Transcript AP Physics Ch 31 ppt

Lecture PowerPoint
Chapter 31
Physics: Principles with
Applications, 6th edition
Giancoli
© 2005 Pearson Prentice Hall
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Chapter 31
Nuclear Energy; Effects and
Uses of Radiation
Units of Chapter 31
• Nuclear Reactions and the Transmutation of
Elements
• Nuclear Fission; Nuclear Reactors
• Nuclear Fusion
• Passage of Radiation through Matter;
Radiation Damage
• Measurement of Radiation – Dosimetry
Units of Chapter 31
• Radiation Therapy
• Tracers and Imaging in Research and Medicine
• Emission Tomography
• Nuclear Magnetic Resonance (NMR) and
Magnetic Resonance Imaging (MRI)
31.1 Nuclear Reactions and the
Transmutation of Elements
A nuclear reaction takes place when a nucleus
is struck by another nucleus or particle.
If the original nucleus is transformed into
another, this is called transmutation.
An example:
31.1 Nuclear Reactions and the
Transmutation of Elements
Energy and momentum must be conserved in
nuclear reactions.
Generic reaction:
The reaction energy, or Q-value, is the sum
of the initial masses less the sum of the
final masses, multiplied by c2:
31.1 Nuclear Reactions and the
Transmutation of Elements
If Q is positive, the reaction is exothermic, and
will occur no matter how small the initial kinetic
energy is.
If Q is negative, there is a minimum initial kinetic
energy that must be available before the reaction
can take place.
31.1 Nuclear Reactions and the
Transmutation of Elements
Neutrons are very
effective in nuclear
reactions, as they nave
no charge and therefore
are not repelled by the
nucleus.
31.2 Nuclear Fission; Nuclear Reactors
After absorbing a neutron, a
uranium-235 nucleus will split
into two roughly equal parts.
One way to visualize this is to
view the nucleus as a kind of
liquid drop.
31.2 Nuclear Fission; Nuclear Reactors
The mass distribution of the fragments shows
that the two pieces are large, but usually
unequal.
31.2 Nuclear Fission; Nuclear Reactors
The energy release in a fission reaction is quite
large. Also, since smaller nuclei are stable with
fewer neutrons, several neutrons emerge from
each fission as well.
These neutrons
can be used to
induce fission in
other nuclei,
causing a chain
reaction.
31.2 Nuclear Fission; Nuclear Reactors
In order to make a nuclear reactor, the chain
reaction needs to be self-sustaining – it will
continue indefinitely – but controlled.
31.2 Nuclear Fission; Nuclear Reactors
A moderator is needed to slow the neutrons;
otherwise their probability of interacting is too
small. Common moderators are heavy water and
graphite.
Unless the moderator is heavy water, the fraction
of fissionable nuclei in natural uranium is too
small to sustain a chain reaction, about 0.7%. It
needs to be enriched to about 2-3%.
31.2 Nuclear Fission; Nuclear Reactors
Neutrons that escape from
the uranium do not
contribute to fission. There
is a critical mass below
which a chain reaction will
not occur because too
many neutrons escape.
31.2 Nuclear Fission; Nuclear Reactors
Finally, there are control rods, usually cadmium or boron,
that absorb neutrons and can be used for fine control of
the reaction, to keep it critical but just barely.
31.2 Nuclear Fission; Nuclear Reactors
Some problems associated with nuclear reactors
include the disposal of radioactive waste and the
possibility of accidental release of radiation.
31.2 Nuclear Fission; Nuclear Reactors
An atomic bomb also uses fission, but the core is
deliberately designed to undergo a massive
uncontrolled chain reaction when the uranium is
formed into a critical mass during the detonation
process.
31.3 Nuclear Fusion
The lightest nuclei can fuse to form heavier
nuclei, releasing energy in the process. An
example is the sequence of fusion processes
that change hydrogen into helium in the Sun.
They are listed here with the energy released in
each:
31.3 Nuclear Fusion
The net effect is to transform four protons into a
helium nucleus plus two positrons, two
neutrinos, and two gamma rays.
(31-7)
More massive stars can fuse heavier
elements in their cores, all the way up to iron,
the most stable nucleus.
31.3 Nuclear Fusion
There are three fusion reactions that are being
considered for power reactors:
These reactions use very common fuels –
deuterium or tritium – and release much more
energy per nucleon than fission does.
31.3 Nuclear Fusion
A successful fusion reactor has not yet been
achieved, but fusion, or thermonuclear, bombs
have been built.
31.3 Nuclear Fusion
Several geometries for the containment of the
incredibly hot plasma that must exist in a fusion
reactor have been developed – the tokamak,
which is a torus; or inertial confinement, which
is tiny pellets of deuterium ignited by powerful
lasers.
31.4 Passage of Radiation Through Matter;
Radiation Damage
Radiation includes alpha, beta, and gamma rays;
X rays; and protons, neutrons, pions, and other
particles.
All these forms of radiation are called ionizing
radiation, because they ionize material that they
go through.
This ionization can cause damage to materials,
including biological tissue.
31.5 Measurement of Radiation – Dosimetry
Radiation damages biological tissue, but it can
also be used to treat cancer and other diseases.
It is important to be able to measure the amount,
or dose, of radiation received. The source activity
is the number of disintegrations per second, often
measured in curies, Ci.
The SI unit for source activity is the
becquerel (Bq):
31.5 Measurement of Radiation – Dosimetry
Another measurement is the absorbed dose
– the effect the radiation has on the
absorbing material.
The rad, a unit of dosage, is the amount of
radiation that deposits energy at a rate of
1.00 x 10-2 J/kg in any material.
The SI unit for dose is the gray, Gy:
1 Gy = 1 J/kg = 100 rad
31.5 Measurement of Radiation – Dosimetry
The effect on tissue of different types of
radiation varies, alpha rays being the most
damaging. To get the effective dose, the dose is
multiplied by a quality factor.
31.5 Measurement of Radiation – Dosimetry
If the dose is measured in rad, the effective
dose is in rem; if the dose is grays, the effective
dose is in sieverts, Sv.
31.5 Measurement of Radiation – Dosimetry
Natural background radiation is about 0.3 rem
per year. The maximum for radiation workers is
5 rem in any one year, and below 2 rem per year
averaged over 5 years.
A short dose of 1000 rem is almost always fatal;
a short dose of 400 rem has about a 50% fatality
rate.
31.6 Radiation Therapy
Cancer is sometimes treated with radiation
therapy to destroy the cells. In order to minimize
the damage to healthy tissue, the radiation
source is often rotated so it goes through
different parts of the body on its way to the
tumor.
31.7 Tracers and Imaging in Research and
Medicine
Radioactive isotopes are widely
used in medicine for diagnostic
purposes. They can be used as
non-invasive scans, or tools to
check for unusual
concentrations that could signal
a tumor or other problem. The
radiation is detected with a
gamma-ray detector.
31.8 Emission Tomography
Radioactive tracers can also be detected using
tomographic techniques, where a threedimensional image is gradually built up
through successive scans.
31.9 Nuclear Magnetic Resonance (NMR)
and Magnetic Resonance Imaging (MRI)
A proton in a magnetic field can have its spin
either parallel or antiparallel to the field.
The field splits the
energy levels slightly;
the energy difference
is proportional to the
field.
31.9 Nuclear Magnetic Resonance (NMR)
and Magnetic Resonance Imaging (MRI)
The object to be examined is placed in a static
magnetic field, and radio frequency (RF)
electromagnetic radiation is applied.
31.9 Nuclear Magnetic Resonance (NMR)
and Magnetic Resonance Imaging (MRI)
When the radiation has the right energy to excite
the spin-flip transition, many photons will be
absorbed. This is nuclear magnetic resonance.
The value of the field depends somewhat on the
local molecular neighborhood; this allows
information about the structure of the molecules
to be determined.
31.9 Nuclear Magnetic Resonance (NMR)
and Magnetic Resonance Imaging (MRI)
Magnetic resonance imaging works the same
way; the transition is excited in hydrogen
atoms, which are the commonest in the human
body.
31.9 Nuclear Magnetic Resonance (NMR)
and Magnetic Resonance Imaging (MRI)
Giving the field a gradient can contribute to
image accuracy, as it allows determining the
origin of a particular signal.
31.9 Nuclear Magnetic Resonance (NMR)
and Magnetic Resonance Imaging (MRI)
Here is a summary of the medical imaging
techniques we have discussed.
Summary of Chapter 31
• Nuclear reaction occurs when nuclei collide
and different nuclei are produced
• Reaction energy or Q-value:
• Fission: heavy nucleus splits into two
intermediate-sized nuclei
• Chain reaction: neutrons emitted in one
fission reaction trigger another, and so on
• Critical mass: minimum needed to sustain
chain reaction
Summary of Chapter 31
• Moderator: slows neutrons
• Fusion: small nuclei combine to form larger
ones
• Sun’s energy comes from fusion reactions
• Useful fusion reactor has not yet been built
• Radiation damage is measured using dosimetry
• Effect of absorbed dose depends on type of
radiation