So, What is Radioactivity?

Radioactivity

Atoms that release this energy are referred to as "radioactive" as is any material that contains atoms that are releasing energy as described above.  Their radioactivity is measured in units of "decays per second (dps)", or multiples thereof (i.e.; a Curie = 37,000,000,000 dps, a Becquerel = 1 dps).

OK, so the definition needs a little defining!  Don't quit yet!

The important concept to grasp to understand the first part of the definition is the "unstable nucleus".

As you know, everything, animal, mineral, and vegetable alike, is constructed from fundamental particles called atoms.  Atoms consist of a central nucleus which contains protons which carry a positive charge and neutrons consisting of no charge, surrounded by orbiting electrons which carry a negative charge.

When an energy imbalance occurs in an atomic nucleus, the nucleus will seek to return to a stable state by "shedding" the excess energy.  It does this by releasing pure energy in high energy photons or by releasing particles from the nucleus. 

As for the second part of the definition, when electrons decelerate (as they do when they are stopped by lead) or when they enter a lower energy orbit or shell, they must give up their excess energy.  This energy is given off in photons called x-rays.  These are essentially the same as the gamma rays that come from the nucleus, except some gamma rays are of higher energy.  The name difference is mainly to denote where they originate.