Development of Compact Accelerator Mass Spectrometry

Accelerator mass spectrometry (AMS) is a high-energy mass spectrometry technique that emerged in the late 1970s. Compared with traditional analytical methods, AMS has become a preferred technique for quantifying long-lived radionuclides at ultra-low natural abundances, owing to its advantages of short measurement time, low sample consumption, and high analytical precision. Early AMS systems were mostly adapted from nuclear physics experimental facilities, with terminal voltages generally no lower than 6 MV. Their high construction costs and low utilization efficiency (as only a fraction of the systems' operating time was dedicated to AMS analysis) greatly limited the development of the AMS application potential. With the advancement of relevant research, specialized AMS with a terminal voltage of 3 MV has achieved commercialization. While satisfying the measurement requirements for the most commonly used radionuclides in routine applications, these specialized AMS instruments have significantly reduced construction costs, thereby greatly advancing the exploitation of AMS application potential. In recent years, due to the discovery of the exponential dissociation characteristics of low-charge-state molecular ions during stripping medium penetration, as well as the advantage that helium gas stripping can significantly reduce energy spread, compact AMS with a terminal voltage of ≤1 MV has emerged as a mainstream trend in the development of AMS. This work focuses on interfering background suppression and transmission efficiency optimization for ¹⁴C analysis by AMS. By analyzing the physical mechanism of AMS technological evolution, this article systematically reviewed the development and the current status of ¹⁴C analysis based on compact AMS worldwide. In terms of interfering background suppression, due to the inability of ¹⁴N to form stable negative ion beams, the design optimization of ¹⁴C-AMS focuses on the suppression of molecular ion interference. High-charge-state (≥+3) molecular ions are unstable; thus, conventional AMS systems use high-charge-state ion analysis to eliminate molecular ion backgrounds. For the design of compact AMS, as the ion beam energy decreases, C⁺ and C²⁺ become the dominant yields of carbon ions after passing through the stripping medium. Therefore, compact AMS must utilize the dissociation property of low-charge-state (+1, +2) molecular ions when passing through the medium to achieve background suppression. Compared with C²⁺-based ¹⁴C analysis, selecting C⁺ not only avoids the ⁷Li₂²⁺ interference but also reduces energy requirements. The Swiss Federal Institute of Technology Zurich (ETH) developed the first compact AMS capable of ¹⁴C dating by selecting C⁺ for analysis. At low energies, the enhanced charge exchange effect makes molecular ion fragments more likely to induce interference signals. Such interference caused by molecular ion fragments mainly occurs in the second stage of the acceleration tube. National Electrostatics Corporation (NEC) developed the first single-stage AMS by eliminating the second-stage acceleration. In terms of beam transmission optimization, improvements in ion transmission efficiency are realized by increasing the stripping yield and enhancing the ion optical transport efficiency. The stripping yield of ion beams is related to ion energy, nuclear charge number, and target materials. For carbon ions of a specific energy passing through a specific stripping medium, the stripping yield is a fixed value. The ion optical transport efficiency is associated with the phase space distribution of ion optics and the acceptance of the device. The phase space of the ion beam is inversely proportional to energy E², and the angular scattering effect is proportional to 1/E. Compact AMS balances background suppression and beam loss by adopting a stepped stripper tube design, which features a wide outer layer and a narrow inner layer. The stripping medium has been gradually upgraded from argon to nitrogen and helium. Taking advantage of the small nuclear charge number and stable dissociation cross-section of helium, the efficiency of ion optical transport is enhanced. With the assistance of ion optical transport matrices and Monte Carlo simulation tools, precise matching between the beam phase space and the acceptance of optical components is achieved. The maximum ion optical transport efficiency thus reaches 95%. The development trend of AMS is miniaturization and commercialization. Through decades of technological iteration, compact AMS has continuously evolved. Up to now, there are nearly 160 AMS worldwide, among which compact AMS systems accounts for more than 60%, and some devices occupy an area of less than 10 m². In China, the number of AMS facilities has grown from 3 at the beginning of this century to 21, including 17 compact systems. The China Institute of Atomic Energy has successfully developed the first domestic single-stage AMS and low-energy tandem AMS, realizing the measurement of nuclides such as ¹⁴C, ³H, ¹²⁹I, and ²³⁹Pu. With the optimization of AMS measurement techniques and a substantial reduction in the construction and maintenance costs of AMS, numerous small and medium-sized laboratories have been able to establish their dedicated AMS analysis platforms, thereby further unleashing the application potential of AMS. Benefiting from the advantages of compact size and low cost, compact AMS is adaptable to various research scenarios and has demonstrated excellent application performance in multiple fields, including biomedicine, environmental climatology, and geoarchaeology.