Ionising Radiation and Half Life

Ionising Radiation
1. Alpha Particles are helium nuclei
  1. An alpha particle is two neutrons and two protons the same as a helium nucleus
    1. They are relatively big, heavy and slow moving
  2. They therefore dont penetrate very far into materials and are quickly stopped even when travelling through air
  3. Because of there size they are strongly ionising which just means they bash into a lot of atoms and knock electrons off them before they slow down which creates lots of ions
2. Beta particles are electrons
  1. Beta particles are in between alpha and gamma in terms or their properties
  2. They move quite fast and they are quite small (they are electrons
  3. They penetrate moderately into materials before colliding have a long range in air and are moderately ionising too
  4. For every beta particle emitted a neutron turns to a proton in the nucleus
  5. A beta particle is simply an electron with virtually no mass and a negative charge
3. Gamma rays are very short wavelength EM waves
  1. Gamma rays are the opposite of alpha particles in a way
  2. They penetrate far into materials without being stopped and pass straight through air
  3. This means they are weakly ionising because they tend to pass straight through air
  4. Gamma rays have no mass and no charge
4. Alpha and beta particles are deflected by electric and magnetic fields
  1. Alpha particles have a positive charge, beta particales have a negative charge
  2. When travelling through a magnetic or electric field both alpha and beta particles will be deflected. They are deflected in opposite directions because of their opposite charges
  3. Aplha particles have a larger charge than beta particles and feel a greater force in magnetic and electric fields. But they are deflected less because they have a much greater mass
  4. Gamma radiation is an electromagnetic (EM) wave and has no charge so it doesnt get deflected by electric or magnetic fields
Half-Life
1. The radioactivity of a sample always decreases over time
  1. Each time a decay happens and an alpha, beta or gamma is given out it means one more radioactive nucleus has disappeared
    1. As the unstable nuclei all steadily disappear the activity (number of nuclei that decay per second will decrease. So the older a sample becomes the less radiation it will emit
      1. How quickly activity drops varies. For some it takes a few microseconds before nearly all the unstable nuclei have decayed whilst for others it can take millions of years
      2. The activity never reaches 0 which is why we have to use the idea of half life to measure how quickly the activity drops of
  2. A short half life means the activity falls quickly because lots of the nuclei decay quickly
  3. A long half lif means the activity falls more slowly because most nuclei dont decay for a long time they just sit there basically unstable but biding their time
2. Half life is the average time it takes for the number of nuclei in a radioactive isotope sample to halve
3. The activity of a radioisotope is 640cpm (counts per minute) two hours later it has fallen to 80cpm. Find the half life
  1. Initial count: 640 /2 after one half life: 320/2 after two half lives: 160/2 after three half lives: 80
    1. Two hours represents three half lives so the half life is 120 mins/3 = 40 mins
4. The half-life is found from the graph by finding the time interval on the bottom axis on the vertical axis

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Ionising Radiation and Half Life

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