When a β− and a β+ particle meet and interact they are annihilated.
Every matter has an antimatter. An antimatter of a particular particle is another particle that has exactly the same properties as the initial particle but carries an opposite charge.
If we look at β− and β+ particles, we will notice that they are matter and antimatter. The interaction of matter and antimatter leads to annihilation. The energy produced by annihilation of matter and antimatter is enough to destroy the tissue.
So, when β− and a β+ particle meet and interact they are annihilated.
