1. NAME AND TITLE

DCTDOS: Neutron and Gamma-Ray Penetration in Composite Duct Systems.

2. CONTRIBUTORS

National Bureau of Standards, Gaithersburg, Maryland

3. CODING LANGUAGE AND COMPUTER

Fortran 77, IBM PC.

4. NATURE OF PROBLEM SOLVED

Estimates are made of neutron and gamma-ray fluence rate (flux density), dose, and spectral features due to penetration through a series of duct segments--straight sections and bends--in arbitrary combinations; and the resulting composite can include computations for a room at the end, if there is one. This method was developed for the rapid estimates that are needed for analysis of protection problems against nuclear weapons, but the concepts are more generally applicable.

5. METHOD OF SOLUTION

The method, called ``orders of reflection,'' involves the following steps: (1) The penetration of the radiation (neutrons or gamma rays) through single straight ducts of various dimensions is first calculated in a one-velocity treatment, assuming an albedo with an angular dependence given by Chandrasekhar's theory, and assuming isotropic scattering. This penetration probability is calculated for the case of 0, 1, 2, ..., n reflections from the walls; (2) An estimate of the energy and angular characteristics of the radiation transmitted after 0, 1, 2, ..., n reflections is then made by using the results of a Monte Carlo calculation in which the energy and angular characteristics of the multiply-reflected radiation is computed in a space-independent treatment; (3) The results of calculations (1) and (2) are combined to obtain a transfer function which relates the intensities, direction, and energy characteristics of the radiations entering and leaving a single straight duct or duct segment.

Using the output of one such calculation as the input for the next, the transmission of the radiation through a sequence of ducts can readily be calculated for any spatial configuration of successive duct legs. The approximations made in the orders-of- reflection method are particularly applicable to neutrons. However, the method, with some adaptation, has also been found to be applicable to gamma rays.

An extensive data base of neutron and gamma-ray albedo data has been incorporated into the system which allows very rapid calculations of radiation penetration of complex duct configurations. Trial calculations have indicated very good agreement with published Monte Carlo calculations. The method may also be applied to shielding problems pertaining to medical radiation sources and accelerators, but the code has not been tested for these applications.

6. RESTRICTIONS OR LIMITATIONS

The neutron calculations use 20 reflections from the duct walls while the gamma-ray calculations use 9 (maximum values of ``n'' mentioned above).

7. TYPICAL RUNNING TIME

Run times for the sample problems take a few minutes on an IBM PC.

8. COMPUTER HARDWARE REQUIREMENTS

The minimum hardware requirements for DCTDOS: an IBM PC or compatible with 2 floppy drives and 640 K Random Access Memory. An 8087 math coprocessor is desirable.

9. COMPUTER SOFTWARE REQUIREMENTS

The Microsoft Version 3.31 was used to compile the programs under PC/DOS 3.1. The Microsoft linker LINK.EXE was used to create two executable files. One (160 K bytes) requires the 8087 coprocessor and the other (277 K bytes) does not. The executable files are included in the package.

10. REFERENCES

L. V. Spencer, DCTDOS: Neutron and Gamma Penetration in Composite Duct Systems, NBSIR 87-3534, National Bureau of Standards (Feb. 1987).

11. CONTENTS OF CODE PACKAGE

Included are the referenced document, the source code files, two executable files, sample case input and output, and a README.DOC file. Three 5.25-in. DSDD diskettes are needed to transmit the package.

12. DATE OF ABSTRACT

November 1987.