Schematic representation of radioactive dating
Nuclear chemistry is the study of reactions involving changes in the structure of the atomic nuclei.
Modern nuclear chemistry has become very interdisciplinary in its applications, ranging from the study of the formation of the
elements in the universe to the design of radioactive drugs for diagnostic medicine. Nuclear research resulted in a promise of
abundant nuclear energy and medical treatment for diseases like cancer. Positron Emission Tomography (PET) is a nuclear medicine
imaging technique which is useful in diagnosis of cancer metastasis (i.e., spreading to other sites) and is just one of the many
application of radio isotopes in medical diagnosis and therapy.
Production of radioactive 14C
Reactions occurring in sun release the neutron into atmosphere. The atmospheric nitrogen (N2)
gas takes up the thermal neutron to form 14C with the emission of proton. This 14C produced is taken up by
plants during photosynthesis and enters the food web making the way for estimation of age.
Applications of nuclear chemistry:
Radio carbon dating:
When high energy cosmic rays collide with the atoms in the earth's upper atmosphere, they bring about nuclear transformations.
For example, the nuclei of the nitrogen atom and high–energy neutrons combine to produce a new isotope of carbon that is
radioactive and is known as C–14.
Carbon is a naturally abundant element found in the atmosphere, in the earth, in the oceans, and in every living creature.
C–12 is by far the most common isotope, while only about one in a trillion carbon atoms is C–14.
Radiocarbon oxidizes (that is, it combines with oxygen) and enters the biosphere through natural processes like breathing and
eating. Plants and animals naturally incorporate both the abundant C–12 isotope and the much rarer radiocarbon isotope into
their tissues in about the same proportions as the two occur in the atmosphere during their lifetimes. When a creature dies, it
ceases to consume more radiocarbon while the C–14 already in its body continues to decay back into nitrogen. So, if we find
the remains of a dead creature whose C–12 to C–14 ratio is half of what it's supposed to be (that is, one C–14
atom for every two trillion C–12 atoms instead of one in every trillion) we can assume the creature has been dead for about
5,730 years (since half of the radiocarbon is missing, it takes about 5,730 years for half of it to decay back into nitrogen). If
the ratio is a quarter of what it should be (one in every four trillion) we can assume the creature has been dead for 11,460 year
(two half–lives). After about 10 half–lives, the amount of radiocarbon left becomes too miniscule to measure and so
this technique isn't useful for dating specimens which died more than 60,000 years ago. Another limitation is that this
technique can only be applied to organic material such as bone, flesh, or wood. It can't be used to date rocks directly.
Carbon Dating – The Premise:
Carbon dating is a dating technique predicated upon three things:
- The rate at which the unstable radioactive C–14 isotope decays into the stable non–radioactive N–14 isotope.
- The ratio of C–12 to C–14 found in a given specimen.
- And the ratio C–12 to C–14 found in the atmosphere at the time of the specimen's death.
Radio tracers
Artificial Radioactivity – Radioactive Traces.
Radioactivity can also be detected and measured using a device known as a Geiger counter. The operation of a Geiger counter is
based on the ionization of matter caused by radiation. The ions and electrons produced by the ionizing radiation permit conduction
of an electric current. The basic design of a Geiger counter is shown in adjacent figure.
It consists of a metal tube filled with gas. The cylinder has a “window” made of material that can be penetrated by
alpha, beta or gamma rays. The center of the tube consists of a wire. The wire is connected to one terminal of a source of direct
current, and the metal cylinder is attached to the other terminal. Current flows between the wire and metal cylinder whenever
ions are produced by entering radiation. The current pulse is created when radiation enters the tube is amplified; each pulse is
counted as a measure of the amount of radiation.
Picture showing construction of Geiger counter
Nuclear Transmutations – Artificial Radioactivity:
Early Transmutation experiments: The first recognized transmutation occurred in 1919, when Ernest Rutherford showed that a
particle emitted from radium bombarded atmospheric nitrogen to form a proton and oxygen–17:
By 1926, experimenters had found that a bombardment transmutated most elements with low atomic numbers to the next higher
element, with ejection of a proton.
An unexpected finding in a transmutation experiment led to the discovery of the neutron. When lithium, beryllium and boron
were bombarded with particles, they emitted highly penetrating radiation that could not be deflected by a magnetic or electric
field. Chadwick proposed that, unlike γ radiation, these emissions consisted of neutral particles with a mass similar to
that of a proton, and he named them neutrons. Chadwick received the Nobel Prize in physics in 1935 for his discovery.
Since then, other techniques for producing artificial radio isotopes have been developed. In fact, the majority of the nearly
1000 known radio nuclides have been produced artificially.