ARSC/GEOL 2110
Physical Science of the Earth System
Exponential decay and Radioactivity
Fall 2001

Lab report is due Thursday, November 15, 2001.

Useful resources:  Ch. 8 pp. 162-164. 

Red lab manual handouts “Simulation:  Radioactive Decay” (done with M&Ms on Thurs. 11/1; done as take-home if you missed class) and “Time and Radioactive Decay” (done with barium-137 and Geiger counter on Tues. 11/6) and your own notes and data from these two labs.

Note:  barium-137, Ba-137, and ¹³⁷Ba are notations that all refer to the isotope of the element barium that has a mass number of 137. 

1.        Turn in your parent/daughter graph from the simulation experiment, making sure your assigned decay rate is shown on the page.  Report your answer to #8 in this handout (how old is the sample … see lab manual for full question).  Why do geologists use the parent/daughter ratio for dating instead of the actual number of nuclei?

2.        Turn in your graph of the decay of Ba-137 (the decay you observed with the Geiger counter) as a function of time.  Estimate the half-life of this isotope, using more than one estimate to check your answer.  Report your best estimate of the half-life and show how you determined it on your graph.

3.        Ignoring the jump that occurred when we switched scales (this tells us there is a calibration problem between scales …), what was observed in the decay experiment at long times?  why didn’t the count rate go to zero?

4.        Why did the data from the radioactive decay of Ba-137, observed with the Geiger counter, not form a smooth curve like the M&M simulation?  List as many reasons as you can think of.  (Hint:  not all the reasons should be about the equipment!)

5.        Suppose we conducted the barium decay experiment again, using the same set-up and equipment.  How could you improve the smoothness of the data you collected?  List as many suggestions as you can think of.

6.        Read #3 in the Time and Radioactive Decay handout (about the supernova explosion).  Instead of making your own graph, use the attached graph of the radioactive decay of two uranium isotopes, U-235 and U-238.  The ratio of the two isotopes as a function of time is plotted on the same scale. 

The graph is prepared with a logarithmic vertical axis.  Notice that each major (labeled) unit on the scale represents an increase in the exponent, that is, one more power or multiplication factor of 10.  The minor (unlabeled) divisions mark the linear intervals between the factors of 10.  For example, the marks between 1 x 10^-1 and 1 x 10⁰ represent 2 x 10^-1, 3 x 10^-1, 4 x 10^‑1and so on.  Note that the logarithmic scale means that these are not evenly spaced as they would be on a linear scale (like the other graphs you have made), but the powers of 10 are evenly spaced (which is not true on a linear-scaled graph).

a)       Use the graph to determine how long ago the supernova explosion took place. 

b)       Why do you think scientists use logarithmic graphs for this kind of data?

7.        The rangers in a state park in Colorado’s Front Range would like a geologic map that shows the type and age of the rocks in the park.  They ask you to do the research to produce this map.  Outline a plan for your research, indicating what you will observe and how this information will help build the geologic map.  Use at least two strategies or different types of observations.  Make (and state) whatever assumptions you find necessary to make about the rocks, the equipment available to you, etc.  Your plan should be reasonably specific but does not need to be extremely detailed (a paragraph or two should do the job).