**1. NAME AND TITLE**

GREAT-GRASS: Monte Carlo Radiation Transport Code Systems for Fallout Shielding.

**2. CONTRIBUTOR**

Radiation Research Associates, Inc., Fort Worth, Texas.

**3. CODING LANGUAGE AND COMPUTER **

FORTRAN IV; IBM 360/75/91.

**4. NATURE OF PROBLEM SOLVED**

GREAT designed to calculate the scattered gamma-ray environment in an air-ground geometry resulting from a plane-isotropic monoenergetic gamma-ray source. Statistical estimation is used to determine the flux energy and angle distribution at a point detector in air above a finite annular source located on or near the ground surface. Ground roughness may be simulated by placing the source below the ground surface.

GRASS was written to calculate gamma-ray attenuation in simple cylindrical structures. GRASS calculates the structure-scattered gamma-ray flux energy distribution and exposure angle distribution at a point on the centerline of an upright cylindrical barrier exposed to gamma rays from a plane source located on, or near, the ground surface.

**5. METHOD OF SOLUTION**

GREAT is a Monte Carlo code designed to calculate the energy and angle distribution of the scattered gamma-ray flux at a point detector in an air-ground geometry resulting from a plane-isotropic monoenergetic gamma-ray source parallel to the air-ground interface. The source, which is finite and annular in shape, may be positioned below the smooth ground surface to simulate the effects of a source located on rough ground. The point detector is located on the vertical axis of the annular source. Initial photon parameters, path lengths, interactions, and scattering angles are obtained by random sampling of appropriate probability distributions. Each photon is traced as it scatters within the defined geometry, and estimates are made of the flux contribution at the detector from each collision. Portions of the procedure were taken from CCC-198/COHORT, a general purpose Monte Carlo code.

Gamma-ray interactions considered in GREAT include absorption, Compton scattering, and pair production. The scattering angle distributions are obtained from the Klein-Nishina formula. Statistical estimation is used to compute the flux contribution at the detector resulting from each collision. A gamma-ray history may be terminated on energy, weight, or collision number.

The Monte Carlo method is also used in GRASS. The energy and angle distribution of the flux incident upon the structure is calculated by a related Monte Carlo procedure and is input in a fixed form which describes (1) "uncollided" gamma rays which reach the barrier before having a collision in the air or ground and (2) "scattered" gamma rays incident on the barrier after having at least one collision in the air or ground. GRASS employs a pseudo-source based on probability distributions derived from the incident flux distributions. The source is located on the outer lateral surface of the barrier. The intensity of the pseudo-source representing the uncollided incident flux may be varied in up to 3 steps as a function of height along with barrier wall, while the pseudo-source used to describe the scattered incident flux is assumed to have the same intensity at all heights. Much of the versatility found in larger Monte Carlo procedures was sacrificed for speed and compactness, allowing GRASS to be compatible with the storage available in the IBM 1130 computer and perform gamma-ray scattering calculations in cylindrical barriers with greater efficiency.

**6. RESTRICTIONS OR LIMITATIONS **

None.

**7. TYPICAL RUNNING TIME**

No study has been made by RSIC of typical running times for GREAT or GRASS.

**8. COMPUTER HARDWARE REQUIREMENTS **

GREAT and GRASS were designed for the IBM 1130 system and were converted by RSIC to run on the IBM 360/75/91 system with standard input-output devices.

**9. COMPUTER SOFTWARE REQUIREMENTS **

Standard compilers and operating systems are required.

**10. REFERENCES**

J. H. Price, "GREAT, A Monte Carlo Procedure for Calculating Gamma-Radiation Environments Above Terrain," RRA-T78 (NRDL TRC-68-11) (December 1967).

J. H. Price, "GRASS, A Monte Carlo Procedure for Calculating Gamma-Ray Attenuation of Simple Structures," RRA-T79 (NRDL TRC-68-10) (December 1967).

J. H. Price, "Monte Carlo Programming for Small Computers," RRA-M84 (June 1968).

**11. CONTENTS OF CODE PACKAGE **

Included are the referenced documents and one (1.2MB) DOS diskette which contains the source code and sample problem input and output.

**12. DATE OF ABSTRACT**

February 1972; revised August 1982.

**KEYWORDS**: MONTE CARLO; GAMMA-RAY; FALLOUT; COMPLEX GEOMETRY