Transcript Slide 1

Radioactivity and
Nuclear Reactions
Chapter 18
Section 1:
Let’s Review…
Atoms are made of three
kinds of particles: electrons,
protons, and neutrons.
The 4 Forces
The Strong Force
The strong force causes protons and neutrons
to be attracted to each other.
 The strong force is very short-ranged, so the
protons and neutrons in the nucleus have to
be VERY close together for it to work.
Forces in a Large Nucleus
Protons and neutrons are held together less
tightly in a large nucleus. See pg. 538.
When a nucleus breaks down and gives off
matter and energy.
 All nuclei (plural of nucleus) that have more
than 83 protons are radioactive.
 Many nuclei that have less than 83 protons
are radioactive too!
Isotopes and Stability
The ratio of protons to neutrons has an
effect on the stability of the nucleus.
In elements with small nuclei, a ratio of 1:1 is
In larger nuclei, a ratio of 3:2 (neutrons to
protons) is stable.
Nuclei with too many or too few neutrons are
radioactive and unstable.
The Discovery of Radioactivity
1896 Henri Becquerel
He left uranium salt in a drawer with a
photographic plate. He later developed the plate
and saw an outline of the uranium salt.
 He thought that the
uranium salt had
emitted some
invisible rays that
darkened the film.
Marie Curie
Discovered 2 new elements: polonium and
She named polonium after Poland, where she was
She came up with the term “radioactivity”
2 Nobel Prizes: Physics and Chemistry
Died of aplastic anemia- caused by radiation.
Even today, her papers are considered too
dangerous to handle, they are kept in lead-lined
boxes, and anyone that reads them must wear
protective clothing!
Section 2: Nuclear
Nuclear Radiation
When an unstable nucleus decays, particles
and energy are emitted.
 3 types of nuclear radiation:
Alpha and Beta radiation
are particles.
 Gamma radiation is
a wave.
Note: This is the
atomic weight, which
is the number of
protons plus neutrons
Alpha Particles
88 protons
138 neutrons
86 protons
136 neutrons
n p
p n
a (4He)
2 protons
2 neutrons
The alpha-particle (a) is a Helium nucleus.
It’s the same as the element Helium, with the
electrons stripped off !
Damage from Alpha Particles
Alpha particles are “large” respectively, and
they lose energy quickly.
Alpha particles can be stopped by a sheet of paper.
Alpha particles are dangerous if they are
released by radioactive atoms inside the
human body.
Damage from alpha particles can cause cells to not
function properly, leading to illness and/or disease.
Transmutation-the process of changing one
element to another through nuclear decay.
Beta Particles (b)
6 protons
8 neutrons
7 protons
7 neutrons
We see that one of the neutrons from the C14 nucleus
“converted” into a proton, and an electron was ejected.
The remaining nucleus contains 7p and 7n, which is a nitrogen
Note: the mass # does not change!!!
Damage from Beta Particles
Beta particles travel much faster, and are
more penetrating than alpha particles.
They can pass through paper, but can be stopped
by a sheet of aluminum foil.
Beta particles can damage cells when they are
emitted from radioactive nuclei inside the body.
Gamma rays (g)
Gamma rays are electromagnetic waves with the highest
frequencies and shortest wavelengths.
10 protons
10 neutrons
(in excited state)
10 protons
10 neutrons
(lowest energy state)
A gamma is a high energy light particle that is emitted
From a nucleus when alpha or beta decay occurs.
It is NOT visible by your naked eye because it is not in
the visible part of the EM spectrum.
Gamma Rays
The gamma from nuclear decay
is in the X-ray/ Gamma ray
part of the EM spectrum
(very energetic!)
Uses of Gamma Rays
Sterilizing medical and dental instruments.
 Treatment of cancer
 Sterilizing food.
Food Irradiation
Food irradiation is the process of exposing
food to ionizing radiation to destroy
microorganisms, bacteria, viruses, or insects
that might be present in the food.
Logo (right): used to show a
food has been treated
with ionizing radiation
Radioactive Half-Life
The half-life of a radioactive
isotope is the amount of time
it takes for half the nuclei in
a sample of the isotope to
Radioactive Dating
Used to
determine ages
of rocks, fossils,
and remains.
Section 3: Detecting
Detecting Radioactivity
Cloud Chambers- detects alpha and beta
 Bubble Chambers- holds a super heated liquid
(doesn’t boil because of high pressure)
Particles move and leave a trace of ions behind.
Electroscopes- positive ions formed by alpha
particles in the air cause the leaves of the
electroscope to come together.
Measuring Radiation
Geiger counter- produces an electric current
when it detects a charged particle (alpha or
beta particle).
Background Radiation
Low levels of radiation are bombarding us all
the time.
 Materials such as bricks, wood, and stones
have low levels of radioactive materials.
 SMALL amounts of radioactive isotopes are
found in the food, water, and air consumed by
all animals and plants…so, animals, humans, and
plants contain SMALL amounts of these
isotopes too!
Sources of Background Radiation
Radon gas- produced in the earth’s crust when
Uranium-238 decays.
Radon gas can seep into houses and cause problems
for the people and pets living inside.
Sources of Background
Other sources of radiation
come from cosmic rays
that come in through the
Earth’s atmosphere.
Background radiation
increases with altitude.
Example: someone living in
Denver, CO would have
more background radiation
yearly than someone living
in Atlanta.
Radiation in Your Body
The amount of background radiation you
receive each year depends on many factors,
Type of rocks underground
Type of materials used to build your home
Elevation at which you live
(many many more…)
It is impossible to eliminate background
Section 4: Nuclear
Nuclear Fission History
1930- Enrico Fermi
1938- Otto Hahn and Fritz Strassmann
Thought bombarding a nucleus with a neutron
would create a bigger atom.
Found that when a neutron strikes a nucleus, the
nucleus splits into 2 smaller nuclei.
1939- Lise Meitner
First to offer a theory to explain the above
Nuclear Fission Definition
Nuclear Fission- the process of splitting a
nucleus into several smaller nuclei.
FISSION- means “To Divide”
How Does Fission Work?
Only large nuclei like those of uranium and
plutonium can undergo fission.
Mass and Energy
Chain Reactions
Chain Reaction- the series of repeated fission
reactions caused by the release of neutrons in
each reaction.
Nuclear Fusion
Two nuclei with low masses are combined to
form one nucleus of larger mass.
FUSION- 2 nuclei come together to form a bigger
FISSION- nuclei are broken apart to smaller
For FUSION to occur, EXTREMELY high
temperatures must be achieved
Fusion occurs in the center of stars and the sun
when hydrogen nuclei fuse to form helium!
When this process occurs heat and light are given
Nuclear Reactions in Medicine
“Tracer” radioisotopes are used in the human
body to help doctors study body functions.
Example: PET (Positron Emission Tomography) Scan
Radioactive isotope fluorine-18 injected into
patient’s bloodstream and travels to the brain.
There it begins radioactive decay and doctors can
obtain a picture of how the brain is working.
Treating Cancer with
Radioactive isotopes like gold-198 or iridium192 can be implanted in or near the tumor
 Tumors can be treated from outside the body
by focusing an intense beam of gamma rays on
the tumor for a short time.
 Cancer cells grow faster than normal cells, so
they take up more radiation, but normal cells
can be affected too and patients can have
severe side effects.