bourgouina/Decay-Chain ? reverse-engineered prompt

Build me a small Python project that models a radioactive decay chain and lets me explore how a parent isotope turns into daughter products over time.

I want it to be easy to run from the command line, with the code kept clean in a src folder and any sample input data in a data folder. It should read a simple chain definition, calculate how the amounts change step by step, and print or save the results in a way that is easy to understand. If it makes sense, add a basic plot or simple output file so I can see the decay over time.

Please make it feel complete and ready to use, with sensible defaults, a few example inputs, and clear instructions for how to run it. If you need to check current Python library docs online while building it, go ahead.