1.  The nucleus of the atom that emits ionising radiation is unstable. The elements that emit ionising radiation are called radioactive. The behaviour of radioactive elements cannot be changed by physical or chemical processes.     2. The natural sources of background radiation are: the Sun, rocks, food and outer space            (cosmic rays).     3. The artificial sources of background radiation are: hospital waste, medical uses (X-rays),         nuclear power stations, fallout from nuclear explosions and nuclear accidents.     4. Some rocks, like granite gives off the radioactive gas radon. If this gas is trapped in houses,         it may cause cancer. The danger from this radioactive gas can be minimised by ventilating the         rooms of the house.     5. Background radiation is detected and measured using a Geiger-Muller tube and a ratemeter.