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When a C atom decays, it emits a beta particle, which can be counted in a gas by the electrical pulse it generates.In a liquid scintillation counter, the beta particle excites the emission of light from a complex organic molecule or "scintillant." Because only about 13.5 decays per minute occur in one gram of modern carbon, it was necessary to use fairly large samples of several grams of carbon.To obtain a truly absolute chronology, corrections must be made, provided by measurements on samples of know age.The most suitable types of sample for radiocarbon dating are charcoal and well-preserved wood, although leather, cloth, paper, peat, shell and bone can also be used.For practical reasons, which are discussed later, the value of "modern" is defined by reference to two primary standards of known radiocarbon content.These two standards were measured by many different laboratories to determine the value of the standards relative to "modern." Because the production rate of C is not a constant, we need to make corrections for this effect, as discussed in the following sections.
Accelerator mass spectrometry (AMS) is a technique for direct measurement of the concentration of radioisotopes.A radiocarbon measurement can be obtained on a sample of ~0.5 mg of carbon, and measured to 40 years in uncalibrated radiocarbon age in a measurement time of 3040 minutes on each sample.About one carbon nucleus in a trillion contains two extra neutrons, giving a mass of 14.Radiocarbon ages are then quoted as "years before present" (BP).The formula used for this calculation is: Radiocarbon age (years BP) = -C in 1950 AD (pre-bomb) material.