# Lab exercises, phys 107, energy

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## LAB EXERCISES, PHYS 107, ENERGY

Daniel W. Koon, Fall 2005

PART A: Count rate in a smoke detector

Set up a Geiger counter to measure the radioactive 241Am source in a smoke detector. The instructor will give you specific instructions for setting up the computer program with which you will measure the count rate in counts/sec.

• Is the number of counts per second equivalent to activity, dose, or a dose rate? The raw data from the counter (Remember: it can only count radiation, not determine its type or its tissue destination.) can most appropriately be labeled by which of these units: Becquerel, Curies, rem, rad, Gray, Sievert?

• Record and report the background rate. Record the “raw” number of counts in each time interval defined by the computer, and convert that rate to counts/sec. Show these numbers and your calculation in the report you hand in. Average over at least ten different time intervals.

• Place the detector close to the americium source. Record its activity, averaging over at least three independent intervals. Make sure that you only record the counts for intervals that begin after you change the setup but before you change it again. Average the results and subtract off the background.

• Compare this to the manufacturer’s specs. If there is a large discrepancy, explain why this might be so, other than the manufacturer lying.

• Based on the manufacturer’s specs, calculate the mass of americium inside the smoke detector. It should be a fraction of a microgram. You will need to make use of a textbook expression that relates the number of radioactive atoms and their lifetime to count rate, and you will also use the connection between atomic mass and Avogradro’s number.

PART B: Shielding I: Distance

Place the detector as close as possible to the radiation source.

• Record its count rate, averaging over at least three readings. Include these data -- the individual readings (counts), their average (counts/sec), and the result when the background is subtracted off -- in your writeup.

Place the detector 1 inch, then 2 inches from the radiation source.

• Repeat the same analysis as before for these data. Then, place the final data -- averaged count rate minus background vs distance -- in a table. Comment on the effect of distance on shielding.

Note that I won’t ask you to graph these data because of the difficulty of determining exactly where distance=0 (theoretically infinite count rate) occurs.
PART C: Shielding I: Absorbing materials

With both the Geiger counter and the smoke detector both fixed in place, and close enough to provide a hefty count rate, measure the count rate as a function of the number of pieces of paper between the two -- zero sheets, one sheet, two sheets.

• Include in the notes you hand in all of the following for zero, one, two sheets of paper -- at least three “raw” count numbers, the averaged count rate, and the averaged count rate with the background subtracted. Comment on whether the second sheet of paper is as effective (fractional decrease in count rate) as the first. Explain why it is not, citing the properties of americium.

Replace the paper with a sheet of lead. Measure the count rate. Repeat for glass and for various other materials that the instructor will provide you with.

• Include the same data again in your writeup. Create a data table with the final count rates for the following: no shielding, 1 sheet of paper, 2 sheets, 3 sheets, a piece of lead, a piece of glass, etc. Comment on the relative effectiveness of the various materials.

PART D: Writing up

• In addition to presenting all your data and calculations and addressing all of the bulleted items on this sheet, write up a Conclusion for today’s experiment in which you describe (a) what you did, (b) what the results were, and (c) what you conclude from these results. Pay particular attention to the following questions: what did you learn about the radiation emitted by the americium, and what did you learn about the effectiveness of various shielding strategies.