Lesson 2
Nuclear Decay
and Reactions
Focus Question
How can nuclear reactions be useful?
New Vocabulary
radioactive
alpha decay
beta decay
gamma decay
nuclear reaction
half-life
activity
fission
chain reaction
nuclear fusion
Review Vocabulary
isotope: each of the differing forms of the same
atom that have different masses but have the
same chemical properties; atoms with the same
number of protons, but different numbers of
neutrons
Radioactive Decay
• Nuclei that emit particles and
energy are said to be radioactive.
• A material that emits particles is
also said to decay.
• Nuclei decay from a less stable
form to a more stable form.
• Radioactivity is a natural process.
Types of Radiation
• The emission of an α particle from a nucleus is a
process called alpha decay.
• The mass number (A) of the
decaying nucleus is reduced by 4.
A→A−4
• The atomic number of the
nucleus (Z) is reduced by 2.
Z→Z−2
• The element changes, or
transmutes, into a different
element.
Types of Radiation
• Beta decay occurs when a neutron is changed to a
proton or a proton changes to a neutron within the
nucleus.
• In beta decay, when a neutron
(charge 0) changes to a proton
(charge +1), an electron
(charge −1) also appears.
Types of Radiation
• In beta decay, a nucleus with N neutrons and Z
protons ends up as a nucleus of N−1 neutrons and
Z+1 protons.
• Another particle, an
antineutrino, is also emitted
in beta decay.
• The symbol for the
antineutrino is the
Greek letter nu with
a bar over it. (𝜈).
Types of Radiation
• When a proton changes to a neutron, the process
is called positron emission.
• This process releases a positron and a neutrino. A
positron has the same mass as an electron but a
charge of +1.
• A redistribution of the energy within the nucleus
results in gamma decay. The  ray is a high-energy
photon.
• Neither the mass number nor the atomic number
is changed in gamma decay.
Types of Radiation
• Gamma radiation often accompanies alpha and
beta decay.
• Radioactive elements often go through a series of
successive decays, until they form a stable nucleus.
• The next slide summarizes the three types of
radiation.
Types of Radiation
Three Types of Radiation
Alpha Decay
Beta Decay
Gamma Decay
 particle ()
 particle ()
 particle (photon)
charge +2
charge 1
no charge
least penetration
medium penetration
highest penetration
transmutes nucleus:
AA4
ZZ2
NN2
transmutes nucleus:
AA
ZZ+1
NN1
changes only energy:
AA
ZZ
NN
Nuclear Reactions and Equations
• A nuclear reaction occurs whenever the energy or
number of neutrons or protons in a nucleus
changes.
• Just as in chemical reactions, some nuclear
reactions occur with a release of energy, while
others occur only when energy is added to a
nucleus.
• Nuclear reactions can be described by words,
diagrams, or equations. The following equation is
for the alpha decay of uranium-238.
238U → 234Th + 4He
90
2
92
Nuclear Reactions and Equations
• The symbols used for the nuclei in nuclear
equations make the calculation of atomic number
and mass number in nuclear reactions simpler.
• The total number of nuclear particles (protons
and neutrons) stays the same during the reaction,
so the sum of the superscripts on each side of the
equation must be equal. (For example: 238 = 234
+ 4)
• The total charge also is conserved during the
reaction, so the sum of the subscripts on each
side must be equal. (For example: 92 = 90 + 2)
Nuclear Reactions and Equations
• One form of nuclear reaction is the emission of
particles by radioactive nuclei.
• The reaction releases excess energy in the form
of the kinetic energy of the emitted particles.
• Examples of this are alpha and beta decay.
Nuclear Reactions and Equations
• Another example of transmutation occurs when a
particle collides with the nucleus, often resulting in
the emission of other particles, as in:
12C + 1H → 13N.
1
7
6
Nuclear Reactions and Equations
Use with Example Problem 2.
Problem A
SOLVE FOR THE UNKNOWN
• Write the equation.
Write the nuclear equation for the decay of
87
radioactive 87
37Rb to 38Sr by the emission of
a beta particle and an antineutrino.
87
37
Rb 
87
38
Sr 
0
1
e  00
EVALUATE THE ANSWER
•
The number of nucleons is conserved:
87 = 37 + 0 + 0
Response
SKETCH AND ANALYZE THE PROBLEM
• List the knowns and unknowns.
KNOWN
Initial: 87
37 Rb
Final:
87
38
 e, antineutrino  
Sr, 
0
1
UNKOWN
What is the nuclear equation?
•
The charge is conserved:
37 = 38 + (−1) + 0
Nuclear Reactions and Equations
Use with Example Problem 2.
Problem B
Write the nuclear equation for the decay of
218
radioactive 222
86Rn to 84Po by the emission
of an alpha particle.
SOLVE FOR THE UNKNOWN
• Write the equation.
222
86
Rb 
218
84
Sr  42 He
EVALUATE THE ANSWER
•
The number of nucleons is conserved:
222 = 218 + 4
Response
•
SKETCH AND ANALYZE THE PROBLEM
• List the knowns and unknowns.
KNOWN
Initial:
222
86
Rn
Final:
218
84
Po,  42 He


UNKOWN
What is the nuclear equation?
The charge is conserved:
86 = 84 + 2
Nuclear Reactions and Equations
KNOWN
Use with Example Problem 3.
carbon 
Response
SKETCH AND ANALYZE THE PROBLEM
• List the knowns and unknowns.
13
6
1
0
C
What isotope is formed?
neutron  n
Problem
One type of neutron source used for research
is known as a neutron howitzer. When
beryllium target nuclei absorb energetic alpha
particles from a plutonium source inside the
neutron howitzer, they are transmuted into an
excited form of 13
6C. When some of the
excited 13
6C atoms relax to a stable state, they
transmute again through emission of a
neutron. What new isotope is created?
UNKOWN
SOLVE FOR THE UNKNOWN
• Write the equation for the nuclear process.
13
6
•
C
A
Z
X  10 n
Solve for A and Z.
A  13  1  12
•
Z 66 0
The element with Z = 6 is carbon, so the
isotope is 12
6C.
EVALUATE THE ANSWER
• Releasing a neutron changes A, not Z, so the
isotope changes while the element remains
the same.
Half-life
• The time required for half of the atoms in any given
quantity of a radioactive isotope to decay is the
half-life of that element.
• If you know the original amount of a radioactive
substance and its half-life,
you can calculate the
amount remaining after a
given number of half-lives.
Half-Life
 1
remaining  original  
 2
t
Half-life
• Each particular isotope has its own half-life.
• Half-lives of radioactive isotopes are used to date
objects.
• The decay rate, or number of decays per second, of a
radioactive substance is called its activity.
• Activity is proportional to the number of radioactive
atoms present. Therefore, the activity of a particular
sample is also reduced by one-half in one half-life.
• The activity of a sample is also related to its half-life.
The shorter the half-life, the higher the activity.
• The SI unit for decays per second is a Becquerel (Bq).
Artificial Radioactivity
• Radioactive isotopes can be formed from stable
isotopes by bombardment with α particles, protons,
neutrons, electrons, or gamma rays.
• The resulting unstable nuclei emit radiation until
they are converted into stable isotopes.
• Artificially produced radioactive isotopes are often
used in medicine and medical research. For example:
• PET scan
• Destroying cancer cells
Nuclear Fission
• The division of a nucleus into
two or more fragments is
called fission.
• For example, uranium-235
undergoes fission when it is
bombarded with neutrons,
resulting in the elements
barium and krypton, along
with three neutrons and an
energy release.
1
0
n
235
92
U
92
36
Kr 
 
U  3 10 n  173 MeV
141
56
Nuclear Fission
• Note that each fission releases three neutrons
which can collide with additional uranium-235
nuclei, resulting in more fissions.
• A chain reaction is a continual process of
repeated fission reactions caused by the release
of neutrons from previous
reactions.
Nuclear Fission
• To create a controlled and useful chain reaction,
the neutrons need to interact with the fissionable
uranium at the correct rate.
• Most neutrons released by the fission of U-235
atoms have high speeds and are called fast
neutrons.
• Naturally occurring uranium consists of less than 1
percent U-235 and more than 99 percent U-238.
Therefore, many samples are enriched with more
U-235.
Nuclear Fission
• U-238 absorbs most of the fast neutrons but does
not undergo fission. Few neutrons from the
original fission do cause another fission.
• To compensate for this, the uranium is broken into
small pieces and placed in a moderator that slows
down the fast neutrons.
• These slower neutrons are absorbed more easily by
U-235 than by U-238, increasing the chances of
another fission occurring.
Nuclear Fission
• The type of nuclear reactor used in the United
States, the pressurized water reactor, contains about
200 metric tons of uranium sealed in hundreds of
metal rods.
• The rods are immersed in water, which serves as a
moderator.
• Rods of cadmium metal,
which easily absorb
neutrons, are placed
between the uranium
rods.
Nuclear Fission
• The cadmium rods are moved in and out of the
reactor to control the rate of the chain reaction.
Thus, the rods are called control rods.
• About once a year, some of fuel rods must be
replaced. The old rods are still extremely radioactive
and must be stored in a location
that can be secured.
• Methods of permanently
storing these radioactive waste products
are currently being
developed.
Nuclear Fission
• Water is a moderator, and it also transfers thermal
energy away from the fission of uranium.
• Energy released by the fission heats the water
surrounding the uranium rods.
Nuclear Fission
• The water itself doesn’t boil because it is under high
pressure, which increases its boiling point.
• This water is pumped to a heat exchanger, where it
causes other water to boil, producing steam that
turns turbines.
Nuclear Fusion
• In nuclear fusion, nuclei with
small masses combine to form
a nucleus with a larger mass.
• In the process of nuclear
fusion, energy is released. This
corresponds to a loss of mass.
• An example of fusion is the
process that occurs in the Sun,
where four protons fuse in
several steps to form one
helium nucleus.
Quiz
1. Which is the emission of 42He from a nucleus?
A
alpha decay
C
beta decay
D
positron emission
CORRECT
B
gamma decay
Quiz
2. In which process does a nucleus with N neutrons and
Z protons end up as a nucleus with N−1 neutrons and
Z+1 protons?
A
alpha decay
C
beta decay
B
gamma decay
D
positron emission
CORRECT
Quiz
3. Neither the mass number nor the atomic number is
changed in which process?
A
alpha decay
C
gamma decay CORRECT
B
beta decay
D
positron emission
Quiz
4. Which occurs whenever the energy or number of
neutrons or protons in a nucleus changes?
A
chemical reaction
C
physical change
B
nuclear reaction
D
redox reaction
CORRECT
Quiz
5. Which is the division of a nucleus into two or more
fragments?
A
fusion
C
activity
B
chain reaction
D
fission
CORRECT