Radioactive Decay

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Radioactive Decay
Predictions Chapter 1, Activity 8
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Radioactive Decay

• Some configurations of protons and neutrons in
  the nucleus are not as stable as others.
• When the nucleus is not stable, it will
  spontaneously transform into a more stable
  configuration.
• For example, Carbon-14 is not stable and will
  therefore spontaneously decay into Nitrogen-14.
• The process by which an atom spontaneously
  changes into a more stable one is called
  radioactive decay.

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Radio-Isotopes

• Isotopes are configurations of atoms that have
  the same number of protons, but different numbers
  of neutrons.
• Radio-isotopes are unstable isotopes that go
  through radio active decay.

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Types of Radioactive Decay

• Alpha Decay
• Beta Minus Decay
• Beta Plus (Positron) Decay
• Gamma Decay

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Alpha Decay
Alpha Decay occurs when two protons and two
neutrons (or a helium nucleus) are emitted from
the nucleus of the parent atom.
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Beta Minus Decay
Beta Minus Decay occurs when a neutron decays
into a proton and an electron, where the latter
is emitted from the nucleus.
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Beta Plus Decay
Beta Plus Decay occurs when a proton decays into
a neutron and a positive electron (positron),
where the latter is emitted from the nucleus.
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                                                 A
lpha particles can usually be stopped by a very
thin barrier. Radioisotopes emitting alpha
particles are usually not hazardous outside the
body, but they can cause damage if
ingested. Betas (streams of electrons) can pass
through a hand, but are usually stopped by a
modest barrier such as a few millimeters of
aluminum, or even a layer of clothing. As with
alphas, beta particles are more hazardous if
inhaled or ingested. Gammas can be very
penetrating and can pass through thick barriers.
Several feet of concrete would be needed to stop
some of the more energetic gammas. A natural
gamma source found in the environment (and in the
human body) is 40K, an isotope of
potassium. Neutrons are also very penetrating.
Some elements, like hydrogen, capture and scatter
neutrons. Water is commonly used as a neutron
radiation shield.
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Half-Life

• 1 half-life equals the amount of time required
  for ½ of the original number of parent atoms to
  decay into daughter atoms.

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Half-Life
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Example

• You have 600 grams of iodine-133. The half-life
  of iodine-133 is 21 hours.
• How much of this sample would exist after 21
  hours?
• After 42 hours?
• After 126 hours?
• Remember the amount of radioactive substance
  diminishes by ½ for every half-life.

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Example 1 (cont.)

• Begin by determining the number of half-lives by
  dividing the elapsed time by the half-life.
• 21 hours/21 hours 1 half-life
• 42 hours/21 hours 2 half-lives
• 126 hours/21 hours 6 half-lives

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Example 1 (cont.)

• Since the amount of radioactive material will
  reduce by one-half with every passing half-life,
  we can conclude

Half-Life Divider Pattern
1 2 21
2 4 22
3 8 23
4 16 24
5 32 25
6 64 26
Divider 2n where n equals the number of
half-lifes.
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Example 1 (cont.)

• For every half-life, divide the original amount
  of radioactive material by the divider
  corresponding to the ½-life in question.
• When n 1, the divider equals 2 or 21.
• m 600 g/2 300 g
• When n 2, the divider equals 4 or 22.
• m 600 g/4 150 g
• When n 6, the divider equals 64 or 26.
• m 600 g/64 9.4 g

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Determining the Amount of Radioactive Material
Left

• To determine the amount of radioactive material
  left in a sample after n number of ½ - lifes
• m mo
• 2n
• Where
• m mass after n ½ - lifes
• mo initial mass
• n number of ½ - lifes

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Applications of Radioactivity

• Dating materials
• Carbon-14 (1/2-life 5730 yrs) used for dating
  plants and animals that were formerly alive.
• Uranium-238 (1/2-life 4.47 billion years) used
  for dating rocks.
• Potassium-40 (1/2-life 1.3 billion years) used
  for dating rocks.
• Medical diagnostics.
• Cancer Treatment
• Smoke detectors.

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Carbon Dating

• Carbon-14 is created by cosmic radiation from the
  sun acting on the atmosphere.
• CO2 makes up a small portion of our atmosphere
  that is consumed by plants through
  photosynthesis.
• While all animals and plants are living,
  Carbon-14 is being constantly replaced.
• When a living organism dies, the replenishment of
  carbon-14 no longer exists and the decay process
  takes over.
• Every 5,730 years, the amount of Carbon-14 will
  decrease by ½ into Nitrogen-14.

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CARBON DATING
The rate at which 14C decays is absolutely
constant. Given any set of 14C atoms, half of
them will decay in 5730 years. Since this rate is
slow relative to the movement of carbon through
food chains (from plants to animals to bacteria)
all carbon in biomass at earth's surface contains
atmospheric levels of 14C. However, as soon as
any carbon drops out of the cycle of biological
processes - for example, through burial in mud or
soil - the abundance of 14C begins to decline.
After 5730 years only half remains. After another
5730 years only a quarter remains. This process,
which continues until no 14C remains, is the
basis of carbon dating
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NUCLEAR MEDICINE
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Cobalt- 60 Cancer Treatment

• Gamma ray emitter
• Half life 5.23 years
• Radiotherapy treatment of cancer with radiation
• Cancerous cells more susceptible to damage by
  ionising radiation than normal cells
• Ionising radiation is directed on to the tumour
  from different directions
• Tumour dose is high but normal tissue receives a
  much lower, less harmful dose

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Cobalt - 60

• Gamma rays kill micro-organisms in food
• May also involve inhibiting sprouting,
  controlling ripening and pasteurising foods
• Fears of genetic modification

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Americium-241

• Emitted a-particles ionise the air molecules,
  conduct an electric current between two terminals
• Smoke clings to ionised air molecules and slows
  them down
• Current decreases and a transistor switch
  activates the alarm
• This type of alarm not used these days

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NUCLEAR ENERGY FISSION
Splitting the Uranium Atom Uranium is the
principle element used in nuclear reactors and in
certain types of atomic bombs. The specific
isotope used is 235U. When a stray neutron
strikes a 235U nucleus, it is at first absorbed
into it. This creates 236U. 236U is unstable and
this causes the atom to fission
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Light atoms tend to combine and release energy as
they do so. Heavy atoms tend to split and
release energy as they do so. Uranium and
Plutonium are particularly useful in this regard,
and are the basis of nuclear fission.
Note there are three neutrons released for every
incident neutron. This is the basis of a chain
reaction.
http//www.youtube.com/watch?vJxzPN-vdP_0feature
related
http//www.youtube.com/watch?vXHitaEy-Xtg
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Light atoms tend to combine and release energy as
they do so. Heavy atoms tend to split and
release energy as they do so. Uranium and
Plutonium are particularly useful in this regard,
and are the basis of nuclear fission.
Heavy nuclei break into lighter nuclei and energy
is released.
Light nuclei fuse into heavy nuclei and energy is
released.
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CHAIN REACTION
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NUCLEAR-ENERGY POWER PLANTS
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In a nuclear reactor, however, the last thing you
(and the rest of the world) want is all your
atoms splitting at once. But the reactor core
needs to be slightly supercritical so that plant
operators can raise and lower the temperature of
the reactor. The control rods give the operators
a way to absorb free neutrons so operators can
maintain the reactor at a critical level.To turn
nuclear fission into electrical energy, the first
step for nuclear power plant operators is to be
able to control the energy given off by the
enriched uranium and allow it to heat water into
steam
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