Just how the Tandem-Van-de-Graaf Accelerator commissioned at ETH Zurich in 1963 became an accelerator mass spectrometer is a long story that Hans-Arno Synal, head of the Laboratory for Ion Beam Physics at ETH Zurich, can tell. The apparatus even looks like it has been round the block a few times. However, behind the deceptively nostalgic accelerator lurks state-of-the-art instruments and equipment that are used to carry out precision analyses of ion beams. The sophisticated filter systems connected to the accelerator enable the filtering out and identification of certain atoms. The filters are series-connected and extend the accelerator like an infinite metal arm. “Using this equipment, we can even identify individual atoms”, explains Synal. “The search for the proverbial needle in a haystack is child’s play in comparison.”

Measuring isotope concentrations

The process referred to as Accelerator Mass Spectrometry (AMS) is used to determine the isotope concentration of long-lived radionuclides (unstable atomic nuclei that decompose radioactively) that are generated with cosmic radiation. Radiocarbon dating, for example, measures the ratio of the stable carbon isotope carbon-12 (¹²C) to the radio isotope carbon-14 (¹⁴C). ¹⁴C forms through cosmic radiation, is assimilated by photosynthesis in plants and is ingested by an organism in food. Compared to ¹²C, it only constitutes a fraction of the carbon in organisms. Through the continual intake of ¹⁴C, the ¹⁴C /¹²C ratio remains constant for instance in a living human during the biological cycle – despite the degeneration of about 3,000 ¹⁴C atoms per second. Only when the organism dies does the ¹⁴C clock start ticking. Calibration curves that are used to compare and monitor values measured in samples date back over 15,000 years, explains Synal. They hail from measuring the rings of fossilized trees.

A few milligrams of Ötzi

This was the technique used at ETH Zurich to date renowned objects like the Turin Shroud or Ötzi, the frozen mummified corpse found in the Alps in 1991. Up until now, they have been the most famous objects to be dated at the Laboratory for Ion Beam Physics. To determine how old something is, a few milligrams of the object are burned and a graphite sample is produced from the resulting carbon dioxide. This is then analyzed in the AMS. The ratio of ¹⁴C atoms to ¹²C atoms compared to the initial concentration of ¹⁴C in the atmosphere defines the radiocarbon age which after calibration by the tree ring curves the true age of the sample. Traditional methods measure radioactive degradation. However, as the half-life increases, the decay becomes seldom and much more material is needed to obtain a good signal, explains Synal. The AMS method is three to four orders of magnitude more efficient. Here, the natural isotope ratios have a concentration of between 10^-15 and 10^-12 atoms in the material being examined. There are 10¹⁵ atoms of ¹²C for one ¹⁴C atom. Such a low proportion of ¹⁴C in the ratio therefore has to be measured with extreme precision, which is only possible with the AMS.

Radiocarbon dating is the best known application. However, the instruments in the lab can also be used to measure other radionuclides, such as beryllium-10 (¹⁰Be), aluminum (²⁶Al), chlorine (³⁶Cl), calcium (⁴¹Ca), iodine (¹²⁹I), plutonium and protactinium. These analyses are used for dating purposes in climate research or geology. The point when a historical landslide occurred can be determined with ¹⁰Be, for instance. The radionuclides are produced directly in the stone through the interaction with the cosmic radiation. However, only 5 to 10 ¹⁰Be atoms are produced in one gram of rock per year, explains Synal. The age of the event can thus be determined by the number of beryllium radionuclides that begin to form on the newly exposed rock at the breaking point following a landslide.

The detectable radioisotopes are also used as “natural trace substances” in the life sciences. In addition, so-called ion beam analysis can also be used to conduct relevant material science tests or alter material surfaces with the ion beam. For example, it can be used to imitate the effect of the cosmic radiation on the solar cells of satellites.

AMS method coined

The lab’s task is to conduct research in the field of ion beam physics. In recent years, systematic studies of the physical processes that underlie the procedure have frequently brought new findings to light, facilitated extensive improvements in the measuring methods and yielded new, more efficient instruments. “Our answers to the primary question as to why the things work in the way we observed earned our lab a leading position,” explains Synal, “and have characterized the development of the AMS method worldwide.”

The team from the lab develops its own instruments for research, new methods right up to their application and makes this know-how accessible to its users. In the last two years, for example, a highly sensitive mass spectrometer has been developed and produced for the pharmaceutical industry, with which ¹⁴C can be used as a tracer for the development of new agents. In particular, this enables us to understand the metabolism of a new agent in the human organism in the early development phase of the medication. This should reduce the amount of time between developing the medication to its usage.

The lab’s customers are not only scientists. They also include auction houses or the city of Berne. The latter is currently trying to clear up an age-old dispute among historians. They are using the ¹⁴C dating method to find out whether a 13^th century document, with which the Hohenstaufen King Frederick II is supposed to have granted the city of Berne the rights to the town, really is the original.

Since the January 1, the Laboratory for Ion Beam Physics in the Department of Physics and its approx. thirty staff members has been the lab for dating in Switzerland. Previously provided by the PSI, this transferred the funding of the lab activities’ basic task to ETH Zurich. About two thirds of the lab expenses are generated through analysis costs that the lab invoices itself. The lab is carried by a board of trustees where its partners – Eawag, Empa and PSI, as well as the Department of Geosciences – make financial contributions. This not only guarantees funding but also the quality of the research, as funds have to be granted for every analysis beforehand in the form of a research contract.