In my fictional universe antimatter projectiles are regularly used in space warfare. I'm not asking, whether it is practical or not, but have three specific questions:
Impulse shaped charge: The idea is, that a warhead containing antimatter in it (like a photon torpedo) would approach the target at high velocity, and detonate. The conservation of impulse dictates, that the photons resulting from the annihilation, and traveling forward, have to have higher frequency, than those traveling backwards, thus the majority of the energy hits the target, instead of being dispersed in the space. By detonation at 0.1c how many percent of the energy would go into the forward hemisphere, in a case of pure electron-positron annihilation, and how many in the case of antihydrogen annihilation?
Naked antimatter projectile. The second question concerns a macroscopic sized projectile, constructed from solid antimatter, as suggested in the comments. How long could an 10 kg mass of antiwater ice/anti-iron/anti-depleted uranium travel in the interplanetary medium at the orbit of the Earth (which is not full vacuum) before losing half of it's mass, assuming 10000m/s velocity? And if I fire it in te interstellar medium? Or in the upper atmosphere of the Earth on 300 km altitude?
It seems to me, that it is quite hard to make up physically plausible forcefield shields. But I can imagine using magnetic field for a specific defensive purpose: Could a strong magnetic field be utilized to break the antimatter containment on enemy missiles, and thus made them explode far away? Or could they be easily coated?
If naked antimatter projectiles described in question 2. would be used regularly, would 'gas shields' mean a viable countermeasure? The idea is, that if the computer senses an incoming antimatter projectile, it emits gas from dedicated vents (or the maneuver thrusters in emergency cases) to annihilate and evaporate it before it hits.