EJ-426 is an efficient detector for thermal neutrons with low sensitivity to gamma radiation. The detector has the form of a flat, white, thin sheet consisting of a homogeneous matrix of fine particles of lithium-6-fluoride (6LiF) and zinc sulfide phosphor (ZnS:Ag) compactly dispersed in a colorless binder. The lithium is enriched in 6Li to a minimum of 95 atom %.