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内容記述 |
In neutron capture cross-section measurements by the time-of-flight (TOF) method using pulsed neutron beams, some of the neutrons scattered within the sample are captured by structural materials around the detector, resulting in the emission of capture gamma-rays. The emitted gamma-rays induce background events (hereinafter referred to as “scattered-neutron background”), and this background must be subtracted from TOF data in analysis to accurately derive the capture cross-section. The Accurate Neutron-Nucleus Research Measurement Instrument (ANNRI) is installed at beamline No.4 of the Materials and Life Science Experimental Facility in the Japan Proton Accelerator Research Complex to measure nuclear reaction. The ANNRI is equipped with large NaI detectors and has been used to perform many capture cross-section measurements. Recently, the authors have attempted to measure the capture cross-sections of structural materials such as iron, nickel, and chromium, which have very large neutron scattering resonances compared to capture cross-section. As expected, the scattered-neutron background of these nuclides has a significant impact on applying the conventional correction method. Through Monte-Carlo simulations using the PHITS code, it was confirmed that neutrons with energies above 100 keV can induce background events with a time-delay between scattering in the sample and detection up to 100 us. This means that neutrons scattered at the resonance form a background in the TOF region slower than the resonance. Without considering the time delay in the resonance scattering, the scattered-neutron background in the resonance region is overestimated and the scattered background in the slow TOF region is underestimated. In addition, the delayed scattered-neutron background with higher energies has also to be considered. Therefore, the conventional correction method for scattered-neutron background cannot be applied, and it is necessary to develop the new methodology that accounts for this time delay. In this presentation, we will report on our current efforts to implement a new correction method to the capture cross section measurement data at ANNRI. Moreover, preliminary results of the Cr-50 neutron capture cross-section using this new methodology will also be presented. |
