Novel technique to study nuclear isomers via atomic processes

Nuclear isomers can decay through multiple processes; in many cases the dominant mechanism is internal conversion (IC). This is an excitation of an atomic-electron resulting in ionization and the creation of atomic vacancies. These vacancies are propagated towards the outer-shells by the emission of X-rays and Auger electrons. Auger-electron emission increases the ionic charge of decaying ions thus affecting the charge-state distribution. We propose a novel technique to identify and study nuclear isomers by investigating the charge-state distribution of residual ions following isomeric decays. This technique is based on the fact that the residual charge-state distribution is sensitive to the internal conversion coefficient (number of IC events occurring), which could be used to extract useful information about nuclear isomers. As a proof of concept, the technique has been applied to study nuclear isomers in 144Cs. The residual charge-state distribution of 144Cs following isomeric decays has been measured using the Lohengrin fission fragment mass spectrometer at the Institut Laue-Langevin, France [1]. Based on a level scheme proposed in Ref. [1], we simulated the nuclear cascade decays and the subsequent emissions of X-rays and Auger electrons in 144Cs using an Auger cascade model [2]. The simulations started in pre-ionized ions since an equilibrium charge-state distribution (q - 20) was established after the ions passed through a nickel foil 0.3 mm away from the 235U target. The simulation provides very good agreement with the experimental measurement and has enabled the extraction of a limit on the isomeric lifetime, an isomeric ratio, and deduction of the unknown level X [1] to be a state at 92.2 keV.