1. NAME AND TITLE

FANTOM: Monte Carlo Calculation of the Response of an External Detector to a Photon Source in the Lungs of a Heterogeneous Phantom.

2. CONTRIBUTOR

Bhabha Atomic Research Centre Hospital, Bombay, India.

3. CODING LANGUAGE AND COMPUTER

Fortran IV; BESM-6.

4. NATURE OF PROBLEM SOLVED

FANTOM calculates the response of a 20-cm-diameter phoswich (3 mm NaI(Tl) primary detector) to a source of low energy photons distributed in the lungs of a heterogeneous MIRD phantom, approximating ICRP Reference Man. The program considers the trunk region of the MIRD phantom which is made up of three types of tissues with different densities: skeletal tissue (1.85), lung tissue (0.3) and soft tissue (1). Each organ in the thorax region is described by simple quadratic equations, with respect to a Cartesian coordinate system (X,Y,Z), the origin of which is located at the center of the base of the trunk, with positive Z-axis, Y-axis, and X-axis directed toward the head, posterior, and left side of the phantom, respectively.

5. METHOD OF SOLUTION

FANTOM is designed for a ²⁴¹Am source (60,000 disintegrations) uniformly distributed in the lungs of the phantom. Monte Carlo techniques are employed to generate photons and trace their fates in the thorax of the phantom. FANTOM starts by tracing 18,000 histories (9,000 in each lung) of photons belonging to each of the four main energies of the ²⁴¹Am nuclide. Each photon is assigned a weight in proportion to its yield. The acceptable points of photon interactions (photoelectric and Compton) in skeletal, lung, and ordinary tissue are determined by the Coleman technique. The calculations yield the exit photon energy spectrum, which is then smeared with an experimentally determined Gaussian resolution function to convert into the pulse-height spectrum observable with the detector. The program has provisions for incorporating the effects of the iodine K X-ray escape as well as the variable intrinsic efficiency of the detector.

6. RESTRICTIONS OR LIMITATIONS

The calculated calibration factors are valid only for body builds typical of the MIRD phantom. There appear to be certain discrepancies regarding the estimates of the body-build parameters for the MIRD phantom. There also appear to be wide variations between experimental observations made on human volunteers and the results of Monte Carlo calculations on the MIRD phantom.

7. TYPICAL RUNNING TIME

No study has been made by RSIC of typical running times for FANTOM.

8. COMPUTER HARDWARE REQUIREMENTS

The code is designed to execute on the BESM-6 computer.

9. COMPUTER SOFTWARE REQUIREMENTS

A Fortran IV compiler is required.

10. REFERENCE

S. Bhati, R. C. Sharma, and S. Somasundaram, "Monte Carlo Calculations of the Response of an External Detector to a Photon Source in the Lungs of a Heterogeneous Phantom," Health Phys. 37, 145-159 (July 1979).

11. CONTENTS OF CODE PACKAGE

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

12. DATE OF ABSTRACT

February 1982; revised February 1983.