How to Measure Radioactive Decay With Helium
Helium can be used to help determine the age of any mineral that contains uranium or thorium by measuring its rate of radioactive decay. This method is informally known as helium dating and the following steps will show how it is performed.
- Difficulty:
- Challenging
Instructions
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1
Study the radioactive decay sequence of uranium-235, uranium-238 and thorium-232. These elements emit alpha particles that are the nuclei of helium atoms: two protons and two neutrons. These particles will quickly retain two electrons to become helium atoms.
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2
Examine rocks containing uranium and thorium that can retain helium for a long period of time. Their helium content will increase over time and because radioactive decay occurs at a constant rate, the ratio of helium to uranium and thorium can tell us the age of that rock. This specific method is called uranium-thorium-helium or (U-Th)/He dating.
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3
Look at rocks containing uranium and thorium that do not retain helium for a long period time. The radioactive decay of uranium and thorium only produces helium-4, so the rock can still be dated by measuring its ratio of helium-3 to helium-4. This particular method is called alpha-helium dating.
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4
Use (U-Th)/He dating on younger rocks with a very high helium retention. Basalt that is no older than the late Cenozoic period is the best choice for (U-Th)/He dating.
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5
Utilize (U-Th)/He dating to study a rock's thermal history. It is important to handle the specimen properly when performing this test so that the helium does not leak out during grinding or other steps.
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