Background: The $\beta$ decays of fission products produced in nuclear fuel are important for nuclear energy applications and fundamental science of reactor antineutrinos. In particular, nuclear reactor safety is related to the decay modes of radioactive neutron-rich nuclei, primarily via the emission of $\gamma$ rays, neutrons, and electrons. Additionally nuclear reactors are the most powerful man-made source of antineutrinos emitted during the $\beta$ decay of fission products. These antineutrinos are used to inspect fundamental properties of leptons as well as informing reactor operation. However, the majority of data on complex decays of fission products collected in the evaluated nuclear data repositories like Evaluated Nuclear Structure Data File (ENSDF) and Evaluated Nuclear Data Files (ENDF) are based on low-efficiency and often incomplete measurements resulting in questionable reference reactor antineutrino flux predictions, see the analysis by [Nichols, J. Nucl. Sci. Technol. 52, 17 (2015)]. Various assessments like the one done under the auspices of the [Yoshida et al., Assessment of Fission Product Decay Data for Decay Heat Calculations: A report by the Working Party on International Evaluation Co-operation of the Nuclear Energy Agency Nuclear Science Committee (Nuclear Energy Agency, Organization for Economic Co-operation and Development, Paris, France, 2007), Vol. 25], as well as by [Sonzogni, Johnson, and McCutchan, Phys. Rev. C 91, 011301(R) (2015)] and [Dwyer and Langford, Phys. Rev. Lett. 114, 012502 (2015)], list the $A=142$ isobars with high cumulative fission yield among the important nuclei where data for reactor decay heat and/or antineutrino production should be verified and/or improved.

Purpose: Our goal is to improve the quality of $\beta$-decay measurements and evaluate the impact of modified decay schemes on reactor decay heat and antineutrino energy spectra, for fission products along the $A=142$ isobaric chain. This work is an in depth follow-up on [Rasco et al., Phys. Rev. Lett. 117, 092501 (2016)]. which presented briefly the impact of the corrected decay scheme of ${}^{142}\mathrm{Cs}$. Here, we extend the data to full isobaric decay chain including the daughter nuclei, ${}^{142}\mathrm{Ba}$ and ${}^{142}\mathrm{La}$, and present more details on the ${}^{142}\mathrm{Cs}$ results.

Method: The decays of neutron-rich isobars of mass $A=142$ produced by means of proton-induced fission of ${}^{238}\mathrm{U}$ were measured using the Modular Total Absorption Spectrometer (MTAS) array on-line at the mass separator and Tandem accelerator at Oak Ridge National Laboratory.

Results: The $\beta$-decay schemes for ${}^{142}\mathrm{Cs}$ and ${}^{142}\mathrm{La}$ were modified with respect to the nuclear data repositories. A small $\beta$-delayed neutron branching ratio for ${}^{142}\mathrm{Cs}$ emitter was remeasured as $0.{10}_{-3}^{+5}$ %. Improved precision on the measured half-lives is reported. Small corrections to the low-energy decay of ${}^{142}\mathrm{Ba}$ are made. The $\beta$-decay patterns for ${}^{142}\mathrm{La}$ and ${}^{142}\mathrm{Cs}$ are presented. The decay heat release and cross section for the detection of reactor antineutrinos are deduced and compared to earlier results.

Conclusions: The $\beta$-feeding pattern for ${}^{142}\mathrm{Cs}$ having decay energy value $Q_{\beta }$ of over 7 MeV was substantially modified with respect to the current ENSDF entry. Smaller changes were encountered for ${}^{142}\mathrm{La}$, but since this $A=142$ isobar also has a large cumulative yield in fission, the changes influence both decay heat and the antineutrino spectra. The previously known $\beta$ intensities for ${}^{142}\mathrm{Ba}$ decay ($Q_{\beta }$ value of 2.2 MeV) were verified and slightly modified. Overall, increased decay heat values and lower flux of antineutrinos interacting with matter are presented.

• Received 13 October 2022
• Accepted 14 February 2023

DOI:https://doi.org/10.1103/PhysRevC.107.034303