an improved radiation detector containing a crystalline mixture of laf 3 and cef 3 as the scintillator element is disclosed . scintillators made with from 25 % to 99 . 5 % laf 3 and the remainder cef 3 have been found to provide a balance of good stopping power , high light yield and short decay constant that is equal to or superior to other known scintillator materials , and which may be processed from natural starting materials containing both rare earth elements . the radiation detectors disclosed are favorably suited for use in general purpose detection and in positron emission tomography .