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.
Beta particles are electron particles ejected from an atom through a nuclear decay event- either Beta+ decay (positron) or Beta- decay (electron). They otherwise behave exactly as electrons do, so I will refer to electrons from now on.
Any moving charged particles interacts with all matter at all time through the electrical (Coulomb) forces between them and the individual nuclei/electrons of every atom. Essentially, the charged particle in motion excites the electrons of the atom, resulting in a transfer of energy ("soft" collision). If the charged particle in motion goes sufficiently close to a nucleus, it can forcibly eject many atomic electrons out of their atom ("hard" collision) or even interact directly with the nucleus (knock-on collision, producing "braking radiation" or bremsstrahlung)
For electrons (Beta particles), we use the "continuously slowing down" approximation (CSDA). A rule of thumb at higher energies that the CSDA range is roughly ~0.5 cm/MeV in water-equivalent material.
What matters for electron interactions is essentially the density of the material, and the atomic number. You want the highest density but the lowest atomic number possible. Usually these are at odds with each other. Electrons can interact through Bremstrallung and produce x-rays when the attenuating material Z is higher, but these materials are also denser so you can pack more in a lower volume. It's a trade-off. Aluminum is considered to be a low-Z metal, and so is Beryllium. Lead is what is typically used when you want to maximize density.
Beta particles are typically created at very low energies (<1 MeV). At that energy, the CSDA range is sub-millimeter to just a few mm. You wouldn't even need substantially thick aluminum - a sheet of aluminum foil ought to do it for many isotopes. Typically in an x-ray tube they use ~2-3 mm of beryllium for this reason - it minimally impacts the x-ray beam, but prevents electron contamination from reaching the patient.