Penetrating Power

There is a significant difference in the penetrating abilities of alpha, beta, and gamma radiation. Alpha particles are the easiest to block and can be stopped by something as thin as a sheet of paper or even human skin. Beta particles are more penetrating and require materials with greater thickness and density, such as plastic, glass, or a few millimeters of aluminum, to be effectively absorbed. Gamma rays, however, are the most penetrating of the three and can pass through many materials. Dense substances like lead or several centimeters of concrete are typically needed to reduce their intensity.

Objective   

 To explore and calculate the relationship (if any) between the intensity of radiation and shielding.

Procedure

Part 1:

1. Set Geiger counter to the most readable scale and turn power on. Allow the instrument to warm up for a few minutes.

2. Prepare a spreadsheet graph (thickness vs. number of counts) to track all of your data using Google Sheets.

3. Record background activity.

4. For this part, we will be starting with the alpha source (Am-241). Make sure it's in a stable position and the Geiger counter is placed in a constant spot, the only thing changing will be the material placed in between the source and the counter. 

5. Polyethylene and aluminum foil will be the only 2 materials used for shielding alphas. Go in order from least to most dense and record this data in your spreadsheet. taking 3 trials for each. When observing the Geiger counter, wait until the number is stabilized to reduce incorrect variables. 

Part 2:

1.  Make another section within your spreadsheet to track beta radiation this time.

2.  Instead of the alpha source, use the beta source (Sr-90) and place the Geiger counter.

3. You will be using the plastic, aluminum, and lead materials to shield and record data. Again, go from least dense to most dense.

4. Record all the data using your spreadsheet.

Part 3:

1. Make a section of your spreadsheet for the gamma source. Replace the beta source with the gamma source (Co-60).

2. Use the lead material in the absorber kit to perform this part of the experiment. Start with the least dense and move your way up to the most dense. 

3. Record all of the data you collect within the spreadsheet.

Questions:

1. Why allow time for the Geiger counter to warm up?

2. Is it necessary to take background readings every day? Explain.

3. What type of results would you expect if the same alpha sample was doubled in quantity?

4. Is polyethylene an effective shield for beta radiation? Is lead? Explain.

5. What results would you observe if you used aluminum for shielding beta rays?

6. From your graph, what thickness of lead is needed to absorb Sr-90 beta rays? For Co-60 gamma rays?

7. What materials, other than lead, are effective for shielding gamma radiation?

8. What amount of lead is needed to reduce gamma radiation to one-half of a previous intensity count?

9. What general statements can you make about the thickness of the absorbing material on the count rates?

Optional: 

1. If the density of air is 1.3 mg/cc, then how long a column of air would be needed to absorb beta rays in the source Sr-90?

2. How long would a column of air need to be to reduce the intensity of Co-60 gamma rays by a factor of 10? Explain.

3. Make a chart comparing alpha, beta, and gamma radiation, including such information on charge, mass, penetration, speed, and energy.

To access data from our own trials, click here.