The energy equivalence of mass 

The mass:energy relationship ... E = mc2 ... allows the energy equivalent of the electron mass to be calculated in Joules, electron Volts, MeV etc.

Energy = mc2 / (1.6x10-19) eV

= 9.11x10-31 x (3.00x108)2 / (1.6x10-19) eV

= 51x10(-31 + 16 + 19)

= 0.51 MeV ... for an electron.

Electron/positron annihilation creates a single gamma ray photon with energy hf.

hf = 1.02x106 x 1.6x10-19 Joules

The gamma ray wavelength is given by l = v/f as....

l = 3.00x108 x 6.63x10-34 / (106 x 1.6x10-19)

= 12x10(8 - 6 - 34 +19)

= 1.2x10-14 meters

... four orders of magnitude shorter than X-rays.