The neutron total cross-section spectrometer (NTOX) applied on the Back-n beamline at the China Spallation Neutron Source (CSNS) is based on a multicell fission chamber and utilizes \(^{235,238}\textrm{U}\) for neutron detection. To reduce the experimental uncertainty in the resonance energy region of \(^{235,238}\textrm{U}\) and improve the neutron detection efficiency, a fast scintillator-based neutron total cross-section (FAST) spectrometer was designed. A prototype based on a large-area square \(^{6}\textrm{Li}\) -enriched \(\textrm{Cs}_{2}\textrm{LiLaBr}_{6}\) (CLLB) scintillator was constructed and beam-tested. The size of the CLLB scintillator was 50.8 mm \(\times\) 50.8 mm \(\times\) 6 mm, and its side was coupled to an array of 1 \(\times\) 8 S14160 MPPC to avoid the irradiation from the high-intensity neutrons and \(\gamma\) -rays. The beam test was performed using a broad-energy pulsed neutron and the time-of-flight (TOF) technique on the Back-n beamline. The results demonstrate that the prototype exhibits good neutron/ \(\gamma\) discrimination capability under strong \(\gamma\) -flash irradiation. The prototype was applied to measure the neutron total cross-section of \(^{\textrm{nat}}\textrm{Pb}\) and the result was compared with that obtained using the NTOX. The two results were consistent in the energy region of 0.3 eV to 1 keV, and the prototype showed a higher detection efficiency and did not exhibit fission resonance effect. This type of spectrometer can be used as a complement to the NTOX in the low-energy range and provides a technical reference and framework for developing the FAST spectrometer on the Back-n beamline.
