Simulation Analysis of Scintillation in Sodium Iodide Detector

The work presented here investigates the use of a new type of integrated circuit as a replacement for photomultiplier tubes (PMTs) that are mainly used in radiation detection and for radiation spectroscopy. PMTs are typically large, fragile, and have complex power delivery requirements and high-energy consumption. While integrated circuit technology has advanced, circuits on these chips are becoming increasingly sensitive to radiation damage and therefore may fail to function properly when exposed to radiation. Researchers at A&M have designed integrated circuits that contain both radiation sensitive and radiation resistant sectors on the same chip. These specialized chips are extremely sensitive to charged particles (such as betas) that are produced at the photocathode in a PMT. With the use of photocathodes along with a scintillator and these integrated circuits, it is possible to detect gamma radiation and perform radiation spectroscopy, and realize advantages that are known for the semiconductor technology. In order to effectively use these integrated circuits as a PMT replacement, it is essential that an understanding be developed regarding the location and amount of betas produced on a photocathode by the optical photons. For understanding this scintillation process, the transport of optical photons was modeled in a 2” x 4” x 16” sodium iodide crystal using Geant4. Simulations were performed using a mono-energetic gamma source of 70 keV and 1 MeV with source modeled either as a pencil-beam or an isotropic emitter (1 ?Ci). In addition, simulations with a mono-energetic gamma source of 70 keV, 140 keV, 230 keV, 388 keV, 511 keV and 661.64 keV were performed. In these cases the source was modeled as an isotropic emitter (1 ?Ci). These analyses were performed to observe the optical photon distribution across all six-detector faces and to characterize the distribution based on gamma energy. It was observed that optical photons are denser on the surface through which the gammas enter the sodium iodide. Future work will include analyses of these optical photons on the photocathode to assess the light collection efficiency and ultimately, the capability of replacing a PMT with specialized integrated circuits.