Techniques for ²⁶Al Measurement Based on a Compact Accelerator Mass Spectrometry System

In recent years, ²⁶Al has become an important radionuclide in Earth and environmental sciences owing to its unique physical properties and environmental behaviors. The ²⁶Al/²⁷Al atomic ratio in typical natural samples is usually on the order of 10⁻¹⁴. For samples at such ultra-trace levels, accelerator mass spectrometry (AMS) is currently the most effective and reliable analytical technique. In 2023, the China Institute of Atomic Energy (CIAE) developed a 0.3 MV compact AMS system. This instrument was designed by integrating multi-nuclide measurement requirements with beam-optical principles for high-efficiency transmission. Key components, including the ion source, injection system, acceleration and stripping system, high-energy analysis system, and detection system, were optimized and structurally integrated. The established AMS platform is compact and stable, occupying a floor area of approximately 26 m², with a maximum terminal voltage of 260 kV. High-sensitivity measurements of ¹²⁹I, ²³⁶Pu, ²³⁶U, and ¹⁴C have been successfully achieved on this facility. Based on this compact AMS facility, ²⁶Al measurement techniques were systematically investigated. This study focused on methods for efficient transmission of Al ions within a compact AMS system. By optimizing the operating parameters of key components, including the injection electrostatic analyzer, injection magnet, accelerator, quadrupole lenses, analyzing magnet, electrostatic analyzer, and detection magnet, stable and efficient delivery of the ²⁷Al beam to FC7 was realized. Among these parameters, the accelerator terminal voltage, charge-state selection, and stripping gas pressure were identified as the dominant factors affecting beam transmission efficiency. Systematic experiments were conducted to investigate these key parameters. The charge-state distributions of Al ions after helium gas stripping were studied under varying terminal voltages. The transmission efficiency of the 1+ charge state reached approximately 60%, while the combined transmission efficiency of Al¹⁺, Al²⁺, and Al³⁺ exceeded 85%. The thickness of the stripping gas was also found to significantly influence transmission efficiency, and the transmission efficiencies of different charge states were measured as functions of stripping gas pressure. To achieve high sensitivity for ²⁶Al measurements, three charge states with relatively high stripping probabilities were investigated with respect to their transmission characteristics and background behavior. Molecular background suppression was studied for ²⁶Al¹⁺, while detectors with range discrimination capability were investigated for ²⁶Al²⁺. However, the attempt was unsuccessful due to insufficient ion energy. Further improvements to the detector will be pursued in future work. Through comprehensive comparison, an analytical technique using the 3+ charge state was established. Measurements of blank and standard samples demonstrated an abundance sensitivity of 10⁻¹⁵ and a measurement precision of approximately 5%. These results indicate that the compact AMS system provides a reliable platform for high-precision ²⁶Al/²⁷Al analysis and supports further applications of ²⁶Al in related research fields. In future work, rapid alternating measurement methods will be implemented on this AMS system to further improve measurement precision and abundance sensitivity, providing more reliable technical support for the analysis of complex samples.