$$^{235}\text {U(n}_{\text {th}}\text {,f)}$$ mass yield evaluation with associated covariance matrix

Abstract The study of fission yields has a major impact on the characterization and understanding of the fission process and its applications. Mass yield evaluation represents a key element in order to perform the best estimation of independent and cumulative fission product yields. Today, the lack of analysis-based correlation matrix between the different fission observables induces many inconsistencies in the evaluations. In particular, the mass yield uncertainties are drastically overestimated in comparison to chain yields while these two quantities only differ from the emission of delayed neutrons. In this work, a new consistent process of mass yield evaluation is proposed taking into account the description of the covariance matrix. For $$^{235}\text {U(n}_{\text {th}}\text {,f)}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mn>235</mml:mn> </mml:mmultiscripts> <mml:msub> <mml:mtext>U(n</mml:mtext> <mml:mtext>th</mml:mtext> </mml:msub> <mml:mtext>,f)</mml:mtext> </mml:mrow> </mml:math> mass yields, existing data cover a large range of produced masses which makes it possible to propose an evaluation of mass yields independently of any model with a precision closed to $$1.5\%$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>1.5</mml:mn> <mml:mo>%</mml:mo> </mml:mrow> </mml:math> for the high yields. This new precision brings the possibility to discuss the origin of the structures in the post-neutron mass yields, in particular the consistency with the pre-neutron mass yields and the prompt neutron multiplicity per mass. The work is the first part of the $$^{235}\text {U(n}_{\text {th}}\text {,f)}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mn>235</mml:mn> </mml:mmultiscripts> <mml:msub> <mml:mtext>U(n</mml:mtext> <mml:mtext>th</mml:mtext> </mml:msub> <mml:mtext>,f)</mml:mtext> </mml:mrow> </mml:math> thermal neutron induced fission yield evaluation which will be included in the new JEFF-4 library.