Speaker
Description
Evaluation of Neutron Data Standards have been a critical component of ENDF/B library releases which is currently coordinated by the IAEA. The latest Neutron Data Standards were released in 2017 [1] and were adopted for the ENDF/B-VIII.0 library [2]. Since very early on, the Neutron Data Standards have been derived using the Bayesian GMA code originally developed by W. Poenitz with an extensive database of vetted experimental data. These data include the simplest integral data, namely spectrum averaged cross sections (SACS) measured in the reference 252Cf(sf) reference neutron field.
Note that the reference Cf(sf) neutron field has been originally evaluated by Mannhart and has been validated as a reference neutron field in IRDFF-II library [3]. We can use the reference Cf(sf) neutron spectra to convolute evaluated Neutron Standard cross sections, to derive SACS and corresponding uncertainties. At the same time Mannhart evaluated the experimentally measured SACS in the reference Cf(sf) spectrum [4]. A comparison of fission SACS measured in the Cf(sf) spectrum for dosimetry reactions U235(n,f), Np237(n,f), U238(n,f), and Pu239(n,f) vs fission SACS derived from Neutron Standards is given in table 1.
Table 1: Fission SACS calculated in a Cf-252(sf) reference neutron spectrum
IAEA STD 2017 [3] Mannhart Eval. [4] Ratio STD/Mannhart
U235nf 1.2267E+03 (1.21) 1.210E+3 (1.2) 0.986
U238f 3.2154E+02 (1.29) 3.257E+2 (1.6) 1.013
Pu239f 1.7978E+03 (1.25) 1.812E+3 (1.4) 1.008
Np237f 1.3598E+03 (1.70) 1.361E+3 (1.6) 1.001
Table 2: Spectral indexes (SI) in a Cf-252(sf) reference neutron spectrum
IAEA STD 2017 [3] Mannhart Eval. [4] Ratio STD/Mannhart
Pu9f/U5f 1.465 1.4975 0.978
U8f/U5f 0.262 0.269 0.973
Np7/U5f 1.108 1.125 0.985
U-235 fission SACS derived from the IAEA STD are -1.2% lower than Mannhart evaluation while Pu-239 fission SACS are +0.8% higher. Those ratios result in a 2.2% underestimation of the Pu9f/U5f spectral index calculated with STD cross sections compared to those values derived from Mannhart evaluation. A similar effect is seen for the U8f/U5f SI.
A similar discrepancy was observed by Casperson [5] in fission reaction rates in critical assemblies. In particular, the C/E Godiva (HMF001) data and the C/E Flaptop-25 (HMF028) data are listed in Tables 3 and 4, where the underestimated SI are highlighted in cyan..
Table 3. Godiva SI – HMF001 (exp. data from ENDF/B-VII.1 paper [6], exp B evaluated by Hansen)
Measured Calc. C/E
Pu9f/U5f 1.4152(1.0%) 1.3841(1.7%) 0.978
Np7/U5f 0.8516(1.4%) 0.8338(2.1%) 0.979
U8f/U5f 0.1643 (1.0%) 0.1584(1.7%) 0.972
Table 4. Flaptop-25 – HMF028 (exp. Data from ENDF/B-VII.1 paper [6], exp B evaluated by Hansen)
Measured calc C/E
Pu9f/U5f 1.3847 (0.9%) 1.3615 (1.7%) 0.985
Np7f/U5f 0.7804 (1.3%) 0.7751 (2.1%) 0.9932
U8f/U5f 0.1492 (1.1%) 0.1450 (1.7%) 0.972
Finally note that published NIFFTE PU9f/U5f [7] and U8f/U5f [8] cross-section ratios are larger than corresponding STD ratios by about 2% which is like above discussed discrepancies. Further investigation of the existing SACS experiments and of the SACS evaluation is warranted. A new evaluation of measured SACS by using Mannhart code with updated inputs from experimental data will be presented; results will be compared to above described calculations.
[1] A.D. Carlson, V.G. Pronyaev, R. Capote et al., Evaluation of the Neutron Data Standards, Nucl. Data Sheets 148, 143-188 (2018)
[2] D.A. Brown et al.: “ENDF/B-VIII.0: The 8th Major Release of the Nuclear Reaction Data Library with CIELO-project Cross Sections, New Standards and Thermal Scattering Data”, Nuclear Data Sheets 148 (2018) 1–142.
[3] A. Trkov et al, IRDFF-II: A New Neutron Metrology Library, Nucl. Data Sheets 163, 1-108 (2020)
[4] W. Mannhart in International Reactor Dosimetry File 2002 (IRDF-2002), Tech. Report Series 452, IAEA (2006); numerical data: https://www-nds.iaea.org/irdf2002/.
[5] R.J. Casperson, “The benefit of adjusting with criticality and reaction rate data”, mini-CSWEG 2021.
[6] M.B. Chadwick, M.W. Herman, P. Oblozinsky et al., ENDF/B-VII.1 nuclear data for science and technology: cross sections, covariances, fission product yields and decay data, Nucl. Data Sheets 112, 2887 (2012)
[7] L. Snyder et al., “Measurement of the 239Pu(n,f)/235U(n,f) Cross-Section Ratio with the NIFFTE fission Time Projection Chamber, arXiv 2107.02881 [nucl-ex] (2021).
[8] R.J. Casperson et al, Measurement of the normalized U-238(n,f)/U-235 (n,f) cross section ratio from threshold to 30 MeV with the NIFFTE fission Time Projection Chamber, Phys. Rev. C97 (2018) 034618