Open-space aliens drop a piece of equipment in 0.4% of kills (elites 2%). Stronger aliens drop the better tiers more often. 4% of those pieces are Epic.
Dungeon aliens drop a piece in 3% of kills, its tier as above.
Rift Cache, won in the Rift: 30% of them hold a piece of this tier.
Upgraded at the Engineer up to +9 with Iridite, from +6 also Reactor Core. Chance of each step: +1 100%, +2 95%, +3 85%, +4 70%, +5 55%, +6 40%, +7 30%, +8 20%, +9 12%. A failed step past +3 takes one plus off.