Internal Dose Time Dependence—Biological Half-Life

6 INTERNAL DOSE TIME DEPENDENCE-BIOLOGICAL HALF-LIFE 

Radioisotopes may enter the body by inhalation, drinking, eating, injection, or through broken skin (wound). If the radiation source is inside the body, the exposure is internal and more damaging. No attenuation is provided by skin or 

A radioisotope is rejected by the body at a rate that depends upon the chemical properties of the element. All isotopes of the same element are rejected at the same rate, whether they are stable or not. For most radioisotopes, the rate of rejection is proportional to the amount of the isotope in the body. This leads to an exponential elimination law as a result of the combination of decay and rejection. Let 

N t) = number of radioactive atoms at time t = radiological decay constant Ag = biological decay constant 

The rate of change of MO is dN t)/dt = —AgNit) — AgNUX with solution 

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