Radioactivity

Radioactivity

1. Alpha Radiation

   • Emits 2 protons and 2 neutrons
     • Same as helium nucleus
   • Forms a new element
     • Alpha decay causes both the mass and the charge of the nucleus to decrease
   • Mass number = 4 Atomic number = 2
   • Absorbed by skin and paper
   • Can only travel a few cm unless in a vacuum
   • Deflected less than beta in magnetic fields
   • Slower than gamma - varying speeds
   • Attracted to the negative plate in electric field as they are positively charged
   • Highest ionising power

2. Beta Radiation

   • Emits an electron
   • Beta particles are high speed electrons ejected from the nucleus as a neutron changes into a proton
     • A neutron turns into a proton in order to stabilise the nucleus
   • Forms a new element because a new proton has been added
     • Beta decay doesn't cause the mass of the nucleus to change but does cause the charge to change
   • Absorbed by thin aluminium
   • Slower than gamma - varying speeds
   • Ionises less than alpha but more than gamma
   • Attracted to the positive plate in electric field as they are negatively charged
   • Can travel 1m approx
   • Mass number = 0 Atomic number = -1
   • Deflected more than alpha in magnetic fields because it is lighter

3. Gamma Radiation

   • Excess energy that comes out of nucleus after alpha / beta decay
   • Emitted as a high-energy light wave
   • Doesn't change in mass or atomic number
     • Because no mass / atomic number
   • They are less ionising than alpha / beta particles
   • Electromagnetic radiation from nucleus
   • Absorbed by lead and concrete
   • Not deflected
   • Fastest radiation type ( 3 x 10 to the power of 8 m/s)
   • Infinite range

4. Discovery of the Nucleus

   • The Plum Pudding Model
     • Negative electrons / 'plums' randomly distributed in the atom / positive 'dough'
   • Rutherford's Scattering Experiment
     • Rutherford's Model = electrons circling a positive nucleus
     • The experiment: Rutherford fired atoms at thin gold foil.
       • He thought that ALL of the atoms would go through.
       • The gold was positively charged in one area
       • The results:
         • 1. Most alpha particles passed straight through the foil (these were particles that passed through far from nucleus)
         • 2. About 1 in 2000 were deflected (these were particles that passed closer to the nucleus)
         • 3. About 1 in 10000 were deflected by angles more than 90 degrees (these were particles that passed very close / head-on to nucleus)
       • The equipment has to be in a vacuum, otherwise the air would stop the alpha particles from moving
       • Thick lead stops all but a narrow beam from entering chamber
       • A momentary flash could be seen on the screen that he used when radiation was detected
       • A radioactive source emits the alpha particles. It has a long half life.
     • He knew that: atoms of certain elements were unstable, there were 3 types of radiation and alpha particles consisted of fast moving positive particles

5. Keywords

   • Half Life
     • the time it takes............
       • for the number of nuclei of the isotope in a sample to halve
       • for the count rate / activity of the isotope in a sample to fall to half its original value
   • Count Rate
     • the number of decays recorded each second by a detector
     • number of counts per second
   • Activity
     • The activity of a radioactive source is the number of unstable atoms in the source that decay per second
     • Activity is measured in Becquerels (Bq) - 1 decay per second
   • Geiger counters = used to detect radioactivity

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