Lab E2: Factors that Affect the Detection Rate of a Geiger Counter                         

revised 6/11/07

 

Reading

For how the Geiger counter works, Serway 14(9).
For conservation laws, Serway 13(5). 
For understanding the interaction of abg's with matter, particularly when analyzing the results, Serway 14(7).
For background radiation and interaction of abg's with humans, Serway 14(9).


Prelab:   Read this lab handout and write a goal statement in your lab book.   Copy the table below into your lab book and fill in all of the columns except the last one.  
Use Serway 13(5) and Appendix B as a reference in filling out the table, although it may not have all the isotopes listed in the table below   You can also query the web site   http://atom.kaeri.re.kr/ton/nuc7.html for nuclear properties; type the nucleus (for example, type Po-210) desired in the Nuclide box at the upper left.


Equipment:   In this investigation, you will use the same equipment as for lab E1.   Refer to and follow the set up instructions for lab E1. NOTE ABOUT EFFICIENT USE OF TIME IN THIS LAB: You will make many measurements of count rates in this lab. In order to finish the lab during the class period, we suggest counting for 30 seconds for each measurement. Set your software to count for 30 consecutive 1-second intervals.

                     

Part A:   Effects of Source/Detector Positioning and Background Radiation

 

1) Place the Geiger tube above a source and observe the count rate.   What things could you do to make a significant change in the count rate?

Try them to see

 

2) Adjust the height of the tube above the source to give a count rate of at least 25 per second if possible.

In this part and the next part (3), you'll need to use the Radiation vs. Time graph to have the computer calculate the mean and standard deviation of your data.  

Record the mean and standard deviation.

 

3)Now remove the source from under the Geiger tube.   Make sure all radioactive sources are far away.  

Watch the count rate for about thirty seconds.  

What sources of particles that might be causing your Geiger counter to register counts even when your source is removed?

List the most likely ones first. (if you're not sure, you can consult Serway or Walker)
You are now measuring what is called the background rate.  
Record the mean and the standard deviation for the background rate.

 

In the remainder of the lab (and in all future radioactivity experiments), it is important to realize that the background rate is always there.  
Therefore, any measured count rate from a radioactive source must be significantly (say, 5 - 10x) above the background count rate to be considered real. 

 

Part B:   Effects of Intervening Objects on the Detection Rate of  a, b, and  g rays


4) In sections 7 - 11, you will use one alpha, one beta and one gamma source. You will only need one source at a time. You should have already reproduced the following table in your lab book.   Record the half-lives for the radioactive isotopes you actually use in the right-most column.
 

Source

Type

Radio-active

Parent

Isotope

Electric charge of Parent

Isotope

Nucleus

Atomic

Mass No. of   Parent

Isotope

Particle

Emitted

Charge

Of

  Particle

Emitted

  Atomic Mass

No. of Particle Emitted

Daughter   Isotope

Charge

of

Daughter   Isotope

Atomic

  Mass No. of daughter isotope

Half life (T1/2)

of   Parent

Isotope

Nucleus

a

210Po

 

 

 

 

 

 

 

 

 

a

241Am

 

 

 

 

 

 

 

 

 

b-

204Tl  

 

 

 

 

 

 

 

 

 

b-

90Sr

 

 

 

 

 

 

 

 

 

g

137Cs

 

 

 

 

 

 

 

 

 

g

60Co

 

 

 

 

 

 

 

 

 

 

5) a) Obtain a strong gamma source. Adjust the height of the Geiger counter on the ring stand and use the wooden 1.0-cm spacers so that there is 1.0 cm of space between the top of the source and the bottom of the detector; place the source on top of three spacers, so that there is also 3.0 cm of space between the source and the base of the ring stand.  

 

b) Make sure that a count rate significantly higher (at least 10 times) than the background is obtained; if not, use a different source. Once you have an acceptable count rate, mark the location of your source as follows: put a blank piece of scrap paper on the base of your ring stand and tape it, so that the paper is flat and will not move. Center the source and wooden spacer as accurately as you can directly underneath the detector window. Next trace the outline of your spacer on the paper, using a pen, and trace the outline of the source on the spacer. In the remainder of the lab, do not move the detector, and make sure the source is always at the same location whenever you take data. Finally, record the average count rate with your source at this location.

 

c) In turn, remove the wooden spacers, one-by-one, recording the count rate each time you do so.  Make sure that the source remains directly underneath the detector window (use the tracings you made to keep the source in the same horizontal position relative to the detector).  Record the new count rate.  At the end, you should have 4 different count rates as a function of 4 different distances of the gamma source from the Geiger counter.

 

6) a) Now, make a large table in your lab journal (covering 2 complete pages) with alpha, beta, and gamma as the columns and Observations 7 - 11 (see below) as rows for each column.  You do NOT have to do experiments 7  11 in ascending order; you can do them in decreasing order or random order. We do not have enough squares of material for each group to hoard their own supply of plastic, aluminum, copper, or lead.

 

Obtain a radioactive source. Leave the source 2 cm or more from the detector (as long as you get a large count rate relative to the background) for the remainder of the experiment.

It does not matter which source you do first; you want a strong source, and we have only a limited amount of them. NOTE: Unlike the beta and gamma sources, the alpha source is exposed and can fall out of the red/blue plastic holder.   For this reason, you must obtain the alpha source directly from the instructor or work service helper, and you must return it directly to them (not to any other lab group) when you are finished. A polonium alpha source is superior to an alpha americium source.

 

Do #6b if you have an 241Am source ONLY: b) If your source is 241Am do the following. Place a single piece of paper between the source and the detector. Measure the count rate with the paper in place, and record this number clearly at the top of your data table. You will need to SUBTRACT this number from every measurement you make with the source. (241Am also emits gamma rays, which we do not want to count here, thus we are subtracting them). Show clearly in your data table any calculations when you use the 241Am source.

 

Do #7 - #11 for each source:

 

7) Place a single piece of paper between the source and detector. Measure and record the count rate with the paper in place. Does the count rate decrease noticeably?

 

8) Place two squares of black plastic between the source and the counter. (2 squares of black plastic are 1.50 mm thick). Record the new count rate. How many pairs of plastic are required to reduce the average count rate to approximately 10% of its original (i.e., without any intervening plastic) value? If you run out of space to add more plastic, just end this experiment and just report the maximum number of pairs of plastic you used, and the fraction by which the count rate was reduced. Report all the results you try (even if they produce less than or more than the targeted 10%).

 

9) Now repeat #10 with aluminum. However, you will need 5 squares of aluminum to have a barrier that is 1.50 mm thick; so always use the aluminum in units of 5 squares each , so that you are adding (or subtracting) the same barrier thickness each time. Measure/record the new count rate. How many aluminum sheet sets reduce the count rate to approximately 10% of the original count rate. Report all the results you try (even if they produce less than or more than the targeted 10%).

 

10) Repeat the experiment with copper. Copper squares are .45 mm thick, so the copper should be used in sets of 3.

 

11) Repeat the experiment with the thin lead squares. Lead squares are .55 mm thick, so the lead should also be used in sets of 3.

 

Analysis Questions

 

12) Why were you instructed in step 3 to make sure all other sources were far from the tube?  To what source does this instruction particularly apply?     Remember this lesson in future experiments with the Geiger tube.

 

13) Why isn't the alpha source enclosed in plastic as are the beta and gamma sources?

 

14) If the source emits radiation equally in all directions, how should the intensity of the count rate measured by the detector depend on the detector's distance from the source? (Hint: we have studied this type of dependence of a measured flux/strength/intensity on distance in several other contexts in physics.)   Plot your data from part 5 above (count rate as a function of distance), and then do an appropriate fit to your data. Plot expectations are in your lab guide.

 

15) An important lesson from the above series of activities has to do with radiation protection. List at least three factors (other than shielding and counter/source separation distance) that influence the total amount of radiation one's body might encounter from a radioactive source (i.e., the total number of decay particles that hit your body). Explain why each of the factors influences the amount of radiation you receive.

 

16) Explain why the a's are more penetrating than the b's and also why the g's are the most penetrating of all; use the properties of the individual rays and the impulse-momentum version of Newton's 2nd law to do this.   There are at least two separate reasons why a's are the least penetrating rays and totally separate reasons why the g's are the most penetrating rays

 

17) Give two reasons (again using Newton's 2nd law) why a lead sheet is more or less effective at shielding than an equal thickness of copper.

 

Conclusion, as always