Module
1: Radiation Properties
This module provides information
about the following topics:
(diagram courtesy of the University of Michigan Student Chapter of the Health
Physics Society)
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.
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. Radioisotopes are arranged by A and Z
in the chart 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:
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·
microwaves
·
visible light
·
radio waves
·
TV waves
·
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 radioisotopes most
commonly used in research at Princeton University.
(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
radioisotopes 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)
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|
1
Becquerel (Bq) = 1 disintegration per second (dps).
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Obviously, 1 Curie is a large amount
of activity, while 1 Becquerel is a small amount. In the typical
Princeton University 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
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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
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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
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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 quantities.
(diagram courtesy of the University of Michigan Student Chapter of the Health
Physics Society )
Radioactive materials decay at
exponential rates unique to each radioisotope. 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 half-life values for
radioisotopes vary widely. For example, the following table shows
half-lives for radioisotopes commonly used at Princeton University:
|
Radioisotope
|
Half-Life
|
|
Hydrogen-3
|
12.3
years
|
|
Carbon-14
|
5730
years
|
|
Phosphorus-32
|
14.3
days
|
|
Phosphorus-33
|
25.3
days
|
|
Sulfur-35
|
87.6
days
|
|
Iodine-125
|
60.1days
|
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