Calibrated age dating

It is vital for a radiocarbon laboratory to know the contribution to routine sample activity of non-sample radioactivity.

calibrated age dating-76

By measuring the activity of a background sample, the normal radioactivity present while a sample of unknown age is being measured can be accounted for and deducted.

In an earlier section we mentioned that the limit of the technique is about 55-60 000 years.

This is calculated through careful measurement of the residual activity (per gram C) remaining in a sample whose age is Unknown, compared with the activity present in Modern and Background samples. Thus 1950, is year 0 BP by convention in radiocarbon dating and is deemed to be the 'present'.

You can get an idea of the relationship between C14 and age at the Carbon Dating calculator page. 1950 was chosen for no particular reason other than to honour the publication of the first radiocarbon dates calculated in December 1949 (Taylor, 19).

All D14C values are normalized to the base value of -25.0 per mille with respect to the standard carbonate (VPDB).

D14C is calculated using: Figure 1: Decay curve for C14 showing the activity at one half-life (t/2).If a sample age falls after 1950, it is termed greater than Modern, or Where Aabs is the absolute international standard activity, 1/8267 is the lifetime based on the new half life (5730 yr), Y = the year of measurement of the appropriate standard.This is an expression of the ratio of the net modern activity against the residual normalised activity of the sample, expressed as a percentage and it represents the proportion of radiocarbon atoms in the sample compared to that present in the year 1950 AD.In order to make allowances for background counts and to evaluate the limits of detection, materials which radiocarbon specialists can be fairly sure contain no activity are measured under identical counting conditions as normal samples.Background samples usually consist of geological samples of infinite age such as coal, lignite, limestone, ancient carbonate, athracite, marble or swamp wood.Standard errors released with each radiocarbon assay (see below) are usually rounded by convention (Stuiver and Polach, 1977).

Tags: , ,