Isotopes with unstable nuclei are __1__ and are called __2__. The __3__ of radioisotopes decay to __4__nuclei plus emission of large amounts of __5__ . The radiation may be alpha, __6__, or gamma. __7__ radiation consists of alpha particles (positively charged __8__ nuclei) that are easily stopped by a sheet of paper. Beta radiation is composed of fast-moving particles, which are __9__. Beta radiation is more penetrating than alpha radiation; it is stopped by __10__. __11__ radiation is electromagnetic radiation. Gamma radiation has no __12__ or electrical charge. It is extremely penetrating. __13__ bricks and __14__ reduce the intensity of gamma radiation but do not completely __15__ it.