Monte Carlo investigations of a high resolution Small Field-Of-View gamma camera

At the time of writing there have been no publications describing Monte Carlo simulations tracking low energy gamma and X-ray photons (less than 200 keV) through Small Field-Of-View (SFOV) photon detection systems. A literature search using the Web of Science (1970-2019) and Scopus (1960-2019) with the keywords “gamma, camera” AND “monte carlo” AND “small” OR “compact” showed 200 and 215 results respectively, but without any photon tracking studies. Two SFOV systems were modelled using PENELOPE v2008 Monte Carlo: the Portable Imaging X-ray Spectrometer detector, a pre-scintillator detector system for non-medical use, and a thallium doped caesium iodide scintillator based Compact Gamma Camera used for medical imaging. Each system uses an electron multiplying charge coupled device modelled as an 8 mm x 8 mm x 5 ?m thick monolithic silicon detector. These simulations demonstrated the Fano-limited energy spectrum, and that the modelled fluorescence do not record some of the caesium and iodine Kα and Kβ? fluorescence photons if the source event originated closer to the boundary of the Monte Carlo accumulator. The corroborative experimental response of the PIXS detector using cadmium-109 showed broadening of the Ag Kα, Kβ? peaks, consistent with the energy resolution being broadened owing to incomplete charge collection, drift and transfer through the shift and gain registers, and also due to noise from the detector readout. As the distribution of photoelectrons from the EMCCD output is stochastic, a premise of distinguishing between zero and single photoelectron as an input, with thresholding using noise peak plus 5? worked well for a gain potential difference ?HV between 33.5 V and 39.5 V, with the system cooled to 256.0 ? 0.1 K. Finally, a GEANT4 v10.5 simulation of caesium iodide crystal comprised of columns 100 ?m x 100 ?m x 1500 ?m thick demonstrated a greater number of optical photons propagating by internal reflection to the 5 ?m silicon detector, when laterally wrapped with 1 ?m aluminium compared either to an unwrapped columnar crystal, laterally wrapped monolithic or unwrapped monolithic crystals.