Study on UF^-₂ Extraction in Accelerator Mass Spectrometry Measurement of ²³⁶U

This study extracts UF^-₂ ions from UF₄ sample material for the first time so as to improve the measurement accuracy and sensitivity for accelerator mass spectrometry (AMS) measurement of ²³⁶U. Compared to the commonly used UO^-/UO₂ (or U₃O₈) combination, the UF^-₂/UF₄ approach brings a higher beam current of extracted U-containing ions and lower interference from U isotopes (²³⁵U in particular). The UF₄ prepared with the procedures developed in this work can provide a higher ratio of F^- /O^- and therefore lower interference from O-containing ²³⁵U and ²³⁸U molecular ions, compared with that from the UF₄ made by conventional liquid-phase reaction. The AMS experiment was carried out on the AMS system at China Institute of Atomic Energy (CIAE), where only a simple surface barrier detector was used to record ions and a reference ²³⁶U sample with a ²³⁶U/²³⁸U ratio of 10^-10 was analyzed. The result shows that the measurement sensitivity of the UF^-₂/UF₄ approach is lower than 10^-10 and that the reference ²³⁶U sample result is in agreement with the reference value within the uncertainty limits, with the relative uncertainty only 4%. In comparison, the measurement sensitivity of the UO^- /U₃O₈ combination approach is 10^-9 , and it cannot give a concrete value for the same reference sample using the same AMS system. If the sophisticated 500-ps-resolution time-of-flight detection system is used in combination with the UF^-₂/UF₄ approach, a sensitivity of 10^-13 (or lower) is expected.