✔ 最佳答案
At the same time you could ask why are different types of radiation stopped by these barriers. Alphas (flying He nuclei) can be stopped by most anything, paper, skin. They deposit their energy in a short space. Betas go a little further because they are less massive, water will stop them. Gammas are massless and are very energetic. They are not spoken of as stopped, but as attenuated. Lead, being very dense as a barrier, can't stop gammas, only attenuate them. This means cut down the intensity of the beam of gammas. The importance of knowing the stopping ability is in shielding designs for nuclear reactors. A combination of shielding material is used to protect workers and the public in general.
參考: I was a radiation worker years ago.