Design of a High-Speed Neutron Imager Using a Boron-Loaded Organic Glass Scintillator

Upgrades to the Spallation Neutron Source at Oak Ridge National Laboratory will provide a second target station and an increased neutron flux, which will offer a more powerful tool for neutron reflectometry. To fully utilize the higher neutron flux, a high-speed, neutron-imaging device (an imaging plane) specifically for reflectometry is required. The neutron imaging plane requires a detection efficiency of 60% for 2 Å neutrons for event rates exceeding 2 Mcps/cm ² with less than 10% dead time. A novel organic glass scintillator material loaded with a boron compound demonstrated good neutron sensitivity. Simulations show that the detection efficiency for thermal neutrons can reach 60% with a 500- $\mu \text{m}$ thickness, if the glass is loaded with 99% enriched $^{10}\text{B}$ compound at 10% wt. A lower concentration of 95% enrichment at 5% wt loading can provide 60% efficiency with a thickness slightly above 1 mm. The decay time of the scintillator is less than 100 ns, providing a fast response for high rate counting. The instrument design is based on detecting the neutron with a fast scintillation material and integrating silicon photomultipliers (SiPMs) with high-speed digital electronics. Traditional pulse sampling using a high-speed analog to digital converter (ADC) to conduct pulse shape discrimination is not viable for imaging, and a new technique has been formulated for this material using a time over threshold (TOT) method to isolate neutrons from gamma-ray interactions. The gamma-neutron peak separation is greater than two sigma, and with a thin scintillator, the gamma-ray rejection per detected neutron should meet the design specification of $10^{-6}$ . Using the TOT method, the digital information to be processed by an interfacing ADC is reduced allowing for readout per SiPM, providing a position resolution close to 1 mm.