The Model 243, along with a pulse height analyzer system, is used for gamma ray assay and wipe test analysis. The Model 243 is shielded with 1.3 cm (0.5 in.) of lead to improve sensitivity by reducing background and other external radiation. The detector is an integrated 4.5 x 5.1 cm (1.8 x 2 in.) (Dia x thickness) NaI (Tl) crystal scintillator optically coupled to a 5.1 cm (2 in.) diameter, 10-stage photomultiplier tube.