Doctoral Dissertations

Date of Award

6-1987

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Nuclear Engineering

Major Professor

H. L. Dodds

Committee Members

J. E. Brau, P. F. Pasqua, R. B. Perez, P. N. Stevens

Abstract

The purpose of this work is the development and testing of a Monte Carlo code system for calculating the response of an ionization chamber to a mixed neutron and photon radiation environment. The resulting code system entitled MICAP - a Monte Carlo Ionization Chamber Analysis Package - determines the neutron, photon, and total responses of the ionization chamber to the mixed field radiation environment. The Monte Carlo method performs accurate simulations of the physical processes involved in detecting radiation using ionization chambers, and eliminates limitations inherent in approximate methods.

The calculational scheme used in MICAP follows individual radiation particles incident on the ionization chamber wall material. The incident neutrons produce photons and heavy charged particles and both primary and secondary photons produce electrons and positrons. As these charged particles enter or are produced in the chamber cavity material, they lose energy and produce ion pairs until their energy is completely dissipated or until they escape the cavity. Ion recombination effects are included along the path of each charged particle rather than applied as an integral correction to the final result. ENDF/B-V partial cross section data have been incorporated in the neutron transport module to account for all processes which may contribute to the output signal. The transport modules utilize continuous angular distribution and secondary energy distribution data when selecting the emergent direction and energy of a particle. Furthermore, reactions are treated as discrete and allowed to occur with any of the constituent nuclides comprising a mixture. Finally, MICAP incorporates a combinatorial geometry package and input cross section processors to eliminate restrictions in the modeling capability of the code system with respect to geometry, physical processes, nuclear data, and sources.

To evaluate MICAP, comparisons were made with results obtained using other code systems and with experimental results. Separate comparisons with other code systems verified the validity of the neutron, photon, and charged particle transport processes and the nuclear models used to describe the individual neutron reactions, respectively. Comparisons with mono-energetic photon calibration experiments and with mixed neutron and photon radiation experiments verified the applicability of MICAP for analyzing the response of ionization chambers to mixed field radiation environments.

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