A resilient locking and sealing arrangement for a multi-channel reflective cavity scintillation detector useful in a CT scanner. The absorption coefficient of the detector window is minimized in order to enhance detector efficiency, by forming such window of layers of woven graphite fiber bonded together with epoxy. The detector window includes resilient sealing means positioned to engage the leading edge of unit cell elements assembled to form a detector array. Resilient lock means force the plate into a reference position and into contact with the resilient sealing means thereby to accurately locate the plate in the array, and also to provide an effective light seal.