Radioactivity

Radioactivity

1. Radioactive emissions

   • Elements have typically one or two stable isotopes. The rest tend to be unstable. Nuclei of unstable isotopes break down at random, to release excess energy to become more stable. These unstable isotopes are said to be radioactive (radioisotopes).
     • Can't predict when a nucleus will decay . Decay is not affected by physical factors, e.g temperature
   • Radiation=any type of energy that spreads from its source, e.g sound, heat
   • Types of radiation
     • Alpha
       • Helium nuclei (2 protons and 2 neutrons)
         • Big, heavy, slow moving, so have weak penetrating power and are stopped quickly
           • Strongly ionising due to their size - they collide frequently into atoms, knocking electrons off them, creating lots of ions
             • As they're electrically charged, they're deflected by magnetic and electric fields
               • When this is emitted, atomic number decreases by two, and mass number decreases by 4
                 • Short range in air, 3-5cm
     • Beta
       • A beta particle is an electron emitted from the nucleus of atom, when a neutron decays to form a proton and an electron
         • Number of protons increase by 1, mass number stays the same
           • Move quite fast, quite small
             • Moderate penetration, and are moderate ionisers
               • As they're charged (negatively), they're deflected by electric and magnetic fields
                 • Moderate range in air, 15cm
     • Gamma
       • No mass, just a high frequency electromagnetic wave
         • Very good penetration, can go a long way without being stopped
           • Weakly ionising, as they tend to pass through rather than collide. Eventually they do hit something and do damage
             • No charge, so not deflected by electric or magnetic fields
               • Always happens after beta or alpha decay
                 • No effect on atomic or mass numbers of the isotope
                   • Long range in air
   • Radioactive sources
     • Sealed when not in use (lead lined boxes)
       • Hold it with tongs, away from your body and people around you
         • Cover exposed skin
           • People should stand a safe distance from the source

2. Detecting radiation

   • Ionisation - occurs when radioactive emissions collide into atoms, and knock electrons off them
     • Geiger-Muller detector can detect ionising radiation
       • Activity(Becquerels)=no.of counts per unit time(seconds/minutes)
         • Activity decreases over time (as more unstable nuclei decay less unstable nuclei, so lower activity)

3. Properties of Radiation

   • Each collision reduces the amount of energy an emission has until it's absorbed
     • More collisions there are, the shorter the distance it'll travel
       • Greater the size of the particle, the more likely it is it'll collide, ionise and be absorbed

4. Background radiation - Radiation that occurs naturally all around us

   • Cosmic rays, rocks at surface/within Earth, air (Radon gas), building materials, food, nuclear tests/accidents
     • More background radiation at higher altitudes
       • In radiation experiments, you measure the radiation without a source, then measure it with a source and subtract the first reading from the second

5. Half-life - average time taken for half of the unstable nuclei to decay/time taken for activity to drop by half from its initial value

   • Large variation in half-lives for isotopes - for some, just a few hours, and others can last millions of years
     • The shorter the half-life, the greater the initial activity, but the shorter the time the activity is significant
       • The longer the half-life, the lower the activity, but the longer it lasts

6. Radioactive dating

   • Archaeological specimens
     • Production of C-14 in atmosphere
       • Absorbed by living things
         • Constant ratio of C-12 to C-14 in living things
           • Decay of C-14 after death
             • Measurement of C-14 remaining to estimate time of death
               • Similar approach to rocks - uranium to lead

7. Dangers of radioactivity

   • Ionisation can damage living cells and cause cancer
     • Higher doses kill cells
       • If source is outside body, alpha is least dangerous - unlikely to reach living cells
         • Alpha most dangerous inside the body (strongly absorbed by living cells)
   • Contamination - Radioactive particles attaching themselves to objects
     • Irradiation - Objects near a source are irradiated (exposed to it. Irradiating something does NOT mean making it radioactive)
     • Especially dangerous, because these particles can get inside your body if you're contaminated
   • Disposal
     • High level waste can stay highly radioreactive for tens of thousands of years
       • Often sealed into glass blocks, and then sealed in metal canisters. Then they could be buried deep underground
         • Need to find suitable places - geologically stable, as if the canisters are disturbed, radioactive material could leak into the ground, and could contaminate groundwater, contaminating soil, plants, rivers, even our drinking water

8. Uses of radioactivity

   • Medical tracers - beta/gamma, short half-life.
     • So initial levels are high enough to be detected, but quickly disappear
       • Alpha can't be used - can't penetrate body's tissues, so would never be detected externally, and is really harmful inside the body
   • Radiotherapy - Gamma, long half-life.
     • Carefully directed to zap the tumour, killing the cancerous cells, and minimising exposure
       • Long half-life maintains dose of radioactivity
   • Sterilisation - Gamma, long-half-life
     • Kills microbes, and is useful because it doesn't involve high temperatures, so can be used on fresh fruit/plastic instruments without being damaged
   • Smoke detectors
     • Alpha, long half-life
       • Long, so it doesn't need to be replaced frequently
         • Alpha radiation is measured by a detector. Alpha radiation is absorbed by smoke particles.
           • Detector can no longer measure it, so it triggers the alarm
   • Paper and metal thickness control - Beta, long half-life
     • If detected radiation level changes, paper is coming out too thick or thin, so control int adjusts rollers to give the correct thickness
   • Locating pipe leaks
     • Gamma, short-half life
       • Source flows down the pipe and a detector is used above ground. Gamma is used as it can pass through any rocks/earth surrounding the pipe. If there's a crack, radiation will collect outside the pipe, and the detector will show extra high radioactivity at that point
         • Short half life so it doesn't cause a long term hazard if it collects somewhere

9. Atom structure

   • Nucleus of an atom has protons and neutrons, which make up most of mass of atom, but taking up virtually no space
     • Atomic number - number of protons in nucleus
       • Total number of protons and neutrons - mass number or nucleon number
         • Electrons are negatively charged and really small
           • Atoms are neutral, so number of protons-number of electrons

10. Emitting an alpha particle decreases proton number by 2 and mass number by 4

    • Emitting beta particles number of protons increases by 1 (a neutron is converted to a proton and an electron)
      • Emitting a gamma particle doesn't affect atomic number or mass number

11. Nuclear fission and fusion

    • Nuclear fission chain reaction used by nuclear power stations
      • Nuclear fission is the splitting of an atom, which releases energy. Can be spontaneous, but in a nuclear reactor it's made to happen, e.g to uranium-235
        • If a slow moving neutron is absorbed by a uranium-235 nucleus, the nucleus can split
          • Each time this happens, it spits out a small number of neutrons. These might go on to hit other uranium-235 nuclei, causing them to split as well and so on. This is a chain reaction
            • When uranium-235 splits in tow, it will form two new daughter nuclei, both lighter elements than uranium
              • These new nuclei are usually radioactive, which is the big problem with nuclear power - radioactive waste
                • Each nucleus splitting gives out a lot of energy - this energy is in the kinetic energy stores of the fission products (daughter nuclei and neutrons)
                  • In a reactor, this energy is transferred to thermal energy stores to produce steam to drive a turbine
    • Nuclear reactors have to work safely
      • Neutrons released by fission reactions in a nuclear reactor have lot of energy. In order to be absorbed by uranium nuclei and sustain the chain reaction, they need to be slowed down
        • The moderator, usually graphite or water, slows down neutrons
          • Control rods, often made of boron, limit rate of fission by absorbing excess neutrons
            • The high-energy neutrons and gamma rays (energy) released in fission are highly penetrating ionising radiation. Shielding has to be used to absorb the ionising radiation. The shielding is usually a thick concrete structure, which may also contain lead or other metals
              • A substance (e.g CO2) pumped round the reactor transfers the energy (by heating) to the water in the heat exchanger. The water turns to steam, which turns a turbine, which turns a generator and generates electricity
    • Nuclear fusion - joining small nuclei
      • Opposite of nuclear fission. In nuclear fusion, two light nuclei collide at high speed and join (fuse) to create a larger, heavier nucleus. E.g hydrogen nuclei can fuse to produce a helium nucleus
        • This heavier nucleus doesn't have as much mass as the two separate, light nuclei did. Some of the mass of the lighter nuclei is converted to energy. This energy is then released by radiation
          • Fusion releases a lot of energy (more than fission for a given mass of fuel) - all the energy released in stars comes from fusion
            • Fusion only happens at really high pressures and temperatures (about 10,000,000 degrees Celsius). This is because the positively charged nuclei have to get very close to fuse, so need to be moving very fast to overcome the strong force due to electrostatic repulsion
              • So far, scientists haven't found a way of using fusion to generate energy for us to use. The temperatures and pressures needed for fusion are so high that fusion reactors are really hard and expensive to use