PHYSICS 320 LABORATORY

RADIATION SHIELDING

EQUIPMENT

Pasco interface and Data Studio software• Nuclear Sensor• Base and Support Rod• Radioactive Sources (alpha, beta, gamma)*• Right Angle Clamp• shielding material, metal, 1 x 1" lead• shielding material, paper, 2" diameter circles • shielding material, plastic, 1 x 1" squares• shielding material, metal, sheet with hole• vernier caliper• micrometer

* The sources are Polonium-210 (labeled Ra DEF, alpha, beta), Thallium-204 (beta), and Cobalt-60 (beta, gamma), camp lantern mantles, Fiestaware plate.

PURPOSE

The purpose of this laboratory activity is to investigate the penetrating ability of three common types of nuclear radiation and the ability of different materials to absorb the energy associated with nuclear radiation.

THEORY

Radioactive decay is strange and mysterious for several reasons. Besides the obvious fact that none of our senses can detect individual decay events, the nuclear decay process seems at the same time to be random yet predictable. It is impossible to say which nucleus will become unstable enough to decay next; however, it is fairly easy to use a Geiger counter to count the number of nuclei which do decay per second at all the locations in a radioactive sample. If you recorded the nuclear decay of a radioactive sample with a Nuclear Sensor and plotted counts per time interval over a period of time, the results would look like a standard distribution curve.

Because in many ways nuclear radiation behaves as though the radiation were tiny "bullets", it makes sense that different materials absorb the energy of nuclear radiation in different ways. The nature of the material through which nuclear radiation moves influences how much energy is absorbed.

The Chart of the Nuclides is useful in determining the decay mechanisms and routes for a source.

PROCEDURE

PART I: Decay Schemes

  • The Ra DEF source is really Pb-210 (Ra D, T1/2=22 yrs) and Bi-210 (Ra E, T1/2=5 days) that has partially converted into Po-210 (Ra F, T1/2=138 days).  Follow this link to see more information about the uranium sequence.  Use the Chart of the Nuclides to write the decay schemes for Polonium-210 (Ra DEF) starting with Pb-210.      What emissions would you expect from this source?
  • Do this as well for Thallium-204 and Cobalt-60.  What emissions would you expect from these sources?

In the second part of this activity, the Nuclear Sensor measures background radiation. In the third part of this activity, the Nuclear Sensor measures radiation from four different sources that are shielded with different thicknesses of four different materials. The measurements are made from a fixed distance over equal intervals of time.

The Data Studio program records and displays the counts of radiation over each time interval. You will determine how well each kind of shielding material absorbs each type of nuclear radiation.

PART II A: Computer Setup – Background Radiation

1. Connect the Data Studio interface to the computer, turn on the interface, and turn on the computer.

2. Connect the Nuclear Sensor Adapter Cable stereo phone plug into Digital Channel 1 on the interface.

3. Open the Data Studio and connect the virtual Geiger counter to the interface box.

4. Make Table display of Counts per Time Period. The Statistics area is open at the bottom of the Table.

5. The Sensor Properties are under the General Tab: Slow and 15s between samples.  Under Sampling Options use Automatic Stop = Time at 61 seconds. Each time period for the Nuclear Sensor is now 15 seconds.

PART II B: Sensor Calibration & Equipment Setup – Background Radiation

• You do not need to calibrate the Nuclear Sensor.

1. Carefully remove the plastic protective cap from the end of the Nuclear Sensor.

2. Use the base and support rod, the right angle clamp, and the tube clamp that came with the Nuclear Sensor to arrange the Nuclear Sensor vertically.

3. Plug the Nuclear Sensor power cord into an electrical outlet.

4. Connect the modular phone plug on the end of the Nuclear Sensor signal cord into the Nuclear Sensor Adapter Cable.

PART II C: Data Recording – Background Radiation

•Prepare to measure the average background radiation count.  Move all radiation sources away from the Nuclear Sensor, or behind shielding.

1. Click the START button to begin collecting data.

• Data collecting will automatically stop after 60 seconds. The Table display will show the number of counts for each fifteen second interval. Run #1 will appear in the Data List in the Experiment Setup window.

2. Click on the Table to make it active. Record the Mean as the average background radiation count (per 15 second interval) in Table 1 in the Data Section.

3. After you record the Mean, delete Run #1. Click on Run #1 in the Data List in the Experiment Menu and press the "delete" key on the keyboard. (An alert box will open to make sure you want to delete the data run.)

PART III A: Computer Setup – Radiation Shielding

• Use the same computer setup as in Part I.

PART III B: Sensor Calibration & Equipment Setup – Radiation Shielding

• Use the same equipment setup as in Part I. The only difference in Part II is that you will be placing different radiation sources and different shielding materials directly below the Geiger-Müller tube that is at the bottom end of the Nuclear Sensor.

PART III C: Data Recording – Radiation Shielding

1. Position the  alpha source under the Geiger-Müller tube at the bottom end of the Nuclear Sensor.

• NOTE: Be very careful when handling any radioactive source as it may be harmful to your health. Avoid long term exposure and always make sure that you wash your hands after handling any radioactive source.

2. Repeat steps 1.,2.,3., from PART II C above to collect Counts per 15 second Intervals after the alpha source radiation passes through:
. . . 0.,1.,2.,3.,4., and 5 layers of paper (You may need to use more sheets to get a significant decrease in the counts for the alpha and beta sources.  If it will take an extremely large number of sheets, continue on to plastic.).
. . . 0.,1.,2.,3.,4., and 5 layers of plastic.
. . . 0.,1.,2.,3.,4., and 5 layers of lead.

Next repeat the entire process of the two previous steps with the alpha source replaced with the beta source.

Repeat the entire process of the two previous steps with the beta source replaced with the gamma source.

Repeat the entire process of the two previous steps with the beta source replaced with the camp lantern mantle.

Repeat the entire process of the two previous steps with the beta source replaced with the Fiestaware plate.  You will need to use a large sheet of lead with a hole cut in it in order to cut out radiation from the entire plate.

PART IV: ANALYZING THE DATA

DATA TABLE #1: Background Radiation

Average background counts = ________

DATA TABLE #2: Radiation Counts per Fifteen Second Intervals

ALPHA SOURCE:

# absorber layers

zero

one  

two  

three

four

 five

 

paper

 

 

 

 

 

 

 

plastic

 

 

 

 

 

 

 

lead

 

 

 

 

 

 

 

BETA SOURCE:

# absorber layers

zero

one  

two  

three

four

 five

 

paper

 

 

 

 

 

 

 

plastic

 

 

 

 

 

 

 

lead

 

 

 

 

 

 

 

GAMMA SOURCE:

# absorber layers

zero

one  

two  

three

four

 five

 

paper

 

 

 

 

 

 

 

plastic

 

 

 

 

 

 

 

lead

 

 

 

 

 

 

 

 CAMP LANTERN MANTLE:

# absorber layers

zero

one  

two  

three

four

 five

 

paper

 

 

 

 

 

 

 

plastic

 

 

 

 

 

 

 

lead

 

 

 

 

 

 

 

FIESTAWARE:

# absorber layers

zero

one  

two  

three

four

 five

 

paper

 

 

 

 

 

 

 

plastic

 

 

 

 

 

 

 

lead

 

 

 

 

 

 

 

 

In the DATA ANALYSIS for each source, graph the Intensity (# Counts - #Background Counts) versus the Absorption (mm's of Shielding) with all three types of absorbing material plotted on the same graph.

In the DATA ANALYSIS for the beta source, also graph ln [Intensity] (ln [# Counts - #Background Counts]) versus the Absorption (mm's of Shielding) with all three types of absorbing material plotted on the same graph.  For a source with single energy beta emissions, the absorption decrease is expected to be exponential and thus linear on a semi-log plot.


QUESTIONS

1.      Which type of radiation is the most penetrating?

2.      Which type of radiation is the least penetrating?

3.      What generalization can you make about the effect of the thickness of the shielding material on the count rate?

4.      What generalization can you make about the effect of the density of the shielding material on the count rate?

5.      Since the energy of the radiation is absorbed by the shield, what effect does the absorbed energy have on the shield?

6.      Why is there a difference in the penetrating ability of the three basic radiation types?

7.      How effective are other shielding materials such as air or water in stopping radiation?

8.      What material is the most effective in absorbing the energy of nuclear radiation?

9.      Describe the construction and operation of a Geiger-Muller tube.

10.  Perform an internet search on the radioactivity of camp lantern mantles.  Describe the radiation emitted by a lantern mantle.

11.  Perform an internet search on the radioactivity of Fiestaware.  Describe the radiation emitted by Fiestaware.