Electron annihilation radiation

Electron annihilation radiation is the radiation caused by the annihilation of electrons by anti-matter.

Electron-positron annihilation produces two gamma rays with precisely an energy of 511 keV. Which means this is a dead giveaway for antimatter use. As you zip along in your antimatter powered rocket, everybody within a couple of light-years will be able to see a fool broadcasting the fact that their rocket contains militarily significant amounts of antimatter.

The gamma-ray flux from an antimatter annihilation can be strong enough to transmute some elements into radioactive isotopes. This happens by the photoneutron process. The cross-section of this is quite low, but the gamma-ray flux can be quite high. The charged pions may be short-lived, but they have a high cross-section and will do all sorts of interesting things to atomic nuclei. The higher the mass of the element transmuted, the longer lived it is as a radioisotope.