HyperNI: A High-Rate, Neutron Pixel Detector System

Neutron reflectometry is a powerful tool for studying the interfaces of artificial or biological materials where the majority of chemical and physical interactions occur. Even though major discoveries have been made in the field, an upgrade at the Spallation Neutron Source will enhance future measurements by increasing the neutron flux, where advanced neutron imaging systems will need to be developed to handle the higher event rates. To handle the high event rates, RMD will develop a composite scintillator material specifically designed for thermal neutron detection using materials with inherently fast decay times. The scintillator will be optically coupled to a silicon photomultiplier, yet the signal processing will be done primarily in the digital domain, where deleterious signal effects due the high rates can be identified and systematically removed using unique signal processing methods for identifying neutrons and omitting background. The Phase I effort resulted in the fabrication of an unique organic glass scintillator loaded with boron, providing neutron sensitivity. Simulations indicate 500-um of the material will provide a 60% detection efficiency for thermal neutrons. The material has multiple decay times, each less than 100 ns, supporting high event rate (> 1 Mcps) counting for neutron imaging. We demonstrated a method to provide gamma- neutron separation using a time over threshold technique. The technique could provide neutron imaging information with gamma rejection using one logic gate, which is critical for a compact readout using a field programmable gate array (FPGA) and for achieving high-spatial resolution (1-mm). The Phase-II effort will develop the organic glass to provide the thin, high-efficiency scintillator. The front- end readout electronics will be prototyped and tested in the lab for establishing requirements on the single, time-over-threshold logic gate. A prototype circuit designed and fabricated in a CMOS process will be done to provide a high-density readout for the imager, and the FPGA coding will be developed to provide a prototype imager for testing at the Spallation Neutron Source. The technology developed in this project is a high-speed neutron imager at rates reaching 20 mega counts per second per centimeter squared. The detector is a tool that can be used for scientific exploration involving neutron detection, which can span from planetary sciences to nuclear medicine, nuclear material management and nuclear security.