knowledge-19 R

• In 1897 J.J Thompson discovered that atoms contain a negatively charged particle , given the name electrons. As a result scientist believed that electrons were embedded inside a sphere of positive charge. (plum pudding model).
• In the modern model of the atom has electrons occupying energy levels at specific distances from the nucleus.
• In 1911, the plum pudding model was replaced by the nuclear model of the atom, where the nucleus contains positive charge of the atom and the negative electrons orbit the nucleus. Most the nucleus and most of the atom is empty space.
• Alpha particle scattering:

  The nuclear model was based on the results of an experiment done in a laboratory run by Ernest Rutherford. A beam of positively- charged alpha particles were fired at a very thin sheet of gold.
• Alpha particle scattering (observations):

  The following observations could only be explained by the nuclear model:

  • most of the particles went straight through the foil- most of an atom is empty space.
  • A few particles were deflected at large angles- the nucleus is positively charged so will repel alpha particles.
  • very few particles were deflected backwards- the mass of the atom is concentrated in a tiny space of the atom, so direct collisions between alpha particles and nucleus are very rare.
• Alpha particle scattering setup:

  • a fixed metal foil in front of lead box
  • alpha source in a lead box with a narrow hole
  • happens inside a evacuated chamber
  • a detector that consists of a microscope focused on a small glass plate. each time an alpha particle hit the plate a spot of light was observed.
• Alpha particle scattering:

  • The apparatus was in a vacuum chamber to prevent air molecules absorbing the alpha particles.
  • The detector was moved to different positions , At each position , the number of spots of light observed in a certain time was counted.
• Nuclear radiation:

  Nuclear radiation is emitted by unstable atomic nuclei to become more stable. Nuclear radiation is known as ionizing radiation because it can produce ions by knocking electrons out of atoms it collides with.
• Types of nuclear radiation:

  An unstable nucleus can decay by emitting an alpha particle , a beta particle , a gamma ray or sometimes a neutron.
• Identifying nuclear radiation:

  The different types of radiation can be identified experimentally using their properties. Penetrating power or absorption by different materials can be measured using specific equipment. Penetrating power is used in industry to control the thickness of sheets of aluminum foil , paper and steel.
• Identifying nuclear radiation:

  equipment used to to measure penetrating power:

  • source in sealed container
  • absorber
  • Geiger tube
  • electronic counter
• Hazards of exposure:

  Ionizing radiation is dangerous because it can damage the atoms or molecules in living cells. High doses of ionizing radiation can kill living cells, and even low doses can cause cell mutation and cancerous growth.
• Hazards of exposure:

  • alpha radiation is dangerous inside the body as it affects all surrounding tissue , but outside the body it only affects cells in the skin and eyes.
  • gamma and beta radiation are dangerous inside and outside the body as they can reach cells through out the body.
• Uses in medicine:

  • Tracers and cameras - a radioactive isotope is injected into or swallowed by the patient - radiation detected outside the body is converted to information by a computer.(gamma)
  • radiotherapy (external)- beam of gamma radiation from outside the body is aimed at cancerous cells.(gamma)
  • Radiotherapy (internal)- small seed or rod of radioactive material is implanted inside cancerous tumor. (beta or gamma)
• Safe handling of radioactive source:

  people who work with ionizing radiation must take precautions to protect against irradiation and contamination , including:

  • keeping as far away as possible from the radiation source.
  • spending as little time as possible in the Prescence of the radioactive source.
  • Shielding themselves from the radiation using concrete barriers or lead plates.
• Background radiation:

  The process by which an unstable nuclei emits radiation to become more stable is radioactive decay. This is a random process- it is not possible to predict when a particular nucleus will decay.
• Background radiation , measuring radioactive decay:

  The activity ,A ( unit becquerel) , of radioactive isotope is the number of nuclei that decay every second. Activity is equal to the change in number of nuclei of the radioactive isotope and is proportional to the number of nuclei N remaining at time t:

  • A = change in N / change in time = - wavelength x N
• Background radiation is radiation that is around us all the time , the level of background radiation is dependent on location.
• Background radiation comes from:

  • naturally occurring in radioactive isotopes present in the air, ground.
  • cosmic rays in space
  • human activity such as medical use of radioactive isotopes.
• Background radiation measurements should be taken and removed from the results of the experiment.
• the inverse square law is:

  • I = k /r^2
  • where k is a constant
• the intensity of radiation is the radiation energy passing through an area 1m^2 normal to radiation per second.
• Gamma radiation spreads out in all directions from the source and its intensity varies with the inverse of distance r from the source.
• the decay constant is the probability of an individual nucleus decaying per second.
• Radioactive decay can be modelled by other events that have a constant decay probability.
• the half life of a radioactive isotope is the time it takes for its activity to half.
• Half life is find by:

  • t (half life) = ln2 /decay constant
• half-life can be found from graphs of decay data such as activity against time or number of radioactive nuclei remaining against time.
• radioactive isotopes with a short half life are useful as radioactive tracers in medicine because they remain in the body for a short amount of time.
• An example of safe handling of radiation:

  People who are around moderate levels of radiation every day may wear a badge with a film that changes colour depending on the amount of radiation exposed to it. To determine different types it is covered in different types of materials and then the film.