Speaker
Description
The thermal neutron constants (TNC) of the major fissile nuclides and the neutron multiplicity of the Cf-252(sf) anchor the neutron data standards. The adopted TNC 2017 standards [1] relied on the iterative Generalized Least Squares (GLS) evaluation of Axton [2]. The TNC 2017 fit combined the Axton's microscopic database with the Lounsbury/Beer [3,4] and Adamchuk [5] alpha (fission to capture ratio) measurements within the GMAP fit but did not revisit the Axton database itself. Since then, additional measurements -several predating the 2017 evaluation- have been brought into consideration, and some assumptions are now better understood, in particular the distinction between bound-atom (SCA) and rolled-metal (SCR) scattering cross sections. In this contribution, we report on a new fit including mean values and uncertainties. We first elaborate on the reproduction of Axton 1986 with our own implementation of the iterative GLS approach. We then propagate the database revisions agreed during recent standards consultations: additional alpha, capture, and total cross-section measurements, SCA/SCR reclassification of total cross section data, and the use of comprehensive bound-coherent scattering lengths with a clean determination of SCA. We discuss the resulting shifts, their consistency with the 2017 standards, with Duran recommendations [6] and with capture cross sections derived independently from R-matrix fits in current evaluated libraries, and the open questions, particularly for Pu-241.
[1] A.D. Carlson et atl, Nucl. Data Sheets 148 (2018) 143-188
[2] E.J.Axton, "Evaluation of the thermal constants of U-233, U-235, Pu-239, Pu-241 and the fission neutron yield of Cf-252", EC JRC Geel, CBNM, Report GE/PH/01/86.
[3] M. Lounsbury, R.W. Durham, G.C. Hanna, “Measurements of Alpha and Fission Ratios for 233U, 235U
and 239Pu at Thermal Energies,” in Proc. Second Int. Conf. organized by IAEA (Helsinki, 15-19 June 1970), Nuclear Data for Reactors,Report STI/PUB/259, Vol. 2, 287 (1972).
[4] M. Beer, M.H. Kalos, H. Lichtenstein, H.A. Steinberg, E.S. Troubetzkoy, “Monte Carlo Analysis of 2200 m/sec Alpha Values of Fissile Nuclides,” Trans. Am. Nucl. Soc. 23, 509 (1972); see also “A Monte Carlo Analysis of a Chalk River Experiment on Cross Sections of Fissile Nuclides,”Report EPRI-NP-163,Electric Power Research Institute (1975).
[5] Yu.V. Adamchuk, M.A. Voskanyan, G. Georgiev, A.L. Kovtun, G.V. Muradyan, N. Stancheva, N. Chikov, N. Yaneva, Yu.G. Shchepkin, “Measuring of 235U alpha value at a Thermal Point,” At. Energiya 65, 434 (1988) (in Russian).
[6] I. Duran, R. Capote, and G. Schnabel, "Integral References for neutron-induced reactions on 233,235U and 239,241Pu at thermal and resolved-resonance ranges", Nuclear Data for Science and Technology conference NDST2025, Madrid, June 2025. To be published.
| Session | Evaluation of Nuclear Data (theory, models, codes) |
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