A positron (
) is a particle that has the same mass as an electron and has a charge of 
. A positron will interact with an electron to form two 
-ray photons.
Assuming that the kinetic energy of the positron and the electron is negligible when they interact,
suggest why the two photons will move off in opposite directions with equal energies,
calculate the energy, in 
, of one of the -ray photons.
.
