This is because the particles or rays become more spread out. The thicker the substance, the more the radiation is absorbed. Alpha, beta and gamma radiations penetrate materials in different ways.

alpha particles are helium nuclei consisting ... leaves mass number and atomic number unchanged; gamma rays produce the least ionization.

The Rays are getting Shane McClanahan back from ... Not every team is willing to get its alpha dog. The Red Sox, for a while, were one of the teams wringing its hands and biding its time.