Here is the full list of all 9 Alien Investigator achievements.
Reach investigator level 10
Reach investigator lvl 20
Reach investigator level 30
Find your first alien completing an investigation.
Complete 20 investigations
Upgrade your walkie talkie to the maximum level.
Visit the cornfield farm
Upgrade your ultraviolet light to the maximum level.
Upgrade your Geiger counter to the maximum level.