To use radioactive dating for a substance
Typically the substance comes out of the gas and is absorbed on to a solid or liquid, which is then analyzed by a detector.
This means that it is barely radioactive, less so than many other isotopes in rocks and sand.
It wasn't until well into the 20th century that enough information had accumulated about the rate of radioactive decay that the age of rocks and fossils in number of years could be determined through radiometric age dating.
This activity on determining age of rocks and fossils is intended for 8th or 9th grade students.
Nevertheless it generates 0.1 watts/tonne as decay heat and this is enough to warm the Earth's core. The nucleus of the U-235 atom comprises 92 protons and 143 neutrons (92 143 = 235).
When the nucleus of a U-235 atom captures a moving neutron it splits in two (fissions) and releases some energy in the form of heat, also two or three additional neutrons are thrown off.
2) To familiarize students with the concept of half-life in radioactive decay.