Anti-matter is not something you want in any sort of tactical weapon. It has a tendency to make very big booms. A mere 100 grams (.1 kilograms), for comparison the M67 frag grenade has a filling of a ~180 grams, of anti-matter being annihilated would yield...
E=MC^2
E=(.1)(300,000,000 m/s)^2
E=(.1)(90,000,000,000,000,000)
E=9E15 joules
A kiloton is equal to 4.184 terajoules, that is 4.184E12 joules. Therefore the complete annihilation of the 100 grams of anti-matter would cause cause an explosion of...
9E15 / 4.184E12 = 2151 kilotons, or 2.2 megatons. However, nothing is 100% efficient, let us assume just 60% efficiency. That gives us 1.4 megatons.
Congratulations, your grenade sized anti-matter bomb just wiped-out the aliens along with your entire team, as well as all civilians and structures for kilometres around. If you're going to be flinging nukes it would be much better to use very small nuclear weapons, probably shot from recoil-less rifles, as combat damage won't accidentally initiate them and they are far cheaper. Of course, if you are willing to level an area with sub-kiloton warheads then why bother sending in a ground troops at all? That's what bomb trucks filled with conventional explosives are for.
Incidentally, we could employ nuclear tipped air-to-air and ground-to-air missiles to down UFOs. The real world has had those since the late 50s or early 60s. No need for direct hits, close enough is good enough :twisted: .
(I'm joking of course, PHALANX would probably be scared shitless of nuking the aliens because the aliens might just nuke back.)