From Last Time…
Radioactive decay
• Nucleus is small, tightly bound
system of protons & neutrons.
• Said last time that some nuclei are more
stable than others.
• Unstable nuclei can decay by emitting some
form of energy.
• Three different types of decay observed:
• Proton number determines the element.
• Different isotopes have different # neutrons.
• Some isotopes unstable, radioactively decay
• Nucleus held together by the strong nuclear force
Alpha decay
Beta decay
Gamma decay
– Stronger than coulomb force,
– But much shorter range than coulomb force.
• Strong force actually between quarks, internal
constituents of the neutron/proton.
‘Leaks’ out to appear as an attractive force.
Mon. Nov 22
(First three letters of greek alphabet).
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Mon. Nov 22
Penetrating power of radiation
• Alpha radiation very weak
• Beta radiation penetrates farther
• Gamma radiation hardest to stop
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Is the radiation charged?
• Alpha radiation positively charged
• Beta radiation negatively charged
• Gamma radiation uncharged
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Mon. Nov 22
Alpha radiation
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A new element
• Alpha radiation now known to be a helium nucleus
(2 protons, 2 neutrons)
• When a nucleus emits an alpha-particle,
it loses two neutrons and two protons.
• It becomes a different element.
Alpha particle
• Example:
Heavy nucleus spontaneously
emits alpha particle
238
92
92 protons
146 neutrons
Mon. Nov 22
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Mon. Nov 22
U→42 He+ 23490Th
2 protons
2 neutrons
90 protons
144 neutrons
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A random process
• This is a quantum-mechanical process
– It has some probability for occurring.
• For every second of time,
there is a probability that the nucleus will decay
by emitting an alpha-particle.
• The alpha-particle
quantum-mechanically
tunnels out of the nucleus.
Radioactive half-life
• Example of random decay.
• Start with 8 identical radioactive nuclei
• Suppose probability of decaying in one second is 50%.
Every second, half
the atoms decay
Undecayed
nuclei
The half-life is one
second
t=0
Mon. Nov 22
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Mon. Nov 22
More physical half-life
has a half-life
of 14 billion years
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Why?
– All nucleons (neutrons & protons)
attracted by short-range strong force
– Protons forced apart by long-range Coulomb force.
– Smaller nuclei will be more stable
• Sample initially
contains 1 million
232Th atoms
• Every 14 billion years,
the number of 232Th
nuclei goes down by
a factor of two.
• Why is the ejected piece an alpha-particle,
and not something else?
– Helium nucleus is much more stable than other light nuclei
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Mon. Nov 22
• Nucleus emits a helium nucleus
2 neutrons, 2 protons.
• Only heavy nuclei will alpha-decay.
• Decay is by quantum mechanical tunneling.
Tunneling is sensitive
to the nuclear barrier.
Decay probability
(and hence half-life)
varies dramatically
Can vary
20 orders of magnitude
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Decay sequence of
Half-lives of alpha-decay
Mon. Nov 22
t=3
sec
• Why does a piece come out of the atom?
232Th
Mon. Nov 22
t=2
sec
238 U
Number of protons
•
t=1
sec
Mon. Nov 22
Number
of neutrons
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Radon
• One of the daughters in the
238U decay series is radon.
Detecting radiation
• A Geiger counter
• Radiation ionizes (removes electrons) atoms
in the counter
• Radon is an alpha emitter
that presents an
environmental hazard
• Inhalation of radon and its
daughters can ionize lung
cells enough to increase the
risk of lung cancer.
Leaves negative
electrons and
positive ions.
• Health risk depends radon
concentration, which
depends on concentration of
uranium and other
radioactive elements.
These are attracted
to the
anode/cathode,
current flow is
measured
Mon. Nov 22
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Mon. Nov 22
Other radioactive decay
• Beta decay
– Now know to be an electron.
– Radioactive nucleus emits an electron
• How can this be?
– There are no electrons in the nucleus!
– Has only neutrons and protons.
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Beta decay
• Nucleus emits an electron
(negative charge)
• Must be balanced
by a positive charge
appearing in the nucleus.
• Particles are not forever!
– Charge is forever, energy/mass is forever
– But particles can appear and disappear
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This occurs as a neutron
changing into a proton
Mon. Nov 22
Quarks
Not just neutrons & protons
• Nucleons have an
internal structure.
• Made up of different
types of quarks.
• In this sense,
neutrons and
protons not so
different.
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• Six quarks (also
six antiquarks)
• All have
electrical
charge, spin,
• Also have other
quantum
nujmbers..
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Quark structure of nucleons
• Proton =
up+up+down
Neutron into proton
• Think about this in terms of quarks.
• Neutron becomes proton by changing 1 quark.
• Charge =
2/3+2/3-1/3=+1
proton
• Proton is a
composite object,
as are many other
particles.
• Neutron =
up+down+down=
2/3-1/3-1/3=0
Mon. Nov 22
neutron
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Down quark changes to up quark by
emitting W-, decays to electron
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Example of beta decay
Neutron decay
•
radioactive form of carbon, decays by
beta decay (electron emission).
• Carbon has 6 electrons, so six protons.
• 12C has (12-6)=6 neutrons.
• 14C has (14-6)=8 neutrons.
• Can be clearer to
visualize this
diagramatically.
14C,
Beta decay:
Now have a new element (changed # protons)
Element with 7 protons in nucleus is Nitrogen
14
6
Mon. Nov 22
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Mon. Nov 22
−
C →14
7 N+ e
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Carbon dating
• This decay is used to time since biological
substance has died.
• 14C naturally present in the atmosphere
as a result of transmutation of 14N.
Cosmic ray proton shatters
atmospheric gas nucleus,
producing atmospheric neutrons.
Neutron knocks proton
out of 14N nucleus, becoming
Mon. Nov 22
14 C
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14 C
•
to
12 C
ratio
14C
has a half-life of ~6,000 years,
continually decaying back into 14N.
• Steady-state achieved in atmosphere,
with 14C:12C ratio of 1:1 trillion (1 part in 10 12)
As long as
biological
material alive,
atmospheric
carbon mix
ingested (as CO2),
ratio stays fixed.
Mon. Nov 22
After death, no exchange
with atmosphere. Ratio
starts to change
as 14 C decays
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Other isotopes of carbon
Antimatter
• Every particle now known to have an antiparticle.
• Even antimatter has been generated.
• Lighter forms of carbon are observed to emit
a particle that looks like an electron, but has
a positive charge!
• This is the antiparticle of the electron.
• Called the positron.
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What is going on?
•
Photons are created so
that energy is
conserved.
Mon. Nov 22
has more
neutrons than the
most stable form 12C.
– So it decays by
electron emission,
changing neutron
into a proton.
Guessing the decay route
Light elements:
Excess neutrons->
change neutron to
proton by electron
emission
• Other isotopes of
carbon have fewer
neutrons
– Decays by emitting
positron, changing
proton into neutron.
Too few neutrons>change proton to
neutrons by
positron emission
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Mon. Nov 22
Detecting radiation
• A Geiger counter
• Radiation ionizes (removes electrons) atoms
in the counter
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60 Cobalt
• Is radioactive by measurement, half-life 5.3 yr.
• By what mechanism will this decay?
• From the periodic table, Co has 27 electrons.
– Stable form of Co is 59 Co, with 32 neutrons
– 60Co has 33 neutrons, one more than most stable form
Leaves negative
electrons and
positive ions.
• Likely to decay by changing one of the neutrons
into a proton
These are attracted
to the
anode/cathode,
current flow is
measured
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• Can often make good guesses regarding decay route
• Only heavy nuclei decay via alpha emission
• Nuclei decay in a way that produces a more stable nucleus
14C
Mon. Nov 22
Matter and antimatter
annihilate when in
close proximity.
– Charge conservation says electron must be emitted
– One more proton means it becomes Nickel ( 60 Ni)
– 60Ni is a stable isotope, with 26% natural abundance.
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Turning lead into gold
Radioactive decay changes one element into another
by changing the number of protons in a nucleus.
This can also be done artificially by neutron
bombardment.
Nuclear fission
• In some cases, the effect of neutron
bombardment is more dramatic.
• Leads to nuclear fission, where a heavy
nucleus is split apart into two smaller ones.
• The transmutation of platinum into gold accomplished
by a sequence of two nuclear reactions
• first: 198Pt + neutron --> 199Pt
• second: 199Pt --> 199Au + subatomic particle
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Mon. Nov 22
Fission chain reaction
• Neutrons are released in this
process, leading to more fission
events
• Chain reaction can result
Mon. Nov 22
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Nuclear Fusion
• ‘Opposite’ process
also occurs, where
nuclei are fused to
produce a heavier
nucleus, but requires
large initial energy
input.
• Called nuclear fusion.
Mon. Nov 22
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