Evaluation of the Performance of the Gamma Densitometers of Atomic Energy of Canada Limited (Part A)

Abstract

<p> The Three Beam Gamma Densitometers developed by AECL-WNRE for measurement of void fraction in flowing steam/water mixtures were investigated to determine the major causes of drift and slow response to changes in void fraction. Available remedies were evaluated. The study indicated the following: (1) Using the existing current mode system, considerable reductions in drift and response time may be possible through the following inexpensive modifications: (a) design changes in the photo multiplier tube high voltage circuit, (b) replacement of the detectors with Cadmium Telluride semi-conductor detectors, (c) replacement of the scintillation phosphor with one which exhibits no afterglow such as Bismuth Germanate or Cesium Fluoride, (d) matching voltages of photo multiplier tube outside wall and photocathode, (e) improved stabilization of high voltage and detector temperature. (2) Drift would probably be reduced to less than 0.1%/hour if the existing current mode system were replaced with a simple integral counting mode system at an approximate cost of $5,200 per densitometer. (3) Additions to the above counting mode system of a single channel analyser (S.C.A.) and an automatic gain control unit which uses a reference light would probably completely eliminate drift as well as substantially improve system response. This would require an additional $4,900 per densitometer. </p> <p> It is recommended that these modifications be implemented in the same order as above, on a trial basis, as far as further expenditure is justified.</p>