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The benchmark experiment on slab iron with D-T neutrons for validation of evaluated nuclear data
Annals of Nuclear Energy 132 (2019) 236–242
Contents lists available at ScienceDirect
Annals of Nuclear Energy journal homepage: www.elsevier.com/locate/anucene
The benchmark experiment on slab iron with D-T neutrons for validation of evaluated nuclear data Yanyan Ding, Yangbo Nie ⇑, Jie Ren, Xichao Ruan, Hanxiong Huang, Jie Bao, Haicheng Wu Key Laboratory of Nuclear Data, China Institute of Atomic Energy, Beijing 102413, China
a r t i c l e
i n f o
Article history: Received 24 December 2018 Received in revised form 16 March 2019 Accepted 24 April 2019 Available online 28 April 2019 Keywords: Iron Benchmark experiment Neutron leakage spectrum TOF
a b s t r a c t An experimental system for benchmark validation of nuclear data with slab samples has been set up at China Institute of Atomic Energy (CIAE). Neutron leakage spectra in the range of 0.8–16 MeV from iron slab samples were measured by time-of-flight technique using a D-T neutron source with the measured angle at 60° and 120°. The thicknesses of the slabs were chosen to be 5 cm, 10 cm and 15 cm. The experimental results were compared with the calculated ones by MCNP-4c simulation, using the evaluated data of iron from the CENDL-3.1, ENDF/B-VII.1 and JENDL-4.0 libraries. From the comparison of neutron spectra and the C/Es between the measured and the calculated results, it has been found that some discrepancies in the major reaction channels including (n, p), (n, n0 ) and (n, 2n) are existing. Detailed information about the neutron product distribution with the evaluated data has been presented to explain the discrepancies which turn out to the data of the above three libraries that needed to be improved as required in different energy ranges. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction The benchmark experimental study on the fusion neutronics plays an important role for validating the accuracy of the evaluated nuclear data, especially the elements that are of interests in nuclear devices, fission and fusion reactors for technologies. Fe is one of such elements that can be used as structural material of nuclear device (Oyama et al., 1993). However, its available experimental data are limited and benchmarking of the evaluated nuclear data libraries is necessary. In order to validate currently available nuclear data files (CENDL-3.1 (Ge et al., 2011), ENDF/B-VII.1 (Chadwick et al., 2011) and JENDL-4.0 (Shibata, et al., 2011)) for iron, the leakage neutron spectra from iron slab samples were measured and calculated at CIAE by using the integral benchmark facility, which has been successfully used in a series of benchmark experiments, such as Uranium (Nie et al., 2010), natural Gallium (Han et al., 2015), Beryllium (Nie et al., 2016), Tungsten (Lin et al., 2016). The neutron leakage spectra from iron samples with the area of 30 cm 30 cm and the thickness of 5 cm, 10 cm and 15 cm were measured at 60° and 120° by time-of-flight technique with a BC501A scintillation detector from 16 MeV down to about 0.8 MeV. The results were compared with the calculations using a Monte Carlo code MCNP-
⇑ Corresponding author. E-mail address: [email protected] (Y. Nie). https://doi.org/10.1016/j.anucene.2019.04.041 0306-4549/Ó 2019 Elsevier Ltd. All rights reserved.
4c (Briesmeister, 2000). The comparison was made in both the leakage neutron spectrum and the calculation-to-experiment ratio (C/E) of the spectrum integrated over four regions. Description of the experimental arrangement, the measurements, the simulations and the results will be given in the following sections.
2. Experimental setup The experiment was performed at the pulsed neutron generator at CIAE. A schematic view of the experimental arrangement is shown in Fig. 1. Deuteron (D+) beam was accelerated by a 300 keV CockcroftWalton type accelerator and bombarded on a Tritium-Titanium (T-Ti) target to produce neutrons by the T(d, n)4He fusion reaction. A silicon detector positioned at 135° with respect to the D+ beam (Dash 1) was used to monitor the neutron yield by counting the associated 4He particles from D-T reaction. The average neutron yield was about 2 109n/s. A stilbene scintillation crystal (Monitor 1) of 5.08 cm in diameter was placed at about 8 m from the T-Ti target along with the D+ beam line for monitoring the neutron pulse shape spectra. Another BaF2 scintillation crystal (Monitor 2), which was located at about 4 m from the T-Ti target in a direction perpendicular to the D+ beam, was used to match the stilbene scintillation crystal to monitor the neutron source pulse shape. The neutron spectra leaking from slab samples were detected by a BC501A liquid scintillation
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Fig. 1. Experimental arrangement for measuring the neutron leakage spectra from iron slab.
detector (Neutron detector), which was located behind the concrete wall at about 8.15 m flight path (the distance between the D+ beam line to the detector) in a direction perpendicular to the D+ beam line. The detector was installed in a lead house with a size of 5.08 cm in diameter and 2.54 cm in length. In front of the neutron detector, a collimator, which was embedded inside the concrete wall of 200 cm thick with a hole of 10 cm diameter, was set to shield the neutron detector from background neutrons. A pre-collimator, which was made of iron, polyethylene and lead, was also placed between the sample and the detector to reduce the neutron background. Another 90 cm long shadow bar of Cu was also installed to eliminate the direct fast neutrons from the T-Ti source. Using such a heavy shielding and collimating system, a high foreground/background ratio has been achieved (Cai et al., 2013). Before the iron experiment, a group of
Fig. 3. Experimental neutron leakage time spectra measured at 60°.
Fig. 2. Leakage neutron spectrum from polyethylene sample at 60°.
experiments with polyethylene samples were performed to ensure the experimental system was reliable. Fig. 2 shows the measured leakage neutron TOF spectrum comparing to the calculated one with a polyethylene sample at 60°. This result indicates that the experimental apparatus and the data analysis procedures work well. The iron slab samples with surface area of 30 cm 30 cm were used in the experiment and the slab thicknesses were chosen to be 5 cm, 10 cm and 15 cm, corresponding to 1, 2 and 3 mean free path for 14.5 MeV neutron, respectively. The sample purity and density were 99.9% and 7.86 g/cm3. The measuring angle can be changed by moving the sample along the Dash 2 line in Fig. 1. In order to make the background correction, two runs of measurement with sample in (foreground) and sample out (background) were per-
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formed for each set up. The neutron leakage spectra measured at 60° are shown in Fig. 3. The time spectra with sample in and sample out are shown respectively. The background corrected spectrum was obtained by subtracting from foreground to background. A simplified block diagram of the electronic circuit for the present experiment is shown in Fig. 4. All events from the detector were recorded by a list mode in event by event basis using a CAMAC data acquisition system. For each event, there are three parameters Pulse Height (PH), Pulse Shape Discrimination (PSD) and Time of Flight (TOF). PH and PSD are used for the detection threshold determination and n-c discrimination, respectively, in the offline analysis. The uncertainties of the experiment mainly consist of statistical and systematic errors. The statistical uncertainties were mainly caused by the normalization calibration (2%). The systematic errors were mainly caused by the relative neutron detection efficiency (3%), and the scattering angle ambiguity (1%). Therefore overall uncertainty is 5% in addition to the statistical errors. 3. Monte Carlo simulation MCNP is a general-purpose Monte Carlo N-Particle code developed by Los Alamos National Laboratory for simulating the transport of neutron, photo and electron through matter. For comparison of the experimental results with the calculation, the neutron leakage spectra were simulated by the MCNP-4c code using the evaluated data of iron from the CENDL-3.1, ENDF/B-VII.1 and JENDL-4.0 libraries. The ACE libraries were generated by the nuclear data processing code NJOY99 (Macfarane and Muri, 2000). In the MCNP simulation, the experimental configuration was modeled in detail, and the detailed experimental parameters were taken into account. These parameters included the neutron energy distribution of the source neutron, the neutron detection efficiency, and the time structure of the pulsed beam. A ring detector estimator was used to tally the leakage neutron. The calculations for two angles of 60° and 120° were performed with the model shown in Fig. 5. The left sample was filled with air when the simulation was performed for 60°, while the right sample was filled with air for 120° simulation. For the background simulation, both samples were filled with air. The neutron histories adopted were 109 and the statistical uncertainties of each time bin were smaller than 1%. Both of the angular distribution and angle dependent energy distribution of the source neutrons were calculated by the TARGET
code (Schlegel, 2005) with the 300 keV deuteron beam energy and the real target geometry, the calculated results were incorporated with MCNP calculation. The efficiency of the detector was calculated with NEFF code (Dietze and Klein, 1982), using a known light output function which was calibrated at the HI-13 Tandem Accelerator by pulsed 9 Be (d,n)10B reaction source at CIAE. The calculated efficiency was verified by measuring the neutron time-of-flight spectrum from a 252Cf spontaneous fission neutron source. Fig. 6 shows the efficiency curves at different detection thresholds used in present work. The time response (TME) was obtained by a combination of the neutron time-of-flight spectrum and the c time-of-flight spectrum, which were measured by the stilbene detector and BaF2 detector, respectively. 4. Results and discussion Fig. 7 shows the measured and calculated neutron leakage spectra at 60° and 120°. It seems that the calculation results with different libraries are quite different from the experimental results in the high-energy region. In this work, since we are focusing on the neutron emission channels, the contribution of different reaction channels to the total reaction cross sections are plotted as a function of the leakage neutron energy for iron slab sample. As shown in Fig. 8, at 12– 16 MeV, the contribution to the total energy spectra mainly comes from the elastic scattering; The (n, n0 ) cross section includes the discrete level inelastic scattering dominates at 8.5 MeV < En < 12 MeV, and the continuum inelastic scattering at 3–8.5 MeV, respectively. At 0.8–3 MeV, the compound nucleus contribution of the (n, 2n) process is dominated and the continuum inelastic scattering show minor contributions. Fig. 9 shows the leakage neutron spectra calculated with different evaluated data libraries comparing with the experimental results. Ration of calculation-to-experiment (C/E) for integrated neutron fluxes over four energy regions are plotted in the bottom of the same figures which can also be seen in Table.1. From these results, the following observations are made: 1. In the 12–16 MeV energy range, the major contribution in the neutron spectrum comes from the elastic scattering and a sharp elastic peak is observed. The simulated yield of the elastic scattering neutrons depends on the choice of evaluated nuclear libraries. The C/Es of the integrated neutron in this range are
Fig. 4. Block diagram of the electronic circuit for the present experiment.
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Fig. 5. Model for the MCNP calculations.
the experimental ones in less than 8% of discrepancies. The calculated spectra with the ENDF/B-VII.1 library show a much better agreement with the experimental ones in less than 3% of discrepancies at 60°, but significantly overpredict by 37% at 120°. The calculated spectra with the JENDL-4.0 library are higher than the experimental ones.
Fig. 6. The detection efficiency with different thresholds.
quite different as shown in Fig. 9(e)–(h), where the calculated spectra with the evaluated data of the ENDF/B-VII.1 are largely overestimated, especially at 120°, while those of the JENDL-4.0 are underestimated. The results from the CENDL-3.1 show better agreements in less than 5% of discrepancies. 2. In the 8.5–12 MeV neutron energy range, the contribution originates from discrete inelastic scatterings. As shown in Fig. 9(e)– (h), the C/Es of three libraries differ greatly from the experimental results. The results from the CENDL-3.1 library agree with
Fig. 8. The contributions to the total energy spectra from the elastic, inelastic and (n, 2n) for iron at incident neutron energy of 14.5 MeV in ENDF/B-VII.1.
Fig. 7. Leakage neutron spectra measured in the experiment and simulated by MCNP code from iron slab of 5-cm-thick, 10-cm-thick and 15-cm-thick. (a) at 60°; (b) at 120°.
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Fig. 9. Comparison of experimental and calculated neutron leakage spectra (a)–(d) and ratio C/E (e)–(h). (a) and (e) with 143 keV electron equivalent threshold at 60°; (b) and (f) with 143 keV electron equivalent threshold at 120°; (c) and (g) with 477 keV electron equivalent threshold at 60°;(d) and (h) with 477 keV electron equivalent threshold at 120°.
3. In the 3–8.5 MeV neutron energy range, the calculated spectra with the JENDL-4.0 library give agreement with the experimental ones at 60°, but slightly underestimate by 13% at 120°. The predictions based on both of the ENDF/B-VII.1 and CENDL-3.1 are better than those of the JENDL-4.0 in this region. 4. In the 0.8–3 MeV, the (n, 2n) reaction makes contribution to the neutron spectra. As shown in Fig. 10, the C/Es are obviously affected by the change of the energy threshold. When the energy threshold is 0.7 Cs (147 keV), the simulated values of the ENDF/B-VII.1 and JENDL-4.0 libraries are in good agreement
with the experimental values within 2% and 3%, respectively, while those of the CENDL-3.1 slightly overpredict by 7%; when the energy threshold turns to be 1.0 Cs (477KeV), the calculated results with the ENDF/B-VII.1 and CENDL-3.1 libraries are higher than the experimental ones, especially overpredict by 19% at 120°. In order to find out the reason for discrepancies between the calculated spectra with the different evaluated data libraries and the measured ones, the total and several partial cross sections in
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Table 1 The C/E values of the spectra integrated over four energy regions for iron slab of 10cm-thick at 60° and 120°. Energy (MeV)
C/E(ENDF/B-VII.1)
C/E(CENDL-3.1)
C/E(JENDL-4.0)
60° 12.0–16 8.5–12 3–8.5 0.8–3
1.247 ± 0.606 0.962 ± 0.039 1.131 ± 0.047 1.111 ± 0.046
1.044 ± 0.042 0.871 ± 0.032 0.988 ± 0.036 1.241 ± 0.058
1.000 ± 0.039 1.224 ± 0.063 1.016 ± 0.038 1.072 ± 0.043
120° 12.0–16 8.5–12 3–8.5 0.8–3
1.394 ± 0.078 1.359 ± 0.077 1.061 ± 0.041 1.127 ± 0.047
0.986 ± 0.039 0.949 ± 0.038 0.947 ± 0.033 1.197 ± 0.053
0.836 ± 0.028 1.274 ± 0.068 0.870 ± 0.028 1.019 ± 0.038
Fig. 11. The angular distribution of elastic cross section for 56Fe at the incident neutron energy of 14.5 MeV retrieved from ENDF/B-VII.1 (line), CENDL-3.1 (dash) and JENDL-4.0 (dot) libraries.
Fig. 12. The neutron spectra of 56Fe(n, n0 )C at incident neutron energy of 14.5 MeV reactions retrieved from ENDF/B-VII.1 (line), CENDL-3.1 (dash) and JENDL-4.0 (dot) libraries. Fig. 10. The variation of the C/E values with energy threshold for iron slab at 60°. (a) 5-cm-thick; (b) 10-cm-thick; (c) 15-cm-thick.
the evaluated nuclear data libraries are studied. The emission neutrons of the (n, el), (n, n0 ) and (n, 2n) reaction channel make contributions to the leakage neutron spectra of the present work. 1. The angular distribution of the neutron elastic scattering for 56 Fe at the incident neutron energy of 14.5 MeV in the evaluated data libraries are shown in Fig. 11. The cross sections from the ENDF/B-VII.1 are higher than those from the CENDL-3.1 and JENDL-4.0 at both 60°and 120°, which caused the differences in elastic scattering energy region of the neutron spectra. These observations indicate that the discrepancies of the elastic scattering neutron yields in the MCNP simulations using the ENDF/ B-VII.1, CENDL-3.1 and JENDL-4.0 libraries originate simply from the differences in the angular distribution of elastic cross section at 60°and 120° in these libraries. 2. The contributions to the total energy spectra from the continuum inelastic scattering for 56Fe at incident neutron energy of 14.5 MeV in the ENDF/B-VII.1, CENDL-3.1 and JENDL-4.0
Fig. 13. The neutron spectra from (n, 2n) reactions at the incident neutron energy of 14.5 MeV retrieved from ENDF/B-VII.1 (line), CENDL-3.1 (dash) and JENDL-4.0 (dot).
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libraries are shown in Fig. 12. At the energy range of 8.5– 11 MeV, there is a small but clear peak observed in the JENDL-4.0 libraries, which is quite different from another two libraries. The JENDL-4.0 cross sections are considerably higher than the ENDF/B-VII.1 and CENDL-3.1, which caused the discrepancies between the measured spectra and the calculated ones in 8.5–12 MeV energy range. 3. The contributions to the total energy spectra from the (n, 2n) reaction for 56Fe at incident neutron energy of 14.5 MeV in the ENDF/B-VII.1, CENDL-3.1 and JENDL-4.0 libraries are shown in Fig. 13. The contribution of (n, 2n) reaction in the CENDL-3.1 is considerably higher than the ENDF/B-VII.1 above 1.2 MeV and lower from 0.5 to 1.2 MeV, where the calculations with the CENDL-3.1 library give significant disagreement with the experimental ones.
overall, further adjustment for the CENDL-3.1 evaluated library will be performed in the future. Acknowledgments The authors gratefully thank the operational staff in CockcroftWalton type accelerator, China Institute of Atomic Energy, for their excellent operation of the D-T neutron source. This work is supported by the National Natural Science Foundation of China (Grant No. 11505297) and (Grant No. 11775311). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.anucene.2019.04.041. References
5. Summary In this work, the validations of the evaluated nuclear data for iron were performed, using the leakage neutron spectrum measured by time-of-flight technique. The measured neutron energy was from 0.8 to 16 MeV. The total uncertainty for the experimental data was below 5%. The theoretical calculations were carried out by MCNP-4c Monte Carlo code using the evaluated nuclear data of the CENDL-3.1, ENDF/B-VII.1, and JENDL-4.0 libraries. The measured spectra can be well reproduced as a whole by the simulations with the three evaluated nuclear data libraries, while some discrepancies in the four separated energy ranges, corresponding to (n, el), (n, n0 ) and (n, 2n) reactions, are found from the comparison. From the comparison, it has been found that in general the calculated neutron spectra with the CENDL-3.1 library show better agreement with the measured ones than those with ENDF/B-VII.1 and JENDL-4.0 from the energy range about 3 to 16 MeV. While in the energy range from 0.8 to 3 MeV, the spectra with the CENDL-3.1 were overestimated. These discrepancies are considered as caused by the improper evaluation of the angular distribution and secondary neutron energy distribution of the elastic, inelastic scattering and (n, 2n) reaction channels in evaluated nuclear data libraries. In conclusion, the data of the three libraries need to be improved as required in different energy ranges. This work provides valuable data for benchmarking the evaluated nuclear data for 56Fe, since the iron data from the CENDL-3.1 coincide with the experiment data most
Briesmeister. J, 2000. MCNP-A general Monte Carlo N-particle transport code system, Version 4C, Report LA 13709-M(2000) Los Alamos. Cai, Xinggang, Nie, Yangbo, et al., 2013. Design of a pre-collomator system for neutronics benchmark experiment. Nucl. Tech. 01, 8–13. Chadwick, M.B., Herman, M., et al., 2011. ENDF/B-VII.1 nuclear data for science and technology: cross sections, covariances, fission product yields and decay data. Nucl. Data Sheets 112, 2887–2996. https://doi.org/10.1016/j.nds.2011.11.002. Dietze, G., Klein, H., 1982. NRESP4 and NEFF4 Monte Carlo Code for the Calculation of Neutron Response Functions and Detection Efficiencies for NE213 Scintillation Detectors. Physikalisch-Technische Bundesanstalt, Braunschweig, Germany. Ge, Z.G., Zhao, Z.X., Xia, H.H., et al., 2011. The updated version of Chinese evaluated nucleqar data library(CENDL-3.1). J. Korean Phys. Soc. 59, 1052–1056. https:// doi.org/10.3938/jkps.59.1052. Han, R., Wada, R., et al., 2015. Fast neutron scattering on Gallium target at 14.8 MeV. Nucl. Phys. A 936, 17–28. Lin, Zuokang, Nie, Yangbo, et al., 2016. Benchmarking of 232Th evaluation by a 14.8 MeV neutron leakage spectra experiment with slab samples. Ann. Nucl. Energy 96, 181–186. Macfarane, R.E., Muri, D.W., 2000. NJOY99.0 Code System for Producing Point wise and Multigroup Neutron and Photon Cross Section from ENDF/B Data. Los Alamos National Laboratory, LA-12740-M. Nie, Yangbo, Bao, Jie, et al., 2010. Benchmarking of evaluated nuclear data for uranium by a 14.8 MeV neutron leakage spectra experiment with slab sample. Ann. Nucl. Energy 37, 1456–1460. Nie, Y., Ren, J., et al., 2016. The benchmark experiment on slab beryllium with D-T neutrons for validation of evaluated nuclear data. Fusion Eng. Des. 105, 8–14. Oyama, Yukio, Kosako, Kazuaki, et al., 1993. Measurement and calculation of angular neutron flux spectra from iron slabs bombarded with 14.8mev neutrons. Nucl. Sci. Eng. 115, 24–37. Schlegel, D., 2005. Target User’s Manual. Physikalisch-Technische Bundesanstalt, Braunschweig, Germany. Shibata, K., Iwamoto, O., et al., 2011. JENDL-4.0: a new library for nuclear science and engineering. J. Nucl. Sci. Technol. 48, 1–30. https://doi.org/10.1080/ 18811248.2011.9711675.
Contents lists available at ScienceDirect
Annals of Nuclear Energy journal homepage: www.elsevier.com/locate/anucene
The benchmark experiment on slab iron with D-T neutrons for validation of evaluated nuclear data Yanyan Ding, Yangbo Nie ⇑, Jie Ren, Xichao Ruan, Hanxiong Huang, Jie Bao, Haicheng Wu Key Laboratory of Nuclear Data, China Institute of Atomic Energy, Beijing 102413, China
a r t i c l e
i n f o
Article history: Received 24 December 2018 Received in revised form 16 March 2019 Accepted 24 April 2019 Available online 28 April 2019 Keywords: Iron Benchmark experiment Neutron leakage spectrum TOF
a b s t r a c t An experimental system for benchmark validation of nuclear data with slab samples has been set up at China Institute of Atomic Energy (CIAE). Neutron leakage spectra in the range of 0.8–16 MeV from iron slab samples were measured by time-of-flight technique using a D-T neutron source with the measured angle at 60° and 120°. The thicknesses of the slabs were chosen to be 5 cm, 10 cm and 15 cm. The experimental results were compared with the calculated ones by MCNP-4c simulation, using the evaluated data of iron from the CENDL-3.1, ENDF/B-VII.1 and JENDL-4.0 libraries. From the comparison of neutron spectra and the C/Es between the measured and the calculated results, it has been found that some discrepancies in the major reaction channels including (n, p), (n, n0 ) and (n, 2n) are existing. Detailed information about the neutron product distribution with the evaluated data has been presented to explain the discrepancies which turn out to the data of the above three libraries that needed to be improved as required in different energy ranges. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction The benchmark experimental study on the fusion neutronics plays an important role for validating the accuracy of the evaluated nuclear data, especially the elements that are of interests in nuclear devices, fission and fusion reactors for technologies. Fe is one of such elements that can be used as structural material of nuclear device (Oyama et al., 1993). However, its available experimental data are limited and benchmarking of the evaluated nuclear data libraries is necessary. In order to validate currently available nuclear data files (CENDL-3.1 (Ge et al., 2011), ENDF/B-VII.1 (Chadwick et al., 2011) and JENDL-4.0 (Shibata, et al., 2011)) for iron, the leakage neutron spectra from iron slab samples were measured and calculated at CIAE by using the integral benchmark facility, which has been successfully used in a series of benchmark experiments, such as Uranium (Nie et al., 2010), natural Gallium (Han et al., 2015), Beryllium (Nie et al., 2016), Tungsten (Lin et al., 2016). The neutron leakage spectra from iron samples with the area of 30 cm 30 cm and the thickness of 5 cm, 10 cm and 15 cm were measured at 60° and 120° by time-of-flight technique with a BC501A scintillation detector from 16 MeV down to about 0.8 MeV. The results were compared with the calculations using a Monte Carlo code MCNP-
⇑ Corresponding author. E-mail address: [email protected] (Y. Nie). https://doi.org/10.1016/j.anucene.2019.04.041 0306-4549/Ó 2019 Elsevier Ltd. All rights reserved.
4c (Briesmeister, 2000). The comparison was made in both the leakage neutron spectrum and the calculation-to-experiment ratio (C/E) of the spectrum integrated over four regions. Description of the experimental arrangement, the measurements, the simulations and the results will be given in the following sections.
2. Experimental setup The experiment was performed at the pulsed neutron generator at CIAE. A schematic view of the experimental arrangement is shown in Fig. 1. Deuteron (D+) beam was accelerated by a 300 keV CockcroftWalton type accelerator and bombarded on a Tritium-Titanium (T-Ti) target to produce neutrons by the T(d, n)4He fusion reaction. A silicon detector positioned at 135° with respect to the D+ beam (Dash 1) was used to monitor the neutron yield by counting the associated 4He particles from D-T reaction. The average neutron yield was about 2 109n/s. A stilbene scintillation crystal (Monitor 1) of 5.08 cm in diameter was placed at about 8 m from the T-Ti target along with the D+ beam line for monitoring the neutron pulse shape spectra. Another BaF2 scintillation crystal (Monitor 2), which was located at about 4 m from the T-Ti target in a direction perpendicular to the D+ beam, was used to match the stilbene scintillation crystal to monitor the neutron source pulse shape. The neutron spectra leaking from slab samples were detected by a BC501A liquid scintillation
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Fig. 1. Experimental arrangement for measuring the neutron leakage spectra from iron slab.
detector (Neutron detector), which was located behind the concrete wall at about 8.15 m flight path (the distance between the D+ beam line to the detector) in a direction perpendicular to the D+ beam line. The detector was installed in a lead house with a size of 5.08 cm in diameter and 2.54 cm in length. In front of the neutron detector, a collimator, which was embedded inside the concrete wall of 200 cm thick with a hole of 10 cm diameter, was set to shield the neutron detector from background neutrons. A pre-collimator, which was made of iron, polyethylene and lead, was also placed between the sample and the detector to reduce the neutron background. Another 90 cm long shadow bar of Cu was also installed to eliminate the direct fast neutrons from the T-Ti source. Using such a heavy shielding and collimating system, a high foreground/background ratio has been achieved (Cai et al., 2013). Before the iron experiment, a group of
Fig. 3. Experimental neutron leakage time spectra measured at 60°.
Fig. 2. Leakage neutron spectrum from polyethylene sample at 60°.
experiments with polyethylene samples were performed to ensure the experimental system was reliable. Fig. 2 shows the measured leakage neutron TOF spectrum comparing to the calculated one with a polyethylene sample at 60°. This result indicates that the experimental apparatus and the data analysis procedures work well. The iron slab samples with surface area of 30 cm 30 cm were used in the experiment and the slab thicknesses were chosen to be 5 cm, 10 cm and 15 cm, corresponding to 1, 2 and 3 mean free path for 14.5 MeV neutron, respectively. The sample purity and density were 99.9% and 7.86 g/cm3. The measuring angle can be changed by moving the sample along the Dash 2 line in Fig. 1. In order to make the background correction, two runs of measurement with sample in (foreground) and sample out (background) were per-
238
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formed for each set up. The neutron leakage spectra measured at 60° are shown in Fig. 3. The time spectra with sample in and sample out are shown respectively. The background corrected spectrum was obtained by subtracting from foreground to background. A simplified block diagram of the electronic circuit for the present experiment is shown in Fig. 4. All events from the detector were recorded by a list mode in event by event basis using a CAMAC data acquisition system. For each event, there are three parameters Pulse Height (PH), Pulse Shape Discrimination (PSD) and Time of Flight (TOF). PH and PSD are used for the detection threshold determination and n-c discrimination, respectively, in the offline analysis. The uncertainties of the experiment mainly consist of statistical and systematic errors. The statistical uncertainties were mainly caused by the normalization calibration (2%). The systematic errors were mainly caused by the relative neutron detection efficiency (3%), and the scattering angle ambiguity (1%). Therefore overall uncertainty is 5% in addition to the statistical errors. 3. Monte Carlo simulation MCNP is a general-purpose Monte Carlo N-Particle code developed by Los Alamos National Laboratory for simulating the transport of neutron, photo and electron through matter. For comparison of the experimental results with the calculation, the neutron leakage spectra were simulated by the MCNP-4c code using the evaluated data of iron from the CENDL-3.1, ENDF/B-VII.1 and JENDL-4.0 libraries. The ACE libraries were generated by the nuclear data processing code NJOY99 (Macfarane and Muri, 2000). In the MCNP simulation, the experimental configuration was modeled in detail, and the detailed experimental parameters were taken into account. These parameters included the neutron energy distribution of the source neutron, the neutron detection efficiency, and the time structure of the pulsed beam. A ring detector estimator was used to tally the leakage neutron. The calculations for two angles of 60° and 120° were performed with the model shown in Fig. 5. The left sample was filled with air when the simulation was performed for 60°, while the right sample was filled with air for 120° simulation. For the background simulation, both samples were filled with air. The neutron histories adopted were 109 and the statistical uncertainties of each time bin were smaller than 1%. Both of the angular distribution and angle dependent energy distribution of the source neutrons were calculated by the TARGET
code (Schlegel, 2005) with the 300 keV deuteron beam energy and the real target geometry, the calculated results were incorporated with MCNP calculation. The efficiency of the detector was calculated with NEFF code (Dietze and Klein, 1982), using a known light output function which was calibrated at the HI-13 Tandem Accelerator by pulsed 9 Be (d,n)10B reaction source at CIAE. The calculated efficiency was verified by measuring the neutron time-of-flight spectrum from a 252Cf spontaneous fission neutron source. Fig. 6 shows the efficiency curves at different detection thresholds used in present work. The time response (TME) was obtained by a combination of the neutron time-of-flight spectrum and the c time-of-flight spectrum, which were measured by the stilbene detector and BaF2 detector, respectively. 4. Results and discussion Fig. 7 shows the measured and calculated neutron leakage spectra at 60° and 120°. It seems that the calculation results with different libraries are quite different from the experimental results in the high-energy region. In this work, since we are focusing on the neutron emission channels, the contribution of different reaction channels to the total reaction cross sections are plotted as a function of the leakage neutron energy for iron slab sample. As shown in Fig. 8, at 12– 16 MeV, the contribution to the total energy spectra mainly comes from the elastic scattering; The (n, n0 ) cross section includes the discrete level inelastic scattering dominates at 8.5 MeV < En < 12 MeV, and the continuum inelastic scattering at 3–8.5 MeV, respectively. At 0.8–3 MeV, the compound nucleus contribution of the (n, 2n) process is dominated and the continuum inelastic scattering show minor contributions. Fig. 9 shows the leakage neutron spectra calculated with different evaluated data libraries comparing with the experimental results. Ration of calculation-to-experiment (C/E) for integrated neutron fluxes over four energy regions are plotted in the bottom of the same figures which can also be seen in Table.1. From these results, the following observations are made: 1. In the 12–16 MeV energy range, the major contribution in the neutron spectrum comes from the elastic scattering and a sharp elastic peak is observed. The simulated yield of the elastic scattering neutrons depends on the choice of evaluated nuclear libraries. The C/Es of the integrated neutron in this range are
Fig. 4. Block diagram of the electronic circuit for the present experiment.
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Fig. 5. Model for the MCNP calculations.
the experimental ones in less than 8% of discrepancies. The calculated spectra with the ENDF/B-VII.1 library show a much better agreement with the experimental ones in less than 3% of discrepancies at 60°, but significantly overpredict by 37% at 120°. The calculated spectra with the JENDL-4.0 library are higher than the experimental ones.
Fig. 6. The detection efficiency with different thresholds.
quite different as shown in Fig. 9(e)–(h), where the calculated spectra with the evaluated data of the ENDF/B-VII.1 are largely overestimated, especially at 120°, while those of the JENDL-4.0 are underestimated. The results from the CENDL-3.1 show better agreements in less than 5% of discrepancies. 2. In the 8.5–12 MeV neutron energy range, the contribution originates from discrete inelastic scatterings. As shown in Fig. 9(e)– (h), the C/Es of three libraries differ greatly from the experimental results. The results from the CENDL-3.1 library agree with
Fig. 8. The contributions to the total energy spectra from the elastic, inelastic and (n, 2n) for iron at incident neutron energy of 14.5 MeV in ENDF/B-VII.1.
Fig. 7. Leakage neutron spectra measured in the experiment and simulated by MCNP code from iron slab of 5-cm-thick, 10-cm-thick and 15-cm-thick. (a) at 60°; (b) at 120°.
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Fig. 9. Comparison of experimental and calculated neutron leakage spectra (a)–(d) and ratio C/E (e)–(h). (a) and (e) with 143 keV electron equivalent threshold at 60°; (b) and (f) with 143 keV electron equivalent threshold at 120°; (c) and (g) with 477 keV electron equivalent threshold at 60°;(d) and (h) with 477 keV electron equivalent threshold at 120°.
3. In the 3–8.5 MeV neutron energy range, the calculated spectra with the JENDL-4.0 library give agreement with the experimental ones at 60°, but slightly underestimate by 13% at 120°. The predictions based on both of the ENDF/B-VII.1 and CENDL-3.1 are better than those of the JENDL-4.0 in this region. 4. In the 0.8–3 MeV, the (n, 2n) reaction makes contribution to the neutron spectra. As shown in Fig. 10, the C/Es are obviously affected by the change of the energy threshold. When the energy threshold is 0.7 Cs (147 keV), the simulated values of the ENDF/B-VII.1 and JENDL-4.0 libraries are in good agreement
with the experimental values within 2% and 3%, respectively, while those of the CENDL-3.1 slightly overpredict by 7%; when the energy threshold turns to be 1.0 Cs (477KeV), the calculated results with the ENDF/B-VII.1 and CENDL-3.1 libraries are higher than the experimental ones, especially overpredict by 19% at 120°. In order to find out the reason for discrepancies between the calculated spectra with the different evaluated data libraries and the measured ones, the total and several partial cross sections in
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Table 1 The C/E values of the spectra integrated over four energy regions for iron slab of 10cm-thick at 60° and 120°. Energy (MeV)
C/E(ENDF/B-VII.1)
C/E(CENDL-3.1)
C/E(JENDL-4.0)
60° 12.0–16 8.5–12 3–8.5 0.8–3
1.247 ± 0.606 0.962 ± 0.039 1.131 ± 0.047 1.111 ± 0.046
1.044 ± 0.042 0.871 ± 0.032 0.988 ± 0.036 1.241 ± 0.058
1.000 ± 0.039 1.224 ± 0.063 1.016 ± 0.038 1.072 ± 0.043
120° 12.0–16 8.5–12 3–8.5 0.8–3
1.394 ± 0.078 1.359 ± 0.077 1.061 ± 0.041 1.127 ± 0.047
0.986 ± 0.039 0.949 ± 0.038 0.947 ± 0.033 1.197 ± 0.053
0.836 ± 0.028 1.274 ± 0.068 0.870 ± 0.028 1.019 ± 0.038
Fig. 11. The angular distribution of elastic cross section for 56Fe at the incident neutron energy of 14.5 MeV retrieved from ENDF/B-VII.1 (line), CENDL-3.1 (dash) and JENDL-4.0 (dot) libraries.
Fig. 12. The neutron spectra of 56Fe(n, n0 )C at incident neutron energy of 14.5 MeV reactions retrieved from ENDF/B-VII.1 (line), CENDL-3.1 (dash) and JENDL-4.0 (dot) libraries. Fig. 10. The variation of the C/E values with energy threshold for iron slab at 60°. (a) 5-cm-thick; (b) 10-cm-thick; (c) 15-cm-thick.
the evaluated nuclear data libraries are studied. The emission neutrons of the (n, el), (n, n0 ) and (n, 2n) reaction channel make contributions to the leakage neutron spectra of the present work. 1. The angular distribution of the neutron elastic scattering for 56 Fe at the incident neutron energy of 14.5 MeV in the evaluated data libraries are shown in Fig. 11. The cross sections from the ENDF/B-VII.1 are higher than those from the CENDL-3.1 and JENDL-4.0 at both 60°and 120°, which caused the differences in elastic scattering energy region of the neutron spectra. These observations indicate that the discrepancies of the elastic scattering neutron yields in the MCNP simulations using the ENDF/ B-VII.1, CENDL-3.1 and JENDL-4.0 libraries originate simply from the differences in the angular distribution of elastic cross section at 60°and 120° in these libraries. 2. The contributions to the total energy spectra from the continuum inelastic scattering for 56Fe at incident neutron energy of 14.5 MeV in the ENDF/B-VII.1, CENDL-3.1 and JENDL-4.0
Fig. 13. The neutron spectra from (n, 2n) reactions at the incident neutron energy of 14.5 MeV retrieved from ENDF/B-VII.1 (line), CENDL-3.1 (dash) and JENDL-4.0 (dot).
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libraries are shown in Fig. 12. At the energy range of 8.5– 11 MeV, there is a small but clear peak observed in the JENDL-4.0 libraries, which is quite different from another two libraries. The JENDL-4.0 cross sections are considerably higher than the ENDF/B-VII.1 and CENDL-3.1, which caused the discrepancies between the measured spectra and the calculated ones in 8.5–12 MeV energy range. 3. The contributions to the total energy spectra from the (n, 2n) reaction for 56Fe at incident neutron energy of 14.5 MeV in the ENDF/B-VII.1, CENDL-3.1 and JENDL-4.0 libraries are shown in Fig. 13. The contribution of (n, 2n) reaction in the CENDL-3.1 is considerably higher than the ENDF/B-VII.1 above 1.2 MeV and lower from 0.5 to 1.2 MeV, where the calculations with the CENDL-3.1 library give significant disagreement with the experimental ones.
overall, further adjustment for the CENDL-3.1 evaluated library will be performed in the future. Acknowledgments The authors gratefully thank the operational staff in CockcroftWalton type accelerator, China Institute of Atomic Energy, for their excellent operation of the D-T neutron source. This work is supported by the National Natural Science Foundation of China (Grant No. 11505297) and (Grant No. 11775311). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.anucene.2019.04.041. References
5. Summary In this work, the validations of the evaluated nuclear data for iron were performed, using the leakage neutron spectrum measured by time-of-flight technique. The measured neutron energy was from 0.8 to 16 MeV. The total uncertainty for the experimental data was below 5%. The theoretical calculations were carried out by MCNP-4c Monte Carlo code using the evaluated nuclear data of the CENDL-3.1, ENDF/B-VII.1, and JENDL-4.0 libraries. The measured spectra can be well reproduced as a whole by the simulations with the three evaluated nuclear data libraries, while some discrepancies in the four separated energy ranges, corresponding to (n, el), (n, n0 ) and (n, 2n) reactions, are found from the comparison. From the comparison, it has been found that in general the calculated neutron spectra with the CENDL-3.1 library show better agreement with the measured ones than those with ENDF/B-VII.1 and JENDL-4.0 from the energy range about 3 to 16 MeV. While in the energy range from 0.8 to 3 MeV, the spectra with the CENDL-3.1 were overestimated. These discrepancies are considered as caused by the improper evaluation of the angular distribution and secondary neutron energy distribution of the elastic, inelastic scattering and (n, 2n) reaction channels in evaluated nuclear data libraries. In conclusion, the data of the three libraries need to be improved as required in different energy ranges. This work provides valuable data for benchmarking the evaluated nuclear data for 56Fe, since the iron data from the CENDL-3.1 coincide with the experiment data most
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