These errors can be reduced by extending the counting duration: for example, testing a modern benzene sample will find about eight decay events per minute per gram of benzene, and 250 minutes of counting will suffice to give an error of ± 80 years, with 68% confidence.
If the benzene sample contains carbon that is about 5,730 years old (the half-life of To be completely accurate, the error term quoted for the reported radiocarbon age should incorporate counting errors not only from the sample, but also from counting decay events for the reference sample, and for blanks.
This convention is necessary in order to keep published radiocarbon results comparable to each other; without this convention, a given radiocarbon result would be of no use unless the year it was measured was also known—an age of 500 years published in 2010 would indicate a likely sample date of 1510, for example.
In 1970, the British Museum radiocarbon laboratory ran weekly measurements on the same sample for six months.
The results varied widely (though consistently with a normal distribution of errors in the measurements), and included multiple date ranges (of 1σ confidence) that did not overlap with each other.
This is addressed by defining the standard to be 0.95 times the activity of HOx I.
All of this first standard has long since been consumed, and later standards have been created, each of which has a given ratio to the desired standard activity.
Because of the fossil fuel effect, this is not actually the activity level of wood from 1950; the activity would have been somewhat lower.
The fossil fuel effect was eliminated from the standard value by measuring wood from 1890, and using the radioactive decay equations to determine what the activity would have been at the year of growth.
The two solutions provided differ slightly in their approach in this regard.
If you have a fossil, you can tell how old it is by the carbon 14 dating method.
The calculations to be performed on the measurements taken depend on the technology used, since beta counters measure the sample's radioactivity, whereas accelerator mass spectrometers (AMS) determine the ratio of the three different carbon isotopes in the sample.
Another standard is the use of 1950 as "present", in the sense that a calculation that shows that a sample's likely age is 500 years "before present" means that it is likely to have come from about the year 1450.
Carbon 14 is a common form of carbon which decays over time.