Analysis of ²³⁶U and its Isotopes by a Compact Accelerator Mass Spectrometry Device

The long-lived rare radionuclide ²³⁶U has become a powerful tracer in geochemical and oceanographic studies in recent years. In the study, ²³⁶U and its isotopes were analyzed by the compact accelerator mass spectrometry (AMS) device independently developed by the China Institute of Atomic Energy (CIAE). The device mainly divides into ion source and elicitation system, injection system, acceleration and stripping system, high-energy analysis system and the ion detection system, which can measure a variety of nuclides such as ¹⁴C, ²⁶Al, ¹²⁹I, ²³⁶U, ²³⁹Pu and so on under low-energy conditions. Compared with the previous generation of device at CIAE, in the air-insulated AMS, the footprint of the entire system has been halved to (6.8×3.3) m²=22 m² based on the central beamline. Through the optimisation of the system transmission, particle detection and background exclusion, ²³⁶U measurement using the compact AMS device was established, and its measurement performance was investigated. The helium stripping technique was adopted, and the appropriate gas thickness was determined by stripping gas scanning, which realised the efficient transmission of ²³⁸U³⁺ with a transmission efficiency of 35%. A new Bragg-type gas detector was used to measure trace nuclides such as ²³⁶U. The detector fully utilised its signal amplification function, and its signal-to-noise ratio was improved to achieve the detection of low-energy heavy ions. The source of the background for ²³⁶U measurement was thoroughly studied, and the exclusion of molecular and isotopic background was achieved. The background of ²³⁶U measurement was determined by scanning the electrostatic analyser and the magnet on the high-energy side, and the measurement background can be stably less than 2.5×10⁻¹⁴. The abundance sensitivity of ²³⁶U was determined using an indirect measurement method, and the effect of factors such as slit settings was discussed. After the verification of uranium standard samples, the abundance sensitivity of ²³⁶U/²³⁸U during the measurement reaches 10⁻¹⁵, and the measurement accuracy reaches 1%. The results showed that the low-energy compact AMS device has unique advantages in heavy nuclides measurement, such as high sensitivity, good safety, low operating cost, and can rapidly analyse uranium samples with low background and minimal sample usage. This system lays a solid foundation for the application of ²³⁶U in nuclear emergencies, nuclear safeguards and environmental tracing.