AMS

The measurement of radiocarbon activity is now primarily conducted using Accelerator Mass Spectrometry (AMS). Compared to previously used radiometric methods, AMS systems are faster and require much lower sample masses (up to tens of samples weighing less than 1 mg can be measured within 48 hours).

At the beginning of 2022, the Czech Radiocarbon Laboratory commenced its own AMS measurements using the MILEA system (Multi-Isotope Low-Energy AMS, Ionplus). In addition to ¹⁴C, it is adapted to detect ¹⁰Be, ²⁶Al, ⁴¹Ca, ¹²⁹I, U, Pu, and other actinides, and theoretically, additional nuclides. The MILEA system is a compact AMS with a maximum accelerator voltage of 300 kV. It consists of an ion source with a vacuum chamber for solid samples and an interface for gas sample introduction, ion-optical elements including quadrupole lenses, a low-energy filtration system comprising an electrostatic analyzer (ESA), a magnet (M), and an injector, tandetron accelerator with a helium target, high-energy filtration using two magnets and an additional electrostatic analyzer, a chamber with Faraday cups for current detection, and two ionization chambers, i.e., intermediate and terminal.

The hybrid ion source of the MILEA system allows for the ionization of carbon atoms in both solid and gaseous phases, i.e., those in the form of graphite pressed in aluminum cathodes or in the form of carbon dioxide carried by a helium stream through a capillary into the source. Our AMS is equipped with a versatile Gas Interface System for introducing carbon dioxide in glass ampoules, pressure vessels, or produced by burning solid samples in an elemental analyzer with sorption concentration into synthetic zeolite. Regardless of the method of introducing carbon samples into the ion source, the actual measurement of samples occurs in three nested cycles to suppress the influence of random processes affecting current stability. Each sample is measured in dozens of consecutive cycles lasting tens of seconds, as well as in cycles with micro- to millisecond frequency, measuring only one isotope according to the changing pulse energy. The measurement sequence for all samples, including working standards, background samples, and controls, is repeated several times, resulting in a total measurement time of 1–3 hours per sample, depending on sample activity, current size, and other parameters.

AMS MILEA in Rez

The excellent sensitivity of AMS, which enables measuring the isotopic ratio of ¹⁴C/¹²C at the level of 10^-15, is achieved through the synergy of several technological solutions. The first and fundamental solution involves the integration of an accelerator with energy and mass filters, i.e., electrostatic analyzers and magnets. Due to high acceleration, ions have sufficiently distinct energies to separate desired from undesired ions, reducing background by up to eight orders of magnitude compared to classical mass spectrometry. The second solution involves the formation of negative ions in ionization chamber, utilizing an accelerated beam of cesium cations. In the case of AMS carbon 14 detection, this is particularly associated with suppressing ubiquitous isobaric nitrogen, which cannot form stable negative ions and would otherwise essentially exclude sensitive measurements. The third unique solution removes molecular interferences using a collisional device, the so-called stripper. A target is placed in the path of the accelerated beam, causing negatively charged ions to lose electrons, change polarity, and simultaneously dissociate most molecular ions.

The desired outcome of AMS carbon measurement is the isotopic ratios of ¹⁴C/¹²C and ¹³C/¹²C. Due to the vastly different abundances, where the ratio decreases as the isotope mass increases on the order of 99:1:10^-15, the system must cope with their fundamentally different detection methods. Currently, the most commonly used strategy is sequential beam injection. Prior to the accelerator, an injector (also known as a bouncer or pulser) is inserted into the path, cyclically changing the voltage, and thus the energy of passing ions on a micro- to millisecond scale. The voltage is selected to ideally allow only one isotope to pass through the rest of the AMS path. The switching speed must be high to suppress the influence of inevitable beam instabilities. Stable isotopes ¹²C and ¹³C are then measured as currents in Faraday cups, while carbon 14 is usually detected using advanced ionization chambers. Key to suppressing mutual isotope interferences is evaluating the detector signal only within defined time windows corresponding to the passage of the respective isotope through the system.

Diagram illustrating the function of a bouncer or pulser with alternating voltage.