why aplha can't through paper?

Alpha decay is a type of radioactive decay in which an atom emits an alpha particle (two protons and two neutrons bound together into a particle identical to a helium nucleus) and transforms (or 'decays') into an atom with a mass number 4 less and atomic number 2 less.
An alpha particle is identical to a helium nucleus, and both mass number and atomic number are the same. Alpha decay is a form of nuclear fission where the parent atom splits into two daughter products.
Alpha particles have a typical kinetic energy of 5 MeV (that is ≈0.13% of their total energy, i.e. 110 TJ/kg) and a speed of 15,000 km/s. This corresponds to a speed of around 0.05c. Because of their relatively large mass, +2 charge and relatively low velocity, they are very likely to interact with other atoms and lose their energy, so they are effectively absorbed within a few centimeters of air. As any heavy charged particle, alpha particles lose their energy within a very short distance in dense media. So with a piece of paper (cardboard be better) with enough density, alpha particles can be shielded and absorbed.
I hope this can help your understanding.