Development of a large-angle pinhole gamma camera using depth-of-interaction detector for nuclear survey

Pinhole imaging is receiving notice as a method to enable gamma camera for nuclear survey. In our previous work, the pinhole gamma camera with depth of interaction (DOI) based on three layers of monolithic crystals and maximum-likelihood position-estimation (MLPE) algorithm was designed for the environmental monitoring and the methodology for 3D event positioning in the detector was established by using Monte Carlo simulation. The aim of this study was to evaluate the performance of our detector module experimentally. The proposed detector module was developed by stacking three layers of monolithic CsI(Tl) crystals, each of which has a dimension of 50.0 × 50.0 × 2.0 mm³. The bottom surface of the third layer was directly coupled to an 8 × 8 channel position-sensitive photomultiplier tube (PSPMT, Hamamatsu H8500C). The PSPMT was read out using a resistive charge divider, which multiplexes 64 anodes into 8(X) + 8(Y) channels. Gaussian-based MLPE methods have been implemented by using experimentally measured detector response functions (DRFs). To measure the resolution in each DOI layer, the gamma events were generated at 25 different points over one fourth of the detector area with a spacing of 5.0 mm in the X and Y dimensions. The FWHMs obtained from the first, second, and third layers had mean values of 2.86, 2.67 and 2.87 mm, respectively. The layer misclassification rate was measured over all pixels as 21.4% for the DOI direction. In this paper, our new detector proved to be a reliable design to characterize the event position in all three dimensions with high and uniform spatial resolution and the large-angle pinhole camera could be a useful tool in the environmental monitoring.