关键词: 辐射俘获截面, 共振参数, 脉冲高度权重技术, 白光中子源

Abstract: Radiative neutron capture cross-section and the resonance parameter are very important nuclear data in the field of the R&D of accelerator driven systems, the transmutation of nuclear waste and nuclear astrophysics. In the recent years, radiative neutron capture crosssections of important nuclides were mostly measured based on the white neutron source facilities, such as the CERN n_TOF, GELINA, LANSCE and so on. The back streaming white neutron beam line (Backn) of China Spallation Neutron Source (CSNS) is the first white neutron facility in China, which can provide pulsed neutron beam with 25 Hz frequency in the energy region from thermal neutron to hundreds MeV. The main application of Backn is to perform nuclear data measurement, such as crosssection, reaction yield, nuclear structure, and so on. The C6D6 liquid scintillation detector is widely used for radiative neutron capture crosssection measurements on timeofflight facilities, because of its low neutron sensitivity and good time response. A C6D6 detection system with four detectors was installed at Backn to perform radiative neutron capture crosssection measurement, especially in the resonance energy region. In this work, the measurement of 169Tm(n,γ)170Tm reaction was performed with the C6D6 detection system at the Backn. The stable isotope of thulium, 169Tm, is one of the fission product poisons, which makes the accurate neutron radiative capture crosssection of 169Tm be of major significance for fission and fusion reactor design. Besides, the radioactivity induced by the reactions of 169Tm(n,γ)170Tm and 170Tm(n,γ) 171Tm makes the 169Tm to be an ideal spectrumsensitive activation detector for the neutron intensity diagnosis, in which the precise neutron capture crosssection is of critical importance. In the present measurement, the pulse height weighting technique was used to make detection efficiency be proportional to the excitation energy of the compound nucleus. The black filter method was used to determine the experimental background due to scattered neutron and inbeam gamma ray. The saturated resonance method was used to normalize the capture yield. The Rmatrix code SAMMY was used to fit the capture yield of 169Tm to obtain the resonance parameters, such as resonance energy, neutron width, radiative capture width, and so on. Then the radiative capture crosssection was calculated with the measured resonance parameters and ReichMoore approximation. For most energy region between 1 eV and 100 eV, the present measurement result is consistent with the recommended value of ENDF/BⅧ.0, but there were still some discrepancies between the measured crosssection and the evaluated data. According to the current results, the measurement and data analysis methods used in this work are suitable for the measurement of radiative capture crosssection and resonance parameter. However, in order to perform high precision measurement of radiative capture crosssection at Backn, the neutron flux and the energy resolution function still need to be determined more accurately.

Key words: radiative capture cross-section, resonance parameter, pulse height weighting technique, white neutron source