Neutron Energy Spectrum Measurement with a Wing-shaped Lithium Glass Detector at CSNS Back-n

摘要: The back-streaming white neutron beam line (Back-n) constructed at Chinese Spallation Neutron Source (CSNS)hasbeenwidelyusedfornucleardatameasurementsandneutronapplications. Neutron energy spectrum is one of the key parameters for neutron facility. Although the neutron energy spectrum of Back-n has been measured for several times, there is still some room for improvement, such as measurements in the low energy region and with different beam profiles. A Wing-shaped lithium glass detector has been developed in this work for neutron energy spectrum measurement. One of the key designs is to use both 6Li glass and 7Li glass in the measurement. The 7Li glass is used to measure gamma background which is supposed to be the same as that of 6Li glass while it rarely interacts with neutrons, thus the pure neutron interaction contribution in 6Li glass can be identified accurately. Another key design is that the detector is setup in wing shape, which can protect the photomultiplier tubes from being blinded by the γ-flash of Back-n. The detector was tested with a Pu-C neutron source as well as the Back-n neutron beam. The neutron energy spectrum with the smallest beam profile of Back-n was firstly measured with this lithium glass detector. The time-of-flight method was used to obtain the neutron energy and the black resonance filter method was used to determine the background induced by gamma rays. Neutron energy spectrum in the energy region between 0.4 eV and 100 keV was obtained. By comparing the present result with that obtained from the Φ30 beam profile measurement, it was found that the shape of the energy spectrum differed within 5%, while the absorption structures were almost the same. Using a relative measurement method, the neutron intensity under the smallest beam profile configuration of Back-n was evaluated to be 8.04×104 neutrons/cm²/s with a proton beam power of 100 kW.