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Neutron capture cross section measurement of U238 at the CERN n_TOF facility in the energy region from 1 eV to 700 keV

F. Mingrone et al. (n_TOF Collaboration)
Phys. Rev. C 95, 034604 – Published 10 March 2017
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  • INTRODUCTION
  • EXPERIMENT
  • OPERATION AND PERFORMANCES OF THE…
  • DATA REDUCTION
  • RESULTS
  • CONCLUSION
  • ACKNOWLEDGMENTS
    • References

    Abstract

    The aim of this work is to provide a precise and accurate measurement of the U238(n,γ) reaction cross section in the energy region from 1 eV to 700 keV. This reaction is of fundamental importance for the design calculations of nuclear reactors, governing the behavior of the reactor core. In particular, fast reactors, which are experiencing a growing interest for their ability to burn radioactive waste, operate in the high energy region of the neutron spectrum. In this energy region most recent evaluations disagree due to inconsistencies in the existing measurements of up to 15%. In addition, the assessment of nuclear data uncertainty performed for innovative reactor systems shows that the uncertainty in the radiative capture cross section of U238 should be further reduced to 1–3% in the energy region from 20 eV to 25 keV. To this purpose, addressed by the Nuclear Energy Agency as a priority nuclear data need, complementary experiments, one at the GELINA and two at the n_TOF facility, were proposed and carried out within the 7th Framework Project ANDES of the European Commission. The results of one of these U238(n,γ) measurements performed at the n_TOF CERN facility are presented in this work. The γ-ray cascade following the radiative neutron capture has been detected exploiting a setup of two C6D6 liquid scintillators. Resonance parameters obtained from this work are on average in excellent agreement with the ones reported in evaluated libraries. In the unresolved resonance region, this work yields a cross section in agreement with evaluated libraries up to 80 keV, while for higher energies our results are significantly higher.

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    • Received 30 November 2016

    DOI:https://doi.org/10.1103/PhysRevC.95.034604

    Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

    Published by the American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Nuclear reactions
    Nuclear Physics

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    Vol. 95, Iss. 3 — March 2017

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    • Figure 1
      Figure 1

      Calibrated amplitude spectra for Cs137 (Eγ=661.7 keV), Y88 (Eγ=898 keV and Eγ=1.836 MeV), and Am/Be (Eγ=4.44 MeV) γ-ray sources. The green lines correspond to the broadened simulations, while the black dots are the experimental points. In the inset the different curves for the channel to deposited-energy calibration are shown.

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    • Figure 2
      Figure 2

      U238(n,γ) capture yield compared with the total background and its individual components discussed in the text. The analytical functions which reproduce the beam-related background and the contribution of in-beam γ rays are superimposed to the measured components.

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    • Figure 3
      Figure 3

      Background yield in the URR obtained by summing the individual background components shown in Fig. 2 (red dots) and by using the black-resonance filters in the neutron beam (blue circles). The bottom panel shows that the ratio deviates by less than 5% from unity, except in the neighbourhood of the two big Al resonances at 34.23 and 84.27 keV, because the amount of aluminum in the windows of the beam line was not precisely known.

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    • Figure 4
      Figure 4

      Statistical distribution of resonance kernel ratios of this work over JEFF-3.2 and this work over ENDF/B-VII.1. The gaussian best-fit curve is also plotted as a dashed line.

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    • Figure 5
      Figure 5

      Experimental capture yield for U238(n,γ) with a resolution of 5000 bin/decade. The sammy calculation based on JEFF-3.2 [39] resonance parameters is represented by the solid line.

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    • Figure 6
      Figure 6

      Resonances at ER=721.68 eV and ER=1211.31 eV. Data from this work are red points, and the sammy best fit is shown as a dotted blue line. Calculations performed using JEFF-3.2 and ENDF/B-VII.1 resonance parameters are shown as continuous-green and dotted-black lines, respectively. The bottom panel shows the residuals of the fit.

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    • Figure 7
      Figure 7

      Resonances at ER=145.66 eV (top left), ER=311.35 eV (top right), ER=488.89 eV (bottom left), and ER=1565.54 eV (bottom right). Data from this work are red points, and the sammy best fit is shown as a dotted blue line. Calculation performed using JEFF-3.2 resonance parameters is shown as a green line for comparison. The bottom panel shows the residuals of the fit.

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    • Figure 8
      Figure 8

      Correction factor for the sample-related effects, i.e., self-shielding and multiple scattering followed by capture, obtained from MCNP6 simulations.

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    • Figure 9
      Figure 9

      U238(n,γ) cross section from this work (red dots) compared to the recently published data by Ullmann et al. [46] (green triangles) and Kim et al. [47] (blue squares), and to the cross section recommended by Carlson et al. [12] (blue circles connected by dotted blue line). The evaluated cross sections from JEFF-3.2 (dark green line) and ENDF/B-VII.1 (cyan line) are plotted for comparison.

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    • Figure 10
      Figure 10

      Measured U238(n,γ) cross section in the energy range 80<En<700 keV: data from this work (red dots) compared to Ullmann et al. [46] (green triangles) and to the cross section recommended by Carlson et al. [12] (blue circles connected by dotted blue line). Also shown are the evaluated cross sections from JEFF-3.2 (dark green line) and ENDF/B-VII.1 (cyan line).

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