Radioactivity is broadly classified into two categories:
a) Natural radioactivity and
b) Artificial or Induced radioactivity.
If a substance emits radiations by itself, it is said to possess natural radioactivity.
If a substance does not possess radioactivity but starts emitting radiations on exposure to rays from a natural radioactive substance, it is said to possess induced or artificial radioactivity.
The first artificial transmutation was caused out by Rutherford in 1919 who bombarded nitrogen gas with alpha particles and obtained hydrogen and oxygen.
The isotope are stable and no further disintegration takes place. Nowadays other bombarding particles (i.e., projectiles) like protons (1H1), deutrons 1H2 electrons can be accelerated to very high speeds by fluctuating electric and magnetic fields in machines such as cyclotron, synchrotron etc. These high speed particles are more efficient in causing nucleus to disintegrate on impact.
The typical transmutations involving various particles are summarised below:
(i) a particle induced reactions
Since a particle is used and a neutron is produced, the reaction is termed as (a, n) reaction.
The first example of radioactivity produced by artificial means is given below:
The isotope produces is itself radioactive.
undergoes decay by positron (b+) emission.
(ii) Deutron induced reactions
(iii) Proton - induced reactions
(iv) Neutron - induced reactions
Thus this phenomenon in which artificial transmutation of a stable non - radioactive nucleus leads to the formation of radioactive isotope is called artificial radioactivity or induced radioactivity.
Some of the isotopes produced as a result of neutron bombardment find applications in different areas.
The features of artificial radioactive isotopes are given below:
i) Artificial radioactive isotopes exhibit behaviour similar to natural radioactive elements and follow some rate law with regard to their disintegration.
ii) Natural radioactivity is shown by elements with high atomic numbers (> 83) whereas artificial radioactivity can be induced in elements with low atomic numbers.
iii) Artificial radioactive isotopes generally have short half-life periods.
iv) They are very rarely found in nature because they decay off as soon as they are formed due to short half-life periods.