1. NAME AND TITLE

AIRTRANS: Monte Carlo Time and Energy-Dependent Three-Dimensional Radiation Transport Code.

AUXILIARY ROUTINES

DATORG: Organized Data Tape Generator.

VANGEN: External Source Generator.

ASP: Anisotropic Secondary Particle Generator.

AIRSCA: Calculates Time-Energy Dependent Fluxes at Point in Atmosphere.

DATA LIBRARIES

81 Pt. Neutron Cross Sections.

81 Pt. Gamma-Ray Cross Sections.

Air Density Tables.

2. CONTRIBUTORS

United Nuclear Corporation, Elmsford, New York.

Lockheed Missile and Space Company, Sunnyvale, California.

3. CODING LANGUAGE AND COMPUTER

FORTRAN IV; IBM 360/75/91.

4. NATURE OF PROBLEM SOLVED

The function of the AIRTRANS system is to calculate by Monte Carlo methods the radiation field produced by neutron and/or gamma-ray sources which are located in the atmosphere. The radiation field is expressed as the time- and energy-dependent flux at a maximum of 50 point detectors in the atmosphere. The system calculates uncollided fluxes analytically and collided fluxes by the "once-more collided" flux-at-a-point technique.

Energy-dependent response functions can be applied to the fluxes to obtain desired flux functionals, such as doses, at the detector point. AIRTRANS also can be employed to generate sources of secondary gamma radiation.

5. METHOD OF SOLUTION

Neutron interactions treated in the calculational scheme include elastic (isotropic and anisotropic) scattering, inelastic (discrete level and continuum) scattering, and absorption. Charged particle reactions, e.g., (n,p) are treated as absorptions. A built-in kernel option can be employed to take neutrons from the 150 keV to thermal energy, thus eliminating the need for particle tracking in this energy range. Another option used in conjunction with the neutron transport problem creates an ``interaction tape'' which describes all the collision events that can lead to the production of secondary gamma rays. This interaction tape subsequently can be used to generate a source of secondary gamma rays.

The gamma-ray interactions considered include Compton scattering, pair production, and the photoelectric effect; the latter two processes are treated as absorption events.

Incorporated in the system is an option to use a simple importance sampling technique for detectors that are many mean free paths from the source. In essence, particles which fly far from the source are split into fragments, the degree of fragmentation being proportional to the penetration distance from the source. Each fragment is tracked separately, thus increasing the percentage of computer time spent following particles at the deep penetrations. Each fragment is assigned a "weight" which is inversely proportional to the degree of fragmentation suffered by the original source particle. All estimates of flux contributions by such a fragment are then multiplied by its assigned weight.

6. RESTRICTIONS OR LIMITATIONS

ND = number of DETECTORS < 50*

NE = number of ENERGY BINS < 50*

NT = number of TIME BINS < 50*

*Subject to 3 ND x NE x NT + 2 NT x NE < 1500-ITED, where ITED = 1464 for neutrons and = 1054 for gammas.

7. TYPICAL RUNNING TIME

Typical running time on the UNIVAC 1108 varies from l/4 to l/2 sec/history/detector. No study has been made for the IBM system.

8. COMPUTER HARDWARE REQUIREMENTS

The code was originally designed for the CDC 1604 using standard I-O and a maximum of 4 tape units. It was converted for use on the UNIVAC 1108 by Lockheed Missile and Space Company, and later (1970) converted by RSIC to run on the IBM 360/370.

9. COMPUTER SOFTWARE REQUIREMENTS

IBM or UNIVAC FORTRAN IV systems.

10. REFERENCES

G. L. Case, "Utilization Manual - AIRTRANS," LMSC-5234 (February 1968).

M. O. Cohen, "AIRTRANS - A Time-Dependent Monte Carlo System for Radiation Transport in a Variable Density Atmosphere and the Ground," UNC-5179 (June 1967).

W. W. Halstead, "Letter of Transmittal for IBM S/360 Version of AIRTRANS," (February 1970).

11. CONTENTS OF CODE PACKAGE

Included are the referenced documents, and one (1.2MB) DOS diskette which contain the source codes, the cross section data libraries and input and output for sample problems.

12. DATE OF ABSTRACT

May 1969; updated July 1981, February 1985, and August 1991.

KEYWORDS: MONTE CARLO; NEUTRON; GAMMA-RAY; AIRBORNE; COMPLEX GEOMETRY; TIME-DEPENDENT