Naturally occurring radioactive nuclei have a physical half life time of ~108 to 1010 years which makes them unsuitable for use in medical imaging.
Thus radioactive nuclei used in medical practice are artificially synthesized. Production of these nuclei involve bombarding stable nuclei with protons, neutrons or alpha particles to disturb their N/Z (neutron/proton) ratio and alter their stability.
Production processes occur in 3 different ways:
A. Nuclear reactor
Involves bombarding stable nuclei with neutrons resulting in radioactive nuclei with neutron excess e.g. Co60 and Mo99 .
The resultant nuclei will have the same atomic number (Z) as the parent nuclei but their mass number (A) will exceed the parent's by one and thus they cannot be separated by chemical means.
27Co59 + n1 ----------> 27Co60 + γ
42Mo98 + n1 ----------> 42Mo99 + γ
Involves bombarding stable nuclei with protons resulting in the production of neutron-deficit radioactive nuclei (of short T1/2).
The resultant nuclei have different atomic number from the parent (increased by one) but have the same mass number. Thus they can be separated by chemical means.
5B11+ 1p1 ----------> 6C11 + n1 + γ
C. Nuclear generators
Use some radioactive nuclei with long half life time to produce other with shorter half life time.
42Mo99 -------67hrs-------> 42Tc99m -------6hrs-------->42Tc99 + γ
NB: γ (Greek letter gamma) represents gamma radiation