Equipment: In this lab, you'll be using a Geiger interface attached to a computer. This allows the computer to count radioactive emissions that are detected in a Geiger-Mueller tube connected to the computer. In using the tube, be careful not to touch or bring anything close to the thin window at the bottom of the tube. This window is very delicate. If broken, the tube could not function. There is a red, plastic cap to protect the window when not in use; remove the cap for measurements and replace it when you are finished using the tube.
The radioactive sources that you will be using are not active enough to be harmful. The beta and gamma sources are completely enclosed in plastic disks so that there is no danger of contacting or ingesting the radioactive materials. The alpha sources, however, while surrounded by plastic (red), are not covered and must only be handled by the plastic. Whenever you finish with a radioactive source, place it in the labeled container.
setting up:
Access the LABNET GEIGER-MUELLER software:
a) stand-alone 286: use the Labnet Geiger-Mueller diskette
b) stand-alone 386/486: access the software on the hard drive
c) networked computer: access the software under Geiger Counters
Plug the power cord of the Geiger tube into a wall outlet.
Place a beta (or gamma) source under the tube. You should notice that a red or orange light inside the Geiger tube is flashing. This indicates that the tube is detecting radioactive emissions. In order to get the computer to count and time these emissions, select Meter Menu and then Meter Mode. (Don't forget to take the red cap off the Geiger counter!)
When you press the space bar, the speaker will begin ticking and the count rate will be displayed.
Collection and Analysis of Data:
1) Adjust the height of the tube above the source in order to give a count rate of about 50 per second. Return to the main menu, select Data Collection, Manual Collection, and 5 second intervals. When you press the space bar, data will be collected for 5 s and recorded in a table. Without making any changes in the source position, take readings for 10 successive 5 s intervals. Record your data in your journal in a table as shown below. Calculate the mean (average) number of counts per 5/s. Then calculate and record the difference between each reading and the mean. (Give absolute values.) Calculate the squares of the differences, find the average of that column, and take the square root of the result. The result is called the standard deviation and is a measure of the variation of the data from the mean value. Your calculator may have a key for finding standard deviation. You may use it in the future. But for now, show results of all calculations in your lab book.
| Point | Total Time (s) | Counts/5 s | Difference | (Difference)2 |
| 1 | ############# | ############# | ############# | ############# |
| 2 | ############# | ############# | ############# | ############# |
| 3 | ############# | ############# | ############# | ############# |
| 4 | ############# | ############# | ############# | ############# |
| 5 | ############# | ############# | ############# | ############# |
| 6 | ############# | ############# | ############# | ############# |
| 7 | ############# | ############# | ############# | ############# |
| 8 | ############# | ############# | ############# | ############# |
| 9 | ############# | ############# | ############# | ############# |
| 10 | ############# | ############# | ############# | ############# |
| Mean ######## | Mean of Diffs squared : | ############# | ||
| Standard Deviation: | ############# |
3) Repeat the data collection part ONLY from step 1, but select 15 s time intervals this time. As soon as you have finished collecting your data, proceed to the instructions in part 7 to get the computer started counting in Histogram mode and then return to this part to finish the calculations. Use the standard deviation function on your calculator this time to compute the mean and the standard deviation (instead of doing the tedious calculation of differences, squares of differences, etc.). To have your TI do all the steps, record your count rate values in a list; then go to STAT, CALC; do 1-Var Stats on the appropriate list; the symbol for standard deviation is .
4) What percentage is the standard deviation of the mean? Why was the ratio of the standard
deviation to the mean different in the two cases above?
5) In your lab book, write the range covered by (mean - 1 standard deviation, mean + 1 standard deviation). How many of your 10 data points fell within that range? Answer for both experiments above.
6) Repeat #5 for "1" replaced by "2." Repeat again for "3."
7) Return to the main menu. Leave the beta source in place and the counter at the same height as it was in part 3. Select Meter Menu and Histogram Mode. Press the space bar. Take care not to move the source as data is collecting. (Let the computer run while you return to part 3, although do keep an eye on the screen from time to time.) Continue collecting data for several minutes while discussing what is happening with your lab partner. Write a complete description of what the computer is doing (include a sketch of the histogram in your lab book) and what you are observing . Tell what is being graphed and what each axis represents. How does the pattern of vertical bars change as time passes? Don't forget to record the mean and standard deviation for the histogram mode at the end!
8) What is the Histogram Mode trying to teach you?
9) In an organized table, summarize, for the three data collection parts, the average (or mean) count rate, the standard deviation, the % that the standard deviation is of the mean, and the percentage of your data points that fell within 3 standard deviations on either side of the mean.
10) Conclusion