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 (235) U 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 Cs-2 LiLaBr6 (CLLB) scintillator considering the y-ray flash and neutron environment on the Back-n beam line. The response of the CLLB scintillator to neutrons and y-rays was evaluated with different Li-6/Li-7 abundance ratios using Geant4. The neutron -y discrimination performance of the CLLB has been simulated considering different scintillation parameters, physical designs, and light readout modes. A cubic Li-6-enriched (> 90% ) CLLB scintillator, which has a thickness of 4-9 mm and side length of no less than 50 mm to cover the Phi 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 -y discrimination performance could provide technical support for other neutron-induced reaction measurements on the Back-n beam line.