Abstract
The minimum detectable activity (MDA) for gamma spectrometric measurements was studied as a function of filling height for two detectors having different background characteristics and for two sample matrices: a water sample with a low content of natural radionuclides, and zircon sand, with high activities of uranium and thorium. The zircon sand is also a material in which the self-attenuation of low-energy gamma photons will be high. The results show that for the water sample it is always favourable to completely fill the sample container. However, due to the inherent content of naturally occurring radionuclides in the zircon sand the MDA may not necessarily improve with the filling height, since an increased amount of sample will increase the background in the spectrum as well.
Highlights
Measurements of radionuclides in environmental and nuclear material are important from both safety and security point of views
One approach to the optimization of the sample filling height of a given sample container is to look for a maximum in the signal, i.e. the count rate, for a fixed activity content per sample unit [2, 3]
The count rate will never decrease, the last incremental addition of sample to the sample container might not contribute to the signal
Summary
Measurements of radionuclides in environmental and nuclear material are important from both safety and security point of views. One of the most common measurement technique serving this purpose is gamma ray spectrometry [1]. An important characteristic of the gamma spectrometric measurement method is minimum detectable activity (MDA) that defines the least amount of activity in the sample one can be confident of measuring. Beside e.g. the measurement efficiency, amount of sample, measurement time and photon emission probability, the MDA [e.g. Optimisation of measurement geometries has been studied earlier [2,3,4]. Those studies aimed at maximizing the detection efficiency multiplied by the amount of sample. Increasing the filling height will result in a reduction of the measurement
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