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

Abstract

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/cm2 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 μm thickness, if the glass is loaded with 99% enriched 10B compound at 10%wt. A lower concentration of 95% enrichment at 5%wt loading can provide 60% efficiency with a thickness slightly above 1mm. 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 with high-speed digital electronics. Traditional pulse sampling using a high-speed analog to digital converter 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 method to isolate neutrons from gamma ray interactions. The gamma-neutron peak separation is greater than 2 sigma, and with a thin scintillator, the gamma ray rejection per detected neutron should meet the design specification of 10-6. Using the time over threshold method, the digital information to be processed by a interfacing ADC is reduced allowing for readout per SiPM, providing a position resolution close to 1 mm.