Radioactivity and particles (pg 66-73)

• The nuclei of unstable isotopes break down at random (radioactive decay).
• Radioactive decay is completely unaffected by physical conditions such as temperature or any sort of chemical bonding.
• When the nucleus of an atom decays, it emits one or more types of radiation - alpha, beta, gamma or neutrons.
• In the process of radioactive decay, the nucleus often changes into a new element.
• There's low-level background radiation around us all the time. It comes from:

  1. Substances on Earth (air, food, building materials, soil, rocks).
  2. Radiation from space (cosmic rays). Mostly from the Sun.
  3. Living things.
  4. Human activity (nuclear explosions or nuclear waste).
• Alpha, beta and gamma are three types of ionising radiation.
• Nuclear radiation causes ionisation by bashing into atoms and knocking electrons off them, creating ions.
• The further the radiation can penetrate before hitting an atom and getting stopped, the less damage it will do along the way and so the less ionising it is.
• Ionising radiation can be detected using a Geiger-Müller detector or photographic film.
• Alpha particles are helium nuclei.

  1. 2 protons and 2 neutrons.
  2. Big, heavy and slow-moving.
  3. They don't penetrate far into materials but are stopped quickly.
  4. Because of their size they are strongly ionising.
  5. Because they're electrically charged (positively), they are deflected by electric and magnetic fields.
  6. Emitting an alpha particle decrease the atomic number of the nucleus by 2 and the mass number by 4.
• A beta particle is an electron which has been emitted from the nucleus of an atom when a neutron turns into a proton and an electron.
• When a beta particle is emitted, the number of protons in the nucleus increases by 1. So the atomic number increases by 1 but the mass number stays the same.
• Beta particles:

  1. They move quite fast and are small.
  2. They penetrate moderately before colliding and are moderately ionising too.
  3. Because they're charged negatively, beta particles are deflected by electric and magnetic fields.
• Gamma rays are very short wavelength EM waves.
• Gamma rays:

  1. They have no mass - they're just energy.
  2. They can penetrate a long way into materials without being stopped.
  3. They are weakly ionising because they tend to pass through rather than collide with atoms.
  4. They have no charge, so they're not deflected by electric or magnetic fields.
• Gamma emission always happens after beta or alpha decay. You never get just gamma rays emitted.
• Gamma ray emission has no effect on the atomic or mass numbers of the isotope. If a nucleus has excess energy, it loses this energy by emitting a gamma ray.
• Alpha particles are blocked by paper, skin, or a few cm of air.
• Beta particles are blocked by thin metal.
• Gamma rays are blocked by thick lead or very thick concrete.
• A Geiger-Müller detector gives a count rate - the number of radioactive particles reaching it per second.
• Geiger-Müller experiment:

  1. Remove the source to measure the background count over a time period. Divide your count by the time period to get a bakground count rate. Do this three times and find the mean. Subtract this from your results.
  2. Replace the source and measure the count rate with no material present three times and take a mean.
  3. Repeat this with different materials.
• Radioactive sources can be dangerous:

  1. They should be kept in a lead-lined box when not in use.
  2. They should only be picked up using long-handled tongs or forceps.
  3. Take care not to point them at anyone, and keep a safe distance from them.
• Uses of nuclear radiation:

  1. Medical tracers use beta or gamma radiation.
  2. Treat cancer.
  3. Sterilising food and equipment with gamma rays.
  4. In industry for tracers and thickness gauges.
• Medical tracers:

  1. A source which emits beta or gamma radiation is injected into the patient (or swallowed).
  2. The radiation penetrates the body tissues and can be detected externally.
  3. As the source moves through the body, the radiographer uses a detector and a computer to monitor its progress on display.
• Doctors use medical tracers to check whether the organs of the body are working as they should.
• Radiotherapy kills the cancer cell and stops them dividing - it involves a high dose of gamma rays, carefully directed to zap the cells in the tumour while minimising the dose to the rest of the body.
• The radioactive source for medical tracers has to have a short half-life, so that the initial levels are high enough to be easily detected, but the radioactivity inside the patient quickly disappears.
• Food and medical equipment can be irradiated with a high dose of gamma rays to kill al microbes and sterilise it. Irradiation is a good method of sterilisation because it doesn't involve high temperatures.
• The radioactive source for sterilisation needs to be a very strong emitter of gamma rays with a long half-life so that it doesn't need replacing too often.
• Gamma emitting tracers are used in industry to detect leaks in underground pipes.
• Gamma emitting tracers:

  1. The source if allowed to flow down the pipe and a detector is used above ground. (Gamma is used because it can pass through any rocks or earth surrounding the pipe).
  2. If there's a crack in the pipe, more radiation will collect outside the pipe, and the detector will show extra high radioactivity at this point.
  3. It should have a short half-life so as not to cause a long-term hazard if it collects somewhere.
• Beta radiation is used in thickness control. It needs to be a beta source, because then the paper will partly block the radiation.
• Risks from nuclear radiation:

  1. Ionising radiation can damage cells and tissues.
  2. Irradiation.
  3. Contamination.
  4. Radioactive waste.
• Beta and gamma can penetrate the skin and soft tissues to reach delicate organs inside the body. This makes beta and gamma sources more hazardous than alpha when outside the body. If they get inside, their radiation mostly passes straight out without doing much damage.
• Alpha radiation can't penetrate the skin, but it's very dangerous if it gets inside the body. Alpha sources do all their damage in a very localised area.
• When radiation enters your body it will collide with molecules in your cells. These collisions cause ionisation, which damages or destroys the molecules. The extent of the harmful effects depends on how much exposure you have to the radiation, and its energy and penetration.
• Lower doses of radiation tend to cause minor damage without killing the cell. This can cause mutations in cells which then divide uncontrollably - this is cancer.
• Higher doses tend to kill cells completely, causing radiation sickness if a large part of your body is affected at the same time.
• Objects near a radioactive source are irradiated (exposed) by it. Irradiating something does not make it radioactive.