Speaker
Description
Accurate nuclear data for neutron capture reactions of minor actinides (MA) are required to estimate the production and the transmutation rates due to their preponderant role in the study and design of transmutation systems related to nuclear waste management. Since its construction in 2009, continuous research efforts have been carried out to measure neutron-induced reactions at the Accurate Neutron Nucleus Reaction Measurement Instrument (ANNRI) of the Japan Proton Accelerator Research Complex (J-PARC). Neutron capture cross section experiments have been performed involving the complementary use of the two detector setups installed at ANNRI, a large array of high-purity Ge detectors and a NaI(Tl) spectrometer.
Nonetheless, ever since the J-PARC accelerator was switch onto double-bunch mode, neutron capture cross section measurements have become unfeasible using keV neutron. In the keV region, the 0.6 µs time difference between neutron generating events, i.e., spallation reactions, is not negligible. Hence, the double-bunch mode introduces serious ambiguities in keV-neutron-induced reactions due to the overlapped contribution of the two neutron-generating events.
A neutron filtering system has been installed at the ANNRI beamline in order to bypass the doublet structure of the incident neutron beam in the keV region. Thick cylindrical slabs of $^{nat}$Fe, $^{nat}$Si and $^{nat}$Cr which share the characteristic of a sharp minimum in the neutron total cross section, were separately introduced in an intermediate stage of the beamline, before the NaI(Tl) spectrometer experimental area. The filtered neutron beams were analyzed by means of both neutron capture and transmission experiments. Moreover, further information about the neutron energy distribution within the filtered peaks was derived through the use of Monte-Carlo simulations with the PHITS code [1].
This presentation will provide an overview of the neutron filtering system implemented at ANNRI. The main characteristics of the filtered neutron beams for $^{nat}$Fe and $^{nat}$Si, already published here [2]; and $^{nat}$Cr will be presented together with 197Au standard experimental results to assess the performance of the neutron filtering system in neutron capture cross section experiments.
This work is supported by the Innovative Nuclear Research and Development Program from the Ministry of Education, Culture, Sports, Science and Technology of Japan
[1] T. Sato et al. J. Nucl. Sci. Technol. 55, 684 (2018)
[2] G. Rovira et al., Nucl. Instr. Meth. 1003, 165318 (2021)