Half life

Half life

1. The unit for measuring radioactivity is the becquerel (Bq). 1 Bq means one nucleus is decaying per second.

2. The radioactivity of a sample always decreases over time (half life).

   • HALF LIFE IS THE AVERAGE TIME IT TAKES FOR THE NUMBER OF NUCLEI IN A RADIOACTIVE ISOTOPE TO HALF.
     • In other words its the time it takes for the count rate for a sample containing the isotope to fall by half its initial level.
   • When radioactive decay occurs an alpha, beta or gamma particle is released. This means that one more radioactive nucleus has disappeared.
   • As the unstable nuclei all steadily disappear, the activity (the number of nuclei that decay per second) will decrease.
     • So the older a sample becomes the less radiation it will emit.
   • How quickly the activity drops off varies a lot.
     • For some substances it takes just a few microseconds before nearly all the unstable nuclei have decayed, for others it can take millions of years.
   • A short half life means the activity falls quickly, because lots of nuclei decays quickly.
   • A long half life means the activity falls slower because most of the nuclei don't decay for along time.

3. Do half life question step by step

   • A typical question: The activity of a radioactive isotope is 640 cmp (counts per minute). Two hours later is has fallen to 80 cpm. Find the half life.