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.
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.
|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|
|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|
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.