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Chapters 39 & 40
Radioactivity and Nuclear
Physics
The Atomic Nucleus
• Nucleons—the protons (+ charge) and neutrons (0 charge) in the nucleus of an atom.
– Neutrons and protons have close the same mass (neutron is slightly larger)
– Nucleons are 2,000 times more massive than electrons.
– Protons and electrons have charges that are equal in magnitude but opposite in sign.
– The # of protons = the # of electrons.
• The # of protons determines the chemical properties of the atom.
• Strong force—the attractive force that holds the nucleus together.
– Strong only over short distances
– Neutrons increase the attractive strong force and prevent protons from electrically repelling one another.
• The more protons there are, the more neutrons that are needed
Radioactive Decay
• Radioactive—term used
to describe the
spontaneous decay of
atomic or subatomic
particles.
– The more protons that are in a nucleus, the more neutrons you need to hold it together.
– For elements over 83 protons, the addition of extra neutrons cannot stabilize the nucleus
Types of decay
• All elements above bismuth (83) decay in some way.
• There are three types named after the first three letters in the Greek alphabet.
• Alpha α
• Beta β
• Gamma χ
Neutron stability
• One factor the effects stability of nucleus is the instability of the neutron
• A lone neutron will spontaneously decay into a proton and an electron.
• If you have a bunch of neutrons, about half of them will decay in about 11 minutes.
• A lone neutron is radioactive
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Alpha Rays
• Have a positive
charge
• a stream of
particles made of
two protons and
two neutrons (He)
– Alpha particles
• Can be stopped by
a sheet of paper
Alpha Decay
• Parent atom is the original atom
• It breaks into the daughter
nucleus through a process
called transmutation
• This can happen over and over
until a stable nucleus is found.
Beta Ray
• A stream of
electrons
• emitted from a
nucleus when a
neutron forms a
proton and an
electron
• can be stopped
by a sheet of Al
Gamma Ray
• Basically massless energy--photons
• High frequency electromagnetic energy
• Emitted when nucleons jump in nuclear energy levels
• Stopped by a thick layer of lead
Radioactive Isotopes (39.4)
• Isotopes—atoms of an element that differ
in their number of neutrons.
– Hydrogen has 3 isotopes
• The common isotope—1 proton
• Deuterium—1 proton, 1 neutron
– Heavy water—water molecules that contain
deuterium
• Tritium—1 proton, 2 neutrons
– Tritium is unstable and undergoes beta decay.
Summary
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Isotope Symbols (p. 613)
• Information is about the nucleus only
(nothing about the electrons).
• Bottom number is protons.
• Top number is protons + neutrons.
Isotopes -
Same Protons
Different Neutrons
Radioactive Half-Life (39.5)
• Half-life—the time needed for
half of the radioactive atoms to
decay.– Rates of radioactive decay appear to be
absolutely constant.
– ½ life can be calculated from the rate of
disintegration.
• The shorter the ½ life, the faster it
disintegrates.
• Geiger counters measure the rate of
disintegration.
– Radium-226 has a ½ life of 1,620 years.
– Uranium-238 has a ½ life of 4.5 billion years.
Large Nuclei – what
happens to them?
Half-Life
• The “pile” doesn’t get smaller.
• The unstable nuclei don’t disappear, they
change into another type of nucleus.
• The pile becomes less radioactive, not
smaller (unless the new element is a gas!)
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Half Life Lab- Carbon 14 Dating
Isotope Symbols (p. 613)
• Information is about the nucleus only
(nothing about the electrons).
• Bottom number is protons.
• Top number is protons + neutrons.
Transmutation of
Elements
• When you give off an alpha or
beta particle, a new element is
made.
• Consider this for U-238
Natural Transmutation of Elements
• Transmutation—the changing of one
element into another.
– The emission of an alpha or beta particle
from the nucleus is one cause of
transmutation.
– Alpha particle emission causes the
atomic # to decrease by two
– Beta particle emission causes the atomic
# to increase by one.
• Consider the transmutation of
Uranium into Thorium
Alpha Decay
• Figure p. 617; decay of uranium 238 into
thorium 234
– Number of P ↓2
– Mass Number ↓4
– Number of Neutrons ↓2
– Notice that the mass numbers balance and the
atomic numbers balance.
• A helium ion spontaneously emitted to
achieve a more stable nucleus (state)
Beta Decay
• The thorium above, is also radioactive and
transmutes into protactinium (bottom p.617)– Number of P ↑1
– Mass Number = (stays the same)
– Number of Neutrons ↓1
• A neutron emits an electron (a beta particle)
becoming (changing into) a proton
• Still get a new element
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U-238 cycle (shown on next slides)
1. Undergoes alpha emission to
become Th-234
2. Th-234 undergoes beta
emission to become Pa-234
3. Pa-234 undergoes beta
emission to become U-234
• U-234 Th-230 etc.
1. Decay of U238
• Uranium is
element 92
• Alpha decay –
loses 2P and 2N
• New atomic
number is 90 (this
is Thorium)
• New mass is 234
p. 619
2. Decay of Th234
• Thorium is
element 90
• Beta decay – 1P
becomes 1N & 1e-
• New atomic
number is 91 (this
is Protactinium)
• Mass is still 234
3. Decay of Pa234
• Protactinium is
element 91
• Beta decay – 1P
becomes 1N & 1e-
• New atomic
number is 92 (this
is Uranium)
• Mass is still 234
1. Decay of U234(a new example)
• Uranium is
element 92
• Alpha decay –
loses 2P and 2N
• New atomic
number is 90 (this
is Thorium)
• New mass is 230
p. 619
Writing Nuclear
Equations
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Some Other Useful Symbols
He4
2
e0
1−
n1
0
Alpha Particle
Beta Particle
Neutron
What has to remain the same on the two
sides of a chemical equation?
Rules
• Total Number of Nucleons
remains constant
• Charge is Conserved
Alpha Decay of Pu-239
Beta Decay Gamma Decay
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Carbon Dating (39.8)
• The earth’s atmosphere is bombarded by
cosmic rays (mostly protons).
– Most protons capture an electron to form
hydrogen atoms in the upper atmosphere.
– Neutrons travel longer distances and may be
captured by the nucleus of a nitrogen atom.
+N14
7 C14
6 H1
1+n1
0
– Less than one-millionth of 1% of the Carbon in the
atmosphere is carbon-14.
Carbon Dating– C-14 joins with oxygen to form carbon dioxide,
which is taken up by plants and organisms
that ultimately consume them.
– C-14 is a beta-emitterC
14
6 N14
7 e0
-1+
– A fixed ratio of C-14 to C-12 is maintained in an organism’s
body as long as it is alive.
– The longer an organism is dead, the less C-14 left in its
remains.
– The ½ life of C-14 is 5,730 years.
– Fluctuations in the production of C-14 result in dates that have
an uncertainty of 15%.
Radioactive Tracers
• Radioactive isotopes of all
elements have been produced by
bombarding the element with
neutrons and other particles.
• Tracers are radioactive isotopes
that can be used to measure the
rate of some process of interest.
– Uptake of fertilizer by plants
– Metabolic processes within the body.
Radiation and You (39.11)
• Radioactive decay warms the center of the earth.
• Helium comes from alpha particles that were once shot out of radioactive nuclei.
• Most radiation we are exposed to comes from outer space.
– The atmosphere deflects much of this radiation.
– We are bombarded most by neutrinos.
• The most common high-speed particles
• Have near zero mass and no charge
• Billions pass through your body each second
– About once per year a neutrino triggers a nuclear reaction in your body.
• Most pass completely through the earth
• Gamma radiation is the most dangerous and comes from radioactive materials
– Causes genetic mutation
• Beta particles also can cause genetic mutation.
Nuclear Fission (40.1)
• Nuclear fission—the splitting of atomic nuclei.
• In all known nuclei, the nuclear strong forces dominate over the repulsive electrical force.
• If a uranium nucleus is elongated, the electrical force takes over causing it to split.– The absorption of a neutron by a uranium nucleus
supplies enough energy to cause this elongation.
– Between 2 and 3 neutrons are produced in most nuclear fission reactions.
– These neutrons can cause the fissioning of 2 or 3 other nuclei, releasing between 4 and 9 additional neutrons.
– This may lead to a chain reaction.
Chain Reactions
– A chunk of U-235 smaller than a baseball would still not fission.• Too many neutrons would find their way to the surface
before striking a U-235 atom.
– Critical mass—the amount of mass for which each fission produces, on average, one additional fission event.• Subcritical mass—one in which the chain reaction dies
out.
• Supercritical mass—one in which the chain reaction builds up explosively.
n1
0 Kr91
36 +
• Fission occurs mainly for U-235.
– Makes up 0.7% of the uranium in pure uranium metal.
– U-238 absorbs neutrons without fissioning.
+ U235
92 Ba142
56 + 3( n1
0 )
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Chain Reaction
The Nuclear Fission
Reactor (40.2)• About 21% of electric energy in the U.S. is
produced by nuclear fission reactors.
• 3 main components to a fission reactor
– Nuclear fuel combined with a moderator to slow down
neutrons.
• Fuel = Uranium, with its fissionable isotope U-235 enriched
to ~3%.
• Moderator = graphite, a pure form of carbon, or water.
– Control rods
• Usually made from cadmium or boron, which readily absorb
neutrons.
• Control how many neutrons from each fission event are
available to trigger additional fission events.
– Water used to transfer heat from the reactor to the
generator.
Nuclear Fission ReactorWhy doesn’t this happen
in Uranium Deposits?
• This type of chain reaction only
occurs with the rare U-235 (0.7%
of natural U)
• U-238 will absorb the neutrons
not allowing the chain reaction.
• U-238 can “snuff out” the
reaction
Energy Released
• The energy released by an atom of U is about 7 million times that of a molecule of TNT
• KE of fragments and neutrons, and gamma radiation.
Explosions
• If the chunk of U-235 were the size of
a baseball, an enormous explosion
would result.
• If the chunk were smaller, there
would be a chance that many
neutrons would escape the surface
before hitting another.
• We have a critical mass that we
need.
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Nuclear Bomb!!!
• Start with two subcritical masses.
Neutrons reach the surface too
readily to have an explosion.
• Now force the two masses together
in a small area (use TNT)
• the combined mass is supercritical
and fission occurs.
• BOOOM!!!!
Plutonium
• U-239 is created when U-238 absorbs a
neutron.
• U-239 emits a beta particle and forms
Neptunium-239 )(1/2 life = 2.3 days).
• Np-239 emits a beta particle to form
Plutonium-239 )(1/2 life = 24,000
years).
• Pu-239 can be easily separated from
uranium.
The Breeder Reactor
• When mixed together, the fissioning of Pufrees neutrons that convert U-238 into more Pu-239.– Produces useful energy
– Breeds more fission fuel
• After a few years of operation, breeder reactors breed twice as much fuel as they start with.
• This is like refilling a gas tank with water
and making gas.
Mass-Energy Equivalence
(40.5)• Mass and energy are equivalent.
– “E = mc2”
– Mass is like a super storage battery.
– When mass decreases, the stored up energy is released.
• A nucleon inside a nucleus has less mass than its rest mass outside the nucleus.
– For Uranium, the difference = 0.7%
– The binding energy is greatest for iron
• The mass difference is related to the “binding energy” of the nucleus.
– This represents the amount of work it would take to disassemble the nucleus.
Nuclear Fusion
• The steepest part of the hill is from H-->Fe
• If we could stick two atoms together, the release in energy would be enormous!!
• Nuclei are positively charged. They would have to combine at a very high speed to stick (aka very high temp)
• thermonuclear fusion
Fusion
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The SunFusion vs Fission
• Fusion does not have chain
reactions to control (no big
boom)
• No pollution
• Produced He (yay balloons!)
• No deadly products from
reaction
• The next slides contain
additional information for
chapter 40
Radiation and you!
• It is all around you!
• Most of it comes from nature,
cosmic and minerals.
• X-rays can give a great portion
• Nuclear fallout, power plants,
etc.
• Nothing to really worry about.
Fission
• The Splitting of
Atomic Nuclei
• Think about the
forces involved
in the nucleus
• Strong force &
electromagnetic
force
Nuclear Fission
Reactors
• 21% of US power
is nuclear
• One Kg of
Uranium is as
affective as 30
freight-car loads
of coal.
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Nuclear Reactor Elements of a reactor
• Nuclear Fuel: U-235 enriched to about 3%.
• Moderator to control the reaction (may be graphite or water)
• Control Rods- can be moved in and out of the reactor to control neutron multiplication. Usually made of cadmium or Boron to absorb neutrons safely.
• Water is heated and used to turn generator.
Nuclear Waste
• When U splits into smaller atoms, they still have too many neutrons.
• Said to be neutron-rich.
• This makes them radioactive.
Plutonium
• Can be made from transmutation
of Uranium and separated from
the Uranuim by chemical means.
• Has a very long half-life and is
very dangerous to humans
(cancer)
• Used in Breeder reactions!
One Million Years for the
Energy to Reach the Surface p. 639
p. 640
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Controlling Nuclear
Fusion
• To get things this hot we need a
container to hold them. It would
melt through anything that we
have.
• Use magnetic fields to suspend
dueterium that is heated with
lasers.
