Nuclear Chemistry

Experiment N-5

In experiment N-2 we found that radiation intensity decreases dramatically with distance.  In experiments N-3 and N-4 we found that radiation intensity decreases with time.  Some substances decrease rapidly with short half-lives while others decrease very slowly with long lives.  The half life is a characteristic property for each isotope.  In this experiment we will investigate another way we can protect ourselves from the effects of the high energies released by radioactive events.

When high energy radiation was first discovered on November 8, 1895 by Wilhelm Konrad Röntgen, most scientists still viewed atoms to be indivisible.  And some scientists viewed atoms to be mere imaginary conveniences of chemists, not really existing at all.  The electron had been proposed but the convincing evidence that it was a small constituent of atoms wasn't presented until 1897 by J.J. Thomson.  So no one had any idea about the interior of atoms, or what must be occurring there to create the radiation.  James Clerk Maxwell had proposed that accelerated electric charge produces electromagnetic radiation such as visible light.  A number of scientists were attempting to determine how electricity related to atoms could occasionally cause some atoms to emit light with discrete line spectra.  But Niels Bohr didn't provide his radical explanation of that for another quarter century.

Ernest Rutherford investigated the radiation from uranium, U, and 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.

Experiment

There are two part to this experiment.  In the first part we shall investigate the effect of various thicknesses of Aluminum, Al, on the passage of α radiation.  In the second part we will investigate the effect of different materials on the passage of α radiation

Part I

Since we know distance affects the intensity of radiation (it spreads out), a sample of Polonium, ₈₄Po²¹⁰, imbedded in a plastic disk (orange in the diagram) will be kept about 0.5 cm from the Geiger-Muller tube.  Different number of sheets of 1 mm thick Aluminum will be placed between the ₈₄Po²¹⁰ and the Geiger-Muller tube and the electrical signal recording the detected radiation again stored in a sound file.

1. Because there is constantly natural radiation around us, that background radiation needs to be counted and subtracted from all our measurements.  Select each sound file in the table and listen to each minute of sound, counting the number of clicks.  (Alternately, look at the graphs where the sounds show as sharp spikes.)  Calculate the average background radiation in units of counts per minute.  (Caution: It may take a few seconds to download each 120KB 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.)
   

   Background Radiation

   Measurement #	sound file	visual plot
   1	mp3 file	graph
   2	mp3 file	graph
   3	mp3 file	graph
   4	mp3 file	graph
   5	mp3 file	graph
   6	mp3 file	graph

   
   
2. The table below contains sound files obtained with the Geiger counter close to the ₈₄Po²¹⁰.  Measure the radiation (clicks per minute) with only a short path of air between source and detector as well as with the various thicknesses of Aluminum, Al, atomic # 13.
   

   α Radiation from ₈₄Po²¹⁰

   Shielding	sound file	visual plot
   none	mp3 file	graph
   1 mm Al	mp3 file	graph
   1 mm Al, also	mp3 file	graph
   2 mm Al	mp3 file	graph
   3 mm Al	mp3 file	graph

   
   
3. Correct all measurements by subtracting the background radiation intensity.
   
   
4. Construct a graph of the reduction in radiation verses the thickness of Aluminum.  Is there a correlation between the ability to shield radiation and the thickness of shielding?  How much Aluminum is required to stop α radiation?

Part II

1. The following were recorded at a different time and location so the background radiation might be different. So it needs to be counted and subtracted from all our measurements.  Select each sound file in the table and listen to each minute of sound, counting the number of clicks.  (Alternately, look at the graphs where the sounds show as sharp spikes.)  Calculate the average background radiation in units of counts per minute.
   
   

   Background radiation

   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 α radiation.
   
   
3. Measure the radiation radiation that penetrates each kind of shielding.  Again calculate the counts per minute.  Subtract the background to determine the intensity of α radiation which penetrates the shielding.  Each metal sheet is approximately 1 mm thick and the paper is the 20# printer paper.
   
   

   α radiation with various shielding

   Shielding	sound file	visual plot	Time duration
   none	mp3 file	graph	30 seconds
   none	mp3 file	graph	30 seconds
   1 sheet paper	mp3 file	graph	30 seconds
   1 sheet paper	mp3 file	graph	30 seconds
   1 sheet Aluminum	mp3 file	graph	30 seconds
   1 sheet Copper	mp3 file	graph	30 seconds

   
   
4. Compare the ability of paper (C is atomic #6), Aluminum (Al atomic #13) and copper (Cu atomic #29) to stop α radiation.

As you noticed, α radiation is easily stopped.  Even an inch of air is sufficient to stop essentially all alpha particles.  As a result, sources of α radiation OUTSIDE your body cause of very little harm because air and the dead outer layers of your skill protect your living cells inside.  However inhaling, eating, or drinking a source of α radiation can kill tissue, cause mutations, and cause cancer.

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.