Radiation



Radiation is the emission of energy as either waves (electromagnetic
radiation) or particles (particle radiation).

Radiation is produced by radioactive decay, nuclear fission and nuclear
fusion, chemical reactions, hot objects, and gases excited by electric
currents.

Radiation is often separated into two categories, ionizing and non-ionizing,
to denote the energy and danger of the radiation. Ionization is the process
of removing electrons from atoms, leaving electrically charged particles
(ions) behind.

Many forms of radiation such as heat, visible light, microwaves, or radio
waves do not have sufficient energy to remove electrons from atoms and
hence, are called non-ionizing radiation. In the case of heat, for objects
at room temperature, most of the energy is transmitted at infra-red
wavelengths.

The negatively charged electrons and positively charged nuclei created by
ionizing radiation may cause damage in living tissue. The term radioactivity
generally refers to the release of ionizing radiation.

Radioactive materials usually release alpha rays (particles similar to the
nuclei of helium), beta rays (quickly moving electrons) and gamma rays.
Alpha and beta rays can often be shielded by a piece of paper or a thin
sheet of steel. They cause most damage when they are emitted inside the
human body. Gamma rays are less ionising than either alpha or beta rays, but
protection against them requires thicker shielding. They produce damage
similar to that caused by X-rays such as burns, cancer, and genetic
mutations. Human biology resists germ-line mutation by aborting most mutated
conceptuses.

Sources of Radiation

Natural Background Radiation

The earth, and all living things on it, are constantly bombarded by
radiation from space, similar to a steady drizzle of rain. Charged particles
from the sun and stars interact with the earth's atmosphere and magnetic
field to produce a shower of radiation, typically beta and gamma radiation.
The dose from cosmic radiation varies in different parts of the world due to
differences in elevation and the effects of the earth's magnetic field.

Radioactive material is found throughout nature. It occurs naturally in the
soil, water, and vegetation. The major isotopes of concern for terrestrial
radiation are uranium and its decay products, such as thorium, radium, and
radon. Low levels of uranium, thorium, and their decay products are found
everywhere. Some of these materials are ingested with food and water, while
others, such as radon, are inhaled. The dose from terrestrial sources varies
in different parts of the world. Locations with higher concentrations of
uranium and thorium in their soil have higher dose levels.

In addition to the cosmic and terrestrial sources, all people also have
radioactive potassium-40, carbon-14, lead-210, and other isotopes inside
their bodies from birth. The variation in dose from one person to another is
not as great as the variation in dose from cosmic and terrestrial sources.

Man Made Radiation Sources

Natural and artificial radiation sources are identical in their nature and
their effect. Above the background level of radiation exposure, the NRC
requires that its licensees limit man-made radiation exposure to individual
members of the public to 100 mrem (1 mSv) per year, and limit occupational
radiation exposure to adults working with radioactive material to 5,000 mrem
(50 mSv) per year.

The exposure for an average person is about 360 millirems/year, 81 percent
of which comes from natural sources of radiation. The remaining 19 percent
results from exposure to man-made radiation sources.

By far, the most significant source of man-made radiation exposure to the
general public is from medical procedures, such as diagnostic X-rays,
nuclear medicine, and radiation therapy. Some of the major isotopes would be
I-131, Tc-99m, Co-60, Ir-192, Cs-137, and others.

In addition, members of the public are exposed to radiation from consumer
products, such as tobacco (polonium-210), building materials, combustible
fuels (gas, coal, etc.), ophthalmic glass, televisions, luminous watches and
dials (tritium), airport X-ray systems, smoke detectors (americium), road
construction materials, electron tubes, fluorescent lamp starters, lantern
mantles (thorium), etc.

Of lesser magnitude, members of the public are exposed to radiation from the
nuclear fuel cycle, which includes the entire sequence from mining and
milling of uranium to the disposal of the used (spent) fuel. The effects of
such exposure have not been reliably measured. Estimates of exposure are low
enough that proponents of nuclear power liken them to the mutagenic power of
wearing trousers for two extra minutes per year (because heat causes
mutation). Opponents use a cancer per dose model to prove that such
activities cause several hundred cases of cancer per year.

In a nuclear war, gamma rays from fallout of nuclear weapons would probably
cause the largest number of casualties. Immediately downwind of targets,
doses would exceed 30,000 roentgen/hr. 450 R (more than a thousand times the
background rate) is fatal to half of a normal population. No survivors have
been documented from doses above 600 R.

Occupationally exposed individuals are exposed according to their
occupations and to the sources with which they work. The exposure of these
individuals to radiation is carefully monitored with the use of
pocket-pen-sized instruments called dosimeters. Some of the isotopes of
concern would be cobalt-60, cesium-137, americium-241, and others. Examples
of industries where occupational exposure is aconcern include:

   * Fuel cycle
   * Industrial Radiography
   * Radiology Departments (Medical)
   * Radiation Oncology Departments
   * Nuclear power plant
   * Nuclear medicine Departments
   * National (government) and university Research Laboratories

The Effects of Ionizing Radiation on Animals

We tend to think of biological effects of radiation in terms of their effect
on living cells. For low levels of radiation exposure, the biological
effects are so small they may not be detected. The body repairs many types
of radiation and chemical damage. Biological effects of radiation on living
cells may result in four outcomes:

  1. injured or damaged cells repair themselves, resulting in no residual
     damage
  2. cells die, much like millions of body cells do every day, being
     replaced through normal biological processes
  3. cells incorrectly repair themselves resulting in a biophysical change.
  4. Low levels of ionizing radiation may be beneficial to many types of
     cells; this phenomenon is termed radiation hormesis, see below.

The associations between radiation exposure and the development of cancer
are mostly based on populations exposed to relatively high levels of
ionizing radiation (e.g., Japanese atomic bomb survivors, and recipients of
selected diagnostic or therapeutic medical procedures).

Cancers associated with high dose exposure include leukemia, breast,
bladder, colon, liver, lung, esophagus, ovarian, multiple myeloma, and
stomach cancers. Department of Health and Human Services literature also
suggests a possible association between ionizing radiation exposure and
prostate, nasal cavity/sinuses, pharyngeal and laryngeal, and pancreatic cancer.

The period of time between radiation exposure and the detection of cancer is
known as the latent period. Those cancers that may develop as a result of
radiation exposure are indistinguishable from those that occur naturally or
as a result of exposure to other chemical carcinogens. Furthermore, National
Cancer Institute literature indicates that other chemical and physical
hazards and lifestyle factors (e.g., smoking, alcohol consumption, and diet)
significantly contribute to many of these same diseases.

Although radiation may cause cancer at high doses and high dose rates,
public health data do not certainly establish the occurrence of cancer
following exposure to low doses and dose rates -- below about 10,000 mrem
(100 mSv).

Most studies of occupational workers exposed to chronic low-levels of
radiation above normal background have shown no adverse biological effects.
Even so, the radiation protection community conservatively assumes that any
amount of radiation may pose some risk for causing cancer and hereditary
effect, and that the risk is higher for higher radiation exposures.

The linear dose-response model suggests that any increase in dose, no matter
how small, results in an incremental increase in risk. The LNT hypothesis is
accepted by the NRC as a conservative model for estimating radiation risk.

High radiation doses tend to kill cells, while low doses tend to damage or
alter the genetic code (DNA) of irradiated cells. High doses can kill so
many cells that tissues and organs are damaged immediately. This in turn may
cause a rapid whole body response often called Acute Radiation Syndrome. The
higher the radiation dose, the sooner the effects of radiation will appear,
and the higher the probability of death.

This syndrome was observed in many atomic bomb survivors in 1945 and
emergency workers responding to the 1986 Chernobyl nuclear power plant accident.

Approximately 134 plant workers and firefighters battling the fire at the
Chernobyl power plant received high radiation doses (70,000 to 1,340,000
mrem or 700 to 13,400 mSv) and suffered from acute radiation sickness. Of
these, 28 died from their radiation injuries.

Radiation hormesis

It has never been proven that very low doses of ionizing radiation are
harmful. A small but growing number of studies offer evidence that they may
even have some beneficial effects.

A linear, no-threshold (LNT) dose response relationship is widely assumed to
be valid by most policy makers and many scientists. However, a small but
growing number of scientists hold that this relationship is grossly
misleading, and may be totally wrong. One problem is that this relationship
ignores known cellular repair mechanisms; cells in all organisms have
efficient methods to detect and repair damage.

Some scientists hold that these linear-response curves were created with an
anti-nuclear political and social agenda in mind, and have little or no
scientific validity.

It is easy to show the fallacy of the linear no-threshold relationship:
Background radiation in our everyday environment does not kill people, yet
radiation blasts from nuclear fission events (e.g. worst-case meltdowns or
nearby atomic bombs) can almost immediately kill a person. These deadly
radiation events are nearly a million times more powerful than background
radiation. Compare this to taking one aspirin a day (which we may call
background level); this has been proven to be harmless for most people, and
actually has substantial medical benefits for many people. If one were to
take one million aspirin a day, that person would die immediately. The same
is true of most essential vitamins and minerals; small amounts are harmless,
or even necessary for life. Doses a million times larger are not healthy,
and potentially fatal. No scientist would make a linear graph for these
phenomenon, and then work backwards to prove that aspirin is deadly, or that
vitamins and minerals are deadly. Yet this same flaw in logic is often
applied to radiation, and to radiation alone.

In fact, there are few, and perhaps none at all, linear dose-relationships
in nature that hold true over all dosage scales.

Some scientists point out that when life first arose over 2 billion years
ago, it evolved in an environment that had thousands of times more
background radiation than we are exposed to today. This means that there
must be much more room for life to live safely in a low radiation
environment than once was previously imagined. None of this, of course, is
meant to minimize the risks of higher levels of ionizing radiation.

Minimizing Health Effects of Ionizing Radiation

Although exposure to ionizing radiation carries a risk, it is impossible to
completely avoid exposure. Radiation has always been present in the
environment and in our bodies. We can, however, avoid undue exposure.

Although people cannot sense ionizing radiation, there is a range of simple,
sensitive instruments capable of detecting minute amounts of radiation from
natural and man-made sources.

Dosimeters resemble pens, and can be clipped to one's clothing. They measure
an absolute dose received over a period of time. They must be periodically
recharged, and the result logged.

Geiger counters and scintillometers measure the dose rate of ionizing
radiation directly.

In addition, there are four ways in which we can protect ourselves:

Time: For people who are exposed to radiation in addition to natural
background radiation, limiting or minimizing the exposure time will reduce
the dose from the radiation source.

Distance: In the same way that the heat from a fire is less intense the
further away you are, so the intensity of the radiation decreases the
further you are form the source of the radiation. The dose decreases
dramatically as you increase your distance from the source.

Shielding: Barriers of lead, concrete, or water give good protection from
penetrating radiation such as gamma rays and neutrons. This is why certain
radioactive materials are stored or handled under water or by remote control
in rooms constructed of thick concrete or lined with lead. There are special
plastic shields which stop beta particles and air will stop alpha particles.
Inserting the proper shield between you and the radiation source will
greatly reduce or eliminate the extra radiation dose.

Shielding can be designed using halving thicknesses, the thickness of
material that reduces the radiation by half. Halving thicknesses for gamma
rays are discussed in the article gamma rays.

Containment: Radioactive materials are confined in the smallest possible
space and kept out of the environment. Radioactive isotopes for medical use,
for example, are dispensed in closed handling facilities, while nuclear
reactors operate within closed systems with multiple barriers which keep the
radioactive materials contained. Rooms have a reduced air pressure so that
any leaks occur into the room and not out of it.

In a nuclear war, an effective fallout shelter reduces human exposure at
least 1000 times. Most people can accept doses as high as 100 R, distributed
over several months, although with increased risk of cancer later in life.
Other civil defense measures can help reduce exposure of populations by
reducing ingestion of isotopes and occupational exposure during war time.
One of these available measures could be the use of potassium iodide (KI)
tablets which effectively block the uptake of dangerous radioactive iodine
into the human thyroid gland.

See also: Electromagnetic radiation, Particle radiation, Gamma rays,
radioactivity, radiation therapy, adaptive radiation, fallout shelter,
nuclear war, nuclear weapon, civil defense.

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