HALF-LIFE
Objective
chapter 16
lesson 3
You will be able to perform half-life calculations.
Penetrating ability of various materials.
Once a radioactive element has undergone decay it
is changed into something else (transmuted). This
means that the number of parent nuclei is
continually decreasing. A quantitative measure of
longevity of a particular isotope is its half-life. It is
the time it takes one half of a radioactive sample
to decay.
The following is an experiment where the half-life of
protactinium is measured. Record the data and draw a
graph of the results.
Your graph should look like this one.
How do we calculate the half-life from this graph?
The actual half-life of Pa-234 is listed as 70 s.
Formula
N=N0(1/2)n
Example 1:
Plutonium-239 has a half life of 24 000 years.
Calculate the amount of a
1.00 kg mass remaining
after:
a) 24 000 yr
Example 2
b) 12 000 yr
c) 36 000 yr d) 100 yr
Polonium-210 is a radioactive element with a half-life of
20 weeks. After 52 weeks 0.250 kg remains. How
much polonium-210 was originally present?
Example 3
The half-life of a radioactive isotope is 23 days. What
percentage of the substance will remain after 100 days?
Solving for the number of half-lives requires the use of
logarithms.
log(N/No) = n log(1/2)
Example 4:
4.0 g of a neptunium is produced on Monday. On
Tuesday of the following week it was tested and found
that 0.25 g were remaining. Calculate the half-life of
neptunium.
Assignment
Read p.811-817
Do SNAP p. 373 all
Answers to even:
2: 56%
4: 0.020 g
6: graph
5
8: 2.0 x 10 Bq
10: 5.18 x 107 Bq
3
3
12: 5.8 x 10 cm
14: graph