Edexcel A Level Physics:复习笔记11.5 Background Radiation

• Radiation is a natural phenomenon, with radioactive elements having always existed on Earth and in outer space
  • However, human activity has added to the amount of radiation that humans are exposed to in various ways

   

• Background radiation is defined as:

Low levels of radiation from environmental sources, which are always present around us

• Radiation is measured in counts per second in a unit called Becquerel (Bq)
• Different amounts of radiation are present in different places around the world, including in the UK.

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

   

Background radiation is the radiation that is present all around 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

   

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 commonly used in 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 increases 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

   

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

   

Detecting Radiation

• When alpha or beta radiation pass close to an atom, they can deliver enough energy to remove electrons, ionising the atom
• Radiation detectors work by detecting the presence of either 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

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 and detector are far enough apart, the radiation is absorbed by the air before reaching the Geiger-counter
  • Results after 1 metre should 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