Radioactivity

Radioactivity

1. Becquerel's key showed that Ur radiation passed through paper, but not metal

   • Lead aprons block x-rays - used as radiation shields
   • Becquerel and Curie discovered radioactivity
   • Rutherford discovered the atom - negative electrons orbit the positive nucleus - proved plum pudding theory wrong
     • p = e

2. 3 types of radiation

   • Alpha
     • Stopped by paper // Deflected by magnetic field (positive)
     • An alpha particle --> 2p + 2n
   • Gamma
     • Stopped by Pb // Undeflected (uncharged)
   • Beta
     • Stopped by Al // Deflected by magnetic field (negative)
     • A beta particle --> Produce electrons

3. Radioactive decay

   • Unstable nucleus emits radiation to form a stable nucleus
     • The radiation causes ionisation which kills the cell
       • Alpha radiation causes most ionisation
         • Uranium decay in rocks produces radon gas
   • Random event

4. Half-life

   • Isotope = atoms of an element that contain different numbers of neutrons (p + n)
     • Neutrons different, protons same
   • Time taken for number of atoms decaying to halve
   • Time taken for count rate to halve

5. Radioactivity at work

   • Alpha
     • Used in smoke alarms
       • Half-life of several years
   • Beta
     • Thickness monitoring - paper and foil
       • Half-life of many years
   • Beta and gamma
     • Tracers in medicine
       • Person radioactive for a few hours, so half-life is a few hours

6. Radioactive dating

   • Used to find age of ancient material
   • Carbon dating
     • Count rate of wood/organic materials compared to count rate of living wood (carbon)
       • Half-life is 5600yrs
   • Uranium dating
     • Igneous rocks contain Ur with half life of 4500M yrs --> count number of Pb and U atoms

7. Geiger counter measures radioactivity = number of atoms that decay per second

   • Count rate = number of counts per second