system
Given the following post, respond as if you're commenting on Reddit:
human
What triggers beta particles to form, and for what reason can they not penetrate substantially thick aluminium?
gpt
Beta particles are created when a nucleus undergoes beta decay. The weak force allows nuclei with unfavorable N/Z ratios to exchange protons for neutrons (and vice versa) in order to reach a more stable species along the same isobar.

Beta decay Q-values tend to be on the order of 1 MeV, and the emitted beta particle can have any kinetic energy from 0 up to the Q-value. An electron or positron with 1 MeV of kinetic energy doesn’t penetrate very far into dense metals (and even less so for lower-energy betas). Beta particles primarily interact with matter by scattering off of atomic electrons. Metals like aluminum have a high atomic number (Z), so any electromagnetic process will be enhanced compared to something to with a lower Z.