Mass spectrometry radioactive dating
These vastly different projects have one thing in common: They all use accelerators to measure levels of carbon-14 and other isotopes.
CAMS is one of several dozen labs worldwide that conduct accelerator mass spectrometry, or AMS.
This process, called sputtering, separates neutral, as well as positive and negative ions from the sample surface.
The sample is held at a negative potential, and negatively charged ions are accelerated away from the sample, resulting in a beam of negative ions (Figure 2, below). The low energy (~5-10 ke V) diverging beam that leaves the ion source is accelerated, focused and transported to the accelerator by the injector system. Most AMS systems use sequential injection, a process that switches between stable and rare isotopes via the application of varying voltages to the electrically insulated vacuum chamber of the analyzer magnet.
The negative ions that enter the accelerator are attracted to the high-voltage terminal, which is what accellerates the CAMS LLNL employs a tandem Van de Graaff accelerator, in which a second acceleration of millions of volts is applied.
Transmission through a foil changes with time due to radiation damage and foil thickening, thus gas strippers are used in all modern analyzers due to their increased transmission stability.
Magnetic lenses focus the high energy particles leaving the accelerator into a magnetic dipole, (the high energy analyzing magnet).
Stable isotopes can be collected at off-axis beam stops where secondary focusing lenses and additional analyzing equipment remove unwanted ions and molecular fragments to eliminate background.
At CAMS LLNL, a magnetic quadrupole lens focuses the desired isotope and charge state to a high-energy mass spectrometer which passes The magnetic quadrupole and electrostatic selectors coupled together ensure high selectivity and sensitivity, respectively.
Other detectors commonly found in AMS systems include surface barrier, time-of-flight, gas filled magnets, and x-ray detectors. Less abundant isotopes are preferable in AMS because the decreased flux of ions reduces background and wear on the instrument, which is of particular concern due to the quick deterioration of particle detectors (performance deteriorates at rates higher than a few thousand particles per second Radiocarbon levels in the atmosphere change according to complex patterns which are affected by a variety of fluctuations ranging from the sun’s solar activity and the earth’s magnetic field, to ocean ventilation rate and climate.
The traditional accelerator was first developed in the early 1930s for nuclear physics research.