SOURCES OF RADIATION EXPOSURE
WHAT ARE THE SOURCES OF RADIATION EXPOSURE ?
There are two types or radiation: (1) ionizing and (2) nonionizing:
Ionizing radiation (e.g., x-rays) knocks electrons from atoms producing ions (i.e., ionizations).
Nonionizing radiation (e.g., sunlight) usually does not dislodge the electrons and is considered not as hazardous as ionizing radiation.
Here, the focus is on sources and exposures to the more hazardous ionizing as may arise from a radiological incident.
Radioactive substances are those that give off radiation.
Radioactivity measures how radioactive a substance is. Units for expressing radioactivity are presented in Section 2.
Marie Curie a Polish-born chemist, was the first to coin the word radioactivity, even though the Frenchman Henri Becquerel discovered it in 1896. In 1898, Marie and her husband, Pierre, discovered that uranium gave off radiation and then mysteriously turned into other elements. One of these elements she called polonium, after her homeland. Another she called radium, the “shining” element. A few years earlier in 1895, Wilhelm Roentgen, a German physicist, had discovered x-rays.
The term irradiated refers to the process of being exposed to radiation.
For radiological incident scenarios, one mainly deals with the following types of radiation: (1) alpha radiation, (2) beta radiation, (3) gamma rays, and (4) neutrons.
Alpha Radiation Alpha radiation is made up of helium atoms stripped of electrons. The stripped atoms are called alpha particles.
Emitted alpha particles are positively charged. They can be stopped by a piece of paper and pose a concern mainly when emitted inside the body.
Alpha emitters are isotopes that emit alpha particles. When the alpha emitter is on the surface of the skin, the dead layer of the skin adequately protects the body from harm by trapping the emitted alpha particles. However, if the skin has a wound, the alpha emitter can enter the body through the wound and cause harm.
Beta Radiation Beta radiation is made up of negatively charged electrons. These electrons are called beta particles.
Beta emitters are isotopes that emit beta particles. Beta particles can pass through the dead layer of the skin and cause harm to the body.
Clothing can provide some protection from beta radiation injury to humans when the beta emitter lands on the outside of the clothing. Beta emitters that are inhaled into the lung or ingested into the gastrointestinal tract can cause harm.
Gamma Rays Gamma rays have no charge or mass. Gamma rays can go through the entire body and can damage multiple organs.
Gamma emitters are isotopes that emit gamma rays. Gamma emitters that deposit on the outside of the body and those taken inside the body both can cause harm. Clothing provides little protection from gamma rays.
Neutrons Neutrons have mass but no charge. Neutrons can penetrate deep into the body, and while doing so, can produce gamma rays through their interaction with tissue atoms. Thus, all neutron exposures involve some gamma rays. Clothing provides essentially no protection from neutrons.
Figure 1.1 shows the relative penetrating power of different radiations.
Figure 1.2 shows how ions produced by different radiations are spaced differently in tissue because of the different penetrating power of the different radiations.
People are exposed to radiation in mainly two modes:
From radiation sources outside the body (external exposure).
From radioactive substances that are inhaled or ingested into the body (internal exposure).
Figure 1.3 shows the external and internal models of exposure. Radioisotopes taken into the body deposit in and irradiated different tissue depending on their chemical properties.
Radiation sources can also enter the body via wounds.
There are many different radiation exposure scenarios that can be evaluated. Some examples follow:
External exposure from relatively distant radiation sources (e.g., neutrons and/or gamma rays)
External exposure from nearby radioactive soil
External exposure from radioactive contamination on the outside of the body
Internal exposure from inhaled radioactive substances
Internal exposure from ingested radioactive substances
Combinations of the above
The radiation exposures may be brief as during a nuclear detonation or protracted (i.e., spread over an extended period) as could arise after inhaling or ingesting radioactive substances released from a nuclear accident or radiological weapon.
Exposure to radiation from natural sources also occurs for everyone but has no impact on performance and is more relevant to public health risk assessment.
1.1 Natural Sources
Radiation took part in the big bang, which is the event considered to have created the universe around 20 billion years ago. Since that time, radiation has been present throughout the cosmos. Everything in the cosmos gets exposed to at least small amounts of radiation.
Radioactive substances became part of our planet Earth when it was formed, presumably as a result of the big bang. Throughout the history of Earth, cosmic radiation has fallen on its surface from outer space, and terrestrial radiation has come from radioactive substances in its crust.
The major part of the radiation received by the world’s population comes from natural sources. Although everyone on our planet gets irradiated from natural sources, some people at certain locations get much more radiation than others because of where they live (e.g., locations with particularly radioactive rocks and soils).
All humans were born slightly radioactive because all living tissue contains radioactive substances. The radioactive characteristic is maintained throughout life. However, the normal radioactivity found in humans is nothing to worry about. Furthermore, nothing can be done to eliminate it.
The main radioactive materials in rocks are potassium-40, rubidium-87, and two series of radioactive elements arising from the decay of uranium-238 and thorium-232. Uranium-238 and thorium-232 are long-lived radioactive isotopes that have remained on Earth since its origin. The levels of terrestrial radiation differ from place to place around the world because the concentrations of these materials in the Earth’s crust vary.
The naturally occurring isotopes carbon-14 and tritium are produced by cosmic radiation. Other naturally occurring isotopes of interest include potassium-40, lead-210, polunium-210, radon-222, and radon-220.
Public exposure to radon in homes has received much attention by governmental agencies.
1.2 Nuclear Workplace
The nuclear workplace includes occupational settings such as nuclear power plants, nuclear weapons production facilities, nuclear medicine-related facilities, and radiation therapy facilities. Many workers are routinely exposed to radiation in such facilities. However, their radiation exposures are usually strictly controlled to avoid harmful effects of irradiation.
One nuclear worker population worth mentioning relates to Russians who were involved in producing plutonium for nuclear weapons during the late 1940s through the 1960s. These activities took place at the Mayak plutonium production facility in the Chelyabinsk region near the Urals Mountains. Workers that were involved in using nuclear reactors to make plutonium were exposed to both neutrons and gamma rays. Workers that were involved in extracting the plutonium were exposed to high levels of both alpha and gamma radiations. Some of these exposures caused the loss of lives.
1.3 Nuclear Weapons
Nuclear weapons initially produce both gamma rays and neutrons after detonation (Figure 1.4). These radiations present the primary early exposure hazard. Some weapons are designed to produce many neutrons (neutron bombs). Delayed exposure can arise from the radioactive fallout (from the sky) from fission weapons and involves alpha-, bet
, and gamma-emitting isotopes. Anything on which the fallout lands (e.g., ground, lake, people, etc.) can become highly radioactive. In 1954, Japanese fishermen on a boat (Lucky Dragon) in the Pacific were covered with fallout from a distant U.S. thermonuclear test explosion.
The radioactive cloud that carries the fallout from fission weapons emits radiation. This radiation is called cloud shine. Radiation can also come from fallout that deposits on the ground. This radiation emission is called ground shine.
A useful rule of thumb is the rule of sevens. This rule states that every seven-fold increase in time following a fission detonation (starting after 1 hour), the airborne radioactivity decreases by a factor of about 10. Thus, after 7 hours (following hour 1), the residual radioactivity decreases to about one-tenth its level at 1 hour after the detonation. This rule of sevens corresponds to an approximate t-1.2 scaling relationship. Here, t is time after arrival of fallout in hours.
1.4 Radiological Weapon Incidents
Radiological weapons are explosive devices intended to spread radioactive material over a target area and cause harm (e.g., to people). The radioactive zone can be influenced by environmental conditions such as wind direction and speed as well as by the location (e.g., height) of the detonation. A radiological incident involving a radiological weapon could have serious impacts on health and also on property values. Human exposures to radiation could involve external exposures (e.g., to beta particles from radionuclides deposited on the skin) or internal exposures (e.g., from inhaled radioactive material).
1.5 Nuclear Reactor Accidents/Destruction
Nuclear accidents, such as occurred in 1979 at Three Mile Island in the U.S. and in 1986 at Chernobyl in Russia, lead to public and worker exposures to radiation. Unlike the Three Mile Island accident, the Chernobyl accident led to the loss of many lives. Such accidents can lead to the release of large amounts of alpha-, beta-, and gamma-emitting radionuclides (i.e., radioactive isotopes) into the environment.
As with nuclear weapons, fallout can arise from nuclear power plant accidents. The terms “cloud shine” and “ground shine” are therefore also used in describing radiation sources associated with nuclear accidents.
Nuclear facilities can also be destroyed by weapons leading to similar radiation sources such as those that arise from nuclear accidents.
1.6 Medical Treatment
Radiation sources are used to diagnose and treat cancer. This leads to exposure of both patients and medical personnel. Medical x-rays are routinely administered at most hospitals.
Radiation sources are used in various industries and have led to worker exposures.
Radioactive isotopes occur in the environment as a result of natural processes and normal human activities. Human exposure pathways associated with radioactive isotopes in the environment are presented in Figure 1.5.