Topic 6- radioactivity

• Isotopes of an element have the same amount of protons but different number of neutrons.
• The mass of an alpha particle-

  4
• The charge of an alpha particle-

  +2.
• Alpha particles are the most ionising, but least penetrating.
• The mass of beta particle-

  1/2000.
• The charge of a beta particle-

  -1.
• Beta particles are the median ionising and the median penetrating.
• The mass of a gamma wave-


  0.
• The charge of a gamma wave-


  0.
• Gamma waves are the least ionising but most penetrating.
• Beta minus decay-


  Mass (nucleon) number stays the same but the atomic (proton) number goes up.
• Beta plus decay-


  Mass (nucleon) number stays the same but the atomic (proton) number goes down.
• Alpha decay-


  Mass (nucleon) number falls by 4 and atomic (proton) number falls by 2.
• Thomson's Plum Pudding model proposed that positive charges distributed throughout the atom with negative electrons also dotted throughout.
• The Bohr model (right) is the one we now recognise. A positive nucleus of protons and neutrons is surrounded by electrons that exist in shells.
• Rutherford fired alpha particles at thin layers of gold atoms, most went straight through, some bent by a small amount and a few bounced back. This indicated that most of the atom is empty space, that the nucleus was small and had a positive charge. Bohr showed that electrons exist in shells.
• Uses of radioactivity-


  Cancer diagnosis and treatment, Irradiating food, Sterilising of equipment, Smoke alarm, Gauging thickness.
• Precautions against ionising radiation-

  Increased distance reduces exposure, as does less exposure time and shielding.
• Positron Emitting Tomography (PET scan)-


  Positron emitting isotopes (tracers) are carried to tumours. The positrons released collide with electrons in the body and the gamma photons emitted are detected outside the body using a gamma camera. Combined with a CT scan, this makes cancer easier to spot.
• The half life of a radioactive substance is the time taken for half of the undecayed nuclei to decay.
• Fission-


  The splitting of a large nucleus into smaller nuclei with the subsequent release of energy.
• Control rods-


  Capture neutrons, controlling the chain reaction.
• Moderators-


  Slow down the fast neutrons so that they can be absorbed by U-235 triggering a chain reaction.
• Fusion-


  The joining of two small nuclei into one larger nucleus with the consequent release of lots of energy.