When a sheet of paper is placed in the path of a radioactive source, the radiation emitted, which passes through the paper consists of

Alpha particles are large and heavily charged, so a sheet of paper stops them. Beta particles are small and fast and pass through paper. They are stopped by a thin sheet of aluminium. Gamma rays are very penetrating and pass through paper easily. They need thick lead to block them.

So what passes through the paper is beta particles and gamma rays.