The ⁶LiF-silicon detector array developed for real-time neutron monitoring at white neutron beam at CSNS

A white neutron source for nuclear data measurement has been built and put into operation on China Spallation Neutron Source (CSNS). We developeda beam monitor for real-time online monitoring of neutron flux on the white neutron source. The beam monitor work simultaneously with other nuclear data measurement detectors to provide neutron flux for data analysis. To minimize beam disturbance and reduce the background generated by the monitor, a structure in which neutron conversion layer and Si detectors are separated is employed: A ⁶LiF film with a mass thickness of 360 μg/cm² was deposited on a 10μm aluminum foil and placed inside the beam line. Eight Si-detectors with a size of 20×20 mm² were placed around the film outside the beam to maximize the reception of secondary particles from the conversion layer and to avoid neutron beam irradiation. The simulation of the beam monitor (layout, counting rate, neutron disturbance, etc.), construction and online performance are described in the paper. Good online performance of the monitor was achieved during the CSNS white neutron source flux calibration experiment in March, 2018: The counting rate for the beam line operating at Ф30 mm and Ф60 mm beam pattern modes was normalized to 100 kW proton beam power, and the result is consistent with the simulation. The total uncertainty of the normalized counting rate is less than 1%. Moreover, the measurement of neutron flux in low energy region below 10 keV of the white neutron source by the monitor is discussed.