Nuclear Chemistry

Experiment N-6

In experiment N-5 we found that α (alpha) radiation can be easily stopped by a mere inch of air.  So it is easy to protect yourself from this kind of radiation risk by simply staying more than a few centimeters away from the source!

As mentioned in Experiment N-5, 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.

Experiment

In this experiment we shall investigate Rutherford's second type of radiation, β.  We will measure the effect of various thicknesses of Aluminum, Al, on the intensity of β (beta) radiation.

Since we know distance affects the intensity of radiation (it spreads out), a sample of Strontium-90, ₃₈Sr⁹⁰, imbedded in a plastic disk (orange in the diagram) will be kept 1.0 cm from the Geiger-Muller tube.  Different number of sheets of 1 mm Aluminum and Copper will be placed between the radiation source and the Geiger-Muller tube.  The electrical signal recording the detected radiation again stored in a sound file.

1. Because there is constantly natural radiation around us, the background radiation always needs to be counted and subtracted from all our measurements.  Select each sound file in the table and listen to each recording of the Geiger counter sounds, counting the number of clicks.  (If the sounds come too fast to count, try looking at the graphs where the sounds show as sharp spikes.)  Calculate the average background radiation in units of counts per minute.

   Background radiation
   (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.)

   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. The three tables below contains sound files obtained first with nothing or paper between the source (C₆H₁₀O₅; C = atomic #6, H = atomic #1, O = atomic #8) followed by sheets of Aluminum shielding (Al = atomic #13), and then Copper shielding (Cu = atomic #29).  Measure the radiation (clicks) for each.
   
   
3. Since these measurements were of shorter duration (to be easier to count), be sure to use the time duration to calculate the counts per minute.  Subtract the average background radiation to determine the intensity of β radiation that penetrates each kind of shielding.  Each metal sheet is approximately 1 mm thick and the paper is the 20# printer paper.
   
   

   Sr-90 β radiation with no shielding or paper

   Shielding	sound file	visual plot	Time duration
   none	mp3 file	graph	5+ seconds
   none	mp3 file	graph	5+ seconds
   12 sheets paper	mp3 file	graph	5+ seconds

   
   

   Sr-90 β radiation with Aluminum shielding

   Shielding	sound file	visual plot	Time duration
   1 sheet Al	mp3 file	graph	5+ seconds
   2 sheets Al	mp3 file	graph	20 seconds
   2 sheets Al	mp3 file	graph	10 seconds
   4 sheets Al	mp3 file	graph	20 seconds
   6 sheets Al	mp3 file	graph	5 seconds
   6 sheets Al	mp3 file	graph	20 seconds

   
   

   Sr-90 β radiation with Copper shielding

   Shielding	sound file	visual plot	Time duration
   1 sheet Cu	mp3 file	graph	10 seconds
   1 sheet Cu	mp3 file	graph	20 seconds
   2 sheets Cu	mp3 file	graph	20 seconds
   3 sheets Cu	mp3 file	graph	20 seconds
   4 sheets Cu	mp3 file	graph	20 seconds
   4 sheets Cu	mp3 file	graph	20 seconds
   5 sheets Cu	mp3 file	graph	20 seconds
   5 sheets Cu	mp3 file	graph	20 seconds

   
   
4. Construct a set of graphs of the reduction in radiation verses the thickness of shielding.  Is there a correlation between the ability to shield radiation and the thickness of shielding?
   
   
5. Compare the ability of paper (C is atomic #6), Aluminum (Al atomic #13) and copper (Cu atomic #29) to stop α radiation.

Hundreds of isotopes are known to emit β radiation.  Most also emit γ radiation which we will investigate in the next experiment.  In this experiment we carefully selected ₃₈Sr⁹⁰ because it decays by emitting only two β becoming stable ₄₀Zn⁹⁰.

While the ₃₈Sr⁹⁰ has a half life of 28.1 years, ₃₉Y⁹⁰ has a half life of only 64 hours.  So the source was primarily Strontium-90 governing the decay, with each produced Yttrium-90 producing a second β with higher energy a short time later:

Optional Extension using another Beta emitter, Thallium-204

Thallium-204 is another isotope that only emits β radiation and none of the γ (gamma) common to other radioactive isotopes.  We have made measurements of it so you can do a comparison.

Again, since we know distance affects the intensity of radiation, a sample of Thallium, ₈₁Tl²⁰⁴, imbedded in a plastic disk is kept about 1.5 cm from the Geiger-Muller tube.  Different number of sheets of 1 mm Aluminum are placed between the ₈₁Tl²⁰⁴ and the Geiger-Muller tube and the electrical signal recording the detected radiation again stored in a sound file.

1. Count the Geiger counter clicks caused by the background radiation for each minute and calculate the average counts per minute.  120KB each
   

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

   
   
2. Count the Geiger counter clicks per minute caused by the unshielded ₈₁Tl²⁰⁴ radiation and with the various thicknesses of Aluminum, Al, atomic # 13.
   

   Shielding	Radiation Source	sound file	visual plot
   none	81Tl204	mp3 file	graph
   1 mm Al	81Tl204	mp3 file	graph
   2 mm Al	81Tl204	mp3 file	graph
   3 mm Al	81Tl204	mp3 file	graph
   6 mm Al	81Tl204	mp3 file	graph
   1.2 mm Al	81Tl204	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?

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.