Radioactivity and Particles
4 marks · EasyA radioactive isotope is used in a tracing experiment in a factory. Its measured activity is 5000 Bq at 10:00 am.
State the unit of activity for a radioactive source, and what it measures.
If the activity falls to 1250 Bq by 1:00 pm, how many half-lives have passed?
If the half-life of the isotope is 1 hour, what will its activity be at 4:00 pm?
Explain why radioactive tracers used in industry should have short half-lives.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) becquerel (Bq); number of decays per second
(b) 2
(c) 313 Bq
(d) So that the source decays quickly and does not remain radioactive for long after the experiment.
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Correct unit (becquerel) and what is measured.
B1 (b) · 1 mark · Appropriate number of half-lives calculated.
B1 (c) · 1 mark · Appropriate final activity using number of half-lives.
B1 (d) · 1 mark · Reason for short half-life clearly explained.
Worked solution
a) The unit is the becquerel (Bq), which measures the number of decays per second.
b) The activity halves twice (5000 → 2500 → 1250), so 2 half-lives have passed.
c) 1:00 pm is 3 hours after 10:00 am (1:00 pm to 4:00 pm is 3 more hours = 3 half-lives):
So at 4:00 pm, activity = 156 Bq.
d) Tracers should have short half-lives so they are safe, decaying quickly and not remaining radioactive for long after use.