Quick reminder for people who have forgotten: alpha particles are Helium nuclei. They are big, slow, and are easily absorbed by a couple of centimeters of air.

Quick reminder for people who have forgotten: alpha particles are Helium nuclei. They are big, slow, and are easily absorbed by a couple of centimeters of air. beta particles are high speed electrons (or positrons). They can be stopped by a couple of millimeters of aluminum, but they emit gamma radiation while being slowed. gamma radiation is high energy photons, everything stronger than X-rays. You need a lot of dense material to stop them. Some elements, when absorbing gamma photons, will emit a free neutron. Slow neutrons get absorbed by atoms; fast neutrons knock atoms apart. Either of these can produce unstable isotopes that may themselves emit radiation.