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Introduction
Radiation is an extraordinarily versatile and useful tool in medicine, industry,
and research. It helps saves lives and provides great benefits to mankind
in innumerable ways. Radiation is no different than other tools. If used improperly
it can be hazardous to health or cause injury. But, if proper precautions
are followed radiation can be used safely to achieve superior results. We
will briefly introduce some basic radiation safety concepts and principles
as they apply to the use of portable nuclear gauges. While the human body
can sense and take actions to prevent injury by many physical agents, such
as heat and noise, it cannot sense radiation. Therefore, it is important to
understand the nature of radiation, its sources, and how to protect yourself
and others.
A Brief History
In 1895, a German physicist named Wilhelm Roentgen fortuitously discovered
X-rays while experimenting with evacuated glass tubes through which an electric
current was passed. Roentgen discovered he could take a picture of the bones
in his hand with the mysterious new rays. Henri Becquerel of France discovered
natural radioactivity a year later. In 1898, Pierre and Marie Curie isolated
the first radioactive elements, radium-226 and polonium-210. The momentous
discoveries of these physicists led to a rapid advancement of scientific knowledge
about radiation and radioactivity, as well as to many practical uses.
Types of Radiation
For purposes of radiation safety only radiation with the capability to cause
ionization is of concern. Ionization occurs when electrons are dislodged from
a neutral atom. When this happens an atom becomes positively charged and some
energy is transferred. Ionization is the process by which radiation affects
the human body and by which it can be detected as well. There are four basic
types of ionizing radiation: alpha, beta, gamma and neutron. The main properties
of each type of radiation are briefly discussed below.
Alpha particles consist of two protons and two neutrons and carry a positive
charge. They are emitted with high energy from the nucleus of heavy elements
during radioactive decay, but lose energy rapidly in passing through material.
A couple sheets of paper are sufficient to stop most alpha particles. Since
they cannot penetrate even the outer dead layer of our skin, they are not
an external hazard.
Beta particles are electrons emitted from nucleus of atoms at nearly the speed
of light. They have a very small mass compared to protons or neutrons and
carry a negative charge. Very energetic beta particles can penetrate 1/2 inch
of wood.
Gamma rays are electromagnetic energy waves emitted from the nucleus of atoms
and have no charge. X-rays are the same as gamma rays, except they originate
outside the nucleus from processes involving electrons. Other familiar types
of electromagnetic wave radiation include: visible light, ultraviolet light,
infrared light, microwaves, and radiowaves. These differ from X-rays and gamma
rays only in wave frequency and energy. Gamma rays are much more penetrating
than alpha or beta particles.
Neutrons are elementary particles which are emitted during certain types of
nuclear reactions. Neutrons have no charge and are also highly penetrating.
Units of Measure for Radiation
The primary quantity of interest in radiation protection is dose equivalent.
It so happens that some types of radiation produce greater effects on the
body than others for the same amount of energy absorbed (absorbed dose). To
account for this, a Quality Factor (QF) is assigned to each type of radiation
to express its relative effectiveness in producing damage. Dose equivalent
is the product of the absorbed dose and the QF for that type of radiation.
It expresses the risk of harm resulting from exposure to different types of
radiation on a common scale. The basic unit of dose equivalent is the rem.
Because a rem is relatively large amount, the millirem (1/1000 of a rem) is
often used instead.
Natural Sources of Radiation
Radiation is emitted by radioactive elements naturally present in the soil,
water, and air. The major sources include potassium-40, uranium-238, and thorium-232.
By virtue of their presence in the environment, radionuclides are found all
the way up the food chain to humans. The human body contains a number of radioactive
elements, including potassium-40, radium-226, and carbon-14. Building materials,
like granite, contain radioactive thorium-232. Even the air we breathe contains
small concentrations of the radioactive gas, radon-222, which seeps from the
Earth's crust. Cosmic rays from outer space are another significant natural
source of radiation. The atmosphere screens out most of the cosmic rays, but
some still penetrate to ground level. The dose from cosmic rays increases
with altitude. For example, people living in mile-high Denver receive about
twice as much dose from cosmic rays as people living at sea level. The interaction
of cosmic rays with nitrogen in the atmosphere also produces radioactive carbon-14
and tritium (H-3).
Man Made Sources of Radiation
Manmade radiation is produced directly through the operation of devices like
X-ray machines, particle accelerators, and nuclear reactors. Accelerators
and nuclear reactors may also produce manmade radioactive elements that emit
radiation. Many manmade nuclides are used in medicine, industry, and research.
For example, moisture-density gauges use the manmade sources: cesium-137 (gamma
source), Cf-252 (neutron source), and americium-241:beryllium (neutron source).
Uses of Radiation and Radioactive Materials
Radiation and radioactive materials have many uses in medicine, industry,
education, agriculture, consumer products, scientific research, and many other
fields. Here is a partial list of current uses:
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medical imaging for disease diagnosis |
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disease treatment |
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lightning rods |
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material composition determination |
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f low detection |
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smoke detection |
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medical instrument and food sterilization |
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well logging |
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radiocarbon dating |
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radiotracer studies |
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anti-static devices |
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gemstone coloration |
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vulcanization, cross-linking |
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deep space power source |
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weapon detection, baggage scanning |
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reactor fuel |
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bomb detection |
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emergency exit signs |
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airport runway lights |
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spark gap tubes or glow lamps |
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spark gap irradiators |
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timepiece, instrument, and gunsight lighting |
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physical property measurement (thickness, moisture, density) |
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How Radiation Affects People
Radiation causes ionization in the molecules of living cells. The ions react
with other atoms in the cell causing damage that interferes with vital cell
processes and with cell reproduction. At low doses, such as we receive from
natural background radiation, the cell may be able to repair the damage with
no adverse effect. At higher doses, the cells might not be able to repair
the damage and the cells die or may reproduce abnormal cells that become cancerous.
The primary risk from occupational exposure to radiation is a slightly increased
risk of developing cancer. Several factors influence how much effect a given
radiation dose will have on living cells.
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All cells are not equally sensitive to radiation. Cells that divide rapidly, like blood cells and the lining of the GI tract, are more susceptible to damage than cells that divide slowly, like nerve and brain cells. |
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Dose to the whole body dose carries greater risk than dose to a portion of the body. |
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A given dose received over long time period (years) is less likely to cause an effect than the same dose received over a short time period (hours). |
Radiation Dose Limits
The federal government has set standards for how much radiation can be received
safely. The limit for whole body radiation for persons working in occupations
that involve radiation exposure is 5000 millirem per year. To put this value
in perspective, the average American receives about 360 mrem a year from natural
background radiation.
Protection from Radiation Sources
The radioactive material in portable gauges is in the form of sealed sources,
therefore, there is negligible chance of internal exposure or contamination
from working with a nuclear gauge. The primary concern is external exposure.
The fundamental principle in radiation protection is that all radiation exposures
should be maintained as low as reasonably achievable. This is referred to
as the ALARA principle. The three key factors which influence an individual's
radiation dose from a given source are time, distance and shielding. Control
of these factors, therefore, is the key to keeping radiation dose ALARA.
Time:
The most direct way to reduce radiation dose is to reduce the time
spent working with or in the vicinity of radiation sources. If the exposure
time is cut in half, the dose will be reduced by the same fraction.
Distance:
Distance is one of the most effective means to reduce dose thanks to basic
principles of geometry. When the working distance from a point radiation source
is increased by a factor of two, the dose received from that source will be
reduced by a factor of four. This is referred to as the inverse square law,
i.e., the radiation intensity from a point source decreases with the square
of the distance from the source.
Shielding:
Shielding is any material used to reduce the intensity of radiation by absorbing
or attenuating the radiation coming from the source. Nuclear gauges have a
significant amount of shielding already built in to protect the operator.
Further information about radiation and radiation safety can be found at a
number of sites
on the world wide web.