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The Bohr Model of the atom consists of a central nucleus composed of
neutrons and protons surrounded by a number of orbital electrons equal
to the number of protons.
Protons are positively charged, while neutrons have no
charge. Each has a mass of about 1 atomic mass unit or amu. Electrons
are negatively charged and have mass of 0.00055 amu.
(Diagram courtesy of the University of Michigan Student
Chapter of the Health Physics Society )
The number of protons in a nucleus determines the element of the atom.
For example, the number of protons in uranium is 92 while the number in
neon is 10. The proton number is often referred to as Z.
An element may have several isotopes. An isotope of an
element is comprised of atoms containing the same number of protons as
all other isotopes of that element, but each isotope has a different number
of neutrons than other isotopes of that element. Isotopes may
be expressed using the nomenclature Neon-20 or 20Ne10, where 20 represents
the combined number of neutrons and protons in the atom (often referred
to as the mass number A), and 10 represents the number of protons (the
atomic number Z).
While many isotopes are stable, others are not. Unstable isotopes
normally release energy by undergoing nuclear transformations (also called
decay) through one of several radioactive processes described later in
this module.
Elements are arranged in the periodic table with increasing Z.
radionuclides are arranged by A and Z in the chart of the nuclides.
Go
to a detailed periodic table of the nuclides.
Radiation is energy transmitted through space in the form of electromagnetic
waves or energetic particles. Electromagnetic radiation, like
light or radio waves, has no mass or charge. The following chart
shows the electromagnetic spectrum.
This training is concerned with radiation that has sufficient energy
to remove electrons from atoms in materials through which the radiation
passes. This process is called ionization, and the high frequency
electromagnetic waves and energetic particles that can produce ionizations
are called ionizing radiations. Examples of ionizing radiation
include:
-
alpha particle radiation
-
beta particle radiation
-
neutrons
-
gamma rays
-
x-rays
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Nonionizing radiations are not energetic enough to ionize atoms and
interact with materials in ways that create different hazards than ionizing
radiation. Examples of nonionizing radiation include:
-
microwaves
-
visible light
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radio waves
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TV waves
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ultraviolet light
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The atomic structure for certain isotopes of elements is naturally unstable.
Radioactivity
is the natural and spontaneous process by which the unstable atoms of an
isotope of an element transform or decay to a different state and emit
or radiate excess energy in the form of particles or waves. These
emissions are energetic enough to ionize atoms and are called ionizing
radiation. Depending on how the nucleus loses this excess energy,
either a lower energy atom of the same form results or a completely different
nucleus and atom is formed.
A given radioactive isotope decays through a specific transformation
or set of transformations. The type of emissions, along with the
energy of the emissions, that result from the radioactive decay are unique
to that isotope. For instance, an atom of phosphorus-32 decays
to an atom of non-radioactive sulfur-32, accompanied by the emission of
a beta particle with an energy up to 1.71 million electron-volts.
The following sections describe the radiations associated with the radioactive
decay of the radionuclides most commonly used in research at the University of Washington.
(diagram courtesy of the University of Michigan Student
Chapter of the Health Physics Society )
An alpha particle consists of two neutrons and two protons ejected
from the nucleus of an atom. The alpha particle is identical to the nucleus
of a helium atom.
Examples of alpha emitters are radium, radon, thorium, and uranium.
Because alpha particles are charged and relatively heavy, they interact
intensely with atoms in materials they encounter, giving up their energy
over a very short range. In air, their travel distances are limited
to no more than a few centimeters. As shown in the following illustration,
alpha particles are easily shielded against and can be stopped by a single
sheet of paper.
(diagram courtesy of the University of Michigan Student
Chapter of the Health Physics Society )
Since alpha particles cannot penetrate the dead layer of the skin, they
do not present a hazard from exposure external to the body.
However, due to the very large number of ionizations they produce in
a very short distance, alpha emitters can present a serious hazard when
they are in close proximity to cells and tissues such as the lung.
Special precautions are taken to ensure that alpha emitters are not inhaled,
ingested or injected.
(diagram courtesy of the University of Michigan Student
Chapter of the Health Physics Society )
A beta particle is an electron emitted from the nucleus of a
radioactive atom.
Examples of beta emitters commonly used in biological research are:
hydrogen-3 (tritium), carbon-14, phosphorus-32, phosphorus-33, and sulfur-35.
Beta particles are much less massive and less charged than alpha particles
and interact less intensely with atoms in the materials they pass through,
which gives them a longer range than alpha particles. Some energetic
beta particles, such as those from P-32, will travel up to several meters
in air or tens of mm into the skin, while low energy beta particles, such
as those from H-3, are not capable of penetrating the dead layer of the
skin. Thin layers of metal or plastic stop beta particles.
(diagram courtesy of the University of Michigan Student
Chapter of the Health Physics Society )
All beta emitters, depending on the amount present, can pose a hazard
if inhaled, ingested or absorbed into the body. In addition, energetic
beta emitters are capable of presenting an external radiation hazard, especially
to the skin.
An important consideration in shielding beta particle radiation is the
ability of beta particles to produce a secondary radiation called
bremsstrahlung.
Bremsstrahlung are x-rays produced when beta particles or other electrons
decelerate while passing near the nuclei of atoms. The intensity
of bremsstrahlung radiation is proportional to the energy of the beta particles
and the atomic number of the material through which the betas are passing.
Consequently, bremsstrahlung radiation is generally not a concern for
lower energy beta emitters such as carbon-14 and sulfur-35, but the higher
energy betas from phosphorus-32 can produce significant bremsstrahlung,
especially when passing through shielding materials such as lead.
Lower atomic number materials such as Plexiglas are preferred shielding
materials for high energy emitters such as phosphorus-32.
(diagram courtesy of the University of Michigan Student
Chapter of the Health Physics Society )
A gamma ray is a packet (or photon) of electromagnetic radiation
emitted from the nucleus during radioactive decay and occasionally accompanying
the emission of an alpha or beta particle. Gamma rays are identical
in nature to other electromagnetic radiations such as light or microwaves
but are of much higher energy.
Examples of gamma emitters are cobalt-60, zinc-65, cesium-137, and radium-226.
Like all forms of electromagnetic radiation, gamma rays have no mass
or charge and interact less intensively with matter than ionizing particles.
Because gamma radiation loses energy slowly, gamma rays are able to travel
significant distances. Depending upon their initial energy, gamma
rays can travel tens or hundreds of meters in air.
(diagram courtesy of the University of Michigan Student
Chapter of the Health Physics Society )
Gamma radiation is typically shielded using very dense materials
(the denser the material, the more chance that a gamma ray will interact
with atoms in the material) such as lead or other dense metals.
Gamma radiation particularly can present a hazard from exposures external
to the body.
(diagram courtesy of the University of Michigan Student
Chapter of the Health Physics Society )
Like a gamma ray, an x-ray is a packet (or photon) of electromagnetic
radiation emitted from an atom, except that the x-ray is not emitted from
the nucleus. X-rays are produced as the result of changes in the
positions of the electrons orbiting the nucleus, as the electrons shift
to different energy levels.
Examples of x-ray emitting radionuclides are iodine-125 and iodine-131.
X-rays can be produced during the process of radioactive decay or as
bremsstrahlung radiation. Bremsstrahlung radiation are x-rays produced
when high-energy electrons strike a target made of a heavy metal, such
as tungsten or copper. As electrons collide with this material, some
have their paths deflected by the nucleus of the metal atoms. This deflection
results in the production of x-rays as the electrons lose energy.
This is the process by which an x-ray machine produces x-rays.
Like gamma rays, x-rays are typically shielded using very dense materials
such as lead or other dense metals.
X-rays particularly can present a hazard from exposures external to
the body.
(diagram courtesy of the University of Michigan Student
Chapter of the Health Physics Society )
Quantity
The quantity of radioactive material present is generally
measured in terms of activity rather than mass, where activity is
a measurement of the number of radioactive disintegrations or transformations
an amount of material undergoes in a given period of time. Activity is
related to mass, however, because the greater the mass of radioactive material,
the more atoms are present to undergo radioactive decay.
The two most common units of activity are the Curie or the Becquerel
(in
the SI system).
| 1 Curie (Ci) = 3.7 x1010 disintegrations per
second (dps) |
| 1 Becquerel (Bq) = 1 disintegration per second (dps). |
Obviously, 1 Curie is a large amount of activity, while 1 Becquerel
is a small amount. In the typical University of Washington laboratory,
millicurie and microcurie (or kilo and MegaBecquerel) amounts of radioactive
material are used.
| 1 millicurie = 2.2 x 109 disintegrations per
minute (dpm) = 3.7 x 107 Bq = 37 MBq |
| 1 microcurie = 2.2 x 106 dpm = 3.7 x 104
Bq = 37 kBq |
Intensity
For the purposes of radiation protection, it is not always useful to
describe the potential hazard of a radioactive material in terms of its
activity. For instance, 1 millicurie of tritium a centimeter from
the body poses a much different hazard than 1 millicurie of phosphorus-32
a centimeter from the body.
Consequently, it is often preferable to measure radiation by describing
the effect of that radiation on the materials through which it passes.
The three main quantities which describe radiation field intensity are
shown in the following table:
| Quantity |
Unit |
What is measured |
Amount |
| Exposure |
Roentgen (R)
Coulombs/kg |
Amount of charge produced
in 1 kg of air by x- or gamma rays |
1 R = 2.58 x 10-4 Cb/kg |
| Absorbed Dose |
rad
Gray (Gy) |
Amount of energy absorbed in 1 gram of matter from radiation |
1 rad = 100 ergs*/gram
1 Gy = 100 rad |
| Dose Equivalent |
Rem
Sievert (Sv) |
Absorbed dose modified by the ability of the radiation to cause biological
damage |
rem = rad x Quality Factor
1 Sv = 100 rem |
* An erg is a unit of work.
Coulombs/kilogram, the Gray, and the Sievert are the SI units for these
quantitities.
For
more detailed information about the meaning of these quantities and units.
(diagram courtesy of the University of Michigan Student
Chapter of the Health Physics Society )
Radioactive materials decay at exponential rates unique to each radionuclide.
Half-life
is the time required for a given amount of some radioactive material to
be reduced to one-half of its original activity.
The following table shows half-lives for radionuclides commonly used
in sealed sources at the University of Washington:
| Radionuclide |
Half-Life
|
| Sodium-22 |
2.6 yr
|
| Iron-55 |
2.7 yr
|
| Cobalt-57 |
271.8 days
|
| Cobalt-60 |
5.3 yr
|
| Strontium-90 |
29.1 yr
|
| Cesium-137 |
30.1 yr
|
| Polonium-210 |
138.4 days |
| Radium-226 |
1600 yr |
| Americium-241 |
432.7 yr |
Go
to an on-line calculator that will calculate the activity of these common
radionuclides at any elapsed time.
This is the end of the Radiation Basics Module, which is the first
of seven Sealed Source Radiation Basics modules. The next module
is the Background Radiation & Other Sources of Exposure Module.
Background
Radiation & Other Sources of Exposure Module (Module 2)
Back
to the Sealed Source Training Introduction Page
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