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Regarding the possiblity of a moon-shattering kaboom raised by the square-cube law, consider Tycho crater:

Its a biggun... >80km across, and clearly visible from Earth (possibly with the aid of binoculars, if your eyes aren't great, but even so) but there are plenty of larger craters on the Moon, which remains resolutely intact.

I wasn't able to find much good research on the nature of the Tycho impactor, but having a fiddle with the charmingly old-school-looking lunar crater simulator, I was able to come up with something about 12km across and travelling at about 15km/s for a total kinetic energy of ~1.5 x 10²³J, or ~36 teratonnes TNT equivalent. Even if it were wrong by two whole orders of magnitude, that's still a much bigger boom than the antimatter cooking off, and it'll do a much better job of shovelling regolith and rock around.

A tonne of antimatter will make a fearsome blast, but still small beans compared to a reasonably sized space rock. The moon isn't going anywhere.

An uncontrolled release of bulk antimatter is also going to result in some of it being blasted away from ground zero by radiation pressure without necessarily being annihilated immediately, as you'd want if it were an antimatter rocket or warhead.

Those early charged pions have a kinetic energy of >200MeV, which does exceed the atomic binding energy of most light elements. The interaction cross section with a nucleus will be small, and the chances of the pion scattering off the nucleus will be reasonable, but some proportion of nuclei in the matter surrounding the blast might in fact be fissioned. Similarly, the >200MeV gamma rays coming out of neutral pion decays can also cause photodisintegration of nuclei.

Certainly, nuclei as light as aluminium might be smashed up by this process... possibly slightly heavier nuclei too, but I'm unclear on that. The resulting fragments may be radioactive, and they may be long lived unstable isotopes. The amount of induced radioactivity is obviously going to be nonzero, but it isn't clear how much there will be (and the question might be too hard to answer).

An uncontrolled release of bulk antimatter is also going to result in some of it being blasted away from ground zero by radiation pressure without necessarily being annihilated immediately, as you'd want if it were an antimatter rocket or warhead.

(shown above: pions signified by π, positive negative and neutral, muons signified by μ, positive and negative, electrons and positons signified by e+ and e-, neutrinos of various flavours shown as ν and finally plain old gamma rays as λ)

Those short-lived decay products can and will interact with other matter around them.

For that matter, an uncontrolled release of bulk antimatter is going to result in some of it being blasted away from ground zero by radiation pressure... this means you've got an expanding shell of ionised antimatter that can also interact with surrounding material. Individual antiparticles will eventually interact with a matter particle, but obviously a single antiproton or antineutron cannot annihilate anything larger than a hydrogen-1 atom. Anything larger will end up being being transmuted into a different isotope or element, and the resulting release of energy by the annihilation will be soaked up by spectator nucleons which may in turn cause the atom to fission.

The mostly likely reaction involves the production of some neutral pions (shown as π⁰) which decay almost immediately into high-energy gamma rays, and also some charged pions (shown as π⁺ and π^-) which have a short lifetime but are also travelling exceedingly fast and can therefore interact with nearby matter. The pions decay into charged muons (shown as μ⁺ and μ^-) which are even more stable, and can travel for quite some distance in a vacuum. These, too, can interact with surrounding matter. The pions eventually decay into electrons and positrons (shown as e⁺ and e^- respectively), the latter of which can annihilate an electron either produced by the decay chain or in surrounding matter producing more gamma rays

An uncontrolled release of bulk antimatter is also going to result in some of it being blasted away from ground zero by radiation pressure without necessarily being annihilated immediately, as you'd want if it were an antimatter rocket or warhead.

This means you've got an expanding shell of ionised antimatter that can also interact with surrounding material. Individual antiparticles will eventually interact with a matter particle, but obviously a single antiproton or antineutron cannot annihilate anything larger than a hydrogen-1 atom. Anything larger will end up being being transmuted into a different isotope or element, and the resulting release of energy by the annihilation will be soaked up by spectator nucleons which may in turn cause the atom to fission.