radguide

10 Radiation for Radionuclide Users Activity The quantity which expresses the degree of radioactivity or radiation producing potential of a given amount of radioactive material is activity. The special unit for activity is the curie (Ci) which was originally defined as that amount of any radioactive material which disintegrates at the same rate as one gram of pure radium. The curie has since been defined more precisely as a quantity of radioactive material in which 3.7 x 10 10 atoms disintegrate per second. The International System (SI) unit for activity is the becquerel (Bq), which is that quantity of radioactive material in which one atom is transformed per second. The activity of a given amount of radioactive material does not depend upon the mass of material present. For example, two one-curie sources of 137Cs might have very different masses depending upon the relative proportion of non radioactive atoms present in each source. The concentration of radioactivity, or the relationship between the mass of radioactive material and the activity, is called the specific activity . Specific activity is expressed as the number of curies or becquerels per unit mass or volume. Half-life Each radionuclide decays at its own unique rate which cannot be altered by any chemical or physical process. A useful measure of this rate is the half-life of the radionuclide. The half-life is defined as the time required for the activity of any particular radionuclide to decrease to one-half of its initial value. Half-lives of radionuclides range from microseconds to billions of years. 0.0 0.2 0.4 0.6 0.8 1.0 0 14.3 28.6 42.9 57.2 71.5 Time (Days) Activity (mCi) Half-life decay scheme of 32 P