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

• Published in: IEEE transactions on nuclear science Vol. 69; no. 6; pp. 1317 - 1321
• Main Authors: Johnson, Erik B., van Loef, Edgar, Kaffine, Meghan, Sosa, Charles
• Format: Journal Article
• Language: English
• Published: New York IEEE 01.06.2022; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Analog to digital converters; Boron; Boron compounds; Compounds; Design; Design specifications; Detectors; Digital electronics; Digital signal processing; Efficiency; Engineering; Gamma rays; Glass; High speed; Imaging; Information processing; Neutron flux; neutron reflectometry (NR); Neutrons; Nuclear Science & Technology; organic glass scintillator (OGS); Photomultiplier tubes; Pulse shape; pulse shape discrimination~(PSD); Reflectometry; Research facilities; Scintillation counters; Scintillators; Shape; Spallation; Thermal neutrons; Thickness
• ISSN: 0018-9499; 1558-1578
• DOI: 10.1109/TNS.2021.3130491

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-<inline-formula> <tex-math notation="LaTeX">\mu \text{m} </tex-math></inline-formula> thickness, if the glass is loaded with 99% enriched <inline-formula> <tex-math notation="LaTeX">^{10}\text{B} </tex-math></inline-formula> 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 <inline-formula> <tex-math notation="LaTeX">10^{-6} </tex-math></inline-formula>. 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.