|The following is based on Fallout Tactics and some details might contradict canon.|
The severed head is that of an unfortunate Vault 0 scientist. It can be used on the dental scanner to open the gates to the power plant section of Vault 0.
- It can be acquired from a dead scientist in the Cryogenics section of Vault 0.
- There is also a terminal, which you can use for the same purpose, in the Cryogenics section.