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Nuclaer Physics

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Nuclear Physics:

Radioactive decay:

  • Regardless of any circumstances radiation does not change: temperature, sound, chemical conditions, etc.
  • These particles were good at ionizing electrons: “knocking off” electrons (charging)
  • Element notation:  
  • A=mass number
  • Z= atomic number
  • X= individual element
  • No two elements are the same
  • Isotopes: same element with different mass… atomic number, different mass number
  • Unless otherwise specified, the element will be assumed to be electrically neutral, so there will be the same number of electrons as there are protons.
  • Protons and neutrons: nucleons
  • Nucleus stays together by strong nuclear force.  Sometimes force is overcome and atom is unstable.  Unstable isotopes: RADIOISOTOPES.

This is because when there are more protons, lots of protons repel as the force does not hold all the protons together.

  • Different ionizing radiation: α, β and γ (alpha, beta and gamma)

α Radiation:

  • Most powerful.  Approximately 5MeV ← unit of energy: Mega Electron Volt.  
  • Ionizes things almost straight away
  • Stopped easily: air, paper, skin
  • Very dangerous if inhaled or indigested. (alters DNA, ionizes)
  • Electrical charge of +2
  • Very heavy, approximately a Helium atom’s weight.
  • Is able to travel at up to approximately 5% of c (where c is the speed of light)
  • Radiation is very damaging and very fast, (50 million km/hr)
  • Because it is so fast, generates heat: burning/melting
  • Alpha radiation has no electrons (ionized) and is just a Helium Nuclei.
  • 42He2+
  • eg. 23592U 23190Th + 42α + energy    ) numbers always work out, add up/balance
  • The ‘energy’ is gamma radiation
  • Also can be written as image00.png

23592U                 23190Th

β Radiation:

  • fairly powerful ≈ 1MeV
  • not as damaging as alpha
  • takes a few meters of air, few cm of paper, mm of skin, very thin metal sheet (e.g. aluminum) to stop it
  • charge of -1
  • small particles, weigh as much as an electron
  • travel at 99%c
  • beta radiation is actually highly energetic electrons
  • 0-1e
  • β radiation occurs when a neutron decays into a protons and an electron and a little particle called an anti-neutrino.  
  • Occurs when there are too many neutrons (isotope is too heavy).  Proton remains in nucleus, electron rockets out very fast.

i.e. 10n 11p + 0-1e + νe  

  • 146C 147N + 0-1β + νe + energy

γ Radiation:

  • less powerful ≈ 0.1MeV
  • Can pass through many cm of lead
  • No charge
  • No mass
  • Travels at the speed of light
  • Given out in large quantities (other two aren’t)
  • Gamma rays are actually electromagnetic radiation of high frequency, i.e. high energy light
  • After decays and interactions, nuclei may have excess energy, therefore being unstable.  This is where they release a γ particle with the excess energy.  Often happens with the decay.
  • E.g. 13153I 13154Xe + β- + γ


  • MeV (M=million) is lots of energy, most reactions involved only a few eV.  i.e. a millionth of the energy involved in ionizing radiation.
  • 1eV is 1x10-19 Joules
...read more.


Geiger counter, ionization chamber


  • There are natural isotopes but if humans create them, it is called artificial transmutation. Shooting neutrons at an isotope until it becomes unstable.

Half Life:

  • No external properties have an effect on radioactive decay.
  • Decay is a random process governed by the half-life of the isotope.
  • The half life, T½, is used to state the time in which an isotope has a 50% CHANCE OF DECAYimage03.png
  • Half life is very accurate if there are many billions of atoms.  The bigger the atom, the more accurate the answer is.
  • Two graphical methods:  either the number of radioisotopes left, or the number of decays of radioactive particles.


  • Is measured in Becquerel (Bq) which is A MEASURE OF DECAYS PER SECOND.
  • Therefore 100Bq means that 100 atoms decay every second.
  • Some substances are chosen for their short half life (in the body, medical purposes), decay quickly and give out radiation but don’t last long.  Others are chosen for a long half life, not a lot of radioactivity but last a long time.

Radiation and YOU

  • It is impossible to avoid radiation: cosmic radiation from space (γ radiation)
  • Background radiation is not a risk
  • High levels of radiation can cause permanent damage and even death
  • Ionizing power of radiation:  break atoms apart, destroying body cells.
  • How bad it is depends on the absorbed dose:
  • Absorbed does = energy absorbed ÷ mass of tissue  
  • Absorbed dose measured in JOULES/KILOGRAMS or GRAYS (Gy)
  • Absorbed does – quick way to gauge danger but
...read more.


The difference is called the MASS DEFECT (energy and mass are the same ‘stuff’)Missing mass means that it has just changed due to conservation of mass / conservation of energy, it has turned into conservation of mass-energyThere is energy that binds the electron into the Hydrogen.The difference in masses is because of the binding energy.E=mc2 can tell us how much.    (m in kg, produces answer in J)

Binding Energy

  • Energy of ‘mass difference’ or ‘mass defect’ is the binding energy of an atom.  
  • When protons and neutrons are combined, the energy holds them together,
  • When they separate, energy is released.
  • To compare different atoms we used the averaged binding energy per nucleon.  The larger the value, the more stable it is.


  • The isotope of Iron, Fe-56, is at the top of the “mountain”, most stable.  
  • All elements that are left of Fe-56 can be combined in nuclear fusion with the release of energy
  • All elements that are right of Fe-56 can be split in nuclear fusion with the release of energy.image05.png

This chart shows:

  • As atoms gain more protons, the number of neutrons that are needed for stability increases faster.  For smaller elements, the number of protons is usually similar to the number of neutrons but further up this is no longer the case.
  • ALL atoms greater than Bismuth (83rd element) are radioactive.  And ALL atoms greater than Uranium have been man made and do not occur in nature.
  • Of all the isotopes, majority are radioisotopes
  • Line of stability is the line that curves away from the diagonal N=Z line.  

...read more.

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