# Best isotope to use when dating very old samples

### Best isotope to use when dating very old samples

Any organic material that is available in sufficient quantity can be prepared for radiocarbon dating.

The measurement of the rate of radioactive decay is known as its half-life, the time it takes for half of a sample to decay.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.the field deflects atoms of different masses differently (heavier atoms deflect less).Targets tuned to different atomic weights count the number of c12, c13, and c 14 atoms in a sample.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.The diminishing levels via decay means that the effective limit for using c14 to estimate time is about 50,000 years. Subsequent work has shown that the half-life of radiocarbon is actually 5730 ± 40 years, a difference of 3% compared to the Libby half-life.However, to avoid confusion all radiocarbon laboratories continue to use the half-life calculated by Libby, sometimes rounding it to 5570 years.These so-called "solid-carbon" dates were soon found to yield ages somewhat younger than expected, and there were many other technical problems associated with sample preparation and the operation of the counters.Gas proportional counters soon replaced the solid-carbon method in all laboratories, with the samples being converted to gases such as carbon dioxide, carbon disulfide, methane, or acetylene.AMS technology has allowed us to date very small samples (such as seeds) that were previously undatable.