beta particle
noun /ˈbiːtə pɑːtɪkl/
  /ˈbeɪtə pɑːrtɪkl/
(also beta ray 
 (physics) /ˈbiːtə reɪ/
  /ˈbeɪtə reɪ/
)- a fast-moving electron that is produced when some radioactive substances decay (= start to break down)
- the properties of beta particles
 - beta particle emission