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Radiation NOTES
When we speak of “radiation” we are usually referring to two related concepts:
1) the wave forms of energy represented by electromagnetic radiation; and
2) forms of energy released by radioactive decay (also known as nuclear decay) of certain unstable atoms.
ELECTROMAGNETIC RADIATION and the ELECTROMAGNETIC SPECTRUM:
Electromagnetic Radiation is most simply defined as light, and is broken up in to regions based on the
frequency of the light. The full range of radiation is called the electromagnetic spectrum. Electromagnetic
radiation is naturally transmitted by stars (including our sun), travels at the speed of light, and can vary in
wavelengths from 0.000000000001 meter (Gamma rays) to 10,000 m (television & radio)! The major regions
of the electromagnetic spectrum are radiowaves, microwaves, infrared, visible light, ultra violet (UV), x-rays
and gamma rays.
Radio waves are used to transmit radio and television signals. Radio waves have wavelengths that range from
less than a centimeter to tens or even hundreds of meters. FM radio waves are shorter than AM radio waves.
Radio waves can also be used to create images. Radio waves with wavelengths of a few centimeters can be
transmitted from a satellite or airplane antenna. The reflected waves can be used to form an image of the
ground in complete darkness or through clouds.
Microwave wavelengths range from approximately one millimeter (the thickness of a pencil lead) to thirty
centimeters (about twelve inches). In a microwave oven, the radio waves generated are tuned to frequencies
that can be absorbed by the food. The food absorbs the energy and gets warmer. The dish holding the food
doesn't absorb a significant amount of energy and stays much cooler. Microwaves are emitted from the Earth,
from objects such as cars and planes, and from the atmosphere. These microwaves can be detected to give
information, such as the temperature of the object that emitted the microwaves.
Infrared is the region of the electromagnetic spectrum that extends from the visible region to about one
millimeter (in wavelength). Infrared waves include thermal radiation. For example, burning charcoal may not
give off light, but it does emit infrared radiation which is felt as heat. Infrared radiation can be measured using
electronic detectors and has applications in medicine and in finding heat leaks from houses.
The rainbow of colors we know as visible light is the portion of the electromagnetic spectrum with
wavelengths between 400 and 700 billionths of a meter (400 to 700 nanometers). It is the part of the
electromagnetic spectrum that we see, and coincides with the wavelength of greatest intensity of sunlight.
Visible waves have great utility for the remote sensing of vegetation and for the identification of different
objects by their visible colors.
Ultraviolet radiation has a range of wavelengths from 400 billionths of a meter to about 10 billionths of a
meter. Sunlight contains ultraviolet waves which can burn your skin. Most of these are blocked by ozone in the
Earth's upper atmosphere. Large doses of UV radiation can cause skin cancer and cataracts. Ultraviolet
wavelengths are used extensively in astronomical observatories. Some remote sensing observations of the
Earth are also concerned with the measurement of ozone.
X-rays are high energy waves which have great penetrating power and are used extensively in medical
applications and in inspecting welds. X-ray images of our Sun can yield important clues to solar flares and
other changes on our Sun that can affect space weather. The wavelength range is from about ten billionths of
a meter to about 10 trillionths of a meter.
Gamma rays have wavelengths of less than about ten trillionths of a meter. They are more penetrating than Xrays. Gamma rays are generated by radioactive atoms and in nuclear explosions, and are used in many medical
applications. Images of our universe taken in gamma rays have yielded important information on the life and
death of stars, and other violent processes in the universe.
The wave properties of electromagnetic radiation: Imagine a buoy floating on water. As a boat passes by,
the waves produced will cause the buoy to bob up and down. This wave is a periodic disturbance or
oscillation that passes through space. A wave consists of repeating units called cycles. The vertical motion of
the buoy is caused by the passage of successive crests and troughs as the waves move through the water. The
wave properties of electromagnetic radiation are described by two independent variables.
Frequency: () refers to the number of cycles of
the wave that pass a given point each second. This
is the vertical motion. The buoy will bob up and
down times per second.
Wavelength: () refers to the distance between
two successive points on the wave. Simply
speaking, peak to peak or trough to trough is
typically defined as the wavelength.
Types of radiation are arranged in order by their wavelengths. The wavelength of a wave is the distance
between 2 consecutive points of a wave. Typically we say peak to peak or trough to trough as these points are
very easy to pick out on the wave. But one could look at the middle of the peak to the middle of the next peak
and call that the wavelength. The frequency of a wave may be defined by how often the wave passes a fixed
point in space in 1 second. For example, 1 peak/second = 1 Hz; 10 peaks/second = 10 Hz.
1
The units for frequency are peaks per sec (or
) often called Hertz (Hz).
sec
meters
The units for wavelength are meters per cycle or
cycle
meters
sec
We can get those units by multiplying frequency x wavelength
The speed of the wave is the distance traveled per some period of time. Thus,
Speed of a wave:  x  (units are
meters
)
sec
In a vacuum, the speed of any type of electromagnetic radiation is the same and is defined as
2.9979 x 108 meters/sec. This is defined as the speed of light – and is defined as c – which is a constant.
c=x
2.9979 x 108 meters/sec =  x 
There is an inverse relationship between frequency and wavelength:
Large wavelength = small frequency
Small wavelength = High frequency
For example, if the wavelength is long (red and orange lines for example) the frequency (how many
wavelengths pass a point in a certain period of time) is low. Large wavelengths thus mean low frequency. If
the wavelength is short (blue and purple lines) then the number wavelengths that pass a particular point in a
certain period of time is high. For example, if the above picture is a snapshot of 1 second, the red line has 1.5
wavelengths (or thereabouts) passing a point in a second. The purple line has 14 wavelengths passing in 1
second.
The total amount of energy represented by a wave within the electromagnetic spectrum is proportional to the
frequency of the wave. Thus, higher frequency waves represent greater energy. The amount of energy
represented by a given wave is calculated using Planck’s equation:
E = h
where h = Planck’s constant = 6.626 x 10-34 J•s
 = frequency (as above)
RADIOACTIVITY: RADIOACTIVE DECAY (also known as NUCLEAR DECAY):
Radioactivity refers to the property of some elements (such as uranium) of spontaneously emitting energy in
the form of radiation as the result of the nuclear decay (meaning disintegration or falling apart of the nucleus
of the atom) of certain unstable atoms. These radioactive decay (or nuclear decay) reactions are
spontaneous and occur in any atom with more than 83 protons, or which has an exceptionally small or large
proton to neutron ratio. Radioactivity is also the term used to describe the rate at which radioactive material
emits radiation. Radioactivity is measured in curies (Ci), becquerels (Bq), or disintegrations per second.
In order to describe radioactive decay (nuclear decay), we must first be comfortable with atomic notation, as
derived from the Periodic Table.
Atomic Notation:
A
Z
X
A = Atomic Mass = number of protons + number of neutrons
Z = Atomic Number = number of protons = number of electrons
X = Atomic Symbol = the elements’ letter designation
12
6
C
The upper left hand corner indicates the atomic mass of the atom.
Remember: atomic mass = number of protons + number of neutrons.
The lower right hand corner gives the atomic number of the atom.
Remember: atomic number = # of protons = # of electrons
We will concern ourselves with four main forms of radiation: alpha, beta, gamma, and X-ray.
Alpha particles: An alpha particle is simply a helium nucleus (He) which is ejected with high energy from an
unstable nucleus of a radioactive element. This particle, which consists of two protons and two neutrons, has
a mass number of 4 and a net positive charge (+2). Although emitted with high energy, alpha particles lose
energy quickly as they pass through matter of air and therefore, have low penetrating power and a short
range (a few centimeters in air). The most energetic alpha particle will generally fail to penetrate the dead
layers of cells covering the skin, and can be easily stopped by a sheet of paper. Alpha particles are generally
hazardous only when an alpha-emitting isotope is inside the body. These slow moving particles are generally
the product of heavier elements.
Example : 23892U ----> 42He + 23490Th
Note that the mass number is decreased by 4 and a new element is formed.
Beta particles: Beta particles are identical to electrons and thus have a charge of (-1). This type of decay
process leaves the mass number of the nucleus unchanged. The electron that is released was not present
before the decay occurred, but was actually created in the decay process itself. A beta particle is tiny in
comparison to that of an alpha particle and has about one hundred times the penetrating ability. Where an
alpha particle can be stopped by a piece of paper a beta particle can pass right through. Beta particles may be
stopped by thin sheets of metal (such as aluminum foil) or plastic. Large amounts of beta radiation may cause
skin burns, and beta emitters are harmful if they enter the body.
Example : 3215P ----> 0-1e + 3216S
Note that the mass number is unchanged and a new element is formed. So what was the effect of this
Beta particle production? It actually changed a neutron into a proton, as reflected by the change in the
atomic number.
Gamma Rays: As the name implies, these are not particles but high-energy, short-wavelength,
electromagnetic radiation waves emitted from the nucleus of an atom. They are very similar to X-rays but
have a shorter wavelength and therefore more energy. The penetrating ability of gamma rays is much greater
than that of alpha or beta particles. They can only be stopped by several centimeters of lead or more than a
meter of concrete. In fact, gamma rays can pass right through the human body. Gamma ray emissions often
accompany other processes of decay such as alpha or beta. An example of this was our previous
representation of an alpha particle process, which is more accurately written as.
Example : 23892U ----> 42He + 23490Th + 
A ramification of alpha or beta particle production is that the newly formed nucleus is left in a state of
excess energy. A way for the nucleus to release this excess energy is by emitting gamma rays. Since
gamma rays have no mass, and are waves rather than particles, the elements atomic number does not
change after emission.
X-rays : X-rays are a form of highly penetrating electromagnetic radiation having a wavelength that is much
shorter than that of visible light. These rays are usually produced by excitation of the electron field around
certain nuclei, rather than by the radioactive decay of the nucleus itself. X-rays are high energy, but with a
lower frequency (and hence lower energy) than gamma rays.
Other Types of Nuclear Reactions include:
 In Positron Emission, a positive beta particle is emitted;
 In Electron Capture, an electron in orbit around nucleus is drawn into the nucleus combining with a proton
to produce a neutron. The mass of the nucleus remains constant but the atomic number drops by 1.
 In Neutron Emission, a neutron is emitted from the nucleus and causes the mass to drop by 1.
Nuclear Equations: Nuclear equations are similar to chemical equations in that the total mass must be
conserved. The difference is that the components in the nucleus change. For instance, in the first example
below of beta particle emission by decay of C-14, the mass does not change but the number of protons does
giving a total mass on each side of 14 and a total number of protons as 6, since 6 = 7 + -1.
14
6
C147 N 10
Field Effects: The following two drawings depict what occurs when the 3 main types of radiation pass through
an electric field and a magnetic field. As you can see, 2 are effected and the 3rd is not. The Beta particle’s
flight is skewed the most, as it is the lightest. The alpha’s path is also altered, but due to it being about 7200
times more massive its path is not altered as much as beta’s. The gammas path is not altered at all, why? It
has not charge, and therefore is not effected by either a magnetic field or electric field.
What is Ionizing Radiation?
Ionizing radiation consists of subatomic particles or electromagnetic waves that are energetic enough to
detach electrons from atoms or molecules, ionizing them. Two of the most common forms of ionizing
radiation are gamma rays and X-rays.
Both forms of ionizing radiation are almost identical with exception to their source of origination. Gamma rays
originate from the nucleus of an unstable element undergoing radioactive decay, while X-rays originate in the
electron fields surrounding the nucleus of an atom or are machine produced.
How is Ionizing Radiation Generated?
Ionizing radiation comes from radioactive sources such as cobalt-60 and cesium-137 and non-radioactive
sources such as X-ray tubes. Radioactive sources are unstable materials that generate gamma rays as they
decay. X-rays are generated in a vacuum tube where high voltage is used to accelerate electrons to a high
velocity, that then collide with a metal target, an anode creating X-rays.
How is X-ray and Gamma Ionizing Radiation Different?
There are three primary differences between X-ray and gamma ionizing radiation; frequency, wavelength, and
photon energy. While the first two are used as identifiers to differentiate the various wavelengths, the third,
photon energy describes the energy or speed at which the rays are traveling. This energy equates to
penetration power; the higher the energy the greater the penetration power. For both gamma and X-ray,
energies are emissions in free space. In actual use, where they are confined in a lead chamber, the energies of
both are affected by scattering and fluorescence until actual energy spectrums are difficult
to define. The greater the energy, the more shielding is required for safe operation.
 and  radiation are really not a threat
outside the body. The dead layer of
skin that covers your body is enough to
shield you from them.  and  radiation
are really only harmful if the radiation is
generated from within the body. A
radioactive isotope can be eaten, drank
or breathed in. When this occurs the
isotope can decay. The tissue in the
body has no protective layer and is
easily ionized, killing the surrounding
cells. Or worse, mutating their DNA…
Half-life(t1/2) is defined as the time required for one half of a number of an isotope to decay, into a new
isotope. Half-lives differ greatly from isotope to isotope, they can range from picoseconds to billions of years.
t1/2 is the symbol of half-life.
Radioactive Dating: Radioactive dating uses half-lives to determine how long something has been around. All
objects take in radioactive materials over the course of their lives. When they die they stop taking in these
radioactive materials. The radioactive materials decay over time. If you know how much of a particular
radioactive material is in a live organism you have the No value. You can use a detector to determine the
amount of this radioactive material in the dead organism, which give you N. If you know the half-life you can
calculate how long the organism has been dead.
C-14 dating is most famous isotopes used for dating but it has limitations. The accuracy of half-life dating is
very poor after the isotope has undergone 5 half-life cycles. So, C-14 with a half-life of 5730 years is not very
accurate after 40,000 years. Potassium-40 is used to date farther back in history. K-40 has a half-life of 1.28 x
109 years this allows scientist to date much older objects, meteors, dinosaurs, crater, etc. A table of useful
isotopic decays follows.
Useful Isotopic Decay
System
Material
Half-life/years
Age range/years
C-14
organic remains
5730
200 - 50,000
U-238 to Pb-206 ratio
Minerals
4510 million
10-4500 million
U-235 to Pb-207 ratio
Minerals
704 million
10-4500 million
Rb-87 to Sr-87 ratio
Minerals
48,800 million
60-4500 million
K-40 to Ar-40 ratio
Minerals
1250 million
0.1-3000 million
Sm-147 to Nd-143 ratio
Minerals
110,000 million
1000-4500 million
Decay Series: All radioactive isotopes will release radioactivity from their unstable nucleus until the nucleus is
stable. The radioactive decays change
the nuclei from one isotope to another
depending on which type of radiation
was released. Following an alpha
decay, the mass drops by 4 AMU and
the atomic number by 2. Therefore
when a U-235 undergoes alpha decay
it becomes Th-231. Following a beta
decay, the mass remains constant and
the atomic number rises by 1.
Therefore when a Th-231 undergoes
beta decay it becomes Pa-231. These
are the first two steps in the decay
series of U-235. This is the decays
series for U-235.