The scientific journal is located in the North-West room of the main building in Sector 12.
There is a combination for the safe to the journal from 3 torn pieces of paper from sectors 47, 44 and 32 (spoiler). If not, you could try the following:
1) Use an Explosive to open the wall next to the safe. (2) Have a very strong merc like Mike waiting in the corner and using a lock kit to open the safe, then move one step away. (3) A third merc comes and picks up the journal
If you merged the combination from the scraps of paper, you will get the safe combination and need not do the above steps. Simply put the combined paper in the merc's main hand and apply it to the safe.
After your team retrieve the Journal, the day will END!
Further note: The enemy will burn the journal to crisp if they see or hear you. So like Processing Plant missions, you have to fall back to Silencer-armed short rage pistols. Even if you recover the burnt journal, it will have little effect--Brenda would just say it was hers and you already know from Sector 33 than Santino is creating a Fallow tree sapling which you will find in Sector 1.
The journal is critical in relation to the Find a cure for the Fallow Virus mission as Brenda will say she needs the journal in order to create the antidote from the Pomillia Flowers you may have found from sector 8. When you heard that, you either have to race to get this journal from sector 12 or blow up an infected Fallow Tree/Trees.
Sadly no monetary awards or boost to guards. You may only hear Brenda's joy on retrieving her journal and some notification of the baby fallow free, which you would have heard from Hervos' note in Sector 33.
However, ending the day earlier without any penalty is helpful, as it allows you to load your "mules" and ship a lot of equipment right back to camp.
(Spoiler)The outcome of this quest it will affect the final cut scene, whether you recover it intact, recover it burnt, or fail to recover it at all. Furthermore, recovering it intact will also affect the outcome of the fallow tree recovery line in the same final cutscene.(Spoiler)