# Pozitron-Electron interaction(anihilation)

## Homework Statement

A given pozitron with T kinetic energy is approaching an electron bound to a hard nucleus(in the K energy level, and the ionization energy for this level, Ek(>0), is given).
The energy of the two resulting photons is to be determined.

## Homework Equations

Conservation of energy.

## The Attempt at a Solution

So I wrote the conservation of energy as follows:
2m0c2 + T = 2ε + Ek ( ε is the energy of one photon )
Let me explain my way of thinking: After the anihilation, the electron has vanished, so the atom has become a ion(so it's energy has increased by Ek).

However, the proposers of the problem consider the conservation of energy as follows:
2m0c2 + T + Ek = 2ε;

So who is right?

## Answers and Replies

mfb
Mentor
If Ek is defined to be positive, it should be subtracted on the left side (or added on the right side).
Otherwise you can violate energy conservation: Take a free electron, let it get caught by a nucleus (emitting energy). Add a free positron, and get more than 2*511keV as energy => you gained the binding energy two times.