1. NAME AND TITLE

AIRSCAT: Calculation of Dose Rate for Gamma-Rays Scattered in Air.

2. CONTRIBUTOR

Worcester Polytechnic Institute, Worcester, Massachusetts.

3. CODING LANGUAGE AND COMPUTER

Fortran IV; PDP-10.

4. NATURE OF PROBLEM SOLVED

The AIRSCAT code was developed as a calculation scheme for air-scattered gamma-rays using the single-scattering approximation with no exponential attenuation or buildup. The program has been useful for preliminary design work and estimation when dealing with unshielded detectors. The method has been applied to the calculation of dose rates at source-detector separation distances of from 5 to 100 feet. A complete survey was completed for source energies from 0.6 to 12 MeV and for beam angles with respect to the source-detector axis from 0 to 180 degrees. The report also includes examples of point source calculations dealing with open-top cells.

The main advantages of this type of calculation are: closed form solutions, small computer memory, large time savings, and lower cost as compared to the Monte Carlo method.

5. METHOD OF SOLUTION

AIRSCAT uses the once-scattered component of Compton scattering to determine individual dose rate contributions at various scatter points within the scattering region. These scattering points are defined by a nonuniform scattering grid, which takes advantage of the fact that the gamma-ray photon energy decreases as a function of the scattering angle and distance from the source, thus the scattering region is biased for the most important scatter points. Once the contribution has been established for each individual scatter point, a total dose rate is derived by integration over the entire scattering region.

6. RESTRICTIONS OR LIMITATIONS

The results of AIRSCAT, using the single scattering approximation, with no exponential attenuation or buildup, are useful for preliminary design work and estimation. The results show that the simple approximation is adequate for unshielded detectors at less than 100 feet. If the detector is shielded in any way, however, these results would not be valid, since energy and angular distribution at the shield are important.

7. TYPICAL RUNNING TIME

For a given choice of program parameters, the typical running time is 0.32 CPU seconds.

8. COMPUTER HARDWARE REQUIREMENTS

AIRSCAT is operable on the PDP-10 computer.

9. COMPUTER SOFTWARE REQUIREMENTS

A Fortran IV compiler is required.

10. REFERENCE

M. C. Guile, "Gamma Ray Air Scattering," Thesis, Worcester Polytechnic Institute (April 1978).

11. CONTENTS OF CODE PACKAGE

Included are the referenced document and one (1.2MB) DOS diskette which contains the source codes.

12. DATE OF ABSTRACT

January 1982.