Questions
1. Explain how your data illustrates the idea that the half-life for decay of a given isotope is
constant.
Our data does not actually demonstrate this phenomenon. However, an accurate
graph would show an exponential slope where, for example, the time between 100 counts
and 50 counts would be the same as the time between 50 counts and 25 counts and
between 25 counts and 12.5 counts. Since exponential trend lines come from constant,
exponential increases and decreases, the half-life can be assumed to be a constant
phenomenon. We can use the number of counts to determine half-life because the number
of counts corresponds to the emission of a charged particle into the Geiger-Mueller
Tubes. When a charged particle goes into the tube, it ionizes the gas and causes voltage to
build up. When enough of a charge is built up, it is detected by the G-M tube and
“counted” as an event of radiation. The decrease in the counts corresponds to a decrease
in radioactivity because there are fewer radiation events causing the counts.
2. How does background count affect your results?
The background count is the number of counts that corresponds to the radiation in
the environment and from the Geiger-Mueller tube. If we did not correct for it, it would
cause our results to be consistently higher than they actually were. However, we
subtracted the average background count from each raw count, so it should not have
affected our results.
6. How would having a more active source affect the results? How would having a less
active source affect the results?
Having a more active source would have a higher rate of decay, and therefore a
shorter half-life. A less active source would have a lower rate of decay, and therefore a
longer half-life.
8. Another way to determine the half-life is to make a plot of the ln(N) vs time. Explain how
you could use this graph to determine half-life. What would be the advantage of this plot?
If two values have an exponential relationship, A vs time for example, plotting
ln(A) vs time will give a linear slope. Since the number of counts and time have an
exponential relationship, plotting the ln(N) vs time will give a straight line. The decay
rate will still dictate how fast or slow the number of counts changes over time. You can
measure the half-life in the same way that you would on a graph of Counts VS times.
9. If two different radioisotopes A and B have the same activity but the half-life of isotope A
is larger than the half-life of isotope B, what is the relative magnitude of the numbers of
radioactive nuclei of isotope A to isotope B?
Radioactivity is directly proportional to the number of radioactive particles in the
sample. Therefore, since the half-life of isotope A is larger than the half-life of isotope B,
then isotope A is more radioactive. As a result of a greater radioactivity, isotope A has a
greater number of radioactive nuclei than does isotope B.
Leave a Comment