Physical half life time (Tp)
The time interval required for an amount of certain radioactive nuclei to decay to its half of original value.
Biological half life time (Tb)
The time interval required for the body to eliminate 50% of any substance by normal routes of elimination: metabolic turn over and excretion.
Effective half life time (Te)
The time interval required for the radioactivity of a certain amount of radioactive substance distributed in tissues and organs to decrease to half its original value due to radioactive decay and biological elimination.
The Physical half life time is longer than the biological half life time which is longer than the effective half life time.
1/Te = 1/Tp+1/Tb