Research on Compact Accelerator Mass Spectrometry Measurement Technique for ¹⁴C

Carbon-14 (¹⁴C) is a key environmental tracer nuclide that plays an essential role in the biogeochemical cycling of the Earth system and has been widely applied in environmental science, geochronology, and life sciences. Accelerator mass spectrometry (AMS) is currently the most sensitive analytical technique for ¹⁴C detection, and its development has progressively trended toward low-energy, compact, and highly integrated systems. In this work, comprehensive research on compact AMS measurement techniques for ¹⁴C was conducted using a compact multi-nuclide AMS system independently developed by the China Institute of Atomic Energy (CIAE). Systematic investigations were carried out on critical aspects affecting ¹⁴C measurement performance, including negative ion extraction, beam optics optimization, charge state selection, stripping gas species and pressure, background suppression, and ion detection. An online Zn-Fe graphitization sample preparation system was designed and built to improve sample preparation efficiency, safety, and controllability. By optimizing the reduction tube structure and reaction parameters, the graphitization time was shortened from 10 h to 7 h, while stable graphite samples with ¹²C⁻ ion beam currents of 50-60 μA were routinely obtained, thus providing high-quality ion beams for AMS analysis. Beam transmission optimization was initially performed using ¹³C as a surrogate isotope. Helium was adopted as the stripping gas owing to its low atomic number and favorable ionization properties, which minimizes angular and energy straggling during the stripping process. By optimizing the stripping gas pressure and accelerator terminal voltage (165 kV), a maximum transmission efficiency of 45% for ¹³C⁺ was achieved, a value comparable to those of internationally similar compact AMS facilities. After beam tuning, ¹⁴C⁺ ions were stably delivered to the detector by scaling the magnetic field strengths according to the mass-to-charge ratios. To achieve high-sensitivity ¹⁴C measurements, molecular background interference originating from ions such as ¹³CH⁺ was systematically investigated. Gas stripping was adopted as the primary method for molecular dissociation, and the optimal stripping pressure was determined to be 3.74×10⁻³ Pa, at which the molecular background was suppressed to the 10⁻¹⁵ level. Furthermore, a background correction method based on monitoring the correlated ¹³C⁺(H) signal was established to further eliminate residual molecular interference. Consequently, the compact AMS system achieved an abundance sensitivity of ¹⁴C/¹²C=1.6×10⁻¹⁵ following background correction. These results demonstrate that the developed compact AMS system is capable of rapid, stable, and highly sensitive ¹⁴C measurements under low terminal voltage conditions, with performance reaching the internationally advanced level. This work provides a solid technical foundation for future applications of compact AMS in environmental tracing, nuclear safety, and multi-nuclide analysis.