4.3.1 Background Radiation

• Background radiation is around us all of the time.
• What are the natural sources of background radiation?

  Cosmic rays, radon gas, and radioactive materials in the Earth's crust.
• What are the man-made/artificial sources of background radiation?

  Nuclear accidents, food and medical usage.
• The level of background radiation and radiation dose may be affected by occupation and/or location.
• What is radiation dose measured in?

  Sieverts (Sv).
• 1000 millisieverts (mSv) = 1 sievert (Sv)
• A teacher used a Geiger-Muller tube and counter to measure the number of counts in 60 seconds for a radioactive rock. The counter recorded 819 counts in 60 seconds. The background radiation count rate was 0.3 counts per second. Calculate the count rate for the rock.

  b.r = 60 x 0.3 = 18

  819 - 18 = 801

  801/60 = 13.35

  Count rate = 13.35 per second
• A householder is worried about the radiation emitted by the granite worktop in his kitchen. 1 kg of granite has an activity of 1250 Bq. The kitchen worktop has a mass of 180 kg. Calculate the activity of the kitchen worktop in Bq.

  1250 x 180 = 225,000

  Activity = 225,000 Bq
• Bananas are a source of background radiation. Some people think that the unit of radiation dose should be changed from sieverts to Banana Equivalent Dose. Suggest one reason why the Banana Equivalent Dose may help the public be more aware of radiation risks.

  It is easier to compare the risk of something to the risk of banana consumption.
• Sources of background radiation are either natural or man-made. Which two of the sources listed are natural sources of background radiation?

  • Cosmic rays
  • Medical X-rays
  • Nuclear power stations
  • Nuclear weapons testing
  • Radon gas

  Cosmic rays & radon gas.
• A teacher used a Geiger-Muller tube and counter to measure the background radiation in his laboratory. The table gives three readings taken by the teacher at three different times on the same day. What is the most likely reason for the readings being different?

  Radioactive decay is a random process.

  
• The teacher takes a radioactive source from a storage box. Figure 2 shows the box. Why does storing the radioactive source in the box reduce the risk of radiation exposure to the teacher?

  The lead lining absorbs the emitted radiation.

  
• Figure 3 shows how the teacher used the GM tube and counter to measure the radiation emitted from the radioactive source. The counter was reset to 0. The count after one minute was 159. How should the teacher calculate the counts from the radioactive source?

  Subtract the background count from 159.

  
• The teacher passed the radiation through an electric field. Figure 4 shows the path that the radiation took through the electric field. What type of radiation was being emitted by the radioactive source? Explain the reason for your answer.

  Beta - beta radiation is negative so therefore is attracted to the positive electric field.

  
• The average total radiation dose per year in the UK is 2 mSv. The table shows the effects of radiation dosage on the human body. The average radiation dose from the granite worktop is 0.003 mSv per day. Explain why the householder should not be concerned about his yearly radiation dose from the granite worktop. (1 year = 365 days)

  0.003 x 365 = 1.095

  1.095 << 100

  well below 100 - no links to cancer/death