The use of cadmium telluride γ spectrometers in monitoring activity deposited in nuclear power stations

Abstract

The ability to inspect and test key components and when necessary repair or replace them is a significant factor in the safety case submitted before consent is given for construction of a nuclear reactor. A knowledge of the probable rate of deposition and isotopic composition of radioactive contamination in these areas is necessary and since it is not always practicable to take samples away to the laboratory for analysis, direct gamma ray spectroscopy in the contaminated environment is sometimes used. The experience of direct monitoring with a Cadmium Telluride spectrometer in two such reactor situations is reported here. In the first situation, a remotely cooled Intrinsic Germanium spectrometer was used in equivalent positions and spectra from both systems are presented. The relative merits of the two systems are discussed. In the second situation, measurements were made in an environment at 70 °C in radiation levels of 10 R h-1. An improvised cooling system was used to maintain the Cadmium Telluride at about 20 °C and Pile-up Rejection was used to enable count rates of about 105 s-1 to be handled. It is noted that the usually quoted detector parameters, resolution and crystal volume, are not necessarily the most important in practical spectrometry of mixed isotopes. As with germanium detectors, the most useful parameter is probably the Peak-to-Compton ratio