for the width fluctuation effects, or in terms of standard Hauser-Feshbach theory if many open particle channels cause the fluctuations to cancel. The transmission coefficients of the nuclear potential are calculated with an optical model subroutine. The full gamma-cascade, containing transitions from and between the continuum states as well as between the discrete levels, is included in the calculations providing the gamma-ray spectra and the populations of the discrete levels in the residual nuclei.

Solution method:
Initially the program calculates the tables of the partial wave transmission coefficients and the spin-dependent level densities of nuclei in the decay channels of the composite nucleus. If the preequilibrium option is selected, the geometry dependent hybrid model subroutine HYBRID is used to calculate neutron and proton spectra, separately for each partial wave. The preequilibrium yields are distributed, over the continuum of states and the discrete levels of the residual nuclei, according to the energy, angular momentum and parity conservation laws. In a next step the compound nucleus decay is followed and its contributions are recorded, on local files as described to each residual nucleus, and added to the previously calculated preequilibrium populations. Multiparticle emission is assumed to take place after the nucleus has reached equilibrium. At each stage of a multiparticle emission process the decaying nucleus appropriately replaces its ancestor, and the decay is treated in terms of the transmission coefficients and level densities. This procedure is repeated until the list of particles emitted in the reaction is exhausted. The population of the discrete levels in the residual nucleus and the spectra of particles and photons emitted at each step of the reaction are printed.

Particles considered in the program are neutrons, protons and alphas. In addition deuterons in the entrance channel and gammas in the exit channels are included. The preequilibrium emission does not allow for alpha particles. The maximum energy and number of subsequently emitted particles are not restricted in the code but the energy discretization of the continuum is confined to 120 bins, which puts a limit on the accuracy of the integration procedure. The maximum number of partial waves considered in the calculation is 30. Only E1, M1, and E2 gamma-transitions are allowed to contribute to to the gamma cascade. Up to 50 discrete levels for each nucleus taken into account can be used. In the gamma-decay of these levels up to 11 transitions depopulating each level might be concerned.

Running time:
The running time depends strongly on the composite nucleus excitation energy, the energy integration step, the number of involved channels, partial waves and reaction stages. A typical running time for a (n,2n) reaction, at an energy equal 14 MeV and for an integration step equal 1 MeV, involving 14 partial waves and all allowed competitions, is about 10 min on a CDC CYBER 73 computer.