nuclear

• Rutherford scattering


  Also known as the gold foil experiment, it was a test of the Thomson model of the atom. It involved bombarding a thin gold foil with alpha particles. The experiment disproved the Thomson model and showed that atoms have a nucleus.
• The Rutherford scattering experiment did not end the exploration of the atom
• Some alpha particles were deflected through angles greater than 90°
• When an alpha particle being deflected by nucleus is at its closest point


  • Kinetic energy is at its minimum value, rate of change of momentum, potential energy, acceleration are at their maximum value
• The force of repulsion by the nucleus on an alpha particle is drawn in a straight line from the centre of the nucleus through the alpha particle and away
• The diameter of the nucleus is much less than the diameter of the atom
• It is necessary to remove the air from the chamber in a Rutherford scattering experiment to prevent absorption of the α particle by the air
• The gold foil should be very thin so that the α particles must not be absorbed by the foil and must only be scattered once
• Types of radiation deflected by a magnetic field

  • Alpha
  • Beta
• Types of radiation deflected by an electric field


  • Alpha
  • Beta
• Charges on the three types of radiation


  • Alpha: +2e
  • Beta: -1e
  • Gamma: 0
• Gamma radiation is emitted at discrete frequencies
• Absorbing gamma radiation does not make an object radioactive
• Equation for surface area of a sphere

  Surface area of a sphere = 4πr^2
• For gamma radiation, a graph of I plotted against r would not be a straight line through the origin, but I against r^-2 would be
• Link between inverse square law and safe handling of radioactive sources


  Keep your distance from the source (by using long tongs etc) as doubling the distance from the source reduces the intensity of the radiation to 1/4
• Gamma radiation is the most appropriate radiation for the sterilisation of surgical instruments
• Background radiation


  Radioactivity that is ever present even if there is no source nearby. It cannot be eliminated.
• Sources of background radiation


  • Radon
  • Rocks
  • Cosmic rays
  • Nuclear fallout
  • Medicine
  • The sun
  • Carbon-14
• Corrected count rate


  Background radiation had been measured and deducted
• To ensure the accuracy of the background radiation measurement, measure the background for a long time or take many repeated measurements
• Alpha radiation produces the greatest number of ion pairs per mm in air
• Typical maximum range in air for α and β particles

  • 0.04m
  • 0.4m
• Solid angle subtended


  The reason why gamma radiation intensity reduces with distance is that the solid angle subtended by a detector is smaller as it is moved away from a source, so fewer gamma photons pass through it each second
• The most hazardous aspect of the presence of α-emitting dust to an unprotected human is that the dust may be breathed in causing cells to be made cancerous by ionisation
• Gamma is used for tracing the passage of particular substances through the human body because the radiation needs to pass through the body to be detected
• Radioactive tracers should not have a very short half-life, otherwise the activity of the source becomes too weak during measurements
• Radioactive tracers should not have a very long half-life, otherwise the decaying source may remain in the body for a long time and could cause damage
• Ionising radiations


  Alpha and beta emissions are known as ionising radiations because they remove electrons from atoms by colliding with them
• Why beta particles have a greater range in air than alpha particles

  They have fewer collisions per metre because beta particles are less ionising so lose energy over a larger distance
• Radioisotopes with short half-lives are particularly suitable for use as a medical tracer because they have high activity so only a small sample is needed, they decay quickly so there is less risk to the patient and other people, and the medical test does not last long
• A radioisotope is used to maintain a constant thickness of sheet paper or steel as it is being rolled by using beta particles that pass through the material to a detector, and if the material is thicker fewer particles are detected and the roller pressure is reduced etc.
• Decay constant


  The probability that an atom will decay per unit time
• The molar mass of U-238 is 238g
• The ratio of C-14 to C-12 for wood starts to differ from the atmospheric ratio when the tree dies
• Electron capture


  An inner orbiting electron is captured by a proton in the nucleus and converted into a neutron
• The existence of gamma radiation provides evidence for the existence of energy levels in nuclei
• Cause of gamma radiation


  After an unstable nucleus decays it might be in an excited state where the nucleons are not arranged in their tightest formation. As the nucleus moves to its ground state the excess energy is released as a gamma photon.
• Metastable


  If the excited state is long-lived
• The usefulness of Mo-90 decaying to Tc-99m is that Tc-99m is a pure gamma emitter (unlike most gamma sources which only emit gamma immediately after emitting α or β), so it can be used as a medical tracer