Transcript Nuclear Physics

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Nucleon: anything you find in the nucleus,
includes protons and neutrons
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Number of protons determines element
Symbol: p+
Positive charge: +e
Mass = mp = 1.6726e-27 kg
Number of protons in nucleus = atomic
number, symbol = Z
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Symbol is n0
No charge
Mass = mn = 1.6749e-27 kg
Number of neutrons in nucleus = neutron
number, symbol N
Total number of nucleons (protons +
neutrons) in a nucleus is call the atomic mass
number, symbol A
A=Z+N
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Atoms of the same element can have
different number of neutrons in the nucleus
(even though same number of protons),
called ISOTOPES
Isotopes react almost identically when
compared to each other, but in physics we’re
concerned with different isotopes
Masses on periodic table are weighted
averages based on natural abundances
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Ex, since most carbon is carbon-12, the
number is pretty close to 12
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The number of neutrons strongly affects the
stability of the nucleus
In unstable isotopes, the number of neutrons
partly determines the rate at which the
nucleus decays and releases radiation
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Masses of atoms are sometimes given in
atomic mass units (amu), which has the unit
“u”
Not an SI unit, but measuring small things in
kg can seem silly, so it’s common
Based on neutral carbon-12 atom,
12.000000u
1 u = 1/12 the mass of carbon-12
1 u = 1.660539e-27 kg
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Since the strong nuclear force holds nucleons
together, energy must be added to separate
them… this is binding energy
Separated nucleons have more energy
Nucleons bound in nucleus haven’t had
energy added yet, so they have more energy
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Since separated nucleons have more energy,
they must have more mass (energy is directly
related to mass)
Nucleons bound in the nucleus have less
energy and therefore less mass
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Mass defect = difference between the mass of
the nucleus and its individual nucleons
Directly related to the binding energy added
to break apart the nucleus
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1896: leaves uranium
in a drawer with a
photographic plate and
accidentally identifies
another part of the
electromagnetic
spectrum
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Isolated two other
radioactive elements:
polonium and radium
Put them under different
stresses, but the elements
always emitted radiation, so
concluded radioactivity
comes from deep within the
atom (i.e., the nucleus)
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Results from the
decay of an unstable
nucleus
Decay happens
because it results in a
more stable nucleus
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Ernest Rutherford found 3 distinct forms of
radiation & divided based on ability to pass through
material and deflection in magnetic field
 Alpha (α): could barely pass through a single sheet
of paper. Deflected as a positive particle in a
magnetic field.
 Beta (β): can pass through about 3mm of
aluminum. Deflected as a negative particle in a
magnetic field. *
 Gamma (γ): can pass through several centimeters
of LEAD! Not deflected in a magnetic field.
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Alpha radiation is a Helium atom,
but we call it an alpha particle
since it comes from radiation
 With protons and neutrons leaving
the nucleus it gets smaller, often
more stable
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Alpha particle: charge +2e, since
no electrons
Use conservation of nucleons to
write-out decay
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Total mass of the daughter nucleus plus the
alpha particle is less than the mass of the
original nucleus
 Missing mass was turned into energy : E = mc2
 Works with our understanding of conservation of
mass and energy being interchangeable
 Energy found mostly in kinetic energy of alpha
particle and daughter nucleus moving away from
one another
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A neutron falls apart and becomes a proton
and an electron
 Leaving electron is the beta particle
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That’s why a neutrons mass is a little bigger
than a protons
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Particles emitted are opposite from beta
negative decay
 Positive positron, sometimes called an anti
electron (antimatter version of an electron)
 Same mass as an electron, but positive charge
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Emits a form of EMR, not a particle =>much harder to
stop (it’s pretty high-up in frequency of the EM
spectrum)
Happens most often after alpha and beta decay
Nucleus has been through a lot and needs to release
excess energy
Since it’s a release of energy A and Z stay the same
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Half life of an element: the time it will take
half of the parent atoms to transmutate into
something else
 Through alpha or beta decays, or another process
 Total number of atoms stays constant
 Based on statistics
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The half life of C-14 is 5730 years. Explain
what you would expect to happen over a long
period of time.
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Activity measures
the number of
nuclei that decay
per second
 Measured in Becquerels (Bq) = decays/second.
 Geiger counter clicking in movies measures the
activity of the sample.
 As time passes, the number of nuclei available
decrease and sample activity does too
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have 75 g of lead-212. If it
has a half life of 10.6 hours,
determine how long it will take
until only 9.3 g remains.
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What do you think of when I say nuclear
energy?
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There are 2 types of nuclear reactions that
release energy
 Fission
 Fusion
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The process of causing a large nucleus (A >
120) to split into multiple smaller nuclei,
releasing energy in the process.
 Can start when large nuclei absorbs a neutron,
causing it to become unstable to the point that it
falls apart
 Reaction that we use in nuclear power plants and
early nuclear weapon
 Pretty easy and cheap energy
 Lots of nuclear waste stored for a long time
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The process of causing small nuclei to stick
together into a larger nucleus, in the process
releasing energy.
 Process that drives our sun and all other suns
 We can duplicate in a lab, but use more energy
than we get out
 Left over products are safe, so lots of research
goes into trying to develop fusion reactors
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The most typical fuel used in a fission reactor is
uranium-235.
 1939: 4 German scientists discovered that uranium-
235 would become very unstable if it gained an extra
neutron, forming uranium-236.
 Uranium-236 is so unstable that a fraction of a
second later it will split to form two smaller atoms,
and in the process release energy.
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If one neutron gives rise to another reaction, the
self sustaining reaction that results is called critical.
 Each reaction leads to one reaction afterwards.
 This is a “chain reaction”.
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If 2+ neutrons give rise to more reactions, the
increasing rate of reactions is called supercritical.
 Each reaction leads to multiple reactions afterwards.
 Generations of reactions increase exponentially
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There are a few situations when we want this
to happen...
 Nuclear bomb, since we want one reaction we
kick off to result in a cascade of exponentially
more and more reactions within a split second
 When a nuclear power plant is first being started
up
▪ Then stepped down to a critical reaction.
▪ If the nuclear reactor is melting down then supercritical
reactions are BAD
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You need subcritical reactions
 Less than a neutron gives rise to other reactions
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Reactors use control rods to control the rate
of the reaction.
 Made from elements such as boron and cadmium,
control rods are very good at absorbing neutrons.
 If a reaction is going supercritical, drop the control
rods further into the core to absorb extra neutrons
and the reaction slows.
 If the reaction is going subcritical, pull the control
rods out further, which lets more neutrons react
and get more reactions going again.