The amount of radioactive carbon-14 in a ample is measured using a Geiger counte

Question

The amount of radioactive carbon-14 in a ample is measured using a Geiger counter, which records each disintegration of an atom. Living tissue disintegrates at a rate of about 13.5 atoms per minute per gram of carbon. In 1977 a charcoal fragment found at Stonehenge, England, recorded 8.2 disintegrations per minute per gram of carbon. Assuming that the half life of carbon-14 is 5730 years and that the charcoal was formed during the building of the site, estimate the date at which Stonehenge was built.

Explanation / Answer

Step1 t(1/2) = .6931/k [eq1] ; k is rate constant

Step2 t = (2.303/k) log [13.5/8.2] eq2

Step3 Divide eq2 by eq1

Step4 t/5730 = (2.303/.6931)log (13.5/8.2)

Step5 t= 4122.4 yrs ; 2145.4 BC

Step6 7th Aug 2146BC

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