Neutron beam filter system for fast neutron cross-section measurement at the ANNRI beamline of MLF/J-PARC

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Abstract

A neutron filtering system has been introduced in the Accurate Neutron–Nucleus Reaction Measurement Instrument (ANNRI) beamline in the Materials and Life Science (MLF) facility of the Japan Proton Accelerator Research Complex (J-PARC) in order to produce quasi-monoenergetic neutron beams. The filtered neutron spectrum by the filter assemblies was analyzed by means of capture and transmission measurements and also by Monte Carlo simulations using PHITS. The characteristics of the filtered neutron beam are discussed alongside its viability in future applications for neutron cross-section measurements in the fast neutron region.

Introduction

The recent development of intense pulsed neutron facilities through the use of nuclear spallation reactions has allowed for the measurement of neutron-induced reactions, namely neutron capture, using small amounts of sample. The reason for this is the high number of induced reactions due to the high intensity of the beam, even in the keV region where the capture cross section is rather small as other reaction channels are being opened. The Accurate Neutron–Nucleus Reaction Measurement Instrument (ANNRI) beamline in the Materials and Life Science (MLF) experimental facility of the Japan Proton Accelerator Research Complex (J-PARC) provides one of the most intense neutron beam currently available and was thoroughly designed in order to measure neutron-induced reactions with high accuracy [1], [2]. However, the J-PARC accelerator is operated in a double-bunch pattern in which two proton bunches are injected into the spallation target with a time difference of 0.6μs. Events detected with a specific time-of-flight (TOF) have two different energies as they could have been originated from each of the two different proton pulses. This is particularly important in the continuum region (keV region) where the cross section can be usually expressed as a smooth averaged function, especially for the case of heavy nuclei such as Minor Actinides (MAs). In this region, it is impossible to separate the contribution from each proton pulse and, hence, this mode introduces serious ambiguities into the cross-section measurements.

In order to bypass the doublet structure of the neutron beam, a neutron filtering system has been designed as part of the “Study on accuracy improvement of fast-neutron capture reaction data of long-lived MAs for development of nuclear transmutation systems” project [3]. Filter materials were introduced into the rotary collimator of the ANNRI beamline to produce quasi-monoenergetic neutron filtered beams. The filter materials present sharp minima in the total cross-section due to the interference between the potential and s-wave resonance scattering. Thus, the incident neutron flux can be molded into sharp almost monoenergetic peaks using materials presenting these filtering windows in their total cross-sections. Two elements, Si and Fe, were used separately to produce one quasi-monoenergetic peak of about 24 keV (Fe) and two peaks of about 54 and 144 keV (Si).

In this paper, the characteristics of the filtered neutron beam at ANNRI using Fe and Si obtained from both measurements and simulations are presented. The incident flux was analyzed by means of capture experiments using a NaI(Tl) spectrometer and transmission experiments using Li-glass detectors. Moreover, simulations using the PHITS [4] code were performed in order to assess the performance of the neutron filter materials. Finally, results of the capture cross section of 197Au using the different filter materials are also presented using the NaI(Tl) spectrometer.

Section snippets

Principle

The J-PARC accelerator is operated in double-bunch mode at a 25 Hz repetition rate. Every 40 ms two 0.1μs-wide proton pulses are shot into the Hg spallation target with a time difference of 0.6μs as can be seen in Fig. 1 [5]. This operation pattern is aimed at increasing the thermal neutron beam intensity. For thermal neutrons, the double-timed structure of the neutron beam disappears as the slow-down process in the moderator is considerably longer compared to the time difference of 0.6μs [6].

Experimental analysis

The Fe and Si filter arrangements were tested by means of capture and transmission experiments in order to evaluate the resulting filtered neutron beam and its effectiveness for future cross-section measurements.

PHITS simulation

Simulations were conducted with the Monte-Carlo code PHITS in order analyze the performance of the neutron filtering system together with the experimental results. The results obtained from the PHITS simulations are essential in this study as they can provide information about the neutron energy distribution of any particular TOF gate. Due to the double-bunch mode, the neutron energies of the gated TOF events for a given filtered window cannot be determined directly from the experimental TOF as

Cross-section results

As the final step of the performance analysis of the neutron filtering system, the capture cross-section of 197Au was measured at the energies of 23.5, 51.5 and 127.7 keV using the Fe and Si filter assemblies with 20 cm thickness. The averaged cross-section was determined by dividing the integrated Au neutron capture yield YAu obtained by applying the PHWT, which was corrected by the self-shielding and multiple-scattering effects, by the integrated incident neutron flux ϕn, normalized using the

Conclusions

The neutron filtering system implemented at the ANNRI beamline is able to provide sharp well-defined energy neutron peaks to bypass the double-timed structure of the neutron beam using Si and Fe as filter materials. Three neutron filters assemblies consisting of 20 cm of Fe, 20 cm of Si and 30 cm of Si were analyzed in the ANNRI beamline by means of capture experiments and transmission experiments. Moreover, the experimental results were accurately reproduced by simulations with the PHITS code.

CRediT authorship contribution statement

Gerard Rovira: Conceptualization, Methodology, Software, Data curation, Investigation, Visualization, Writing - original draft. Atsushi Kimura: Supervision, Resources, Investigation. Shoji Nakamura: Supervision, Investigation, Writing - review & editing. Shunsuke Endo: Software, Resources, Data curation, Investigation. Osamu Iwamoto: Supervision, Writing - review & editing. Nobuyuki Iwamoto: Supervision. Tatsuya Katabuchi: Project administration, Funding acquisition, Supervision, Investigation,

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The neutron experiments at the MLF of the J-PARC were performed under the user program (Proposal No. 2018A0213, 2018B0195 and 2020P0100). This work is supported by the Innovative Nuclear Research and Development Program from the Ministry of Education, Culture, Sports, Science and Technology of Japan .

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1

Current address: Institute for Integrated Radiation and Nuclear Science, Kyoto University, 2 Asashiro-Nishi, Kumatori-cho, Sennan-gun, Osaka 590-0494, Japan.

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