Half-Life
Name: _______________________
Date: ___________ Period: _____
I. INTRODUCTION
Some radioisotopes are more unstable than others. We can compare the instability of a radioisotope
by measuring a characteristic time, the half-life. The half-life of a radioactive material is the time
needed for half of a sample of that material to decay.
1. Cesium-137 is a radioactive isotope which is formed as product of nuclear fission, which involves the
splitting of large nuclei like uranium and plutonium. It has a half-life of approximately 30 years and
decays by beta emission into barium-137. Due to its short half-life it poses a large danger to humans.
(a) Write the decay equation for cesium-137.
(b) How much time does it take for half of a
sample of cesium-137 to decay?
(c) Each circle below represents one atom. A shaded circle represents a cesium-137 atom, and a white
circle represents barium-137. After the passage of each half-life the number of each atom is shown.
t = 0 half-lives
t = 1 half-lives
t = 2 half-lives
t = 3 half-lives
Elapsed
Time:
Amount
Left:
II. DEMONSTRATION
2. We can model a radioisotope as a penny and the half-life time as a coin-flip. Each coin flip represents
the passage of one half-life. Since there is a 50% chance that a radioisotope will decay in one half-life, a
coin flip that results in tails can represent a decay. The entire class will now flip pennies. When a penny
lands on tails, it decays, so remove it from the sample. Record and graph the class data below.
No. of
coin flips
No. of
pennies left
Fremont Physics ©Kepple 2014
Nuclear Physics – Page #7
4/27/15
Ac-225: 10 days Th-228: 2 years Pa-224: 1 sec Np-228: 1 min
IV. EXAMPLE PROBLEMS
The half-lives of several radioisotopes are listed above. Use this information to answer each question.
3. A jar is filled with 20 grams of Actinium-225.
How much will be left after 10 days?
4. You measure 16g of Protactinium-224, how
much is left after 3 seconds?
5. Four years ago, you purchased 8kg of
Thorium-228. How much is left today?
6. A sample of Neptunium-228 decays from
20kg to 5kg. How much time has passed?
Cs-137: 30 years I-131: 8 days N-13: 10 min Po-211: 0.5s
V. PRACTICE PROBLEMS
The half-lives of several radioisotopes are listed above. Use this information to answer each question.
7. How much of a 12 gram sample of N-13
remains after 10 minutes?
8. A sample of Iodine-131 is measured to be
12g. How much will remain after 16 days?
9. How long will it take a sample of Polonium211 to decay from 16g to 1g?
10. A sample of Cesium-137 is measured to be
10g. How much was there 90 years ago?
VI. CULMINATING PROBLEM
11. A sample of radium-224 is placed in a sealed
container and left to decay. The mass of the
remaining sample is measured every 2 days.
time
0 days
2 days
4 days
6 days
mass
20.0 g
14.1 g
10.0 g
7.07 g
(a) What is the half-life of radium-224?
(b) How much will remain after 8 days?
(c) During the experiment, helium-4 was
detected. What type of decay occurred?
(d) Write a decay equation which describes the
nuclear reaction that took place inside the
container as the sample decayed.