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8 Ionizing Radiation and Life
Beams of particles are characterized by their intensity (energy per unit area per
unit time) or by their particle fluence rate (number of particles per unit area per unit
time).
8.3 Natural Radiation
At the surface of the Earth, solar radiation is dominated by visible light, but also
contains significant amounts of infrared and ultraviolet radiation. In addition, above
the atmosphere, the Sun exposes the Earth with X-rays and a solar wind made mostly
of a plasma of energetic protons and electrons. Much higher-energy particles come
from extrasolar and extragalactic sources. Our atmosphere and our magnetic field
protects us from much of the damaging effects of these ionizing radiations. The very
energetic protons can penetrate the field, but then get inelastically scattered by the
upper atmosphere, producing a ‘shower’ of particles with less energy, including Xrays, muons, protons, alpha particles, pions, electrons, positrons, and neutrons. The
neutrons, being uncharged, easily reach the ground.
The dose rate from a cosmic-ray shower varies with height and with latitude.
Airline crews receive higher doses of background radiation per year than nuclear
power workers. The dose rate is highest for flights near the polar regions, because
magnetic deflection is least effective there. As a way of comparison, the sea level
annual dose to humans from cosmic rays is about 0.30 mSv per year. The annual
dose, including all natural sources, averages to about 3.4 mSv per year. Thus, cosmic
sources at sea level are only about 1/10th the natural background. An abdominal CT
Scan gives a dose of 6–20 mSv. A single flight from London, England, to Abilene,
Texas, can make a cosmic-ray dose of over 0.03 mSv.
The natural background includes radon inhalation (2.3 mSv/yr), terrestrial
radiation (0.5 mSv/yr), and ingested radiation, such as Potassium-40, Thorium232, and elements from the uranium series (0.29 mSv/yr). Radon-222 and, to a lesser
extent, radon-220, are the largest contributors to the natural background radiation.
They come as a decay product of Uranium-238, Radium-226, and Thorium-232
radioactivity within the Earth. Being an inert gas, radon seeps to the surface. 4 A
square kilometer of soil down 40 centimeters contains about one gram of Radium226, a decay product of uranium. The Radon-222 gas released has a half-life of
only 3.8 days, but it is continually replenished, and is denser than air. Radon
accumulating in basements with poor ventilation is an increased hazard to people
living there. If inhaled into the lungs, it will emit alpha particles and leave other
radioactive nuclides such as polonium, also an alpha emitter, lodged in lung tissue.
After smoking, radon is the second leading cause of lung cancer (causing about
20,000 deaths per year in the United States).
4 Even though uranium atoms are heavier than iron, uranium oxide complexes are less dense,
concentrating in the lower mantle of the Earth and carried to near the surface by magma flow.
8 Ionizing Radiation and Life
Beams of particles are characterized by their intensity (energy per unit area per
unit time) or by their particle fluence rate (number of particles per unit area per unit
time).
8.3 Natural Radiation
At the surface of the Earth, solar radiation is dominated by visible light, but also
contains significant amounts of infrared and ultraviolet radiation. In addition, above
the atmosphere, the Sun exposes the Earth with X-rays and a solar wind made mostly
of a plasma of energetic protons and electrons. Much higher-energy particles come
from extrasolar and extragalactic sources. Our atmosphere and our magnetic field
protects us from much of the damaging effects of these ionizing radiations. The very
energetic protons can penetrate the field, but then get inelastically scattered by the
upper atmosphere, producing a ‘shower’ of particles with less energy, including Xrays, muons, protons, alpha particles, pions, electrons, positrons, and neutrons. The
neutrons, being uncharged, easily reach the ground.
The dose rate from a cosmic-ray shower varies with height and with latitude.
Airline crews receive higher doses of background radiation per year than nuclear
power workers. The dose rate is highest for flights near the polar regions, because
magnetic deflection is least effective there. As a way of comparison, the sea level
annual dose to humans from cosmic rays is about 0.30 mSv per year. The annual
dose, including all natural sources, averages to about 3.4 mSv per year. Thus, cosmic
sources at sea level are only about 1/10th the natural background. An abdominal CT
Scan gives a dose of 6–20 mSv. A single flight from London, England, to Abilene,
Texas, can make a cosmic-ray dose of over 0.03 mSv.
The natural background includes radon inhalation (2.3 mSv/yr), terrestrial
radiation (0.5 mSv/yr), and ingested radiation, such as Potassium-40, Thorium232, and elements from the uranium series (0.29 mSv/yr). Radon-222 and, to a lesser
extent, radon-220, are the largest contributors to the natural background radiation.
They come as a decay product of Uranium-238, Radium-226, and Thorium-232
radioactivity within the Earth. Being an inert gas, radon seeps to the surface. 4 A
square kilometer of soil down 40 centimeters contains about one gram of Radium226, a decay product of uranium. The Radon-222 gas released has a half-life of
only 3.8 days, but it is continually replenished, and is denser than air. Radon
accumulating in basements with poor ventilation is an increased hazard to people
living there. If inhaled into the lungs, it will emit alpha particles and leave other
radioactive nuclides such as polonium, also an alpha emitter, lodged in lung tissue.
After smoking, radon is the second leading cause of lung cancer (causing about
20,000 deaths per year in the United States).
4 Even though uranium atoms are heavier than iron, uranium oxide complexes are less dense,
concentrating in the lower mantle of the Earth and carried to near the surface by magma flow.