Radioactive Shielding

Caution: This experiment should be done under the supervision of a science teacher at a High School Physics laboratory. Handle the radioactive materials with tweezers, tongs, or gloves. You should not consume any food, drink or make-up during this experiment. You should wash your hands with soap and water after doing the experiment.

Materials:

1. Geiger Counter
2. alpha, beta, and gamma sources available from science supply catalogs
3. 5 cm x 5 cm pieces of cardboard, aluminum and lead
4. tweezers, tongs, or gloves
5. stopwatch or electronic counter

Space Settlement Relevance
To distinguish between alpha, beta, and gamma radiation by comparing the ability of these radioactive sources to penetrate different materials

Concepts
Alpha, beta or gamma radiation is released when changes take place in the nucleus of an atom. Alpha particle is a helium nucleus composed of two neutrons and two protons. Beta particle is a high energy electron, and gamma ray is a high-energy photon. The mechanics of radiation absorption vary with the type of radioactive source, the initial energy of the radioactive particle or ray, and type of absorbing material. There is a close relationship between the initial energy of the alpha particle and its penetration of a particular absorbing material. However, because the alpha particle is a helium nucleus with a relative heaviness, an alpha particle is quickly absorbed. Since beta particles have the same mass as the electrons in the absorber, the beta particle is deflected in collisions with other electrons; it does not follow a well-defined path through the material. For beta particles, penetration is inversely proportional to the density of the absorbing material.

While charged particles gradually lose their energy in many collisions, photons lose all their energy in a single collision. Therefore, absorption of gamma rays is defined in terms of the absorption photons in a beam by a certain percentage. Thus, the intensity of a gamma ray is reduced exponentially as it penetrates a given material. In this experiment, you will measure and compare the penetrating ability of alpha, beta, gamma radiation through cardboard, aluminum, and lead absorbers.

Procedure: see caution above

1. Set up Geiger counter according to your teacher's instructions.
2. In order to obtain the many required readings in the short period of time available, you will make counts for only ten seconds. Since radioactive decay is spontaneous and does not always occur at a constant rate, it is possible that some of the measured activities may be a little larger or smaller than expected. Use a stopwatch to time the counting. Turn on the counter and measure the background radiation for ten seconds. Record this value.
3. Place the alpha sample under the Geiger counter. Measure the activity for ten seconds and record the value. Place two sheets of cardboard on top of the alpha source and measure the activity for ten seconds. Record the value. Continue adding layers of cardboard in pairs until the activity drops to that of the background radiation level. Record all values.
4. Remove the cardboard and repeat the process with aluminum sheets. Record the values of activity. Once the measured activity drops to that of the background radiation level, stop measuring.
5. Remove the alpha source and repeat the procedure with the beta source, testing the shielding capacity of cardboard, aluminum, and lead. Record the values of activity. Once the measured activity drops to that of the background radiation level, stop measuring.
6. Replace the beta source with the gamma source. Repeat the procedure with layers of cardboard, aluminum, and lead and record the values.
7. Return the radioactive sources to their appropriate storage containers, as instructed by the teacher.

Data:
Background radiation = _______________ counts per 10 seconds (c/10s)

				Measured Activity (c/10 s)
		Cardboard			Aluminum		Lead
Number of sheets	alpha	beta	gamma	alpha	beta	gamma	beta	gamma
0	...............	...............	...............	...............	...............	...............	...............	...............
2	...............	...............	...............	...............	...............	...............	...............	...............
4	...............	...............	...............	...............	...............	...............	...............	...............
6	...............	...............	...............	...............	...............	...............	...............	...............
8	...............	...............	...............	...............	...............	...............	...............	...............
10	...............	...............	...............	...............	...............	...............	...............	...............

Questions:

1. Which type of radiation is most easily absorbed? Which type of radiation is least easily absorbed?
2. How much and what type of material is required to reduce the various radiation sources to one-half of their activity?
3. Explain how to identify an unknown radiation source.
4. Was it possible in this experiment to eliminate all of the gamma radiation? Explain.
5. How much lead would be required to reduce to one fourth the intensity of radiation from a gamma source?

Author: Tugrul Sezen
sezen@nas.nasa.gov
Date Last Modified: 3/23/99

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