Science 10 - Radiation Chapter Notes!
1. electromagnetic spectrum 9. alpha radiation 17. half-life 25. nuclear fusion
2. visible spectrum 10. beta radiation 18. background radiation 26. nuclear fission
3. electromagnetic wave 11. gamma radiation 19. ionizing radiation 27. chain reaction
4. wavelength 12. Geiger counter 20. cancer 28. nuclear reactor
5. frequency 13. isotopes 21. Gray (Gy) 29. moderator
6. electromagnetic spectrum 14. radioactive decay 22. Sievert (Sv) 30. critical mass
7. radioactive 15. nuclear transmutation 23. X-ray radiographs 31. Albert Einstein
8. nuclear force 16. Becquerel 24. Marie Curie *draw diagrams
RADIATION has numerous forms: e.g. sunlight, the heat from your hand, X-rays, and uranium bombs
all produced different kinds of radiation.
ELECTROMAGNETIC RADIATION is radiation that is in the form of electromagnetic waves. This
includes the light we see with our eyes (called the VISIBLE SPECTRUM, which includes all colours)
plus some invisible types of radiation (radio & TV signals, microwaves, infrared (better known as heat),
ultraviolet, X-rays, and cosmic rays).
Energy can be transferred from place to place in the form of disturbances called waves. There are
different types of waves, but all waves have some things in common.
AMPLITUDE: the distance from the midpoint of the wave crest wavelength
to the trough or crest
WAVELENGTH (): the distance between two successive amplitude
FREQUENCY: the number of crests that pass a given
point per unit time (equals the number of "cycles" per trough
second). Measured in Hertz (Hz) wavelength
The energy of an electromagnetic wave is related to its frequency. As the frequency of the
electromagnetic radiation increases, so does its energy. That is why X-rays and gamma rays, which
have a high frequency and energy (and short wavelength) are dangerous to living things; their waves
have so much energy they can penetrate deeply and can break molecules apart.
WAVE VELOCITY: the speed at which the wave crest appears to move. The speed of a wave will remain
constant unless the medium changes. These terms are related by the following formula:
WAVE VELOCITY = WAVELENGTH X FREQUENCY
v = f = v/f f = v/
Electromagnetic waves all move at the speed of light ("c" = 3.0 x 108 m/s (about 1.1 billion km/hour)
The electromagnetic spectrum is shown below. The sun emits all of these waves, but we only see a
small portion (called the visible spectrum)
R O Y G B I V
T.V./Radi microwav infrared visible U.V. X-Rays gamma
o es rays
longer wavelengths 750 nm 400 nm shorter wavelengths
lower frequency higher frequency
lower energy higher energy
Colour is simply different wavelengths of light:
RED ORANGE YELLOW GREEN BLUE INDIGO VIOLET
700 nm ~500 nm 400 nm
"White Light" is all these colours put together.
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Ever wonder why the sky is blue or rainbows form?
The sky is blue because the sunlight (white light) gets the blue waves are scattered more by the Earth's
atmosphere. The scattered blue waves give the sky its blue colour. The sunlight looks yellow to us
because white light minus the blue part = yellow.
A RAINBOW occurs when white light is split up into its constituent colours by small drops of water in the
Electromagnetic waves are called varying electric field
"electromagnetic" because they have both
an electric part and a magnetic part, moving
at right angles to each other. The electric part
induces the magnetic part, and vice-versa!
varying magnetic field
RADIATIONS FROM ATOMS
Recall that atoms have in their nuclei positively charged protons and neutrons (with no charge). Why
don't the positive charges repel each other and cause the nucleus to break up? The reason is that
the nucleus is held together by a force called the STRONG NUCLEAR FORCE, which is very powerful
but only exerts its effect over a very short range (i.e. the size of the nucleus). There is also a force
called the WEAK NUCLEAR FORCE (which is actually a form of electromagnetic force) that is involved
in the radioactive decay of some atoms.
In 1896, Henri Becquerel (1852-1908) discovered that some atoms have nuclei that are unstable and
give off several types of radiations. Atoms that do this are said to be RADIOACTIVE.
A radioactive element is unstable and will undergo radioactive decay, in which it will give off radiations
from its nuclei. There are three types of RADIOACTIVE EMISSIONS that can be emitted by a nucleus:
1. Alpha Radiation (- rays): heaviest but least penetrating. Like a
2. Beta Radiation (-rays): high energy electrons traveling at high
gamma ray speed.
3. Gamma Radiation (-rays): high energy electromagnetic rays (like
x-rays but can have even more energy. Penetrate the furthest).
Every time an alpha ray is released from an atomic nucleus, the
element gets lighter and changes into another element.
238 4 234
U He Th
92 2 90
Decay will continue with alpha, beta, and gamma rays being emitted until finally an element is formed that
is not radioactive. This stable element is lead (Pb), which has an atomic weight of 206.
Radioactivity can be used to measure time because the rate of decay is very steady and happens at a
constant rate, so it can be used like a clock (indeed, the most accurate clocks use radioactivity to
The term HALF-LIFE refers to the time it takes for half of the atoms in a radioactive sample to
convert to the stable end product.
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This graph shows the decay of a 16 g sample
of Thorium-234. It has a half-life of about 24 days. Radioactive Decay of Thorium-234
So in about 24 days, if you started with 16 grams,
you would have 8 grams. In 48 days you would have
4 grams, and so on. 12
Radioactive decay can not be seen with the 10
naked eye or normal microscopes since the Grams Th 8
radiations are so tiny or invisible, so we have a 6
machine called a GEIGER COUNTER that can 4
detect radiation. 2
ISOTOPES 0 50 100 150
The ATOMIC NUMBER of an element is, by
definition, the number of protons in its nucleus. If the number of protons changes (through alpha
decay, for example), the atom will have a different number of protons, and will have different
characteristics, and will therefore not be the same element any more. Therefore, the atomic number of
an element can never change, because if it did, it would be a different element.
Recall that NEUTRONS have no electric charge. Some elements have different numbers of neutrons.
Since the neutrons have no charge, they don't affect the basic characteristics of the element, though they
contribute to its atomic mass. The MASS NUMBER of an element is the number of protons and
neutrons in its nucleus. This number can change, as it is possible for the same element to have
different numbers of neutrons. Atoms of the same element that have different numbers of neutrons (and
therefore different mass numbers) are called ISOTOPES.
For example, there are 3 isotopes of CARBON:
6 protons, 6 neutrons
6 protons, 7 neutrons
6 protons, 8 neutrons
Carbon-12 is by far the most abundant of the carbon isotopes. It is stable and stays the same.
Carbon-14 is radioactive and is used for finding the age of such things as Egyptian mummies. It
undergoes radioactive decay to form nitrogen.
There are 3 isotopes of hydrogen:
1 2 3
1 H ("regular" hydrogen), 1 H ("Deuterium"), 1 H ("Tritium”). Deuterium is used in nuclear reactors.
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Types of Radioactive Decay
When an unstable nucleus emits radiation, it is said to have undergone radioactive decay.
Again, there are 3 main types of emissions from nuclei, and so there are 3 types of radioactive decay.
1. ALPHA DECAY: the nucleus emits an alpha particle (two protons, two neutrons) and becomes a new,
lighter element. e.g. Uranium-238 decays to Thorium-234 and Helium-4. A new nucleus (Thorium-234)
is thus formed. This new nucleus is called the decay product. When a nucleus changes to another type
of nucleus, it is called a nuclear transmutation.
2. BETA DECAY: the nucleus emits a beta particle (high-energy electron). This happens when a neutron
changes into a proton plus an electron (notice how the charges balance: neutrons can be thought of as
being made of a proton and an electron). When the nucleus loses the electron, it loses one of its
neutrons, but "gains" a proton, As a result, the atomic number increases by one. This also is a
nuclear transmutation. This is what happens to carbon-14 when it decays:
6 C -1e + 14 N
3. GAMMA DECAY: Since gamma rays are not particles, when a parent nuclei undergoes gamma decay, it
retains the same number of protons and neutrons. The nucleus just loses some energy, and therefore
becomes more stable. 56
Ba* 137 Ba + gamma radiation
Radioactivitiy Changes With Time
The ACTIVITY of a sample is the number of nuclei in the sample that undergo radioactive decay each
second. It is measured in becquerels (Bq) after Henri Becquerel. e.g. 10,000 decays in one second is
equal to 10,000 Bq.
Do Review 7.2 on pp 144 - 145 #1 - 8
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