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About AMS

Accelerator Mass Spectrometry (AMS) is a technology that allows us to distinguish and detect different types of atoms on the basis of differences in atomic weights (mass).  Major advances in the application of this technology to radiocarbon dating were made in the 1980s and it has since become the dominant method used in radiocarbon labs worldwide.

Advantage of AMS

AMS has considerable advantage over radiometric (beta counting) methods such as gas proportional counting (GPC) and liquid scintillation (LS) counting, for example:

  • Smaller sample size (e.g. for charcoal, 5mg for AMS versus 5-20g for LS, GPC)
  • Easier to obtain enough sample material
  • Closer association to an archaeological event
  • For sampling, this means less damage to artefacts
  • Shorter measurement times (e.g. 10-15 minutes for AMS versus days to months for LS, GPC)
Principles of Operation – An AMS distinguishes between molecules of similar mass by accelerating them to very high velocities: this is why it is referred to as Accelerator mass spectrometry.

The method used to achieve this acceleration most often involves a tandem accelerato (two stages of acceleration), as is the case for our AMS at 14CHRONO, a NEC compact model 0.5 AMS, Figure 1.

This machine is used to measure the relative abundances of the carbon isotopes, 14C, 13C and 12C. To carry this out, samples that have been converted to graphite are pressed into a holder (called a ‘cathode’) and positioned on a sample wheel of 40 cathodes. The sample wheel (containing unknowns, backgrounds, standards and secondary standards) is then loaded into the (ion) source. During running, a cathode is bombarded with caesium (Cs+) ions and negatively charged carbon atoms (C) are produced. These negative ions are accelerated out of the source and down the beamline.

The beamline has four changes in direction, each associated with either an electrostatic analyser (ESA) or a magnet that can be tuned to alter the alignment/direction of the beam.

The principle objective of the AMS is to optimize detection of the rare 14C isotope while minimize problematic detection of stray, interfering particles; hence each major steering component is designed and tuned to remove unwanted molecules from proceeding further down the line, while steering the maximum possible quantity of 14C to the detector. Generally, electrostatic analysers are used to filter out particles that do not have the correct energy and magnets filter out particles of unsuitable mass.

A key element of the beamline is the main acceleration region (positioned inside the accelerator tank, a large chamber that is filled with electrically insulating SF6 gas).

Negative ions, exiting the injection magnet are accelerated towards a positive potential at the centre of this region. There, they collide with argon atoms (stripper gas) and, as well as any unwanted molecules undergoing dissociation or breakup, electrons get ‘stripped’ from the atoms of interest, converting them from negative to positive ions. Now positively charged, the beam is further accelerated down the beamline (away from the positive potential) to where the dissociated particles can be filtered out by the analysing magnet; this permits only isotopes of interest to continue on and be measured either at Faraday cups (13C, 12C) or in a gas ionization detector (14C) at the end of the beamline. Of note, in an AMS it is not the absolute number of the rare 14C isotope that is measured to provide the date of a sample. Instead, the ratio of 14C to the stable isotopes 13C and 12C are measured and the age is determined based on how these ratios compare with those of known standards (OX2).

Running a wheel- The structure of our sample wheels is the following: a typical wheel has four run groups: a tuning run group followed by three run groups of eight unknowns.

The tuning run group consists of two blanks, two OX2 standards, and two pairs of secondary standards. Each cathode in the tuning run group is run for four 2-minute exposures. A typical exposure of the OX2 standards produces approximately 45,000 counts at the detector. If the AMS machine is deemed to be running satisfactorily at this point, the tuning run group is transferred to the last run group where all cathodes will be run for four more exposures at the end. All cathodes are run for ten 2-minute exposures.

The unknowns in each run group are normalized to the nearest-in-time runs of bracketing OX2 runs to form the ratio to standard. For more details on how analysis is carried out see here.

Ramped Pyroxidation/ Combustion

Sample Types & Considerations