RADIOACTIVE DECAY
NCCS 1.1.4
Radioactive Decay
Radioactive decay is the spontaneous disintegration of a nucleus into a
slightly lighter nucleus, accompanied by emission of particles,
electromagnetic radiation, or both.
Nuclear radiation is particles or electromagnetic radiation emitted from
the nucleus during radioactive decay.
An unstable nucleus that undergoes radioactive decay is a radioactive
nuclide.
All of the nuclides beyond atomic number 83 are unstable and thus
radioactive.
Protons and neutrons are called nucleons.
An atom is referred to as a nuclide.
Radioactive Nuclide Emissions
A nuclide’s type and rate of decay depend on the nucleon content and energy level
of the nucleus.
Below are some examples of the type of radioactive decay that can occur.
Types of Radioactive Decay
Alpha Emission
An alpha particle (α) is two protons and two neutrons bound together
and is emitted from the nucleus during some kinds of radioactive
decay.
4
2
He
Alpha emission is restricted almost entirely to very heavy nuclei.
Types of Radioactive Decay, continued
Beta Emission
A beta particle (β) is an electron emitted from the nucleus during some
kinds of radioactive decay.
To decrease the number of neutrons, a neutron can be converted into a
proton and an electron.
n  p+ β
1
0
1
1
0
-1
The atomic number increases by one and the mass number stays the same.
Types of Radioactive Decay, continued
Positron Emission
A positron is a particle that has the same mass as an electron, but has a
positive charge, and is emitted from the nucleus during some kinds of
radioactive decay.
To decrease the number of protons, a proton can be converted into a
neutron by emitting a positron.
p 
1
1
n+ β
1
0
0
+1
The atomic number decreases by one and the mass number stays the same.
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Types of Radioactive Decay, continued
Electron Capture
In electron capture, an inner orbital electron is captured by the nucleus of
its own atom.
To increase the number of neutrons, an inner orbital electron combines
with a proton to form a neutron.
0
-1
e+ p 
1
1
1
0
n
The atomic number decreases by one and the mass number stays the same.
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Types of Radioactive Decay, continued
Gamma Emission
Gamma rays () are high-energy electromagnetic waves emitted from
a nucleus as it changes from an excited state to a ground energy
state.
Comparing Alpha, Beta and Gamma
Particles
NUCLEAR REACTION PROBLEM
What type of particle is emitted?
4
Po

++ balances
??
Identify212
the
product
the
212
4 that
84 Po  2 He
He
2
following84nuclear reaction.
212
84
Po 
4
2
He +
?
Nuclear Reactions, continued
This is an alpha emission type of reaction
212
84
Po 
4
2
He +
mass number: 212 − 4 = 208
?
atomic number: 84 − 2 = 82
2. The nuclide has a mass number of 208 and an
atomic number of 82,
208
82
Pb.
3. The balanced nuclear equation is
212
84
Po 
4
2
He +
208
82
Pb
Half-Life
Half-life, t1/2, is the time required for half the atoms of a radioactive nuclide
to decay.
Each radioactive nuclide has its own half-life.
More-stable nuclides decay slowly and have longer half-lives.
Potassium-40 Half-Life
Rate of
Decay
Half-Life, continued
Sample Problem B
Phosphorus-32 has a half-life of 14.3 days. How many
milligrams of phosphorus-32 remain after 57.2 days if
you start with 4.0 mg of the isotope?
Sample Problem B Solution
Given: original mass of phosphorus-32 = 4.0 mg
half-life of phosphorus-32 = 14.3 days
time elapsed = 57.2 days
Unknown: mass of phosphorus-32 remaining after 57.2 days
Half-Life, continued
Solution:
Step 1: find number of half lives
Step 2:find amount of phosphorus remaining after time has lapsed
Step 1: 57.2 days = 4 half lives
14.3 days
Step 2: 4 mg x ½ x ½ x ½ x ½ = .25 mg
OR 4 mg (1/2) 4 = .25 mg
FORMULA: (original amount of substance) x (1/2 life) (number of ½ lifes)
FISSION
Fission is a reaction when the nucleus of an atom, having captured a neutron,
splits into two or more nuclei, and in so doing, releases a significant amount of
energy as well as more neutrons. These neutrons then go on to split more nuclei
and a chain reaction takes place.
FUSION
Fusion is a process where nuclei collide and join together to form a heavier
atom, usually deuterium and tritium. When this happens a considerable
amount of energy gets released at extremely high temperatures: nearly 150
million degrees Celsius. At extreme temperatures, electrons are separated
from nuclei and a gas becomes a plasma—a hot, electrically charged gas.
FISSION VS FUSION
Definition
Fission is the splitting of a large atom into two or
more smaller ones.
Fusion is the fusing of two or more lighter atoms
into a larger one.
Natural occurrence of the process
Fission reaction does not normally occur in nature.
Fusion occurs in stars, such as the sun.
Byproducts of the reaction
Fission produces many highly radioactive particles.
Few radioactive particles are produced by fusion
reaction, but if a fission "trigger" is used, radioactive
particles will result from that.
Conditions
Critical mass of the substance and high-speed neutrons High density, high temperature environment is
are required.
required.
Energy Requirement
Takes little energy to split two atoms in a fission
reaction.
Energy Released
The energy released by fission is a million times greater
The energy released by fusion is three to four times
than that released in chemical reactions, but lower than
greater than the energy released by fission.
the energy released by nuclear fusion.
Nuclear weapon
One class of nuclear weapon is a fission bomb, also
known as an atomic bomb or atom bomb.
One class of nuclear weapon is the hydrogen bomb,
which uses a fission reaction to "trigger" a fusion
reaction.
Energy production
Fission is used in nuclear power plants.
Fusion is an experimental technology for producing
power.
Fuel
Uranium is the primary fuel used in power plants.
Hydrogen isotopes (Deuterium and Tritium) are the
primary fuel used in experimental fusion power plants.
Extremely high energy is required to bring two or more
protons close enough that nuclear forces overcome their
electrostatic repulsion.
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