Accelerator mass spectrometry

Atomic nuclei consist of protons and neutrons. The number of protons determines the chemical element. For example, a carbon atomic nucleus always has 6 protons. The number of neutrons can now vary: the majority of carbon in nature has 6 neutrons, i.e. a total of 12 nuclear particles. The number of neutrons therefore determines the isotope, which in this case is noted as carbon-12, or ¹²C.

Approximately one in a trillion carbon atoms in nature has 8 neutrons instead, ¹⁴C. Living organisms absorb this carbon until the end of their life cycle. The ¹⁴C isotope is unstable and decays on average after about 6000 years. If you now determine the number of ¹⁴C atoms in a sample, you can precisely determine the age of this sample: older samples have fewer ¹⁴C atoms because more time has passed and therefore more ¹⁴C atoms have decayed.

As the number of ¹⁴C atoms in the samples is very small, accelerator mass spectrometry (AMS) is needed to separate the ¹⁴C isotope from the much more abundant ¹²C isotope.

Cologne is home to two such accelerators, which are used for AMS measurements: The 6 MV Tandetron machine and the 10 MV FN-Tandem accelerator. The schematic of the 6 MV AMS machine is shown in the next picture.

First, the prepared sample is introduced into a so-called ion source, which triggers the atomic nuclei of the sample and then feeds them into a particle accelerator. The accelerator increases the speed of the atomic nuclei and sends the particles through a thin film that sits in the middle of the accelerator tank. This process removes interfering molecules such as ¹³C-H. This is a very important step, as otherwise such a molecule could later be mistakenly counted as a ¹⁴C particle.

After the particles have left the accelerator tank, they are deflected using a strong electromagnet. This is where the actual separation of the atoms takes place: the slightly heavier ¹⁴C atoms drift outwards a little more than the ¹³C and ¹²C atoms, as shown in this picture:

Other elements of the system ensure that no other particles are accidentally identified as ¹⁴C particles. At the very end of the system, the rare ¹⁴C particles are detected atom by atom in a gas-filled detector. This enables us to detect the few ¹⁴C atoms individually from a sample containing many trillions of conventional carbon atoms.

In addition to carbon isotopes, the AMS method can be used for many other isotopes. This results in applications in various areas of basic research and everyday life.

Below you will find further information on the accelerators (including a virtual tour!) and on current research projects in accelerator mass spectrometry.