Masters Theses

Date of Award

5-1995

Degree Type

Thesis

Degree Name

Master of Science

Major

Nuclear Engineering

Major Professor

L. F. Miller

Committee Members

Jim Turner, Dave Simpson, Peter Groer

Abstract

The Oak Ridge Y-12 Plant currently uses alpha scintillation detectors as part of its alpha (α) personnel monitoring instrumentation (PMI) program. The purpose of this project is to identify and correct problems with these detectors and to improve their performance. The α PMI program uses alarm ratemeter instruments which are calibrated to alarm above a set count-rate. All of the detectors used in this program must have very similar operating characteristics. During field use, changes in the efficiency and increases in the background countrates have caused marked differences within the detector population. The radiological control program depends upon the measurements made by these instruments to assure that there is no spread of contamination. Confidence in these measurements is based upon the accuracy, precision, and reliability of these instruments. This confidence was challenged by sources of error which were being introduced into these detectors. Tests were performed on each component of these detectors, and it was discovered that there were two major error sources. The first was a chronic slow elevation of the background count-rate. The second was a shift in the efficiency of detectors within the instrument population. Modifications in the zinc-sulfide (ZnS) phosphor screens were able to virtually eliminate this background problem. This modification was able to consistently maintain low background count-rates at a fraction of the cost of phosphor replacement. In addition, the study of these detectors resulted in the elimination of sources of electrophonic noise which were adding to the background of these detectors. The shift in the efficiency was corrected by exercising tighter administrative controls. A representative sample of detectors was taken out of the detector population. Replicate background and source counts were taken using both standard and modified phosphors in order to compare this data. The detectors with the modified phosphors had an average background of 1.73 counts per minute (CPM), compared to 9.09 CPM for the standard phosphor. The efficiency of the detector with the modified phosphor was 0.22, compared to 0.207 for the standard detector. Finally the detector with the modified phosphor had minimum detectable amount (MDA) capability of 36.99 disintegrations per minute (DPM), while the standard detector had an MDA of 76.40 DPM using standard normal statistics. The techniques in this study can be easily applied to almost any alpha scintillation detector which uses a ZnS phosphor and a photomultiplier (PM) tube, thereby improving detector capabilities at a considerable cost savings.

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