Radioactivity

Radioactivity

1. nucleus

   • nuclear model of atom proposed by Ernest Rutherford
   • Rutherford's experiment
     • used a narrow beam of alpha particles
       • all same kinetic energy
       • in an evacuated container
         • thin metal foil placed in path of beam
           • alpha particles scattered by foil
             • detected by a detector
               • can be moved round at a constant distance from the point of impact of the beam on the metal foil
               • microsope used to observe pinpoints of light emitted by alpha particles hitting a fluorescent screen
     • observations
       • most particles passed through foil with little or no deflection
       • small percentage of particles deflected through angles more than 90
     • interpretation
       • most atoms mass concentration in small region in centre
         • nucleus
       • nucleus positively charged
         • repels alpha particles

2. experiments

   • ionisation
     • equipment
       • ionisation chamber
         • contains air at atmospheric pressure
       • picoammeter
     • procedure
       • ions created in chamber
         • ions attracted to oppositely charged electrode
           • they are discharged
             • electrons pass through the picoammeter as a result of ionisation in the chamber
               • current is proportional to number of ions per second created in chamber
   • cloud chamber
     • equipment
       • cloud chamber
         • contains air saturated with a vapour at a very low temperature
     • procedure
       • alpha and beta passed through chamber
         • leave visible track of minute condensed vapour droplets
           • due to ionisation of the air
           • air space is supersaturated
             • ions produced trigger the formation of droplets
   • absorption tests
     • equipment
       • Geiger tube
       • counter
       • various absorbers
     • procedure
       • source passes through absorber
         • goes through Geiger tube
           • then to scaler counter
             • each particle entering tube I registered by counter as a single count
             • number of counts in given time used to work out count rate
             • count rate with absorber compared to count rate without
         • source at fixed distance from absorber
       • count rate due to background radiation measured first
         • count rate measured with source at fixed distance fromtube without absorber
           • background count rate subtracted from this value
   • range in air
     • equipment
       • Geiger tube
       • counter
     • procedure
       • count rate measured for different distances between source and tube
         • starting close to tube
         • background count rate measured in absence of source
           • count rate at each distance can be corrected

3. dangers

   • ionising radiation
     • damages living cells
       • destroys cell membranes
       • damages vital molecules
         • DNA
           • can cause cells to divide and grow uncontrollably
             • cause cancerous tumour
           • DNA in sex cells
             • cause mutation
               • passed to future generations
         • directly or indirectly by creating free radical ions
           • these react with vital moelcules
             • normal cell division is affected
               • nuclei are damaged
   • precautions
     • storage
       • in lead-lined containers
     • when using
       • no sources should contact skin
       • sources transferred using handling tools
       • liquid, gas, powder sources sealed in containers
         • avoid spilling or inhaling
       • not used for longer than necessary

4. decay

   • half life
     • time taken for mas of isotope to decrease to half initial mass
   • decay curve
     • mass decreases with time at a slower and slower rate
   • activity
     • number of nuclei of isotope that disintegrate per second
     • proportional to mass of isotope
   • decay is random
     • emission of radiation is unpredictable

5. uses

   • dating
     • carbon
       • measuring activity of dead samples
         • age can be calculated
           • need to know activity of same mass of living sample
     • argon
       • ancient rocks contain trapped argon gas
         • age of rock measured by calculating proportion of argon-40 to potassium-40
   • tracers
     • used to follow path of substance through a system
     • source used
       • emits beta or gamma
         • can be detected outside flow path
       • half life
         • stable enough for measurements to be made
         • short enough to decay quickly after use

6. radiation

   • types of radiation
     • alpha
       • deflected by magnetic field
       • consists of positively charged particles
         • two protons and two neutrons
       • causes strong ionisation
       • range in air
         • few centimetres
       • cloud chamber
         • straight tracks produced
       • absorbed by paper and thin metal foil
       • energy
         • constant for given source
     • beta
       • deflected by magnetic field
       • consists of negatively charged particles
         • fast moving electrons
       • weaker ionising effect than alpha
       • range in air
         • up to a metre or more
       • cloud chamber
         • produce wispy tracks
           • easily deflected as a result of collisions with air molecules
           • not very visible
             • not very ionsisng
       • absorbed by 5mm of metal
       • energy varies up to a maximum for given source
     • gamma
       • magnetic field has no effect
       • consists of high energy photons
       • has the weakest ionising effect
         • photons carry no charge
       • absorbed by several cm of lead
       • range in air
         • unlimited
       • follows inverse square law
       • energy
         • constant for given source
   • ionised air
     • making it conduct electricity