Topic 6: Radioactivity

• Radioactivity comes from the unstable breakdown of an isotope.

  Isotope - an atom with the same number of protons but different number of neutrons
• • When an EM wave hits an electron, it absorbs the energy and is excited to a higher energy level than its current orbit
  • The electron will then fall back after it loses energy and an EM wave will be emitted
  • Ionising radiation (more powerful EM waves) are able to knock electrons off the atom as it gains a lot of energy
• Proton - mass 1, positively charged (1+), found in the atom nucleus

  Neutron - mass 1, no charge (0), found in the atom nucleus

  Electron - negligible mass, negatively charged (-1), found in atom orbit

  • Most of the mass of an atom is concentrated inside the nucleus
• Background Radiation - low level ionising radiation that is ever present in our environment

  • radioactivity is a random process
  • the radioactivity of a source is measured in becquerels (Bq) 1Bq = 1 nucleus dying per second
  • background radiation varies across the UK as places with more granite rock have more radon gas

  
• When ionising radiation enters the GM (Geiger-Muller) Tube, it causes ionisation of a gas inside the tube.

  The ionised gas produces a pulse of current which is amplified and sent to a counter

  
• A positron is an antimatter particle that has the same mass as an electron but is positively charged.

  A positron which is emitted from the decay of an unstable nucleus is known as beta-plus (β+)

  • antimatter --->positron
  • negative electron ---> positive antielectron (positron)
  • positive proton ---> negative antiproton
• β- Decay:

  • an electron is emitted from an unstable nucleus
  • one neutron changes into a proton and an electron

  neutron ---> proton (stays in nucleus) + electron (emitted from nucleus at high speed)
• β+ Decay:

  • a positron is emitted from an unstable nucleus
  • positron changes into a neutron and a positron

  proton ---> neutron (stays in nucleus) + positron (emitted at half the speed of light)
• Nuclear Decay Equations:

  • Alpha - equivalent to helium, decreases mass number by 4, decreases atomic number by 2
  • Beta Negative - mass number doesn't change, increases atomic number by 1
  • Beta Positive (Positron) - mass number doesn't change, decreased atomic number by 1
  • Neutron - decreases mass number by 1, doesn't change atomic number
  • Gamma - doesn't do anything
• Gamma is released when a nucleus rearranges itself.

  As a result of the rearrangement, energy is released as a gamma ray
• Activity - the number of radioactive nuclei decaying per second where the unit is becquerels (Bq)
• Half-Life means the time taken for the activity to half
• Dangers of Ionising Radiation:

  • damage and kill cells/tissues
  • cause mutations
  • lead to cancer
  • radiation poisoning
• Ionising Radiation Precautions:

  • wear protective clothing (gloves, lead-lined aprons)
  • use sources for the shortest time possible to minimise exposure
  • keep as far away from sources as possible to reduce irradiation (tongs to handle sources)
  • keep sources in lead-lined containers when not used
  • never point sources at people
  • wear radiation monitoring badges/equipment to check the person's radiation levels
• Radioactive Contamination - unwanted radiation isotopes end up on other materials which can be transferred

  This is hazardous as the radioactive atoms decay and emit ionising radiation
• Nuclear Fusion - when two small, light nuclei join together to form a larger, heavier nucleus

  As a result, a lot of energy is released
• The problem with creating nuclear fusion on Earth is that it requires a huge amount of energy to overcome the electrostatic forces of repulsion between the positively charged nuclei:

  • very high pressures/density
  • very high temperatures

  Both conditions are present in the Sun but not the Earth.

  Very high temperature lasers compensate for low pressures
• Nuclear Power stations use nuclear fission to generate electricity:

  1. Energy from nuclear fission chain reactions inside the reactor make uranium rods glow red hot
  2. Heat is carried away by a coolant (CO2/water) which is pumped through the reactor
  3. Heat boils water in the heat exchanger to make high pressure steam like in coal/oil powered stations
  4. Steam turns a turbine which causes a generator to make electricity

  nuclear energy ---> heat in coolant ---> potential energy of steam ---> kinetic energy of turbine ---> electrical energy

  
• Nuclear Power station reactor controls:

  • Nuclear fission chain reaction is controlled by movable control rods made of boron or cadmium
  • Boron/cadmium rods absorb neutrons in order to reduce or stop chain reactions completely
  • Graphite or water are included as a moderator to slow down neutrons
  • Fission of an uranium atom works more efficiently with slow neutrons
  • Whole reactor is shielded in steel and concrete to absorb the dangerous gamma-rays
• Nuclear Power Advantages:

  • keeps standards of living as future energy demands increase
  • save fossil fuels
  • less damage to the environment (no CO2 emissions)
  • not a lot of nuclear waste (easy to store/bury)
  • safer than most industries
• Nuclear Power Disadvantages:

  • should save energy instead of using more
  • use renewable sources as uranium is limited
  • produces waste that stays reactive for thousands of years
  • irresponsible to leave waste for future generations
  • consequences of mistakes makes it too dangerous
• Nuclear Fission - where slow moving neutrons are fired at a large unstable parent nucleus which splits into two daughter nuclei and 3 fast neutrons

  Loss mass is converted into radioactive gamma rays
• External Radiotherapy:

  • gamma rays
  • can damage/kill healthy cells which will make patient sick
  • rays are rotated around cancer cells to maximise damage to cancer and minimise killing healthy cells
• Internal Radiotherapy:

  • radioactive source inside the body
  • can damage/kill healthy cells which will make patient sick
  • beta rays
  • more damaging
  • can't pass far through the body
  • can be inside or next to cancer cells
• Medical Tracers:

  • gamma rays (less harmful than alpha and beta)
  • beta rays are used sometimes
  • use isotopes with as short a half-life as possible so they emit radiation for a short period (stops being harmful)
  • track the movement of isotopes around the body (tracking the radiation they emit)
• Benefits and Risks of Radiotherapy:

  • radiotherapy can save a person's life
  • medical tracers can help diagnose diseases
  • minimise the risk of cancer via radiation by using a low dose with a short half-life
  • side effects can make patient sick
  • risky
• PET Scans:

  1. Positron emitting isotope is added to a molecule to make a tracer
  2. Tracer is injected into the patient and travels around the body to the target organ
  3. When positrons are emitted from the tracer and meet electrons, they annihilate each other to create two gamma rays that travel in opposite directions
  4. Gamma rays are detected outside the body

  These tracers must be produced nearby since they have short half-lives and the level of activity decreases rapidly with time.

  Tracers must be administered shortly after being produced
• Radiation Smoke Alarms:

  • an isotope emits alpha particles inside alarm
  • alpha radiation ionises which allows a small electric current to flow between two electrodes
  • when smoke passes through alpha particles, current stops which sets off the alarm
• Radiation Sterilising Medical Equipment:

  • put cleaned piece of material inside an air-tight bag then place it inside a machine that can generate a small field of radiation
  • ionising radiation can penetrate through the bag and kill all bacteria inside
  • bag ensures equipment remains sterilised after being taken out
• Radiation Thickness Control:

  • in paper mills, the thickness of paper is measured by how much beta radiation passes through the paper to a GM tube
  • counter controls the pressure of the rollers to give the correct thickness
  • beta rays are used for plastic and aluminium foil too
• Radiation Sterilising Food:

  • gamma rays kill bacteria, mould and insects in foods before and after they have been packaged
  • prolongs shelf life of food but sometimes changes the taste
  • irradiated gamma rays do more damage in smaller doses and food doesn't become contaminated (safe for human consumption)
• Rutherford's Gold Foil Experiment:

  • Ernest Rutherford fired alpha particles at a thin gold sheet (foil) to learn more about atom structure
  • It was predicted to go straight through
  • detectors showed that some alpha particles scattered while others bounced back towards the source
  • Rutherford concluded that there must've been a positive charged nucleus which changed some alpha particle directions
  • calculated that nucleus was tiny compared to overall atom size