Conceptual design of a Cs2LiLaBr6 scintillator-based neutron total cross section spectrometer on the back-n beam line at CSNS

To reduce the experimental uncertainty in the 235U resonance energy region and improve the detection efficiency for neutron total cross section measurements compared with those obtained with the neutron total cross section spectrometer (NTOX), a dedicated lithium-containing scintillation detector has been developed on the Back-n beam line at the China Spallation Neutron Source. The Fast Scintillator-based Neutron Total Cross Section (FAST) spectrometer has been designed based on a Cs2LiLaBr6 (CLLB) scintillator considering the γ-ray flash and neutron environment on the Back-n beam line. The response of the CLLB scintillator to neutrons and γ-rays was evaluated with different 6Li/7Li abundance ratios using Geant4. The neutron-γ discrimination performance of the CLLB has been simulated considering different scintillation parameters, physical designs, and light readout modes. A cubic 6Li-enriched (> 90 %) CLLB scintillator, which has a thickness of 4-9 mm and side length of no less than 50 mm to cover the Φ 50 mm neutron beam at the spectrometer position, has been proposed coupling to a side readout SiPM array to construct the FAST spectrometer. The developed simulation techniques for neutron-γ discrimination performance could provide technical support for other neutron-induced reaction measurements on the Back-n beam line.