Stalker: Call of Pripyat → Quests → Protecting the Scientists
The quest can be taken by talking to Professor Herman after completing the quest " Latest Developments ".
Walkthrough of the quest "Protecting Scientists"
The easiest option is to talk to the leaders of Duty or Freedom. Both will happily agree to send soldiers to defend the bunker.
In the substation workshops at Zaton, there's a group of hungry mercenaries led by Tesak. After you bring them food (they won't let you near them until then), you can offer them work for the scientists.
Spartak's squad from "Shevchenko." Recruitment is only possible if you sided with the free stalkers in the "Race" quest. Furthermore, Spartak will not agree to guard the bunker until you complete the "Deal" quest (on Boroda's side). Reward
Regardless of your choice, Professor Herman will reward you with 4,000 RU and some medicine.
Source: ru.stalker.wikia.com
See also:
Stalker CoP helmets stats and info