Edexcel IGCSE Physics: Double Science 复习笔记:7.1.6 Detecting Radiation

Edexcel IGCSE Physics: Double Science 复习笔记:7.1.6 Detecting Radiation

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

     

 

 

Geiger-Counter

 

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

 

 

Worked Example

A Geiger-Müller tube is used to detect radiation in a particular location. If it counts 16,000 decays in 1 hour, what is the count rate?

 

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

     

 

 

Exam Tip

If asked to name a device for detecting radiation, the Geiger-Müller tube is a good example to give. You can also refer to it as a GM tube, a GM detector, GM counter, Geiger counter etc. (The examiners will allow some level of misspelling, providing it is readable). Don’t, however, refer to it as a ‘radiation detector’ as this is too vague and may simply restate what was asked for in the question.

Background Radiation

 

  • 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

     

 

8.1.5-Background-Radiation-Chart

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

     

 

Worked Example

A student is using a Geiger-counter to measure the counts per minute at different distances from a source of radiation. Their results and a graph of the results are shown here.

 

4.3.1-Background-exampleDetermine the background radiation count.

 

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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