Accelerator mass spectrometry
The accelerator mass spectrometry or AMS (for English accelerator mass spectrometry ) is a form of mass spectrometry . AMS works with a particle accelerator (almost always a tandem accelerator ) and two mass spectrometers . In 1977 it was first developed at Oxford University as a refinement of radiocarbon dating , which until then was only possible by measuring the radioactive decay of carbon-14. Today the method is used for numerous isotopes, usually long-lived radionuclides. AMS measures the ratio of a ( radio ) isotope to another - mostly stable - isotope of the same element. There are around 84 AMS laboratories worldwide (July 2006). Most of them focus on the measurement of 14 C .
After the sample has been chemically prepared, it is atomized in a sputter ion source using conventional equipment . The negatively charged particles ( anions ) created in this way are sucked off by an electrical high voltage (a few kilovolts). In a subsequent magnetic field, ions of different weights are deflected to different degrees, so that a first mass selection takes place before the accelerator. Radiocarbon dating would not be possible at this point, however, since the signal of the carbon-14 to be measured is still from a background many orders of magnitude higher due to molecular ions of the same mass (e.g. 13 C 1 H - or 12 C 1 H 2 - ) is superimposed. The stable isobar nitrogen-14 does not form stable negative ions and is therefore not a background source. This is also the case for the AMS nuclides 26 Al and 129 I and their stable isobars 26 Mg and 129 Xe. The same applies to very heavy nuclides such as 240 Pu, where stable isobars no longer exist.
After the first mass analysis described above, the ions are injected into a tandem accelerator. Here they are accelerated by a positive high voltage of several million volts to the so-called terminal, where they fly through a thin carbon film or a gas duct. There the ions lose several electrons due to collisions. Above all, the weakly bound, external electrons, which are responsible for the molecular binding, are all stripped off, so that all molecules here are completely destroyed. The now multiple positive charged ions are further accelerated by the same high voltage. After the accelerator, the particle beam is cleaned of the molecule fragments in a second magnetic mass analysis. The ions remaining in the beam are individually detected with an ionization chamber or with a silicon detector, whereby the high particle energies allow a further, drastic background reduction. This also enables measurements of isotopes (e.g. 10 Be, 36 Cl, 41 Ca, 53 Mn, 59 Ni, 60 Fe), whose stable isobars do form negative ions, but which can then be discriminated by suitable detector systems.
The stable isotopes are measured by measuring the current strength of the ion beam. To do this, the beam is caught in a Faraday cup .
With AMS, for example, 14 C / 12 C ratios up to about 10 −15 can be measured. With a typical sample amount of one milligram, this corresponds to radioactivity of about 0.2 μBq, ie one core that disintegrates in two months. For radiocarbon dating, the isotope ratio is usually measured with an accuracy of 0.5%.
AMS detector systems
In the AMS, the radionuclides are usually measured by counting. Individual ions are registered in particle detectors. Different detector systems are used.
Silicon detectors are semiconductor detectors . They provide an energy signal: the kinetic energy of the incident ion can be measured. A typical area of application are radionuclides for which no isobaric background is to be expected. These are z. B. 14 C and 26 Al, the stable isobars 14 N and 26 Mg of which do not form negative ions.
Ionization chambers, which provide information about the energy loss, can be implemented on the one hand via transversely divided anodes (see also ), on the other hand by orienting the electric field in the direction of the beam and measuring the signal rise time. (see Bragg detector )
TOF and ΔTOF
With the time -of-flight mass spectrometer, also abbreviated TOFMS (Time-of-flight mass spectrometer), the mass / charge ratio is determined over the time of flight of the ion.
Post-stripping is an isobar separation process. It is used for the measurement of 81 Kr , for example . The Isobar 81 Br is stable. The ion beam is first accelerated to an energy of several G eV with a cyclotron and then shot through a film. In the film, the ions release further electrons through many collision processes. If the energy is large enough, some of the ions give up all the electrons. The bare atomic nuclei now have a charge that corresponds to the atomic number. Following the post-stripping, the beam is separated according to charge with a magnet. Bromine ions with atomic number 35 can reach a maximum charge of 35e, but krypton ions can reach a charge of 36e. If the magnet and diaphragms are set so that only ions with the charge 36e land in the detector, the background caused by 81 Br can be effectively reduced.
Gas-filled magnetic gaps are an effective means of suppressing isobaric backgrounds such as 53Cr for 53Mn measurements or 60Ni for 60Fe measurements. The different atomic numbers of the elements and the resulting energy and spatial separation in the gas are used. Isobars can be suppressed by several orders of magnitude with a suitable magnetic field - before they even reach the detector (ionization chamber).
Well-known examples of structures with gas-filled magnets are the GAMS (Gas-filled Analyzing Magnet System) in Munich at the MLL or the gas-filled narrow split-pole of the ANU in Australia.
Age determination of fossils and archaeological finds.
- P. Collon et al. a .: Measurement of 81 Kr in the atmosphere. In: Nuclear Instruments and Methods in Physics Research. Section B: Beam Interactions with Materials and Atoms. Volume 123, Issues 1-4, March 2, 1997, pp. 122-127. doi : 10.1016 / S0168-583X (96) 00674-X