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GCSE Questions: Radioactivity

Q11.

(a) 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 0.30 counts per second.

Calculate the count rate for the rock in counts per second.

Background rate is 0.30 counts per second - you need to deduct that from the total count

In 60 seconds 60 x 0.30 counts will be due to background activity = 18 counts

Therefore the counts from the rock in 60 seconds = 819 - 18 = 801 counts

Count rate = 801 ÷ 60 = 13.35 counts per second

[3 marks]

(b) 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.

1 kg granite has an activity of 1250 Bq

180 kg granite has an activity of 1250 x 180 Bq = 225,000 Bq

[2 marks]

The average total radiation dose per year in the UK is 2.0 millisieverts.

The table below shows the effects of radiation dose on the human body.

Radiation dose in millisieverts per year
Effects
10 000
 Immediate illness; death within a few weeks
1000
 Radiation sickness; unlikely to cause death
100
 Lowest dose with evidence of causing cancer

 

The average radiation dose from the granite worktop is 0.003 millisieverts per day.

Explain why the householder should not be concerned about his yearly radiation dose from the granite worktop. (One year is 365 days).

0.003 x 365 = 1.095 mSv

This dose is well below 100 mSv - the lowest dose with evidence of causing cancer.

[2 marks]

(c) 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.

People are more able to compare their willingness to undertake a radiation risk / dose / hazard to the risk they take from the radiation dose they get from (eating) bananas than when confronted with a measurement in a unit they are unfamiliar with.

One BED is roughly correlated to 10−7 sievert (0.1 μSv). But this dose is not cumulative, as the principal radioactive component is 'excreted' when you go to the loo! It is not really a suitable 'unit' - but does highlight the low risk in food.... to risk cancer from consuming banannas you would need to eat a million bananas in one go - which would rather make danger of possible cancer a secondary problem!

[1 mark]

[Total: 8 marks]