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. GaAs/AlGaAs diodes developed in this project, as well as commercial silicon photomultipliers (SiPM), 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. To obtain the radiation tolerance, the GaAs diode is thin (less than 1 ?m thick), while to facilitate operation beyond the breakdown voltage, a triple mesa structure was utilized. Simulations showed a strong suppressing in the electrical field on the edges of the diode, while measurements indicate a strong (multiple orders of magnitude) suppression of leakage current in the triple mesa structure compared to a single mesa structure. To further enhance the performance of a photomultiplier (arrays of the GaAs diodes), the packing density of the diodes is improved by transitioning to a double mesa structure, where simulations show a viable electric field profile for Geiger-mode operation. 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.