Radioactivity and Nuclear Reactions

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Radioactivity and Nuclear Reactions Dr Graeme
Jones
LECTURE 2
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Pioneers of Radioactivity
Marie Curie Nobel Prize for Physics 1903 Nobel
Prize for Chemistry 1911
Pierre Curie Nobel Prize for Physics 1903
Henri Becquerel Nobel Prize for Physics 1903
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What holds the nucleus together

• There are both attractive and repulsive forces in
  the nucleus.
• A small gravitational force attracts the nucleons
• An electrostatic force repels the protons from
  each other
• The strong nuclear force sticks them together at
  short distances but is negligible at long
  distances.
• Overall a total binding energy for the nucleus
  can be calculated. When you do this you realise
  that some of the mass of the nucleons has been
  converted into energy through Einsteins Equation
  
• E mc2
• Typically these binding energies are very large
  and measured in MeV. Thats why when you split
  nuclei so much energy is released.

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Unstable Nuclei

• Atoms with very low atomic numbers have about the
  same number of neutrons and protons and are
  generally stable.
• As Z gets larger more neutrons are needed to
  produce a stable nucleus, the largest stable
  nucleus is Bismuth-209 (Z 83)
• Nuclei with more than 83 protons are all
  unstable, and eventually break up into smaller
  pieces through radioactive decay.
• You can find all the different nuclei known at
  http//www.nndc.bnl.gov/chart/ along with their
  half life.

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Alpha Emission

• An alpha particle (a) is the nucleus of a Helium
  atom which contains 2 protons and 2 neutrons
• Alpha particles do not travel far in air before
  being absorbed.
• Without alpha emitters there would be no smoke
  detectors (Americium-241), balloons that float or
  people speaking in squeaky voices.

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Beta Emission

• In Beta Emission a neutron is converted into a
  proton and an electron and the electron is
  emitted from the atom, termed a beta particle
  (ß-)
• Note the mass does not change, but the number of
  protons is increased by one
• Beta particles are more penetrating than alpha
  particles, having a higher energy, can travel a
  few feet in air and need materials such as
  aluminium foil to stop them

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Use of Beta Emitters

• Iodine-131 is used to treat thyroid disorders,
  such as cancer and hyperthyroidism. 
• Phosphorus-32 is used in molecular biology and
  genetics research. 
• Strontium-90 is used as a radioactive tracer in
  medical and agricultural studies. 
• Tritium is used for life science and drug
  metabolism studies to ensure the safety of
  potential new drugs. It is also used for luminous
  aircraft and commercial exit signs, for luminous
  dials, gauges and wrist watches. 
• Carbon-14 is a very reliable tool in dating of
  organic matter up to 30,000 years old.
• Beta emitters are also used in a variety of
  industrial instruments, such as industrial
  thickness gauges, using their weak penetrating
  power to measure very thin materials.

http//www.epa.gov/radiation/understand/index.html
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Gamma Emission

• Gamma radiation (?) is very high-energy ionizing
  radiation.
• A gamma photon is emitted from the nucleus as a
  way of the nucleus losing energy, often after
  beta emission.
• There is no overall change in the composition of
  the nucleus, Gamma photons travel at the speed of
  light and hence are very penetrating and need
  very dense materials such as lead to stop them.

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Gamma Emission

• Gamma emitters are widely used radiation sources.
  
• Cesium-137
• cancer treatment
• measure and control the flow of liquids in
  numerous industrial processes
• investigate subterranean strata in oil wells
• measure soil density at construction sites
• ensure the proper fill level for packages of
  food, drugs and other products.
• Cobalt-60
• sterilize medical equipment in hospitals
• pasteurize certain foods and spices
• treat cancer
• gauge the thickness of metal in steel mills.
• Technetium-99m
• TC-99m is the most widely used radioactive
  isotope for diagnostic studies. (Technetium-99m
  is a shorter half-life precursor of
  technetium-99.) Different chemical forms are used
  for brain, bone, liver, spleen and kidney imaging
  and also for blood flow studies.
• cobalt-60 or cesium-137 can improve the physical
  characteristics of materials. For example,
  exposure to gamma radiation improves the
  durability of some wood and plastic composites.
  Treated materials can be used for flooring in
  high-traffic areas of department stores,
  airports, and hotels, because they resist
  abrasion and ensure low maintenance.
• Industrial radiography uses gamma radiation to
  inspect metal parts and welds for defects.

http//www.epa.gov/radiation/understand/index.html
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Decay Series

• This series illustrates how unstable radioactive
  nuclei decay back to stable nuclei

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Half Life

• The rate of radioactive decay is characteristic
  of each radionuclide.
• The radioactive half life is the time required
  for the disintegration of one-half of the
  radioactive atoms that are present when
  measurement starts.
• Note this does not represent a fixed number of
  atoms that disintegrate, but a fraction.  
• Radioactive decay is said to follow first order
  kinetics and to be exponential

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Half Life
www.onlinephysicshelp.com/radioactivity.htm
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Half Life of Different Nuclei
                                     
http//www.scienceteacherprogram. org/chemistry/st
evens03.html
                                
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Units of Radioactivity

• The original unit or radiation was a curie (Ci)
  defined as 37 billion disintegrations per second.
  This was originally measured as the activity of
  one gram of radium.
• The SI unit is the becquerel (Bq), defined as the
  activity of a quantity of radioactive material in
  which one nucleus decays per second. It is
  therefore equivalent to s-1.
• 1 Ci 3.71010 Bq or 37 GBq