核データニュース，No.131 (2022)

話題・解説 (II)2021 年度核データ部会賞
奨励賞

Neutron Beam Filter System for Fast Neutron Cross-Section
Measurement at the ANNRI Beamline of MLF/J-PARC
日本原子力研究開発機構
核データ研究グループ

Rovira Leveroni, Gerard
gerard.rovira@jaea.go.jp

1. Introduction
Precise nuclear data for minor actinides (MAs) and ﬁssion products (FPs) are of the utmost
important for the design of advanced nuclear facilities and, hence, incessant experimental efforts are necessary in order to continuously improve the nuclear data accuracy. The Accurate
Neutron-Nucleus Reaction Measurement Instrument (ANNRI) beamline in the Materials and Life
Science Experimental Facility (MLF) building of the Japan Proton Accelerator Research Complex(J-PARC) was designed for such endeavor. Since its inception in 2010, several measurement campaigns have been carried out using the experimental equipment currently installed at ANNRI: (1) a4π high-purity Ge detector array, (2) a NaI(Tl) spectrometer and (3) two Li-glass detectors. Thusfar, the reported accurate experimental results for neutron-induced reactions include data for theneutron total and neutron capture cross section for MAs such as 241 Am [1], 243 Am [2] and 237 Np[3, 4]; FP including 99 Tc [5] and 107 Pd [6]; and structural material, namely 93 Nb [7] and 181 Ta[8]. Current research trends, which are focused on the nuclear transmutation of high-level nuclear
waste (HLW), require further experimental data for MAs in the keV-region [9, 10]. Nonetheless,the present status of the ANNRI beamline does not allow for cross section results to be attained
without substantial ambiguities for keV-neutron induced reactions. The reason for this is the operation pattern of the J-PARC accelerator. In the current condition, the J-PARC accelerator is operated
in the double-bunch mode in which two proton bunches are shot into the spallation target with atime difference of 0.6 µs at 25 Hz. For keV neutron induced reactions, the 0.6 µs time difference
between spallation reactions is not negligible since it is within one order of magnitude of the neutron time-of-ﬂight (TOF), e.g., 100 keV neutrons have a TOF of 6.4 µs and, moreover, it is not– 17 –possible to ascertain whether the detected events were induced by neutrons originated in the primary or the delayed spallation reaction. Thus, it is imperative for the ambiguities introduced by thedouble-bunch mode to be resolved in order for accurate experimental results in the keV region tobe attainable again. In this article, the main viewpoints of the neutron ﬁltering system implemented
at ANNRI are presented. A complete assessment of the neutron ﬁltering system can be found here
[11, 12].

2. Neutron Filtering Technique
The neutron ﬁltering technique has been applied to the ANNRI beamline in order to tailor quasimonoenergetic neutron beams to bypass the double-bunch structure of the incident neutron beam.
This technique is based on employing thick slabs of materials that possess the characteristic of a
sharp minima in the neutron total cross section. By designing the ﬁltering system with the adequate thickness, neutron with energies other than that of the sharp minima are stopped in the ﬁlter
material. Hence, only neutrons with the energies of the sharp minima are able to stream throughthe ﬁlter material creating mono-energetic neutron beams. This technique has been successfully
applied to continuous neutron sources such as nuclear reactors [13, 14, 15] and in one TOF experiment [16]. In the present work, the neutron ﬁltering technique was applied to the ANNRI beamline
to resolve the double-bunch predicament in keV-neutron experiments. Three different ﬁlter conﬁgurations with 20 cm of nat Fe, 20 cm of nat Si and 15 cm nat Cr were analyzed separately withexpected ﬁltered energies of 24 keV (Fe), 54 and 144 keV (Si); and 46 and 136 keV (Cr). The
neutron total cross sections for nat Fe, nat Si and nat Cr from JENDL-4.0 [17] are displayed in Fig.
1 together with the neutron energies of the sharp minima.

Fig. 1 Neutron total cross sections of nat Fe, nat Si and nat Cr from JENDL-4.0.

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3. Experimental Setup
The performance of the neutron ﬁlter was evaluated by means of neutron capture and transmission experiments using the NaI(Tl) spectrometer and the Li-glass detectors installed in the ANNRI
beamline, respectively. The ﬁlter materials were introduced into the rotary collimator of the ANNRI beamline, right before the experimental areas. The position of the neutron ﬁlter relative to thedetectors can be seen in the schematics of the ANNRI beamline in Fig. 2.

Li-glass detectors

Fig. 2 Schematic of the ANNRI beamline.

3.1 Neutron Capture Experiments
The time distribution of the ﬁltered incident neutron ﬂux was experimentally determined bymeasuring the 478-keV γ -rays emitted from the 10 B(n,αγ )7 Li reaction in a measurement of a90%-enriched 10 B sample with a diameter of 10 mm and a thickness of 0.5 mm. This technique
is commonly used in the ANNRI beamline since this reaction emits only one γ -ray and, thus,
detected events from the 10 B(n,αγ )7 Li reaction are easy to isolate. The detected events were ﬁrst
corrected using the simulations with the PHITS code [18] taking into account the 10 B(n,αγ )7 Li
energy dependence and other sample related effects such as self-shielding and multiple scattering
and, then, normalized by means of the saturated resonance method using a measurement of a Au
sample thick enough for the ﬁrst resonance to be completely saturated. Further information about
the neutron capture experimental setup can be found here [3].

3.2 Neutron Transmission Experiments
The neutron capture experiments were complemented with neutron transmission experiments
in order to increase the reliability of the present study since this is the ﬁrst time that the neutronﬁltering technique is applied to the ANNRI beamline. Therefore, the ﬁltered neutron time distribution was experimentally determined with the Li-glass detectors as well. Two Li-glass detectors
with different concentrations of Li were employed. A GS20 detector having a 6 Li > 95% concentration was utilized to measure the ﬁltered neutron TOF spectrum by means of the 6 Li(n, α)3 H
reaction. Moreover, a GS30 detector with a concentration of 7 Li > 99% was also used to estimate
the background contribution due to other particles aside from neutrons. A complete description of
the Li-glass detectors setup of the ANNRI beamline is provided here [1].

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4. Experimental Results
The experimental neutron time distribution results were determined using the NaI(Tl) spectrometer and the Li-glass detectors installed in the ANNRI beamline. As shown in Fig. 3, there is very
good agreement with the results from both detector setups for the three ﬁlter conﬁgurations. The
only difference arises due to the different detector position relative to the spallation target. TheNaI(Tl) spectrometer is situated at a neutron ﬂight path of 27.9 m whereas the Li-glass detectors
are situated at a neutron ﬂight path of 28.6 m and, hence, results from the Li-glass detectors present
some delay relative to the NaI(Tl) spectrometer results. Nonetheless, all the expected ﬁltered peaks
for the three ﬁlter conﬁgurations of 24 keV (Fe), 54 and 144 keV (Si); and 46 and 136 keV (Cr)are clearly visible with both detector setups.
20NaI(Tl) spectrometer
Li-glass detectors

FeNeutron Intensity [n/proton shot/100 ns]

1524 keV

105075 144 keV
50SiNaI(Tl) spectrometer
Li-glass detectors

CrNaI(Tl) spectrometer
Li-glass detectors

54 keV

250136 keV

21.546 keV

10.5
0
4

5

678

9

10

111213

14

15

16

17

18

Time-of-flight [µs]Fig. 3 Filtered neutron time distribution measured with the NaI(Tl) spectrometer (black) andthe Li-glass detectors (red) for the ﬁlter conﬁgurations of 20 cm of nat Fe (top), 20 cm of nat Si(middle) and 15 cm of nat Cr (bottom).

5. Monte-Carlo Simulations
Even though the expected peaks for the three ﬁlter assemblies are clearly distinguishable fromthe background levels, the neutron energy distribution within the ﬁltered peaks cannot be directly
ascertained from the neutron TOF because the direct relationship between neutron TOF and energy
is hampered by the double-bunch mode. In order to determine the neutron energy distributions,Monte-Carlo simulations with the PHITS code were performed to reproduce the experimental neutron time distribution results. The neutron transmission ratios were simulated with PHITS and the– 20 –

results were unfolded by applying the ANNRI resolution function from the work of Kino et al. [19]using the model function proposed by Ikeda and Carpenter [20].

Fig. 4 Simulated ﬁltered neutron time distribution with PHITS (red) compared with the experimental results obtained with the NaI(Tl) spectrometer (black) for the ﬁlter conﬁgurationsof 20 cm of nat Fe (top), 20 cm of nat Si (middle) and 15 cm of nat Cr (bottom).
Two-dimensional results for the neutron time and energy distributions were obtained simultaneously from the Monte-Carlo simulations. With the aim of assessing the reliability of the simulations, the simulated neutron time distribution results were compared to those obtained from theexperimental neutron capture measurements. As can be seen in Fig. 4, the simulated results areable to accurately reproduce the experimental ﬁltered neutron time distribution results obtainedwith the NaI(Tl) spectrometer. At the same time, since neutron time and energy distributions weresimultaneously calculated and, taking into consideration that the simulated neutron time distributions precisely reproduce the experimental results, the simulated neutron energy distributions
are deemed highly reliable and, thus, were employed to determine the neutron energy distribution
within the ﬁltered peaks.
The energy distribution results for the Fe assembly are displayed in Fig. 5. A TOF gate wascreated from 12.6 to 15.0 µs to wrap the simulated events streamed through the 24 keV window
and the results showed a neutron energy distribution within the ﬁltered peak with a centroid energyof 23.5 keV. By using the neutron ﬁltering system it is clear that a much sharper neutron energy
distribution is obtained, eliminating the contribution from higher and lower energy neutrons.

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Fig. 5 (left) Simulated ﬁltered TOF spectrum by the Fe ﬁlter assembly (red) compared to theTOF from the simulation without ﬁlter (black). A TOF gate was created between 12.6 and 15.0µs (light blue). (right) Simulated energy distribution of the gated neutrons from 12.6 to 15.0
µs for the Fe ﬁlter array (blue) and the simulation with no ﬁlter (black).
Similar ﬁltering performances for the remaining ﬁltered peaks from the Si and Cr ﬁlter assemblies were obtained. The Si ﬁlter simulation yielded much sharper neutron energy distributions
with centroid energies of 127.7 keV and 51.5 keV as shown in Figs. 6 and 7. The results of 127.7
keV are much lower than the expected value of 144 keV. This is due to the inﬂuence of the nat Al
present in the beamline. nat Al possesses high values for the neutron total cross section around 140
to 150 keV and, hence, neutrons with that energy are not able to reach the sample position of theNaI(Tl) spectrometer as they interact with the nat Al present in the beamline.

Fig. 6 (left) Simulated ﬁltered TOF spectrum by the Si ﬁlter assembly (red) compared to theTOF from the simulation without ﬁlter (black). A TOF gate was created between 5.4 and 6.4µs (light blue). (right) Simulated energy distribution of the gated neutrons from 5.4 and 6.4 µs
for the Si ﬁlter array (blue) and the simulation with no ﬁlter (black).

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Fig. 7 (left) Simulated ﬁltered TOF spectrum by the Si ﬁlter assembly (red) compared to theTOF from the simulation without ﬁlter (black). A TOF gate was created between 8.6 and 9.8µs (light blue). (right) Simulated energy distribution of the gated neutrons from 8.6 and 9.8 µs
for the Si ﬁlter array (blue) and the simulation with no ﬁlter (black).
750
TOF gate
No Filter
Cr 15 cm Filter (x10)

No Filter
Cr 15 cm Filter

75500
50133.4 keV

250250

Simulated Counts [arb.]

Simulated Counts [arb.]

1000468101214
Time-of-flight [µs]16100000 120000 140000 160000 180000
Neutron Energy [eV]Fig. 8 (left) Simulated ﬁltered TOF spectrum by the Cr ﬁlter assembly (red) compared to theTOF from the simulation without ﬁlter (black). A TOF gate was created from 5.4 to 5.7 and6.0 to 6.3 µs (light blue). (right) Simulated energy distribution of the gated neutrons within 5.4
to 5.7 and 6.0 to 6.3 µs for the Si ﬁlter array (blue) and the simulation with no ﬁlter (black).
Finally, the neutron energy distribution obtained from the simulations of the Cr ﬁlter array arepresented in Figs. 8 and 9. Centroid energies of 133.4 and 45.0 keV were determined from theanalysis of the simulation results. The neutron energy distribution results are summarized in Table
1 including the obtained standard deviations for the neutron energy distribution within the eachﬁltered peak.

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100

1000
No Filter
Cr 15 cm Filter

75750

5045.0 keV

25500

250046810
1214
Time-of-flight [µs]1635000

40000

45000 50000 55000
Neutron Energy [eV]

Simulated Counts [arb.]

Simulated Counts [arb.]

TOF gate
No Filter
Cr 15 cm Filter (x10)060000Fig. 9 (left) Simulated ﬁltered TOF spectrum by the Cr ﬁlter assembly (red) compared to theTOF from the simulation without ﬁlter (black). A TOF gate was created between 9.2 to 10.5µs (light blue). (right) Simulated energy distribution of the gated neutrons from 9.2 to 10.5 µs
for the Si ﬁlter array (blue) and the simulation with no ﬁlter (black).
Table 1 Summary of the performance for the different ﬁlters analyzed.
Filter Element

Centroid Energy [keV]

Std. dev [keV]

Fe

23.5

1.4

51.5

3.9

127.7

12.5

45.0

1.9

133.4

8.1

SiCr

6. Standard Cross Section Measurement
The neutron ﬁltering system performance in neutron capture cross section experiments wasevaluated by means of standard cross section measurements. Evaluated nuclear data from the197

Au(n,γ )198 Au reaction is deemed highly reliable due to the large amount of experimental data

available. Thus, experimental techniques can be assessed by referring the experimental value forthe 197 Au(n,γ )198 Au reaction to the standard evaluated nuclear data. In the present work, the neutron capture cross section of 197 Au was determined at each of the ﬁve ﬁltered peaks tailored by theneutron ﬁltering system with centroid energies of 23.5 keV (Fe), 51.5 and 127.7 keV (Si); and 45.0
and 133.4 keV (Cr). The neutron capture cross section was calculated by creating TOF gates towrap the events of each of the ﬁltered peaks as follows:⟨σAu (Eg )⟩ =YAu (Eg )C(Eg ) 1
ϕn (Eg )nAu(1)

where < σAu (Eg ) > is the averaged neutron capture cross-section for each of the ﬁve TOF gates(Eg ); YAu (Eg ) and C(Eg ) are the Au neutron capture yield and the correction coefﬁcient for
self-shielding and multiple-scattering for each TOF gate (Eg ), respectively. ϕn (Eg ) stands for the– 24 –

normalized incident neutron ﬂux at the TOF gate (Eg ) and nAu sample area density in at/barn.

1.2197
IAEA Standard - 197
Au(n, γ)JENDL-4.0 Au(n, γ)
Lederer (2011)
Massimi (2014)
Present - Si Filter
Present - Fe Filter
Present - Cr Filter

Cross section [b]

10.8
0.6
0.4
0.2
10100
Neutron Energy [keV]

Fig. 10 Neutron capture cross-section results for 197 Au using the ﬁlter conﬁgurations of 15 cmof Cr (45.0 and 133.4 keV), 20 cm of Fe (23.5 keV) and 20 cm of Si (51.5 and 127.7 keV) asﬁlter material compared with the evaluated data from JENDL-4.0 and the IAEA standard datalibrary with uncertainty; and with the experimental data reported by Ledererd and Massimi.
The results for the neutron capture cross-section of 197 Au are shown in Fig. 10 together with theevaluated data from JENDL-4.0 [17] and the IAEA standard cross section[21]: and the reported
data from Lederer [22] and Massimi[23]. The x-axis bars mean the standard deviation displayed inTable 1. The present cross-section results are consistent with the experimental data from Ledererand Massimi and agree within one-sigma uncertainty with both evaluated nuclear data values fromJENDL-4.0 and IAEA standard cross section library. The present results provide evidence that
keV-neutron capture cross section measurements are attainable at the ANNRI beamline by using
the neutron ﬁltering system to bypass the double-bunch structure of the incident neutron beam.

7. Conclusions
A neutron ﬁltering system has been installed at the ANNRI beamline in order to accurately measure neutron-induced cross sections in the keV region avoiding the ambiguities introduced by thedouble-bunch structure of the incident neutron beam. The neutron ﬁltering system consists of aseries of thick cylinders made of materials that possess the characteristic of sharp minima in theneutron total cross section. In the present study, the ﬁltering performance of three ﬁlter conﬁgurations consisting of 20 cm of nat Fe, 20 cm of nat Si and 15 cm of nat Cr was experimentally examined– 25 –by means of neutron capture and transmission experiments. These results were complemented byMonte-Carlo simulations with the PHITS code. These ﬁlter conﬁgurations are able to mold theincident neutron ﬂux into sharp neutron peaks with centroid energies of 23.5 keV (Fe), 51.5 and127.7 keV (Si); and 45.0 and 133.4 keV (Cr). Moreover, the effectiveness in keV-neutron capture
cross section measurements was validated by the good agreement of the present experimental results of the 197 Au(n,γ )198 Au reaction using the neutron ﬁltering system with the evaluated nuclear
data values from JENDL-4.0 and the IAEA standard cross section library, together with the reported data of Lederer and Massimi. Thus, in this work, the neutron ﬁltering technique was proved
to be a suitable solution to avoid the inﬂuence of the double-bunch structure of the incident neutron
ﬂux in order to accurately measure keV-neutron-induced reactions.

8. Acknowledgments
The neutron experiments at the MLF of the J-PARC were performed under the user program(Proposal No. 2018A0213, 2018B0195, 2020P0100 and 2020A0273). This work is supportedby the Innovative Nuclear Research and Development Program from the Ministry of Education,Culture, Sports, Science and Technology of Japan. Grant Number: JPMXD0217942969.

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