*2.1.1.2. The scintillator crystal*

The *γ*-camera crystals are generally composed of NaI(Tl). Features that make this crystal desirable include high mass density and atomic number (Z), thereby effectively stopping *γ* photons, and high efficiency of light output [3, 4]. The most important characteristics of the crystal that must be ensured are: 1) high detection efficiency, 2) high energy resolution, 3). low decay constant time and a light refraction index close to the glass one. Most current cameras incorporate large (50 cm×60 cm) rectangular detectors. While expensive, the larger field of view results in increased efficiency. In early designs, crystals were often 0.5 inches thick, which was well-suited for high energy *γ* photons. In more recent implementations of the *γ*-camera, crystals only 3/8-inch or 1/4-inch thick are used, which is more than adequate for stopping the predominantly low-energy photons in common use today and which also results in superior intrinsic spatial resolution.
