20b Neutron Dose Tolerant Solid-State Photomultiplier for Nuclear Imaging

Neutron probes are a powerful tool in material science, and the Spallation Neutron Source at Oak Ridge National Laboratories is undergoing an upgrade to provide a high flux of neutrons on test samples to promote new discoveries. The higher intensity comes at a price of higher neutron and gamma-ray backgrounds that will damage equipment, specifically photodetectors, a critical component of science instruments. A technology that enables new instruments for discoveries in physics and material sciences is the silicon photomultiplier, yet this photodetector will be damaged rapidly within these high-intensity radiation fields. The project is to design a solid-state photomultiplier that will be significantly more tolerant to the gamma-ray and neutron backgrounds by replacing the silicon with a different semiconductor material. Phase I work focused on GaAs/AlGaAs coupled with the development of a triple-mesa structure for the photodiodes. Prototype units were less than 1 µm in thickness and were optically, electrically and radiologically tested. Our data show that the triple-mesa structure helps in minimizing the surface leakage current. The quantum efficiency for the thin prototype diodes was greater than 30% at 600 nm at 0 V, which is expected based on the prototype design. Both silicon photomultipliers (SiPM) and GaAs/AlGaAs diodes were exposed to 108 and 109 n/cm2 in identical thermal neutron fields. While the SiPMs showed an increase in dark current, the GaAs/AlGaAs diodes showed no increase, demonstrating the feasibility of developing a rad-hard GaAs photomultiplier. An optimized prototype Geiger photodiode (GPD) will be designed, fabricated and tested. We will focus on increasing the quantum efficiency from 300 nm up to 800 nm. Geiger mode performance will be enhanced to provide operation over multiple volts beyond the breakdown voltage while maintaining a low dark current. Once optimized, the GPD will be integrated into a complete solid-state photomultiplier with quenching resistors. Along with physics instrumentation, solid state photomultipliers are used for nuclear instruments in the medical fields and nuclear industry. The technology to be developed in the project will support applications where the detectors will be exposed to high radiation fields, such as those found in the nuclear industry, sciences, and space applications.