Question 20
A counter recording
radioactive decays from a radioactive source gives the following counts in
equal intervals of time.
time
/ min counts
0–10
424
10–20
395
20–30
413
30–40
363
40–50
366
50–60
294
60–70
301
70–80
253
80–90
212
What can be deduced
from these readings?
A that
radioactivity is random and that the half-life is 90 minutes
B that
radioactivity is random and that the half-life is uncertain
C that
radioactivity is spontaneous and that the half-life is 90 minutes
D that radioactivity is spontaneous and that the
half-life is uncertain
Reference: Past Exam Paper – November 2010 Paper 12 Q40
Solution:
Answer:
B.
The half-life is uncertain
because of the randomness of the decay – random counts are obtained in equal
intervals of time.
Half-life is the time for
the count to be halved.
Consider the values 424
(at time 0-10 min) and 212 (at time 80-90 min). If we are considering the lower
limit for the count of 424 (that is, we are considering the 0 min instead of
the 10 min), the same should be done for the count of 212.
Thus, half-life = 80 – 0 =
80 min.
Similarly, if we consider
the upper limit.
Half-life = 90 – 10 = 80
min
So, the tine interval
between the 424 and 212 readings is 80 min, not 90 min.
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