Nuclear Physics

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Nuclear Physics

• Physics 12

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Protons, Neutrons and Electrons

• The atom is composed of three subatomic particles

Particle Charge (in C) Symbol Mass (in kg)
Electron -1.602x10-19 e- 9.109 56x10-31
Proton 1.602x10-19 p 1.672 614x10-27
Neutron 0 n0 1.674 920x10-27
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Atomic Nucleus

• Atom described using
• X atomic symbol
• A atomic mass number (nucleon number)
• Z atomic number
• Number of protons and electrons Z
• Number of neutrons A - Z

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Strong Nuclear Force

• The electrostatic forces inside a nucleus would
  rip it apart if there was not another force
• By the end of the 1930s physicists had
  determined that nucleons attract each other
• This is the strongest force in the known universe

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Stability and the Nucleus

• Although the Strong Nuclear Force is strong
  enough to hold a small nucleus together, as the
  size of the nucleus becomes larger, the
  electrostatic forces begin to become more
  important
• As a result, if we consider various nuclei based
  on their Atomic Number and Neutron Number we get
  the following result

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Stability and the Nucleus
Each black dot represents a stable nucleus, with
the number of neutrons shown on the vertical axis
and the number of protons on the horizontal axis
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Nuclides and Isotopes

• Nuclides are different combinations of nucleons
• Isotopes occur when an element (specific Atomic
  Number) has different numbers of neutrons
  (different Atomic Mass Numbers)
• For example, there are three common isotopes of
  hydrogen

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Nuclides and Isotopes
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Nuclear Binding Energy

• It takes 13.6 eV to separate an electron from a
  hydrogen atom
• However, it takes more than 20 MeV to separate a
  neutron from a helium-4 atom
• The energy to separate all the nucleons in a
  nucleus is called the binding energy

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Larger nuclei are held together a little less
tightly than those in the middle of the Periodic
Table
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Mass Defect

• If you were able to apply the 20 MeV required to
  separate a neutron from helium-4, what would
  happen to it?
• This is dealt with using Einsteins Special
  Theory of Relativity and the fact that mass and
  energy are equivalent ? E mc2
• The mass of helium-4 (2p, 2n) is smaller than
  that of helium-3 (2p, 1n) and a neutron
• The energy that was added to remove the neutron
  was converted into mass
• The difference between the mass of a nuclide and
  the sum of the masses of its constituents is
  called mass defect

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Atomic Mass Unit (u)

• When dealing with nucleons, it is often more
  useful to deal with mass in unified atomic mass
  units (u) instead of kilograms

Particle Mass (in kg) Mass (in u)
Electron 9.109 56x10-31 0.000 549
Proton 1.672 614x10-27 1.007 276
Neutron 1.674 920x10-27 1.008 665
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Binding Energy Example

• Determine the binding energy in electron volts
  and joules for an iron-56 nucleus given that the
  nuclear mass is 55.9206u

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Binding Energy Example

• Determine the binding energy in electron volts
  and joules for an iron-56 nucleus given that the
  nuclear mass is 55.9206u

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Binding Energy Example

• We would expect the binding energy per nucleon to
  be about 8MeV

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Radioactive Isotopes

• In discussing the nucleus, we looked at a plot of
  stable nuclei
• It is also possible to have a nucleus that is not
  stable (meaning that it will fall apart)
• An unstable nucleus will decay following a few
  very specific processes
• We call this decay radioactivity and classify it
  into one of three types

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Radioactive Isotopes
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Alpha Decay

• An alpha particle (a) is a helium nucleus (two
  protons and two neutrons)
• A nucleus that emits an alpha particle will lose
  the two protons and two neutrons
• Large nuclei will emit alpha particles
• They do not penetrate matter well and a sheet of
  paper or 5cm of air will stop most
• They can free electrons from atoms, meaning they
  are a form of ionizing radiation

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Alpha Decay
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Beta Decay

• When a nucleus emits a beta particle (ß), it
  appears to lose an electron or positron from
  within the nucleus
• There are two types of beta decay (ß- and ß)
• Beta particles can penetrate matter to a greater
  extent than alpha particles they can penetrate
  about 0.1mm of lead or 10m of air
• They are also a form of ionizing radiation but
  less damaging than alpha particles

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Beta Decay (ß-)

• In this type of beta decay, a neutron becomes a
  proton and a ß- particle (high energy electron)
  is emitted
• In addition an antineutrino ( ) is emitted
  (antimatter) along with the beta minus particle
• The nucleuss atomic number increases by one
  while the atomic mass number remains the same

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Beta Decay (ß-)
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Beta Decay (ß)

• In this type of beta decay, a proton becomes a
  neutron and a ßparticle (high energy positron or
  antielectron) is emitted
• In addition a neutrino ( ) is emitted along
  with the beta plus particle
• The nucleuss atomic number decreases by one
  while the atomic mass number remains the same

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Beta Decay (ß)
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Gamma Decay (?)

• When a nucleus goes through alpha or beta decay,
  the daughter nucleus is often left in an excited
  state
• In order to reduce the energy of the nucleus, it
  will go through gamma decay (high energy photon)
  to return to the ground state
• Gamma radiation can pass through 10cm of lead or
  2km of air
• It is the most damaging of all due to the energy
  of the gamma particle

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Gamma Decay
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Decay Series

• When a large nucleus decays by alpha and beta
  radiation, the daughter nucleus will be more
  stable than the original nucleus
• However, the daughter nucleus may still be
  unstable and will itself go through alpha or beta
  radiation
• This leads to a decay series

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Rate of Radioactive Decay

• It is impossible to predict when a specific
  nucleus will decay
• You can describe the probability of decay
• The concept of half life is used with radioactive
  decay the time required for half of the sample
  to decay
• Using the half life equation, it is possible to
  determine how much of a sample would remain after
  a given period of time

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Half Life

• N ? sample remaining
• N0 ? original sample
• ?t ? elapsed time
• T ? half life

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Half Life