Background radiation is all around us. We can do little to avoid it. Most background radiation comes from natural sources, while most artificial radiation comes from medical examinations and industrial processes.
Sources of background radiation.
Granite – used for house building – is the major source of radon. In addition, the Earth’s rocks contain a large number of naturally occurring isotopes, such as uranium.
Cosmic radiation from outer space depends on altitude. Flying in an aircraft increases the dose of cosmic rays by a factor of ten.
Isotopes which are made artificially in nuclear reactors are used for a variety of medical and industrial purposes. Notice that nuclear weapons account for a tiny fraction of the background dose. Workers in these places must have their radiation dose monitored to avoid over-exposure. For more on detecting radioactivity, go here.
For more general information on radioactivity, look here.