Study Of The Neutron Energy Spectrum Measurement Of The CSNS Back-n White Neutron Source

The newly built back-streaming white neutron source(Back-n)at China spallation Neutron Source(CSNS)brings breakthrough for the nuclear data measurement in China,which is pushing nuclear measurement toward higher precision,wider energy region,more information,etc.The measurement of the Back-n neutron energy spectrum is not only one of the main topics of the beam characterization,but also the important basis for the nuclear data measurement performed at Back-n.The neutron energy spectrum of Back-n has been measured by ²³⁵U based multi-layer fission chamber combined with the time-of-flight method.However,the cross section of ²³⁵U(n,f)have strong resonances in some energy regions,and the energy spectrum oscillates significantly,which cannot provide accurate data support for high-precision measurements.Therefore,the energy spectrum of the Back-n still needs to be measured by different detectors to further improve the accuracy and reduce the uncertainty.In this thesis,the first measurement of the neutron energy spectrum of the Back-n end station 1(ES#1)under small beam spot(beam spot size is about?20 mm)was carried out based on three neutron spectrum measurement systems at the Back-n experimental terminal with the time-of-flight method.The detectors were used including a fast-multi-layer fission ionization chamber(FIXM),a silicon monitor based on ~6Li F(~6Li-Si),and an array of proton recoil telescopes(?E-E).FIXM was used to measure the spectrum in the full-energy range of 0.5 e V-200 Me V,~6Li-Si was used to measure the spectrum in the energy range below 10 ke V,and?E-E was used to measure the energy spectrum in the high energy region from 10 to 100 Me V.After the analysis and evaluation of 3 detectors'data,the energy spectrum of the Back-n ES#1 with small beam spot were finally obtained.Among them,the measurement results of ~6Li-Si were used in the energy region below 10 ke V,for the energy region from 10 to 100 Me V,the results of both FIXM and?E-E were considered by the weighted average method,and the measurement results of FIXM were used in the rest of the energy regions.The integrated neutron flux in the whole energy region is 1.624×10~6neutrons/cm~2/s at the beam power of 100 k W,which is consistent with the FLUKA simulation results(1.67×10~6 neutrons/cm~2/s within the error of about 3%,besides,the maximum statistical uncertainty of the neutron energy spectrum is kept within 3%except the energy regions less than 0.5 e V and above 100 Me V.The results of this thesis will be an important reference for measurement proposals and data analysis at Back-n facility,and also provide the necessary data support for the nuclear data measurements in China.