Here is the full list of all 20 The Brookhaven Experiment achievements.
Kill four enemies at once with an explosive.
Survive the fourth campaign level.
Survive the third campaign level.
Survive the second campaign level.
Kill 1000 enemies.
Survive the first campaign level.
Kill 100 enemies.
Survive the fifth campaign level.
Survive the sixth campaign level.
Survive the seventh campaign level.
Survive the eight campaign level.
Survive the ninth campaign level.
Survive the final campaign level.
Finish a nighttime campaign level without turning on the flashlight.
Blow the head off of something large and toothy.
Survive ten waves in a survival map.
Acquire all guns and upgrades in campaign mode.
Acquire all guns and upgrades in a survival game.
Acquire 200 throwables in campaign mode.
Survive a nighttime campaign level using only the knife.