3. DESCRIPTION OF PROBLEM OR FUNCTION
The purpose of the code is to analyze time-of-flight cross section data in the resolved and unresolved resonance regions, where the incident particle is either a neutron or a charged particle (p, alpha, d, ...). Energy-differential cross sections and angular-distribution data are treated, as are certain forms of energy-integrated data.
In the resolved resonance region (RRR), theoretical cross sections are generated using the Reich-Moore approximation to R-matrix theory (and extensions thereof). Sophisticated models are used to describe the experimental situation: Data-reduction parameters (e.g. normalization, background, sample thickness) are included. Several options are available for both resolution and Doppler broadening, including a crystal-lattice model for Doppler broadening. Self-shielding and multiple-scattering correction options are available for analysis of capture cross sections. Multiple isotopes and impurities within a sample are handled accurately.
Cross sections in the unresolved resonance region (URR) can also be analyzed using SAMMY. The capability was borrowed from Froehner's FITACS code; SAMMY modifications for the URR include more exact calculation of partial derivatives, normalization options for the experimental data, increased flexibility for input of experimental data, introduction of user-friendly input options.
In both energy regions, values for resonance parameters and for data-related parameters (such as normalization, sample thickness, effective temperature, resolution parameters) are determined via fits to the experimental data using Bayes' method (see below). Final results may be reported in ENDF format for inclusion in the evaluated nuclear data files.
The new features added to SAMMY include:
1. The value of NU for ETA calculations can now be energy dependent.
2. Extensive revisions have been made to the self-shielding multiple-scattering (ssm) module.
3. Tabulated values (from Monte Carlo calculations) can be used instead of SAMMY-generated double-plus scattering corrections.
4. The "simple" resolution function may include a Gaussian whose width is a linear function of energy.
5. Input resonance parameters can now be presented as reduced width amplitudes gamma instead of partial widths GAMMA = 2Pgamma2.
6. For transmission measurements, the sample thickness may be non-uniform.
7. SAMMY now produces a third type of output file from which plots may be made-an ASCII file (with extension "LST") is created.
Please see the home page http://www.ornl.gov/sci/nuclear_science_technology/nuclear_data/ for the ORNL Nuclear Data Group and links from there to the SAMMY homepage.