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GRASS, A MONTE CARLO PROCEDURE FOR CALCULATING GAMMA-RAY ATTENUATION OF SIMPLE STRUCTURES,
RADIATION RESEARCH ASSOCIATES INC FORT WORTH TEX
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The GRASS Monte Carlo procedure was written in 1130 FORTRAN 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. 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. Author
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