T7: radioactivity

• atomic number = number of protons in the nucleus
• mass number = number of protons + neutrons in the nucleus
• isotope = atom of the same element with same number of protons but different number of neutrons
• radioactive decay is when an unstable nucleus spontaneously emits radiation to become more stable
• beta minus particle (β-) - electron
• gamma ray (γ) - high energy electromagnetic wave
• alpha particle (α) - helium ion, contains two protons and two neutrons
• three types of ionising radiation: alpha beta gamma
• ionisation = when an atom loses or gains electrons causing it to become an ion
• ALPHA description:

  • strong ionising power
  • range of a few centimetres
  • stopped by paper or thin card
  • nature = helium nucleus
• BETA description:

  • moderate ionising power
  • range of 1 meter
  • stopped by thin aluminium sheet
  • nature = fast moving electron
• GAMMA description:

  • very weak ionising power
  • range of at least 1 kilometre
  • stopped by 10cm of lead
  • nature = electromagnetic wave
• practical: investigate penetration powers of different radiation types

  • detect using Geiger Müller Tube
  • try three different materials - paper, aluminium, lead
  • count rate will decrease if radiation is stopped
• Alpha decay:

  • 2 protons and 2 neutrons are lost
  • mass number decreases by 4
  • atomic number decreases by 2
• beta decay:

  • 1 neutron is converted to a proton and an electron emitted
  • mass number constant
  • atomic number increase by 1
• gamma decay:

  • energy lost from atom in for of electromagnetic wave
  • mass and atomic number constant
• background radiation sources from earth and space:

  • radon in air
  • granite in rocks
  • cosmic rays
  • medical equipment
  • food and drink
• the activity of a radioactive source decreases over time - measured in becquerels
• a half life = the time taken for the radioactivity of a specific isotope to halve
• uses of radioactivity:

  • gamma radiography
  • medical tracer
  • gauging = coal absorbs radiation
  • radiotherapy - high dose directed to cancer cells
• contamination = occurs when a material that contains radioactive atoms is deposited on materials, skin or clothing
• irradiation = the process in which an object is exposed to radiation
• radiation can cause:

  • mutations in living organisms
  • damage to cells and tissues
• nuclear fission = process where heavy atoms are split into smaller lighter atoms = releases energy
• nuclear fussion = process where lighter atoms are forced to join together to make hevier atoms = releases energy
• nuclear fission of U-235:

  • slow moving electron absorbed by U-235
  • becomes U-236 = unstable
  • splits to form two smaller daughter nuclei, three neutrons and gamma radiation
• chain reaction: the process by which a fission reaction produces more fission products
• chain reaction = three neutrons produced from fission of U-235 = hit another U-235 = repeat process
• chain reaction needs minimum mass of U-235 = critical mass
• moderator = graphite

  • purpose: to absorb some kinetic energy of neutrons to slow them down = slow neutrons are more easily absorbed by U-235
• control rods: made of boron

  • purpose: to absorb neutrons and remove them from fission = helps change rate of nuclear fission
• shielding around nuclear reactor:

  • vessel made of steel
  • surrounded by concrete = at least 5 meters
  • prevents radiation escaping
• fusion = isotopes of hydrogen collide at high speed

  • mass before is greater than mass after
• nuclear fusion is the source of energy for sun and all stars

  • sun = hydrogen undergoing fusion to become helium
• high temperature needed for nuclear fusion = energy needed to overcome repulsive force between positivly charged nucleus of each isotope
• high pressure needed to increase chance of fusion between nuclei