This reference may be useful.
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You may have some difficulty finding a radioactive source with a half-life short enough to measure. Most are used in nuclear medicine, with carefully limited distributiuon.
My son once did a science fair experient like yours, using wadded-up Coleman lantern mantles as a source. They contain radioactive thorium. The half life was too long to measure, so he tried to verify the inverse square law. He used a Heathkit Geiger counter that I once made and still have. Background radiation complicated the experiment.
He did not get an A, was not happy about that, and I have avoided giving Science Fair advice ever since. If you have some specific questions, we have several teachers here who will be glad to help.
I'm working on the "Introduction to radioactivity and half-life experiment". It's harder than I thought. Any references that you know of that might make this easier to understand?
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