You are given a sample which
has been irradiated in a nuclear reactor. To the right, click on
Sample to see an animation of the radiation
process, modeled after the Research Reactor at University of Missouri-Rolla.
You will be able to count this sample, and to radiate it at different levels
for further counting. The counter automatically starts each minute
and counts for a set period of time (10, 20, 40, or 60 seconds), initially
set at 40 seconds. Note that the first reading is NOT
at t = 0.
After viewing the animation, click on Go to
Counter to begin the experiment.
Click on the Please Sign In
button. Enter your name as it will appear on the printed report.
Then enter your code number if one has been assigned - if not, accept the
Click on start
the counter will begin its operation. The window at the top right
shows the count in progress, and the window at the top left shows the count
for the previous period. The times (start of the count) and counts
are recorded in a table below the counter.
Note the initial value (at 2 minutes) of the counts. Divide this
value by 2 (21)
- this is approximately the value that will be observed after
one half-life. Divide the initial value
by 4 (22)
- this is the value to look for after two
half-lives. Divide the initial value
by 8 (23)
to get the value to look for after three half-lives.
Allow the counter to run until the count is less than 1/8 of the initial
value, then click on stop.
Click on prepare for print.
The recorded data is transferred to this page. Inspect the data to
determine the time required for the initial value to be divided by 2
(you must subtract 2 minutes from the time at which this value is reached).
This is t1/2 .
Similarly, determine the time required for the initial value to be divided
by 4 (t3/4)
and by 8 (t7/8).
Enter these values on the data sheet. You should notice that these
values are simple multiples of each other. Click on the go
back button. Do NOT print the data yet.
Click on clear registers.
A dialog box asks if you want to save the data. Click on OK.
If you click on Cancel,
the data will be removed from the data sheet.
Select a different radiation time (short
Repeat steps 3
process can be repeated with different radiation times, and/or different
counting times. The half-life should not be affected by these factors.
10. The half-life can also be
determined by graphing
or linear regression (see
The logarithm of the counts ln(cts)
is plotted vs time. The slope of this line is the rate constant for
nuclear decay (k).
The half-life is equal to 0.693 (ln(2))
divided by k.