Nuclear Chemistry

Experiment N-7

Earlier it was mentioned that Ernest Rutherford investigating the radiation from uranium, U, determined that not all radiation penetrated matter the same.  Part of the radiation was easier to block than the remaining radiation.  He began using the letters of the Greek alphabet to distinguish the different kinds of radiation: α (alpha), β (beta), γ (gamma), etc.

Previously we found that γ (gamma) radiation intensity decreases with greater distance from a source and that α (alpha) radiation is easily stopped and β (beta) radiation intensity decreases with greater thickness materials such as Aluminum and Copper.  It is common knowledge that lead stops radiation.  But it should be clear now that shielding reduces the intensity of radiation in half with a sufficient thickness of material.  Generally higher atomic weight solids provide more shielding than lower atomic weight materials.

Experiment

In this experiment we shall investigate the effect of shielding thickness on γ (gamma) radiation.  In this experiment we place different thicknesses of other materials between the Geiger counter and a small sample of Cobalt-60 (₂₇Co⁶⁰) that produces γ radiation.

Since we know distance affects the intensity of radiation (radiation spreads out), the Cobalt imbedded in a plastic disk (orange in the diagram below) is kept 2.0 cm from the Geiger-Muller tube.  Several sheets of Aluminum, Al, and Copper, Cu, are placed between the ₂₇Co⁶⁰ and the Geiger-Muller tube.  The electrical signals indicating radiation detection are stored in sound files.  By playing the sound files, you can hear the same clicking sounds that the Geiger counter produces.

1. Because there is constantly natural radiation around us, that background radiation needs to be counted and subtracted from all our measurements.  Listen to each minute of sound and count the clicks.  Find the average background radiation counts per minute.  (Also notice how much natural variation there is from one minute to another.)
   
   

   Background radiation
   (Caution: 120KB may take a few seconds to download each file.
   Troubleshooting: If no player appears after a sound file is selected, use visual format.
   If no sound occurs during play, check computer sound volume.)

   Measurement #	sound file	visual plot	time duration
   1	mp3 file	graph	60 seconds
   2	mp3 file	graph	60 seconds
   3	mp3 file	graph	60 seconds
   4	mp3 file	graph	60 seconds
   5	mp3 file	graph	60 seconds
   6	mp3 file	graph	60 seconds
   7	mp3 file	graph	60 seconds

   
   
2. Using the sounds available in the table below, record the radiation detected (count the clicks) with nothing but air between the source and the counter.  Using the time duration, calculate the counts per minute.  Subtract the average background radiation to determine the intensity of γ radiation.
   
   
3. Measure the radiation radiation that penetrates each thickness of shielding.  Again calculate the counts per minute.  Subtract the background to determine the intensity of γ radiation which penetrates the shielding.  Each sheet is approximately 1 mm thick.
   

   γ radiation with various shielding

   Shielding	sound file	visual plot	Time duration
   none	mp3 file	graph	5 seconds
   5 sheets Aluminum	mp3 file	graph	5 seconds
   10 sheets Aluminum	mp3 file	graph	5 seconds
   5 sheets Copper	mp3 file	graph	5 seconds
   7 sheets Copper	mp3 file	graph	5 seconds

   
   
4. Graph on cartesian coordinates (ordinary graph paper) of the reduction in radiation verses the shield thickness.  Is there a correlation between the amount of shielding material and the shielding provided?
   
   
5. With adequate data, a graph on semi-logarithmic graph paper of the reduction in radiation verses the thickness of shielding yields a straight line.  Such a plot can be used to determine the amount of each material that would be necessary to reduce the radiation to half intensity, called the half thickness.
   
   
6. It is not unusual for something to go wrong during an experiment.  If these results aren't ideal, try to propose what should be done differently to improve results.

You may have been surprised to learn that even lead does not totally block γ (gamma) radiation but partially absorbs the radiation.  So if you wish protection from a source of γ (or similar X) radiation, a significant thickness of shielding may be needed.

The penetrating power of γ radiation depends on both the atomic mass of the shielding and the radiation energy.  The energy can be determined from the half thickness.  So larger amounts of shielding are needed if lower atomic mass materials are used.

Communicating technical information such as observations and findings is a skill used by scientists but useful for most others.  If you need course credit, use your observations in your journal to construct a formal report.