Psychic nullifier (Fallout)

A psychic nullifier is a strange device, possibly the product of an alien technology. It blocks telepathic or psychic commands.

Uses
It is placed on the head of a psyker to disable or prevent him or her from using his or her powers. A psychic nullifier also blocks incoming telepathic attacks when worn by a non-psyker, such as those frequently experienced in the Corridor of Revulsion. The Vault Dweller can obtain one to aid in passing through the Corridor of Revulsion on the way to the Master.

Locations
Gideon and the other three psykers each have nullifiers in the game. The Vault Dweller can kill Gideon or persuade him to give the "helm". The others can be obtained either through theft or killing them.