Optimization of useful ion yield for uranium particles in LG-SIMS multi-collection mode and preliminary investigation of ThO+ signal characteristics toward age dating

Abstract

In this study, the useful ion yield of uranium on vitreous carbon planchets was determined using large-geometry secondary ion mass spectrometer (LG-SIMS) in multi-collector mode with IRMM-2329P standard uranium particles. The influences of raster size and primary ion beam current under Gaussian-mode were systematically investigated. Uranium oxide particles fabricated via a hydrothermal method were further adopted to comparatively evaluate the effects of four types of primary ion beams (O₂⁺, O⁻, O₂⁻, O₃⁻) and two substrate materials (vitreous carbon and silicon) on uranium useful ion yield. The measurement results demonstrated that a 20 μm raster size provides a favorable balance between signal collection efficiency, measurement repeatability, and single-particle selectivity, acquiring more than 90% of total ion counts even at a beam current of 50 nA. Under identical primary ion beam conditions, the uranium useful ion yield on vitreous carbon was substantially higher than that on silicon, and negative oxygen ion beams outperformed positive beams in ionization efficiency. Among all tested beams, the O₃⁻ primary beam exhibited the most prominent improvement in yield. The useful ion yield directly determined with IRMM-2329P certified particles under the conventional O₂⁺ beam was 2.03%; scaling by relative total U⁺ ion counts further indicated that the O₃⁻ beam provided the highest estimated yield of 3.27% on vitreous carbon, 61% higher than that of O₂⁺. In addition, using thorium nitrate and CRM U850 powder, we preliminarily investigated the signal characteristics of ThO⁺ ion clusters in view of their potential application to age dating of uranium particles, with ThO₂⁺ evaluated for comparison. The test results indicated that under the conditions of O₂⁺ primary beam and vitreous carbon planchets substrate, ThO⁺ exhibited higher signal intensity and better stability than Th⁺. Additionally, the signal intensity of ThO⁺ could exceed four times that of Th⁺ with the increase of surface oxygen coverage. These findings provide baseline data on ThO⁺ ionization behavior toward the development and optimization of uranium particle age-dating methods.