HOMEWORK #12
Name: _______________________
Date: ___________ Period: _____
Radioactive Decay and Half-Life
REVIEW QUESTIONS – check concepts
1. What forces act on the nucleus of an atom?
2. In order for a nucleus to be stable, what
condition must be met?
3. When a nucleus undergoes an alpha decay
what happens to the atomic number? What
happens to the mass number?
4. When a nucleus undergoes a beta decay
what happens to the atomic number? What
happens to the mass number?
5. Do highly unstable nuclei have a long half-life
or a short half-life?
6. How much of a radioactive sample will
remain after two half-lives have passed?
DECAY EQUATIONS
7. Indicate whether the decay equation is an example of ALPHA DECAY or BETA DECAY.
8
4Be
233
90Th
→ 42He + 42He
→
233
91Pa
+ −10𝑒
8. Fill in the missing pieces for each decay equation.
(a) 224
92
(c) 249
96
U →
Cm →
+
Bk +
4
2
He
(b) 239
92
(d) 233
95
U →
+
Am →
Np +
0
-1
e
9. Write the decay equation for each of the following nuclear reactions.
(a) Beta decay of Lutetium-180.
(b) Alpha decay of Tantalum-157.
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Fremont Physics ©Kepple 2013
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Nuclear Physics – HW-B
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4/22/14
HALF-LIFE PROBLEMS
10. Neptunium-229 has a half-life of 4 min.
How much time will it take for a 10 kg sample
of Neptunium-229 to decay to only 2.5 kg?
11. Thorium-228 has a half-life of 2 years. Four
years ago, Mr. Kepple purchased 12 kg of
Thorium-228. How much of it is left today?
12. A 20.0 g sample of nitrogen-13 is placed in
a sealed container and left to decay. The mass
of the remaining nitrogen-13 is measured
every 5 minutes.
13. A 20.0 g sample of iodine-131 is placed in a
sealed container and left to decay. The mass
of the remaining iodine-13 is measured every
8 days.
time
mass
0 min
20.0 g
5 min
14.1 g
10 min
10.0 g
15 min
7.07 g
time
0 dy
mass 128.0 g
8 dy
64.0 g
16 dy
32.0 g
24 dy
16.0 g
(a) What is the half-life of nitrogen-13?
(a) What is the half-life of iodine-131?
Now suppose 30 minutes have passed.
(b) How many half-lives have passed?
Now suppose only 4 g of iodine-131 remains.
(b) How many half-lives have passed?
(c) How much of the sample remains?
(c) How much time has elapsed?
14. One of the primary components of a hydrogen bomb is hydrogen-3, also known as: tritium. It is
radioactive and decays by beta emission with a half-life of approximately 12 years. In order to maintain
its stocks of nuclear weapons, the United States must constantly produce additional tritium to replace
the tritium that has decayed.
(a) Suppose that during maintenance a certain
nuclear warhead is found to have only onefourth the amount of tritium as a brand-new
warhead would have.
i. How many half-lives have passed?
ii. In what year was this particular warhead
manufactured?
(b) A radioactive isotope is considered extinct
after approximately 10 half-lives. Suppose that
the United States stopped producing tritium
today. Estimate the number of years would it
take for the United States to completely run
out of tritium.