There are three primary sources of background radiation 2 natural and one man-made; Cosmic Radiation, Terrestrial Radiation (both natural), and Man-made Radiation. The units used to measure radiation dose are called REMs. Smaller divisions of this unit are called millirems (mRem) or 1/1000 of a Rem. The average dose from natural radioactive sources range from about 250-450 mRems/year in the United States depending on where you live. There are areas in this country as well as in other countries that are well above these numbers, reaching all the way up to about 2000 mRems/year. The major contribution to this dose in the U.S. is Radon gas, which contributes about 55% of your dose. Between cosmic and terrestrial sources, they contribute about 80% of your natural dose.
The source of cosmic radiation is from outside the solar system. All the energy that was released at the beginning of the universe is still around. The high-energy gamma rays released when stars were formed and colliding hit the earth all the time.
The Sun releases many high-energy particles and gamma rays. These particles interact with the earth and its atmosphere all the time. The Van Allen radiation belts and the magnetic field that surround the Earth, protect the Earth from some of these particles. The atmosphere of the Earth functions as a protective shield from some of these particles and rays as well. This is why one gets more radiation dose as one increases in altitude. At sea level a person receives about 27 mRem per year due to cosmic radiation and this dose doubles for every 6000 ft one increases in altitude.
This Cosmic radiation interacts with the upper atmosphere to create other Radioactive elements. Carbon –14 is produced when the high-energy particles interact with Nitrogen. Hydrogen-3 is produced when these particles interact with nitrogen, oxygen, or lithium in the upper atmosphere. These radionuclides then filter down to the surface of the Earth and get incorporated into the lifecycle of all living things. All living things have some radioactive material in their bodies at all times. Some of this cosmogenic material gets incorporated into the cycle of elements that run through the body daily. This is the basis for C-14 dating. All living things have a constant ratio of radioactive carbon (C-14) to normal carbon (C-12) in their tissues. When an organism dies, be it a person, animal, or plant, there is no more turn over. So all of the radioactive carbon in the tissues starts to decay, then over time this ratio changes at a set rate. So, one can measure this ratio of carbons to determine the period of time that has pasted from when the organism died.
Terrestrial radiation has several sources. Some of this radiation is the result of the decay of 4 main decay chains. The first elements in these decay chains were on the planet earth when it formed. As these radioactive elements decay into other elements, which are also radioactive, they produce a chain of elements that range from the original element down to elements that no longer are radioactive. All of these radioactive elements in these chains are present in specific ratios to the original element. The original radionuclides present are Uranium-235, Uranium-238, Neptunium-237, and Actinium-240. The decay products of these radionuclides include most of the radioactive elements that contribute to the terrestrial exposure of everyone on the planet.
There are two other radionuclides that contribute to the dose of everyone; these were produced independently of the other chains. These two radionuclides are Rb-97 and K-40. K-40 (potassium-40) is responsible for most of the dose received by the human body in everyday activities. The element K is present in every cell in your body and helps with controlling ion concentration gradients and proper nerve function. Some of these K atoms are radioactive and are responsible for your dose. Rb-97 (Rubidium-97) is the element, which is present in some soils, and it accumulates in Brazil nuts, which is the most radioactive food.
Man-made radiation is, contrary to popular belief, the smallest contributor to someone’s radiation dose due to natural sources. Man-made radiation includes the dose received from nuclear weapons testing of the 50’s and 60’s, nuclear power (mostly from the production of the fuel), consumer products, and the main one medical applications. There were 100’s of above and below ground tests of atomic bombs during the decades following WWII. Some of the dust that was introduced into the atmosphere was radioactive dust, which has been circulating in the upper atmosphere for many years. There are many steps involved in the production of the fuel that is used in the cores of nuclear power plants. This includes mining, processing, transportation, and disposal of by-products. Some of these steps introduce naturally occurring radioactive material from where they were dug out of the ground to the surface of the land.
Consumer products such as smoke detectors, lantern mantles, some colored ceramic tiles, electrostatic precipitators, etc. contain radioactive material and contribute a small dose to you. There are also many products that do not contain radioactive materials but used radioactive materials in their production. Products such as some cosmetics, contact lens solution, Teflon pans, and rubber tires are irradiated during their production.
Medical applications are the most familiar to most people. If you ever had an X-ray, a thyroid scan, fluoroscopy, or other procedure that uses radiation, you know about their uses. The dose that you will get from an X-ray, or other medical procedure, will be much greater than the dose that you would get from any of the other man-made sources of radiation.
If you would like to know more about background radiation, what consumer products have or use radiation, or medical applications, please feel free to email me with your questions or to set up a class so I can answer everybody’s questions.