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 
1
0
1
1
p+ β
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.
1
1
p 
n+ β
1
0
0
+1
The atomic number decreases by one and the mass number stays the same.
SUPPLEMENTAL INFORMATION
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.
SUPPLEMENTAL INFORMATION
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.
SUPPLEMENTAL INFORMATION
Mass Stability
The difference between the mass of an atom and the sum of the masses of
its protons, neutrons, and electrons is called the mass defect.
Nuclear Binding Energy
According to Albert Einstein’s equation E = mc2, mass can be converted to
energy, and energy to mass.
This is the nuclear binding energy, the energy released when a nucleus is
formed from nucleons.
The nuclear binding energy is a measure of the stability of a nucleus.
The binding energy per nucleon is the binding energy of the nucleus
divided by the number of nucleons it contains
Elements with intermediate atomic masses have the greatest binding
energies per nucleon and are therefore the most stable. The elements
with an atomic number of 82 or less have stable isotopes
SUPPLEMENTAL INFORMATION
Band of Stability
Alpha
particle
The graph has a plot of
stable elements, this part
is called band of stability.
At the higher end of the
band of stability lies
alpha decay, below is
positron emission or
electron capture, above
is beta emissions and
elements beyond the
atomic mass of 83 are
unstable radioactive
elements.
SUPPLEMENTAL INFORMATION
Alpha
particle
Alpha decay is located at the
top of the plotted line, because
the alpha decay decreases the
mass number of the element in
order to keep the isotope
stable. This is done by using
the element helium (He). An
unstable isotope's protons are
decreased by 2 and its neutrons
are decreased by 4, and
because the isotope was
originally unstable before it
went through alpha decay, the
elements are still considered
unstable.
SUPPLEMENTAL INFORMATION
Beta decay accepts
protons so it changes
the amount of
protons and
neutrons. the
number of protons
increase while
neutrons decrease.
To make things
easier to understand
think of the ratio of
the isotope: there are
too many neutrons
compared to the
number of protons
therefore it is above
the band of stability.
SUPPLEMENTAL INFORMATION
Positron emission
and electron
capture is when the
isotope gains more
neutrons. Positron
emission and
electron capture are
below the band of
stability because
the ratio of the
isotope has more
protons than
neutrons, think of
it as there are too
few protons for the
amount of
neutrons and that
is why it is below
the band of
stability.
SUPPLEMENTAL INFORMATION
The band of stability can be
explained by the relationship
between the nuclear force
and the electrostatic forces
between protons.
Stable nuclei tend to have even
numbers of nucleons.
This is referred to as the even-odd
rule
According to the nuclear shell
model, nucleons exist in
different energy levels, or
shells, in the nucleus.
The numbers of nucleons that
represent completed nuclear
energy levels—2, 8, 20, 28,
50, 82, and 126—are called
magic numbers.
SUPPLEMENTAL INFORMATION
EVEN ODD RULE AND MAGIC NUMBERS
•Nuclides containing odd numbers of both protons and neutrons are the
least stable
•means more radioactive.
•Nuclides containing even numbers of both protons and neutrons are most
stable
•means less radioactive.
•Nuclides contain odd numbers of protons and even numbers of neutrons
are less stable than nuclides containing even numbers of protons and odd
numbers of neutrons.
•In general, nuclear stability is greater for nuclides containing even numbers
of protons and neutrons or both
SUPPLEMENTAL INFORMATION
MAGIC NUMBERS
Magic numbers are natural occurrences in isotopes and are stable. Below is a
list of numbers of protons and neutrons; isotopes that have these numbers
occurring in either the proton or neutron are stable. In some cases there the
isotopes can consist of magic numbers for both protons and neutrons; these
would be called double magic numbers. But the double numbers only occur
for isotopes that are heavier, because the repulsion of the forces between the
protons.
The magic numbers:
proton: 2, 8, 20, 28, 50, 82, 114
neutron: 2, 8, 20, 28, 50, 82, 126, 184
Also, there is the concept that isotopes consisting a combination of eveneven, even-odd, odd-even, and odd-odd are all stable. There are more
nuclides that have a combination of even-even than odd-odd. (See chart.)
SUPPLEMENTAL INFORMATION
Half-Lives of Some Radioactive
Isotopes
SUPPLEMENTAL INFORMATION
Decay Series
A decay series is a series of radioactive nuclides produced by successive
radioactive decay until a stable nuclide is reached.
The heaviest nuclide of each decay series is called the parent nuclide.
The nuclides produced by the decay of the parent nuclides are called
daughter nuclides.
SUPPLEMENTAL INFORMATION
Uranium-238 Decay
SUPPLEMENTAL INFORMATION
Artificial Transmutations
Artificial radioactive nuclides are radioactive nuclides not found naturally on
Earth.
They are made by artificial transmutations, bombardment of nuclei with
charged and uncharged particles.
Transuranium elements are elements with more than 92 protons in their
nuclei.
Artificial transmutations are used to produce the transuranium elements.
SUPPLEMENTAL INFORMATION
Radiation Exposure
Nuclear radiation can transfer the energy from nuclear decay to the electrons
of atoms or molecules and cause ionization.
The roentgen (R) is a unit used to measure nuclear radiation exposure; it is
equal to the amount of gamma and X ray radiation that produces 2  109
ion pairs when it passes through 1 cm3 of dry air.
A rem is a unit used to measure the dose of any type of ionizing radiation
that factors in the effect that the radiation has on human tissue.
SUPPLEMENTAL INFORMATION