HUMAN-MADE SOURCES

Radioactivity is produced by a variety of human activities. The most well known in the early years of atomic research was the fission bomb. Its modern descendant is the vastly more powerful hydrogen fusion bomb. Such a weapon, when detonated, produces an immediate, intense burst of ionizing radiation. The high-speed subatomic particles produced in the initial blast, especially if the explosion occurs at or near the ground, cause large amounts of material to become radioactive. The resulting radioactive dust, called fallout, precipitates back to Earth over a period of time. Some fallout, especially from the largest nuclear bombs, can rise high into the troposphere and enter the jet streams, where it is carried around the globe.
Nuclear fission reactors contain radioactive elements. The heat from the decay of these elements is used to generate electrical power. Some byproducts of fission are radioactive, and because they cannot be reused to generate more power, they represent radioactive waste. Disposal of this waste is a problem because it takes many years, even centuries, to decay. If a fusion reactor is ever developed and put into use, it will be a vast improvement over the fission reactor because controlled hydrogen fusion produces no radioactive waste.
Radioactive isotopes can be produced by bombarding the atoms of certain elements with high-speed subatomic particles or energetic gamma rays. Charged particles are accelerated to relativistic speeds by particle accelerators, also known informally as atom smashers. A linear particle accelerator is a long, evacuated tube that employs a high voltage to accelerate particles such as protons, alpha particles, and electrons to speeds so great that they can alter or split certain atomic nuclei that they strike. A cyclotron is a large ring-shaped chamber that uses alternating magnetic fields to accelerate the particles to relativistic speeds.