Simplify the following text for a Grade 15 native English-speaking person.
Original: Her achievements include the creation of a theory of radioactivity ( a term she coined ) , techniques for isolating radioactive isotopes , and the discovery of two new elements , polonium and radium .
Simplified: She created a theory of radioactivity ( a term made by her and Pierre ) , found different ways for separating radioactive isotopes , and discovered two new elements , radium and polonium .