Digital radiography by counting photons: innovative solution for testing very thick parts

In 2019 we presented the first prototype of a photon counting detector (PCD) specially designed for the inspection of very thick parts requiring the use of high energy (HE) sources (E > 200keV). HE PCD differs from the standard model by the following characteristics: 1) Photoconductor CdTe active layer thickness optimized for HE, with about 30% increased detection efficiency; 2) Completely renewed ASIC allowing selection of energy threshold up to 1300keV. Modification 2) allows to reduce scattered radiation when using gamma sources such as Iridium 192 and Cobalt 60; or pulsed sources like linear accelerators and betatrons. The object-contrast results from this considerably improved. Image quality is comparable to, if not better than, film. We present energy threshold optimization study and performance evaluation results in terms of ISO 19232 IQI (wire and hole), signal-to-noise ratio, and spatial resolution (via duplex IQI) according to ISO 17636_2 (class B). Focus is given to HE PCD/film comparison, which was carried out by means of mock-ups containing artificial and/or real flaws; and, in 2020, for the first time in a nuclear power plant site, for the examination of primary circuit dissimilar welds. Photons energy thresholding successfully addressed also the presence of background radiation. The latter represented 2% of counted photons, while with film, background was responsible of 13% of optical density.