Maharashtra BoardEnglish MediumSTD 11 ScienceChemistryNuclear Chemistry and Radioactivity6 Marks
Question
Explain the term: Radiocarbon dating in detail.
✓
Answer
Radiocarbon dating: The technique is used to find the age of historic and archaeological organic samples such as old wood samples and animal or human fossils.
Radioisotope used for carbon dating is $^{14}C.$
$i.$ Radioactive $^{14}C$ is formed in the upper atmosphere by bombardment of neutrons from cosmic ray on $^{14}N.$
${ }_7^{14} N +{ }_0^1 n \longrightarrow{ }_6^{14} C +{ }_1^1 H$
$ii. \ ^{14}C$ combines with atmospheric oxygen to form $^{14}CO_2$ which mixes with ordinary $^{12}CO_2.$
$iii.$ This carbon dioxide is absorbed by plants during photosynthesis.
$iv.$ Animals eat plants which have absorbed a carbon dioxide $(^{14}CO_2 + ^{12}CO_2).$ Hence, $^{14}C$ becomes a part of plant and animal bodies.
$v.$ As long as the plant is alive, the ratio $^{14}C/^{12}C$ remains constant.
$vi.$ When the plant dies, photosynthesis will not occur and the ratio $^{14}C/^{12}C$ decreases with the decay of radioactive $^{14}C$ which has a half$-$life $5730 \ years.$
$vii.$ The decay process of $^{14}C$ is given below:
${ }_6^{14} C \longrightarrow{ }_7^{14} N +{ }_{-1}^0 e$
$viii.$ The activity $(N)$ of given wood sample and that of fresh sample of live plant $(N0)$ is measured, where, $N0$ denotes the activity of the given sample at the time of death.
$ix.$ The age of the given wood sample, can be determined by applying following Formulae:
$t =\frac{2.303}{\lambda} \log _{10} \frac{ N _0}{ N }$
$\text { where } \lambda=\frac{0.693}{5730 y }=1.21 \times 10^{-4} y ^{-1} .$
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