The Buzz about Antimatter: More about the Cloud Chamber

Conservation of momentum:
Relativistic mechanics shows that the conservation of momentum in gamma ray annihilation requires the presence of a massive nucleus nearby to carry off some of the photon's momentum. Since its mass is very large, this nucleus acquires only a small velocity in this process.

Conservation of energy:
The rest mass energy of the electron and of the positron is .511 MeV (million electron volts). Consequently, the photon must have an energy of at least 1.022 MeV to create this pair. Any additional photon energy goes into the kinetic energy of the electron and positron. Since the massive nucleus nearby acquires only a small velocity compared to the electron and positron, it carries off very little kinetic energy (which is proportional to the square of the speed).

Since the presence of the nearby nucleus is required for the photon annihilation, and since space is an excellent vacuum, gamma rays travel through space with an essentially infinite lifetime. Also, since the minimum photon energy for this annihilation is about 1 MeV, forms of electromagnetic energy with smaller photon energies do not annihilate when passing through matter. Imagine the effect on the history of life if light photons annihilated while passing through the waters of the oceans!