Chemical effects on nuclear decay of $^{235}$U isomer in the uranyl form

TL;DR

This study demonstrates that the nuclear decay half-life of $^{235}$U in uranyl compounds varies with different halide ligands, influenced by molecular orbital formation, revealing a chemical effect on nuclear decay processes.

Contribution

It provides the first experimental evidence that molecular bonding affects nuclear decay rates, linking chemical environment to nuclear stability in uranyl compounds.

Findings

Half-lives vary with ligand type, increasing with electronegativity.

Uranyl fluoride has the shortest half-life due to fewer 6p electrons.

Quantum calculations support the role of molecular orbitals in decay variation.

Abstract

The nucleus of uranium-235 ($^{235}$U) possesses an exceptionally low-energy isomeric state, $^{235m}$U. Unlike most radioactive nuclides, whose nuclear-decay half-lives are constant, the half-life of $^{235m}$U varies with its chemical environment$^{1,2}$ owing to interactions with outer-shell electrons in the internal-conversion (IC) process. However, the mechanism underlying this half-life variation, particularly the role of molecular bonding beyond simple electron-density effects$^{1,2}$, remains unresolved. Here, we investigate variations in the half-lives of $^{235m}$U and the corresponding IC-electron energy spectra for uranyl (UO2$^{2+}$) compounds with different halide ligands. The half-lives of $^{235m}$U are measured to be 25.32(4), 26.05(8), 25.84(3), and 25.44(3) min for uranyl fluoride, chloride, bromide, and iodide, respectively, indicating that the half-life increases with increasing ligand electronegativity, with the exception of uranyl fluoride. The shortest half-life observed for uranyl fluoride is attributed to the smallest number of 6p electrons occupying bonding orbitals, as indicated by the IC-electron energy spectra and quantum chemical calculations. This work provides the first observation of a significant variation in a nuclear decay process driven by changes in molecular orbital formation, paving the way toward a deeper understanding of interactions between a nucleus and electrons involved in chemical bonding.