Accelerator mass spectrometry

The Helsinki Accelerator Mass Spectrometry Facility (HAMS) is the only AMS system in Finland. Most radiocarbon determinations (research and industry) in Finland are done by us. High quality of the results is due to continued process optimization and advanced data-analysis. The sensitivity of the method is currently <10-17 and precision <0.2% for high-precision samples.

Accelerator mass spectrometry (AMS) is an ultrasensitive technique for measuring the concentration of a single isotope. The electric and magnetic fields of an electrostatic accelerator system are used to filter out other isotopes from the ion beam. The high velocity means that molecules can be destroyed and removed from the measurement background. As a result, concentrations down to one atom in 1017 atoms are measurable.

Many rare isotopes, such as 10Be, 14C, 26Al, 36Cl, 129I, and several interesting isotopes of Uranium and Plutonium are measured with AMS internationally. Currently our research concentrates on the isotope 14C, with applications in arhaeological/historical dating of any carbonaceous material, environmental and atmospheric research, and differentiating different sources of carbon. Adding the capability to measure other isotopes is possible.

In 14C-AMS, an electrostatic tandem accelerator and several magnetic and electrostatic analyzers are used to measure the abundance of a rare isotope, 14C in this case. Negative carbon ions are drawn from a graphite sample by cesium-sputtering. The ions are first sorted by a low-energy mass spectrometer, which selects ions of given mass and injects the ions into the accelerator. Due to high velocities, the molecules are destroyed in the center of the accelerator in a column of gas, removing all interfering molecules. After the accelerator, several high-energy spectrometers are used to filter out unwanted background ions from the rare isotope beam. Finally the 14C ions are counted in a semiconductor detector.

Main advantages of AMS as compared to decay counting or laser-based measurements are the high accuracy and sensitivity (0.2% in the best AMS laboratories for a 1-ppt-14C sample) and the small sample size (1 mg for normal samples, can go to < 50 micrograms).
Standard 14C measurements include

  1.     Archaeological and geological samples
  2.     Bioportion measurements for gaseous, liquid and solid fuels, flume gases
  3.     Cultural heritage samples from Finland