Radioactivity refers to the number of decays occurring in a
radioactive substance per one unit of time. The unit of radioactivity,
the becquerel (Bq), equals one disintegration per second. Radiation is
emitted when such decay occurs.
Each radioactive substance has its own half-life, which means the period in which the radioactivity decreases to half of its original value. Half-lives vary from fractions of a second to billions of years.
When reference is made to a radiation dose, what is actually meant is usually the effective dose. The unit for effective dose is the sievert (Sv). In practice, the thousandth and millionth fractions of the dose rate unit Sv/h (mSv/h and μSv/h) are also used.