Activity and count rate, calculating with half-life including net decline, and the crucial difference between being irradiated and being contaminated.
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1.Define half-life.
The time taken for the number of unstable nuclei in a sample to halve, or for the count rate to fall to half its original value.
2.Why is half-life used rather than predicting individual decays?
Decay is random, so individual nuclei cannot be predicted. With very large numbers, the overall behaviour is reliable.
3.A sample has 800 counts per second. What is it after one half-life?
400 counts per second.
4.After two half-lives, what fraction of the original remains?
One quarter.
5.After three half-lives, what fraction remains?
One eighth.
6.A sample of 1600 Bq falls to 200 Bq. How many half-lives have passed?
Three. 1600 to 800 to 400 to 200.
7.The half-life is 5 years. How long for activity to fall to one eighth?
Three half-lives, so 15 years.
8.A source has a half-life of 2 hours. What fraction remains after 8 hours?
Four half-lives have passed, so one sixteenth remains.
9.What is meant by net decline?
The ratio of the remaining activity to the initial activity after a given time.
10.Activity falls from 400 to 50. State the net decline as a ratio.
50 to 400, which simplifies to 1 to 8.
11.Describe the shape of a decay curve.
A curve that falls steeply at first and gradually levels off, never quite reaching zero.
12.How do you find half-life from a decay graph?
Read the initial count rate, halve it, find the time at which the curve reaches that value.
13.Why does the curve never reach zero?
Each half-life removes half of what is left, so some always remains.
14.What is irradiation?
Being exposed to radiation from a source outside the body.
15.What is contamination?
Having unwanted radioactive atoms on or inside an object or a person.
16.What is the key difference between them?
Irradiation stops as soon as the source is removed. Contamination continues because the source is still there, decaying.
17.Does an irradiated object become radioactive?
No. Irradiation does not make an object radioactive.
18.Why is contamination generally more dangerous long term?
The radioactive material stays in contact with or inside the body and continues to emit radiation.
19.Which type of radiation is most dangerous inside the body?
Alpha, because it is highly ionising and all its energy is absorbed by nearby tissue.
20.Which type is most dangerous outside the body?
Gamma, because it is penetrating enough to pass into the body from outside.
21.Give two precautions when handling radioactive sources.
Use tongs to increase distance, limit the time of exposure, and store sources in lead lined containers.
22.Why is limiting exposure time important?
The dose received depends on both the intensity of the radiation and how long you are exposed to it.
23.Why must findings on radiation effects be peer reviewed?
So other scientists can check the methods and conclusions, which builds confidence in advice about safety.
24.Why is a source with a short half-life used as a medical tracer?
Its activity falls quickly, so the patient is not exposed for longer than necessary.
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