Nuclear Physics

• Rutherford Scattering Experiment

  • A beam of alpha particles was directed at a thin gold foil
  • Occurs in a vacuum so that no collisions between air particles and alpha particles can occur
  • The experiment was done in order to determine structure of an atom
• Rutherford Scattering Experiment: Why did most alpha particles passed straight through?

  The atom is made mostly from empty space
• Evidence that the nucleus had a positive charge

  The nucleus repels the alpha particles and caused it to deflect from its original path
• 3 Types of Radiation

  • Alpha
  • Beta (plus and minus)
  • Gamma
• Most ionising radiation?

  Alpha
• Most penetrating radiation?

  Gamma
• What blocks alpha radiation?

  Paper or a few centimetres of air
• When a nucleus decays through gamma radiation, how does the atomic number and mass number change?

  They remain the same as the number of protons and neutrons do not change
• Why is ionising radiation seen as dangerous?

  It can kill or mutate cells, which could lead to mutations and lead to things such as cancer
• Real life use of Alpha radiation

  Smoke Detectors
• Real life use of Beta radiation

  Measuring the thickness of paper or aluminium foil
• Which type of radiation follows the inverse square law?

  Gamma radiation
• Inverse Square law of gamma radiation

  The intensity of the radiation is inversely proportional to the square of the distance from the source
• Units of intensity

  Watts per square metre (W/m^2)
• Background radiation


  Radiation that is constantly in the surrounding from sources such as rocks and food
• Decay Constant

  The probability of a nucleus decaying per second
• Units of the decay constant

  s^-1
• Half life

  The time it takes for half of the unstable nuclei in a substance to decay
• Half life equation

  $T_{\frac{1}{2}}=$$\frac{\ln2}{\lambda}$
• Activity equation

  $A=$$\lambda N$
• Units of activity

  Becquerels (Bq)
• True of False: Radioactive isotopes decay exponentially

  True
• Why is Technetium 99m useful in medicine?

  • It releases gamma radiation
  • It has a short half life therefore it doesn't stay highly radioactive for long
  • Half life of 6hrs: long enough for it to be detatched
  • It can also be made near to the hospital
  • Easy to detect outside the patient
  • 'Clears away' after a few days
• Where on the curve of stability does B- decay occur and why?

  Above the stability line, because the nuclei found there contains too many neutrons. Therefore when beta minus decay occurs the neutron turns into a proton and it becomes more stable
• What type of decay occurs below the stability line and why?

  Beta plus decay. As the isotopes found here often have too many protons. Therefore when beta plus decay occurs the proton turns into neutrons
• How does the heavier nuclei often decay?

  Through alpha decay
• Ways to estimate nuclear radius

  • Distance of closest approach
  • Electron Diffraction
• how is electron diffraction used to determine the diameter of a radius?

  • An electron beam is fired at a thin sheet of the desired atom
  • A diffraction pattern is produced on a screen behind
  • Use $\sin\theta=$$1.22\frac{\lambda}{d}=$$1.22\frac{\lambda}{2R}$
• True or False: The density of a nucleus is independent of its radius

  True
• Equation to convert mass to its energy equivalent

  $E=$$mc^2$
• Mass Defect

  The difference between the total mass of all the nucleons separately compared to the mass of the nucleus
• Why is there a mass defect?

  Energy is needed to bring the constituent parts of a nucleus together, therefore the mass equivalent of the energy is lost and the total mass decreases
• Binding Energy

  The energy required to separate a nucleus into its constituent parts
• Nuclear fission

  • Where an unstable nucleus splits into 2 smaller nuclei
  • Often occurs with larger nuclei
  • The binding energy per nucleon increases when fission occurs therefore the overall process releases energy
• Nuclear Fusion

  When two small nuclei fuse together to create a larger nuclei.

  The new nucleus has a larger binding energy per nucleon than the old nuclei therefore energy is released in the process