Half life can be defined as;
i) The time it takes for one-half of the number of atoms of a radioactive nuclide to decay or
ii) The time required for a given mass of a radioactive isotope to decrease to half its original mass
iii) Time in which one half of the original number of nuclei decay
The rate at which radioctive isotope decays is constant and independent of physical conditions such as temperature and pressure.
The rate of decay of radioactive material is measured in half-life and each radioisotope has a unique half-life.
Some radioisotopes have very short half-lives while others have very long half-lives, half-lives therefore range from microseconds to billions of years. For example. Polonium-215 has a very short half-life of 0.0018 seconds and Uranium-238 has a half-life of 4.5 billion years.
| Radioactive isotope
|1.27 x years
|2.6 x years
|4.5 billion year
Half-life is used in estimating the ages of old archaeological artifacts or oragnic materials, radiocarbon for instance which has a half-life of 5730 years has historically been used to date 500-50000 years old organic materials.
Radioisotopes with longer half-lives such as are used to determine ages of rocks millions of years old ( see radioctive dating in the separate section).
Apart from its application in radioactive dating, half-life is also applied in nondestructive testing (NDT) by radiographers. Here, if radiation intensity is halved, the radiographer will have no choice but to expose the source for a perioddouble the stint when the camera was new. the key concept here is that while the penetrating power of the radiation remains constant, the amount of radiation decreases due to age.
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