Background Radiation (CIE IGCSE Physics)

• 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

• Although most background radiation is natural, a small amount of it comes from artificial sources, such as medical procedures (including X-rays)
• Levels of background radiation can vary significantly from place to place

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

Natural Sources

• Radon gas (in the air)
  • Airborne radon comes from the ground
  • This is from the natural decay of uranium in rocks and soil
  • The gas is tasteless, colourless and oderless but it not generally a health issue 

• Rocks and Buildings
  • 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
  • Natural radioactivity can be found in building materials, including decorative rocks, stone and brick

• 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

• It is important to regulate the exposure of humans to radiation
  • The amount of radiation received by a person is called the dose
• Ionising nuclear radiation is measured using a detector connected to a counter

Count Rate

• Count rate is the number of decays per second recorded by a detector and recorded by the counter
  • It is measured in counts/s or counts/min
• The count rate decreases the further the detector is from the source
  • This is because the radiation becomes more spread out the further away it is from the source

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

Examples of other radiation detectors include:

• Photographic film (often used in badges)
• Ionisation chambers
• Scintillation counters
• Spark counters

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

EXTENDED

• 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