**1. NAME AND TITLE**

GNASH-FKK: Pre-equilibrium, Statistical Nuclear-Model Code System for Calculation
of Cross Sections and Emission Spectra, Version gn9cp0.

**AUXILIARY CODE**

GNXS: Analyze decay chains, retrieve cross sections and spectra, and make
various ENDF-6 formatted files from GNASH output.

**DATA LIBRARY**

STRUCTURE.DAT: Nuclear level energies, spins, parities, and gamma-ray branching ratios.

TRANS.COEF: Particle transmission coefficients from spherical or deformed optical model calculations.

MASS.DAT: Table of ground-state masses, spins, and parities.

**2. CONTRIBUTOR**

Los Alamos National Laboratory, Los Alamos, New Mexico.

**3. CODING LANGUAGE AND COMPUTER**

Fortran 77; Sun (P00125/SUN05/00).

**4. NATURE OF PROBLEM SOLVED**

GNASH provides a flexible method by which reaction and level cross sections, isomer ratios, and
emission spectra (neutron, gamma-ray, and charged-particle) resulting from particle- and photon-induced reactions can be calculated. The September 1991 release of GNASH incorporated an
additional option for calculating gamma-ray strength functions and transmission coefficients by
including the Kopecky-Uhl model. In addition, improvements were made to the output routines,
particularly regarding gamma-ray strength function information. Major improvements in the 1995
FKK-GNASH release include added capabilities: to read in externally calculated preequilibrium
spectrum from, e.g., Feshbach-Kermin-Koonin theory, to do multiple preequilibrium calculations, to
calculate appropriate spin distributions for nuclear states formed in preequilibrium reactions, and to
do incident-photon calculations. In the 1998 release improvements were made in the accuracy of the
exciton model and other calculations, and provision was made for including energy-dependent
renormalization of the reaction cross section and energy-dependent exciton model parameterization
(for data evaluation purposes).

**5. METHOD OF SOLUTION**

GNASH uses Hauser-Feshbach theory to calculate complicated sequences of reactions and includes
a pre-equilibrium correction for binary tertiary channels. Gamma-ray competition is considered in
detail for every decaying compound nucleus. A multi-humped fission barrier model is included for
fission cross-section calculations. Three options for level densities are available.

**6. RESTRICTIONS OR LIMITATIONS**

In its present configuration, each calculation can handle decay sequences involving up to 38
compound nuclei and each decaying compound nucleus can emit a maximum of 5 types of radiation
(neutrons, gamma rays, protons, alphas, etc.). Angular-momentum effects and conservation of parity
are included explicitly. Each residual nucleus in a calculation can contain up to 78 discrete levels
whereas its continuum region can be represented by up to 204 energy bins. The incident-particle types
that are permitted are neutrons, protons, deuterons, tritons, ^{3}He, and ^{4}He. Angular distributions are
not calculated; i.e., isotropy is assumed in the center-of-mass (c.m.) system. Angular distribution
effects can be added in postprocessing utility codes making use of, for example, Kalbach-Mann
systematics. The above restrictions can be easily adjusted by increasing the array dimensions in the
parameter statements in the code.

**7. TYPICAL RUNNING TIME**

The running times typically range from a few seconds to a few minutes per incident energy
depending upon incident particle energy, mass range of the target, number of compound nuclei
included, and the energy bin width that is used.

**8. COMPUTER HARDWARE REQUIREMENTS**

GNASH was developed originally on the CDC 6600 and 7600 then on Cray. The current release
runs on Sun Sparcstations. Using GNASH for ENDF evaluations to 150 MeV, requires more than
100 MB RAM, to include all the necessary decay chains.

**9. COMPUTER SOFTWARE REQUIREMENTS**

At RSICC GNASH-FKK was run on a Sun Sparcstation20 under Solaris 2.6 using f77 Vers. 4.2.
Double precision (-r8) is needed.

**10. REFERENCES**

**a. included in documentation:**

M. Chadwick, "readme.tr98" (January 1998).

P. G. Young, M. B. Chadwick, "Code Input Description, Version T2/PGYC/93GNASH/GN7SRC (Update 17)" (May 1996).

P. G. Young, E. D. Arthur, and M. B. Chadwick, "Comprehensive Nuclear Model Calculations:
Theory and Use of the GNASH Code," (to be published)(1998).

**b. background information:**

J. Kopecky and M. Uhl, "Test of Gamma-Ray Strength Functions in Nuclear Model Calculations,"
*Phys. Rev. C* 41, 1941 (1990).

P. G. Young, E. D. Arthur, M. B. Chadwick, "Comprehensive Nuclear Model Calculations:
Introduction to the Theory and Use of the GNASH Code," LA-12343-MS (July 1992).

**11. CONTENTS OF CODE PACKAGE**

Included are the referenced documents and one 3.5-in. DS/HD (1.44 MB) diskette containing the
source code, data libraries, sample problem input and output in a compressed tar file.

**12. DATE OF ABSTRACT**

March 1984, revised June 1988, March 1989, September 1991, December 1992, January 1996,
March 1998.

**KEYWORDS: ** NUCLEAR MODELS; WORKSTATION; CROSS SECTION
PROCESSING