"Radiation" is litterally any sub-atomic particle moving so fast that it can not be connected to anything else until it collides with something and transfers its kinetic energy. This means all frequencies of light, electons, atomic nuclei, their constituents or the antimatter equivalents of any of these particles are radiation.
In this way you can correctly say that antimatter reactions create nothing but radiation. Proton-antiproton reactions tend to make gamma-rays and pions. (Pions are a class of particles made up of quarks. Basically, quarks don't like to be alone so they always pair up. But when energies are high, they pair up in random ways, ignoring everything about the partner. Instead of trying to list all the different possible combinations, it's easier to just say, "pions".)
And with that kind of particle zoo exploding outwards, any particle you don't capture to use for power will strike a nucleus somewhere and put it in an excited state, making it at least temporarily radioactive. And, in some cases, will take a stable nucleus and set it down the road of a decay chain, making it spew out particles several times before it can get back to a new stable state.
I didn't want to just say, "Yes. M/AM is radioactive." I wanted to describe what that means and why. I hope I succeeded.