Examples inaccurate carbon dating
During the lifetime of an organism, the amount of c14 in the tissues remains at an equilibrium since the loss (through radioactive decay) is balanced by the gain (through uptake via photosynthesis or consumption of organically fixed carbon).However, when the organism dies, the amount of c14 declines such that the longer the time since death the lower the levels of c14 in organic tissue.Pre-treatment seeks to remove from the sample any contaminating carbon that could yield an inaccurate date.Acids may be used to eliminate contaminating carbonates.This discovery meant that there are three naturally occurring isotopes of carbon: Whereas carbon-12 and carbon-13 are stable isotopes, carbon-14 is unstable or radioactive.Carbon-14 is produced in the upper atmosphere when cosmic rays bombard nitrogen atoms.A more recent innovation is the direct counting of c14 atoms by accelerator mass spectrometers (AMS).The sample is converted to graphite and mounted in an ion source from which it is sputtered and accelerated through a magnetic field.
Since there are practical limits to the age range of the method, most samples must be younger than 50,000 years and older than 100 years.
Working with several collaboraters, Libby established the natural occurrence of radiocarbon by detecting its radioactivity in methane from the Baltimore sewer.
In contrast, methane made from petroleum products had no measurable radioactivity.
Libby calculated the half-life of c14 as 5568 ± 30 years.
This means that half of the c14 has decayed by the time an organism has been dead for 5568 years, and half of the remainder has decayed by 11,136 years after death, etc.
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