Objective: What are the major sources of radiation exposure?
It depends on obvious factors such as occupation and area of residence, but it also depends on more subtle factors such as medical history and travel habits. To sort out these factors, the EPA conducted a survey in 1980 and derived average levels in the U.S. for two major categories: natural and artificial sources.
NATURAL SOURCES (also called background levels) sum to about 100 mrems/year.
They include the following sub-categories.
1. Terrestrial sources include all land based sources. U.S. community levels range from about 15 to 100 mrem/year. Globally, levels are as high as 2 rems/year (e.g., parts of Brazil). Various materials contribute radiation, including granite, coal (especially in the western U.S.), and clay (brick homes have typically twice the radiation of wood homes). The most serious radiation exposure from terrestrial sources is usually radon, because it is a gas (i.e., inhalation exposure).
2. Cosmic radiation (also called extra-terrestrial sources) originates from deep space, from such sources as our own sun (a minor contribution) and supernovas. Primary cosmic radiation is mostly protons, some electrons, and various atomic fragments. When these sources hit our atmosphere, their impact creates secondary cosmic radiation, composed of mostly gamma rays, electrons, mesons, and neutrinos. Cosmic radiation varies from 40-160 mrem/year depending on three major factors:
1) altitude (e.g., one plane flight cross country can contribute 1 mrem, and Denver has about twice the cosmic radiation of L.A);
2) latitude (cosmic radiation is attracted to our magnetic poles); and
3) air pressure (as pressure increases, cosmic radiation decreases at ground level).
ARTIFICIAL SOURCES sum to about 80 mrem/year on average.
1. Medical procedures are the major source, with a single chest X-ray contributing an average 200 mrem. Dental x-rays are far less than chest x-rays.
2. Nuclear power plants contribute, on average, about 1 mrem/year. However, this is only for residents closest to the facility. Of course, this does not include other concerns such as nuclear accidents and nuclear waste disposal.
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Types of ionizing radiation:
Direct (charged particles)
1. alpha Helium atoms stripped of their electrons
particles: low penetrating power; strong ionizer
2. beta electron particles
particles: moderate penetrating power; moderate ionizer
3. other charged
particles: single protons, charged fragments, etc.
Indirect
4. gamma rays: electromagnetic radiation from nucleus
high penetrating power; weak ionizer
5. X-rays: electromagnetic radiation from electrons
high penetrating power; weak ionizer
6. neutrons: 1 proton, 1 beta particle, 1 neutrino
Units of Measurement
activity (of source): radioactive decay ("disintegrations")
7. Curie: (Ci) rate from one gm of natural radium-226/second
= 37 billion disintegrations/second
8. Becquerel: (Bq) = 1 disintegration/second
exposure: ionization in air
9. Roentgens: (R) = 1 esu/cc of air = 773,400 esu/kg
10. Exposure
unit: 1 coulomb/kg = 3,789 R = 3 billion esu/kg
absorbed dose: energy absorbed
11. RAD: radiation absorbed dose
(100 ergs/gram of absorbing material)
12. Gray: (Gy) = 100 RADS
dose equivalent: biological effect
13. REM: roentgen equivalent man = RADs x RBE
14. RBE: relative biological effectiveness (a ratio)
15. LET: linear energy transfer (energy transferred/unit length)
16. Sievert: (Sv) = 100 REMs
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Objective: To provide a few examples of the positive applications of our
knowledge of radiation.
1. Iodine-131 tracers: "radioactive cocktail"
A. beta radiation
1/2-life = about 8 days
B. used to study thyroid activity and thyroid treatment
(thyroid concentrates iodine)
note: 99.9% eliminated in about 80 days
2. NAA (neutron activation analysis): "atomic fingerprint"
A. bombard sample material with neutrons
B. measure frequency and intensity of resultant gamma radiation
C. used to measure trace quantities of pollutants
(e.g., heavy metals)
3. Americium-241
A. beta radiation
1/2-life = about 400 years
B. on end of lightning rods, increases attraction
C. also used in smoke detectors:
ionizes air between two electrodes (creates current)
smoke interferes with current, thereby activating alarm
risk of cancer is negligible and outweighed by risk of fires
4. Cobalt-60
A. cheapest form of gamma radiation
B. used in cancer treatments, food irradiation
5. Carbon dating (Carbon-14)
A. how carbon-14 is created in air:
cosmic rays include neutrons:
N + N ---> C + H
in air, C-14 is at stable concentration in CO2
exception -- 20th century activities:
atomic bomb testing
increased CO2 levels
B. how Carbon-14 is used to date materials (e.g., paper)
plants take in CO2 (photosynthesis)
when plants are alive, in equilibrium with environment
upon plant death, C-14 undergoes beta decay (e.g., paper)
1/2-life = 5,760 years
carbon dating accurate up to 50,000 years
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1. Non-ionizing electromagnetic energy
radiation: that cannot ionize,
but may cause health effects
Starting with the highest frequencies:
2. Ultraviolet
exposure: sun, industrial equipment, tanning booths (!)
effects: sunburn, skin cancers
fever, nausea
cataracts, retina damage
controls: clothing, sunglasses, suntan lotions
3. Visible light
exposure: normally no risk
eclipse can cause retinal burn
lasers (light amplification by stimulated
emission of radiation)
effects: burn hole in retina (even at low quantities)
controls: various filters, glasses
4. Infrared
exposure: quickly detected
effects: burns, cataracts, retina damage
controls: special clothing and shielding
5. Microwaves
exposure: radar, T.V., radio, microwave ovens
effects: heating (cooked tissue),
interferes with pacemakers,
association w/ cataracts, cancer, birth defects
controls: distance, materials
6. Electromagnetic fields
exposure: power transmission lines
electric blankets, toasters, hair dryers,
T.V., video display terminals, etc.
effects: not proven !
controls: prudent avoidance (?)
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