Masters Theses

Date of Award

12-1991

Degree Type

Thesis

Degree Name

Master of Science

Major

Nuclear Engineering

Major Professor

L. F. Miller

Abstract

Providing accurate neutron dosimetry for a variety of neutron energy spectra is a formidable task for any dosimetry system. Unless something is known about the neutron spectrum prior to processing the dosimeter, the calculated dose may vary greatly from that actually encountered; that is until now. The entrance of bubble detector technology into the field of neutron dosimetry has eliminated the necessity of having an a priori knowledge of the neutron energy spectra. Recently, a new approach in measuring personnel neutron dose equivalent was developed at Oak Ridge National Laboratory. By using bubble detectors in combination with current thermoluminescent dosimeters (TLDs) as a Combination Personnel Neutron Dosimeter (CPND), not only is it possible to provide accurate dose equivalent results, but a simple four interval neutron energy spectrum is obtained as well. The components of the CPND are a Harshaw albedo TLD (two TLD-600/700 pairs one covered by cadmium, the other by ABS plastic) and two bubble detectors with theoretical energy thresholds of 100 keV and 1500 keV (BD-100R and BDS-1500 from Bubble Technology Industries, Canada).

The original CPND methodology has been modified with the goal of improving the spectrometric capabilities and the resulting dosimetric accuracy. The foci of the modification were: 1) refinement of the BD-100R and BDS-1500 response functions, 2) reevaluation of the TLD-600 thermal neutron sensitivity, 3) redefinition of the energy intervals for which the neutron spectrum is described, and 4) introduction of a matrix algorithm for neutron spectrum deconvolution and dosimetric determination. The effectiveness of the modifications was assessed by reevaluating the original raw data from a series of radioisotopic source and in situ measurements and comparing them with the original CPND results.

The results of the modified CPND demonstrate significant improvements in the spectrometric and dosimetric accuracy have been realized, relative to the original CPND characterization. This is evidenced by an overall increase in dosimetric accuracy of 2% for the in situ and 28% for the radioisotopic measurements. Individually, the modified version outperformed the original in eight of the ten measurements, while of the remaining two, one was the same and for the other the original results were better by 2%. The final neutron dose equivalent results were within 11% of the reference values for the five in situ spectra and within 2% of the reference values for the radioisotopic source spectra. Presented are: 1) a synoptic history surrounding emergence of bubble detector technology, 2) a brief overview of the current theory on mechanisms of interaction, 3) the data and analysis process involved in refining the response functions, 4) performance evaluation of the original CPND and a reevaluation of the same data under the modified method as presented in this work, 5) the procedure used to determine the reference values of component fluence and dose equivalent for field assessments, 6) analysis of the after-modification results, 7) a critique of some currently held assumptions, offering some alternative explanations, and 8) my personal thoughts concerning potential applications and directions for future research. Also provided in an appendix is a technical note detailing the organic nexus between the response characteristics of a neutron dosimeter, the fluence-to-dose equivalent factor and the neutron spectrum being measured, which includes several radioisotopic V source sensitivities and calibration factors calculated for the BD-100R and TLD-600.

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