Development of a retarding-field type magnetic bottle spectrometer for studying the internal-conversion process of 235mU

Abstract The nuclear half-life of 235m U (the first metastable nuclear excited state of 235U) varies depending on chemical environments because the nucleus interacts only with outer-shell electrons in the nuclear internal conversion (IC) process due to its extremely low excitation energy at 76.737 ± 0.018 eV. Elucidating the mechanism of the half-life variation of this unique isomer would largely contribute to understanding interactions between nuclei and shell electrons. For elucidating the mechanism, we aim at measuring both half-lives and IC electron energy spectra of 235m U in various chemical environments. In this study, we developed a retarding-field type magnetic bottle spectrometer for measuring the half-lives and energy spectra with high signal to noise ratio ( S ∕ N ) and high energy resolution. As an evaluation of the fabricated apparatus, the collection efficiency of electrons emitted from a 235m U sample to a channeltron electron detector was measured as a function of the strong and weak magnetic fields and the collection voltage applied to a mesh placed on the entrance of the channeltron detector. Electron collection efficiencies of almost 100% were confirmed under several conditions. Due to this high efficiency together with the low dark noise of the channeltron detector, high S ∕ N measurements have been achieved successfully. An IC-electron energy spectrum of 235m U recorded under the most efficient condition showed an energy resolution ( Δ E ∕ E ) of ∼ 2%, which is sufficiently high to observe small peak shifts and splittings in the IC-electron energy spectra of 235m U compounds. This developed apparatus allows high precision measurements of half-lives and IC-electron energy spectra of 235m U in various chemical environments even with small amount of 235m U and low emission rate of IC electrons.