Extension of the Linear Depth Attenuation Method for the Radioactivity Depth Analysis Tool (RADPAT)

This paper relates to the determination of contamination depth in concrete media via the attenuation of radiation being emitted by the contamination. Previous research has shown that contamination depth can be derived by comparing the relative attenuation of the x-to- γ-ray photo-peak intensities of a given caesium-137 spectrum to that suggested by a derived attenuation model. Once given some initial calibration, a single cadmium zinc telluride (CZT) detector taking measurements over the surface of a concrete wall can ascertain the distribution of depth of the absorbed contamination. The work detailed in this paper provides the natural extension to the research on caesium so that it can be ex tended to other isotopes, such as cobalt-60. In particular the paper details the procedure used to obtain a new set of measurements which demonstrate that, whilst the method be applicable to obtain the depth of contamination for isotopes other than caesium-137, in the case of cobalt-60 it has important limitations. The CZT detector detector used for these measurements utilizes a bespoke pre-amplifler specifically matched to a CZT type detector. This device connects directly to a standard analogue signal processing chain, in stark contrast to previous measurement attempts where bespoke hardware was used. Therefore the output from the detector was of an inherently analogue nature which was supplemented by a digital oscilloscope when the measurements were taken; a step necessary due to the requirement that an instrument based on this method is portable. The oscilloscope, set to acquire at 500 Msps, logged data directly to a standard personal computer for subsequent analysis. This paper details the effect of the digital method and evaluates its performance on final result in comparison to the standard analogue technique. Finally, insight on the prototype design of an instrument based on this approach is provided which, by using the data obtained during the investigation, is calculated to require an embedded system based on a field-programmable gate array, capable of acquiring data at around 50 MSa/s.