Activity

     Activity is the number of unstable nuclei that decay* per second.

       *decay: break down, gives out radiation and change into new stable nucleus.

       Example: Carbon-14 decays into nitrogen-14 and beta radiation is given out.

       The activity of radioactive elements decreases with time.

    Half-life

      Half-life of a radioactive material is the time it takes half the unstable nuclei in a sample to decay.

        or ...

        Half-life of a radioactive material is the time it takes the count rate from a sample to half.

      Higher tier only:

        Calculations of half-life can be used to date materials.