Physics Worksheet - Annual Examinations For Secondary Schools, 2015 Page 8
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1210. This question is about radioactivity and half-life.
a) Figure 12 shows two instruments used in measuring the activity of radioactive substances.
i) Name instrument Q.
R
Counte
Radioactive
________________________________[1]
source
Q
ii) Name instrument R.
________________________________[1]
iii) Which one (Q or R) is used to detect
radiation emitted by unstable nuclei?
______________
[1]
Figure 12
iv) Which one (Q or R) gives the number of unstable nuclei decaying (breaking down) over a
period of time? ______________
[1]
b) Gregg found that the rate of decay of a radioactive isotope X is 1025 counts/minute. He notices
that when the radioactive isotope X is not present, the reading drops to 25 counts/minute.
i) Why is there still a count rate when the radioactive isotope X is removed?
____________________________________________________________________________ [1]
ii) Name two sources of this radiation.
_______________________________________________________________________________
____________________________________________________________________________ [2]
c) Gregg starts filling the table below while investigating the half-life of radioactive isotope X.
Gregg knows that the corrected count rate = total count rate – 25 counts/minute .
Time / (hours)
0
0.5
1.0
1.5
2.0
2.5
Total count rate / (counts/min.)
1025
865
625
525
445
Corrected count rate / (counts/min.)
1000
840
710
600
420
i) Complete the missing values in the above table.
[2]
ii) Complete the following statement about half-life:
Half-life is the time taken for __________ the nuclei of a sample of a radioactive substance to
__________.
[2]
iii) Use the table to calculate the half-life of the radioactive isotope X.
_______________________________________________________________________________
____________________________________________________________________________ [2]
iv) Calculate the mass of radioactive isotope X which remains after 3 half-lives given that the
initial mass of this isotope was 8 g.
8 g → ________ →_________ →_________
[3]
Physics – Form 5 Secondary – Track 2 – 2015
Page 8 of 12
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