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Modern Physics
Model of the atom
Radioactivity
Introduction - Today we expand our discussion of
explaining what happens at the nuclear level atoms.
• Radioactivity (substances that give of invisible radiations)
and the types of radiations.
• What happens when an atom undergoes radioactivity
decay
• Uses for radiation
• The types of reactions that cause nuclear reactions
• The affects of nuclear reactions on energy
Nucleons
• Are particles occupying the nucleus
• Consist of + charged protons and neutral
neutrons
• Have almost 2000 time the mass of electrons
• Are made of quarks and leptons (building
blocks of matter, elementary particles)
Atom
Atomic mass = protons + neutrons
(number of nucleons)
Atomic number = number of protons
Isotopes
• Atoms of the same element with different
numbers of neutrons (different masses)
Radioactive Isotopes
• Has an unstable nucleus
• Spontaneously emits a particle and decay into
another element.
Marie Curie – Nobel prize winner
• The word radioactivity was
first used by Marie Curie in
1898.
• She used the word
radioactivity to describe the
property of certain
substances to give off
invisible “radiations” that
could be detected by films.
Radioactive Decay
• Three different kinds of
radiation given off by
radioactive materials:
– Alpha rays
– Beta rays
– Gamma rays
• called “rays” because the
radiation carried energy and
moved in straight lines, like
light rays.
• Radioactivity comes
from the nucleus of the
atom.
• If the nucleus has too
many neutrons, or is
unstable the atom
undergoes radioactive
decay.
• decay - to "break
down."
Atomic Decay
• Alpha decay: the nucleus ejects two protons and two
neutrons.
• Beta decay: a neutron in the nucleus splits into a proton
and an electron.
• Gamma decay occurs because the nucleus is at too high
an energy. The nucleus falls down to a lower energy state
and, in the process, emits a high energy photon.
• Radioactive decay gives off energy.
• The energy comes from the conversion of
mass into energy.
• Because the speed of light (c) is such a
large number, a tiny bit of mass generates
a huge amount of energy.
• Radioactivity occurs because everything
in nature tends to move toward lower
energy.
Radiation
• The flow of energy through space.
• Forms of radiation:
– Light
– Radio
– Microwaves
– X-rays
• Many people mistakenly think of radiation as
only associated with nuclear reactions.
X-ray machines
• X-rays are photons
• Used to produce
images of bones and
teeth on x-ray film.
• X-ray film turns black
when exposed to xrays.
X-Rays Uses
• High level therapeutic
x-rays are used to
destroy diseased tissue,
such as cancer cells.
• The beams are made to
overlap at the place
where the doctor wants
to destroy diseased
cells.
CAT scan
• Computerized Axial
Tomography
• Produced by a computer that
controls an x-ray machine as it
takes pictures of the body from
different angles.
• Produces three-dimensional
images of bones and other
structures within the body.
Radiation Detection
The Geiger counter is
a type of radiation
detector invented to
tell when radiation is
present and to
measure its
intensity.
Half-life
• The time it takes for half the mass of a
radioactive sample to decay.
• Ranges from a fraction of a second to billions
of years.
• Is not affected by external conditions.
Fusion reactions
• Nuclear reaction that
combines, or fuses, two
smaller nuclei into a
larger nucleus.
• It is difficult to make
fusion reactions occur
because positively
charged nuclei repel each
other.
Fission reactions
• A fission reaction splits
up a large nucleus into
smaller pieces.
• A fission reaction
typically happens
when a neutron hits a
nucleus with enough
energy to make the
nucleus unstable.
Nuclear Reactions and Energy
• A nuclear reaction is any process that
changes the nucleus of an atom.
• Radioactive decay is one form of
nuclear reaction.
Nuclear Reactions and Energy
• If you could take apart a nucleus and separate
all of its protons and neutrons, the separated
protons and neutrons would have more mass
than the nucleus did.
• The mass of a nucleus is reduced by the energy
that is released when the nucleus comes
together.
• Nuclear reactions can convert mass into energy.
Nuclear Reactions and Energy
• Both these nuclear reactions release a small
portion of the mass as large amounts of
energy.
• Nuclear fusion is what powers a modern
nuclear warhead (hydrogen bomb)
• Nuclear fission (less powerful) occurs in an
atomic bomb (like the ones used against Japan
in WWII), or in a nuclear power plant.
Mass Energy Equivalence
The energy released can be calculated using the equation:
E = mc2
Where:
E = energy released (J)
m = mass difference (kg)
c = speed of light in a vacuum (3 x 108 m/s)