When we say radioactive material, we mean the material emits ionizing radiation, capable of producing ion pairs in biological materials. Examples are alpha particles, beta, gamma, X-rays and neutrons. So in all cases, the radiation produces electrical interactions in the absorbing material. X-rays are also produced artificially in machines by rapid slowing down of an electron beam.
Everything around us is radioactive. There is cosmic radiation and terrestrial radiation. Cosmic radiation comes from deep space. Its origin is uncertain and the radiation is highly energetic. It is a mixture of many different types of radiation. All these highly energetic radiation particles interact with the atmosphere and as a result cosmic radiation at ground level becomes primarily muons, neutrons, electrons, positrons and photons. Exposure of humans to this cosmic radiation is unavoidable. The resultant dose is called external dose. Depending upon the elevation from the sea level, the dose rate due to exposure of population is different, and varies considerably. During the interaction, gaseous radioactive isotopes (radionuclides) such as Tritium (H-3) and Carbon-14 are also produced. These gaseous radionuclides get mixed up in the air we are breathing. Over time, they get incorporated in the bio-sphere.
Terrestrial radiation comes from the naturally occurring radioactive material present in rocks, in the ocean, in the atmosphere and in living organisms. Humans are also exposed to the radiation emitted by these terrestrial radioactive materials. Examples of these long-lived radionuclides are: Potassium-40, uranium and thorium series (their decay products such as radioisotopes of radium, polonium and lead). The radionuclides are also found in our diet and they get incorporated in the body. One of the radioactive decay products in the uranium and thorium series is gaseous Radon. The two important radionuclide are Radon-220 (from thorium series) and Radon-222 (from uranium series). Being gaseous, they come out of the solid matrix (like: rock/soil/bricks) and get mixed up in the air we are breathing. These inhaled radionuclides result in internal dose to the body.
The unit of radioactivity is Becquerel (Bq). It represents the quantity of radioactive material that emits one one disintegration per second. The older unit is a Curie (Ci) and 1 Curie is equal to 37 billion Bq. Thus, Bq is a very small unit and prefixes of kilo, mega and giga are use for thousand, million and billion respectively are used to express the amount of radioactive material generally used. Curie is a very big unit and hence prefixes of milli, micro and nano are used to express the amount: one-thousandth, one-millionth and one-billionth of a Curie.
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