Edexcel IGCSE Physics 复习笔记 7.1.6 Detecting Radiation

• It is important to regulate the exposure of humans to radiation
• The amount of radiation received by a person is called the dose and is measured in sieverts (Sv)
• One sievert is a very big dose of radiation
  • It would cause acute radiation poisoning

   

• People would normally receive about 3 mSv (0.003 Sv) in one year
• To protect against over-exposure, the dose received by different activities is measured
• Radiation can be measured and detected using a photographic film or a Geiger–Müller tube

Photographic Film

• Photographic films detect radiation by becoming darker when it absorbs radiation, just like it does when it absorbs visible light
  • The more radiation the film absorbs, the darker it is when it is developed

   

• People who work with radiation, such as radiographers, wear film badges which are checked regularly to monitor the levels of radiation absorbed
• To get an accurate measure of the dose received, the badge contains different materials that the radiation must penetrate to reach the film
  • These materials may include aluminium, copper, paper, lead and plastic

   

• The diagram shows what a typical radiation badge looks like:

A badge containing photographic film can be used to monitor a person’s exposure to radiation

• The badge shows the amount of different types of radiation that the radiographer has been exposed to
• Different areas of the film are exposed to different types of radiation
  • Alpha radiation is unlikely to be detected at all as it will be absorbed / stopped by the paper
  • Beta radiation is absorbed by the aluminium
  • Gamma (or X-rays) affect all areas of the film but the lead will reduce some of the gamma radiation

   

Geiger-Müller tube

• The Geiger-Müller tube is the most common device used to measure and detect radiation
• Each time it absorbs radiation, it transmits an electrical pulse to a counting machine
• This makes a clicking sound or displays the count rate
• The greater the frequency of clicks, or the higher the count rate, the more radiation the Geiger-Müller tube is absorbing
  • Therefore, it matters how close the tube is to the radiation source
  • The further away from the source, the lower the count rate detected

   

A Geiger-Müller tube (or Geiger counter) is a common type of radiation detector

Step 1: Identify the different variables

• • The number of decays is 16 000
  • The time is 1 hour

   

Step 2: Determine the time period in seconds

• • 1 hour is equal to 60 minutes, and 1 minute is equal to 60 seconds

   

Time period = 1 × 60 × 60 = 3600 seconds

Step 3: Divide the total counts by the time period in seconds

Counts ÷ Time period = 16 000 ÷ 3600 = 4.5

• • Therefore, there are 4.5 decays per second

   

• It is important to remember that radiation is a natural phenomenon
• Radioactive elements have always existed on Earth and in outer space
• However, human activity has added to the amount of radiation that humans are exposed to on Earth
• Background radiation is defined as:

The radiation that exists around us all the time

• There are two types of background radiation:
  • Natural sources
  • Man-made sources

   

Background radiation is the radiation that is present all around in the environment. Radon gas is given off from some types of rock

• Every second of the day there is some radiation emanating from natural sources such as:
  • Rocks
  • Cosmic rays from space
  • Foods

   

• Man-made sources of radiation increase the background radiation levels, examples include:
  • Fallout from nuclear weapons testing and nuclear accidents
  • Exposure from medical testing

   

Natural Sources

• Radon gas from rocks and soil
  • Heavy radioactive elements, such as uranium and thorium, occur naturally in rocks in the ground
  • Uranium decays into radon gas, which is an alpha emitter
  • This is particularly dangerous if inhaled into the lungs in large quantities

   

• Cosmic rays from space
  • The sun emits an enormous number of protons every second
  • Some of these enter the Earth’s atmosphere at high speeds
  • When they collide with molecules in the air, this leads to the production of gamma radiation
  • Other sources of cosmic rays are supernovae and other high energy cosmic events

   

• Carbon-14 in biological material
  • All organic matter contains a tiny amount of carbon-14
  • Living plants and animals constantly replace the supply of carbon in their systems hence the amount of carbon-14 in the system stays almost constant

   

• Radioactive material in food and drink
  • Naturally occurring radioactive elements can get into food and water since they are in contact with rocks and soil containing these elements
  • Some foods contain higher amounts such as potassium-40 in bananas
  • However, the amount of radioactive material is minuscule and is not a cause for concern

   

Man-Made Sources

• Medical sources
  • In medicine, radiation is utilised all the time
  • Uses include X-rays, CT scans, radioactive tracers, and radiation therapy

   

• Nuclear waste
  • While nuclear waste itself does not contribute much to background radiation, it can be dangerous for the people handling it

   

• Nuclear fallout from nuclear weapons
  • Fallout is the residue radioactive material that is thrown into the air after a nuclear explosion, such as the bomb that exploded at Hiroshima
  • While the amount of fallout in the environment is presently very low, it would increase significantly in areas where nuclear weapons are tested

   

• Nuclear accidents
  • Accidents such as that in Chernobyl contributed a large dose of radiation into the environment
  • While these accidents are now extremely rare, they can be catastrophic and render areas devastated for centuries

   

Corrected Count Rate

• Background radiation must be accounted for when taking readings in a laboratory
• This can be done by taking readings with no radioactive source present and then subtracting this from readings with the source present
  • This is known as the corrected count rate

   

Step 1: Determine the point at which the source radiation stops being detected

• • The background radiation is the amount of radiation received all the time
  • When the source is moved back far enough it is all absorbed by the air before reaching the Geiger-counter
  • Results after 1 metre do not change
  • Therefore, the amount after 1 metre is only due to background radiation

   

Step 2: State the background radiation count 

• • The background radiation count is 15 counts per minute

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