A radiation detector head assembly is provided that includes a detector housing and a rotor assembly. The detector housing defines a cavity therein. The rotor assembly includes a detector unit, a body, and a sealing member. The body defines an opening oriented in the imaging direction. The body is disposed at a distance from the detector housing within the cavity defining a passageway extending axially along the body. The sealing member includes a body extending across the opening. The sealing member is coupled to at least one of the shielding unit or the collimator, and is mounted within the cavity to provide a gas-tight seal along the imaging direction between the passageway and the detector unit.