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Radiometric dating methods detect beta particles from the decay of carbon 14 atoms while accelerator mass spectrometers count the number of carbon 14 atoms present in the sample.Both carbon dating methods have advantages and disadvantages.Methuselah's tree ring sequence near its core -- when it was a young tree -- was matched to the sequence found in pieces of nearby trees which had died previously. Henry Michael of the University of Pennsylvania's Museum Applied Science Center for Archaeology (MASCA) scanned the area almost every summer for over three decades.His goal was to find sections of dead trees whose rings could be pieced together to extend the samples as far back in time as possible.Burning the samples to convert them into graphite, however, also introduces other elements into the sample like nitrogen 14.When the samples have finally been converted into few milligrams of graphite, they are pressed on to a metal disc.Ions from a cesium gun are then fired at the target wheel, producing negatively ionized carbon atoms.
The bristlecone pine series, the longest built from a single species in a single location, stretches back to 7040... It is based on thousands of rings from 81 living trees and 118 dead trees, and Michael found more than three-quarters of the samples.
Other times, he had to fit together pieces of tree like a jigsaw puzzle.
According to Tom Gidwitz: "Dendrochronologists [scientists who study tree rings] have built sequences for a number of tree species, including German, Irish and Polish oaks, Patagonian cypresses, Lebanese cedars, pine, yew, spruce, and chestnut.
Thanks to nuclear physics, mass spectrometers have been fine-tuned to separate a rare isotope from an abundant neighboring mass, and accelerator mass spectrometry was born.
A method has finally been developed to detect carbon 14 in a given sample and ignore the more abundant isotopes that swamp the carbon 14 signal.
At this stage, other negatively charged atoms are unstable and cannot reach the detector.