A positron -electron pair form a hydrogen like system. Applying Bohr's theory to this system and ignoring any relativistic effects, the distance (a) between the positron andelectron in the ground state is given by

For the position -electron system, the mass of the position = mass of the electron, much less than that of a proton . the distance in the ground state > aB (hydrogen atom)A careful calculation shows a ≃1.06 A0 ( = 2 aB)