IB DP Physics: HL复习笔记7.1.5 Background Radiation

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:

Background radiation describes the low level of radiation present in the surroundings at all times

  • 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

Origins of Background Radiation

  • Background radiation can come from natural sources on Earth or space and man-made sources

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

Detecting Radiation

  • When radiation passes close to an atom, it knocks out electrons, ionising the atom
  • Radiation detectors work by detecting the presence of these ions or the chemical changes that they produce
  • Examples of radiation detectors include:
    • Photographic film (often used in badges)
    • Geiger-Muller (GM) tubes
    • Ionisation chambers
    • Scintillation counters
    • Spark counters


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