1. NAME AND TITLE

COHORT-II: General Purpose Monte Carlo Radiation Transport Code System.

AUXILIARY ROUTINES

EXECT: Main Control Code.

INPUT: Input Data Processor.

SORCE: Primary Source Data Generator.

HTA: Particle Tracking and Scoring Code.

OUTPUT: Edit and List Code.

SECGAM: Secondary Gamma-Ray Source Data Generator.

RAND: FORTRAN Random Number Generator.

The original design of the COHORT code dates back to work done at General Dynamics, Fort Worth, Texas, in 1962. Development continued in the COHORT series at Radiation Associates (RRA), Fort Worth, and Brown Engineering (BE), Huntsville, Alabama, for NASA Marshall Space Flight Center (1964). COHORT was further developed by RRA for Aerojet Nuclear Systems for CCC-191/ACOH. The BE work resulted in CCC-169/CAVEAT. COHORT-II is a major revision and modernization of the original COHORT (1966).

2. CONTRIBUTOR

NASA Lewis Research Center, Cleveland, Ohio.

3. CODING LANGUAGE AND COMPUTER

FORTRAN IV; IBM 7094.

4. NATURE OF PROBLEM SOLVED

COHORT-II is a Monte Carlo general purpose shielding code which tracks neutrons and photons through flexible shield geometry configurations.

Printed output includes number fluxes that are averaged over regions and fluxes that are at arbitrarily located point detectors. The volume averaged region fluxes can be sorted according to arbitrary energy bins. The point-detector fluxes list uncollided (for isotropic sources only) and scattered flux contributions separately and can be sorted according to their polar and azimuthal angles of arrival at the detector as well as their energies. An estimate of the statistical precision is included for both volume and point detectors. Energy integrated quantities such as dose rates, heating rates, reaction rates, and fluxes greater than a given energy can also be computed for both volume and point detectors by entering suitable response functions. Where only one type of detector is of interest, the computation associated with the other type can be omitted to save time. All scoring, both for volume and point detectors is performed by statistical estimation.

5. METHOD OF SOLUTION

Primary particles are generated from previously known spatial, angular, and energy distributions supplied by the user. The conventional code method is employed where individual particles are generated, tracked, and analyzed. The shielding geometry allowed is flexible with space divided into homogeneous regions bounded by surfaces describable by a general quadratic equation. Spherical, cylindrical, and rectangular parallelepipeds are allowed for the source regions. Biasing options include selection of source particles from preferred locations, energies, and directions. The exponential transform can also be used during particle tracking. Secondary gamma rays can be generated and placed on tape during a neutron problem to be tracked during a subsequent computer run. Variable dimensioning is employed for efficient use of computer storage.

The program does not require the user to handle tapes except when a leakage or a secondary gamma-ray tape has been requested. A complete analysis of a typical reactor-shield problem would require three separate Monte Carlo problems to be run on the computer. These runs would be used to track and analyze, respectively, primary neutrons, primary photons, and secondary photons. A physical source tape for the secondary gamma problem would be generated during the primary neutron run.

6. RESTRICTIONS OR LIMITATIONS

The code requires input cross-section data in its own specialized format and cannot accept multigroup cross-section data as used in conventional transport codes.

7. TYPICAL RUNNING TIME

No study has been made by RSIC of typical running times for COHORT-II.

8. COMPUTER HARDWARE REQUIREMENTS

COHORT-II is operable on the IBM 7094 computer.

9. COMPUTER SOFTWARE REQUIREMENTS

A FORTRAN IV compiler is required as well as the Overlay feature with a main link and five dependent links. Variable dimensioning has been incorporated.

10. REFERENCE

L. Soffer and L. Clemons, Jr., "COHORT-IIA Monte Carlo General Purpose Shielding Computer Code," NASA TN D-6170 (April 1971).

11. CONTENTS OF CODE PACKAGE

Included are the referenced document and one (1.2MB) DOS diskette which contains the source codes, data libraries, and sample problem input and output.

12. DATE OF ABSTRACT

September 1975; revised September 1982.