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Physics 12
Mr. Jean
January 13th, 2012
The plan:
• Video clip of the day
• Finish BBC video
• Types of radioactive decays.
Friday the 13th in 2012…
• There are 3 Friday the 13th’s in 2012
• Each Friday the 13th is exactly 13 weeks a
part from each other.
• Spooky?
Symbols Used for
Nuclear Reactions:
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Neutron
Proton
Electron
Alpha particle
Gamma ray
Beta particle
Nuclear Reactions:
• Nuclear reactions are somewhat different than
chemical reactions. In chemical reactions, the
equation is balanced when the number of each of the
different elements on the reactant side equals the
number of the different elements on the product side.
• In nuclear reactions, the atomic number and the mass
number for each element must be balanced on both
sides (in addition to the number of elements).
– We say that the mass number and atomic number must be
conserved. The effect of balancing the atomic number is to
actually balance the charge of the reactant and product.
Alpha Decay:
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Alpha particles. The symbol for the alpha
particle is .
 particles are helium nuclei. Each alpha
consists of 2 protons and 2 neutrons.
's have a positive charge (+2).
's are only slightly deflected by a magnetic
field (because of their large mass).
They are stopped easily by a sheet of paper.
Alpha Decay Fact:
• All nuclei with more than 83 protons
spontaneously begin to decay.
• They are always trying to get back to a more
stable substance.
• The most stable substance in the world is iron…
there’s a million things I can say about iron, stars
and spectrum analysis.. So let’s leave this for
another day….. Ok.. Here’s a teaser…
• Another measure of stability is binding energy,
the amount of energy needed to overcome the
strong nuclear force and pull apart a nucleus.
The binding energy per nucleon for the most
stable isotope of each naturally occurring
element is illustrated in Figure 2.
• Binding energy reaches a maximum of 8.79
MeV/nucleon at 56Fe.
• As a result, there is an increase in stability when
much lighter elements fuse together to yield
heavier elements up to 56Fe and when much
heavier elements split apart to yield lighter
elements down to 56Fe, as indicated by the
arrows.
Alpha Decay:
• Alpha decay: In alpha decay, an unstable nucleus
produces a daughter nucleus and releases an  particle.
• The reason alpha decay occurs is because the nucleus
has too many protons which cause excessive repulsion.
In an attempt to reduce the repulsion, a Helium nucleus
is emitted. The way it works is that the Helium nuclei are
in constant collision with the walls of the nucleus and
because of its energy and mass, there exists a nonzero
probability of transmission. That is, an alpha particle
(Helium nucleus) will tunnel out of the nucleus.
• Key:
– The atomic mass of the original element has
decrease by 4 because helium has an atomic
mass of 4.
– The atomic number of the original element
has decreased by 2 because helium has an
atomic number of 2. (2 Protons)
Beta Decay:
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There are three types of Beta Decay:
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Beta particles. The symbol for the beta particle is
.
's are electrons, so they have a have a negative
charge (-1).
They can be greatly deflected by a magnetic field
(because of their small mass and negative charge).
's penetrate matter a greater distance than 
particles, but they still aren’t very penetrating. They
can be stopped by a layer of metal foil.
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Beta Decay:
• Beta decay occurs when the neutron to proton
ratio is too great in the nucleus and causes
instability. In basic beta decay, a neutron is
turned into a proton and an electron. The
electron is then emitted. Here's a diagram of
beta decay with hydrogen-3 going to Helium-3:
• Beta decay: In  decay a neutron in the
nucleus of the unstable radioactive parent
decays and becomes a proton as it emits a 
particle (an electron).
• Example: Thorium-234 has one of its neutrons
become a proton - this increases the atomic
number by one, but has no effect on the mass
number since a neutron and a proton are both
nucleons. A beta particle is also produced.
• Note that the atomic number on the left is equal to the
total atomic number on the right.
• During electron capture, the atomic number of
the daughter decreases by one, there is no
change to the mass number.
• Note that in this reaction, you could produce gold from
mercury. (When you build a machine that can do this let
me know, I want in…)
Positron Emission:
• There is also positron emission when the
neutron to proton ratio is too small. A proton
turns into a neutron and a positron and the
postiron is emitted. A positron is basically a
positively charged electron. Here's a diagram of
positron emission with carbon-11:
Electron Capture:
• The final type of beta decay is known as
electron capture and also occurs when the
neutron to proton ratio in the nucleus is too
small. The nucleus captures an electron which
basically turns a proton into a neutron. Here's a
diagram of electron capture with beryllium-7:
Danger from Beta Decay:
• Electron fired out of the reactions at very
high velocity.
– Luckily can be blocked relatively easily.
• High energy particles are also releases.
– Gamma Ray
– This is because your element is going from a
high particle state to a lower energy states (ie:
energy is released)
Gamma Decay:
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Gamma rays. The symbol for gamma rays is .
Very short wavelength, high frequency photons.
Gamma rays have no charge.
They are not deflected by magnetic fields
They are the most penetrating form of radiation.
Stopping ’s requires great thicknesses of heavy
materials such as lead or concrete.
Gamma Decay:
• Gamma decay occurs because the nucleus is
at too high an energy. The nucleus falls down
to a lower energy state and, in the process,
emits a high energy photon known as a
gamma particle. Here's a diagram of gamma
decay with helium-3:
Finishing the BBC Video:
• If there is time..
• There is still 10 minutes left to the video
from earlier on this week.