Cesium iodide crystal activated with thallium (CsI(Tl)) is used as radiation sensor because of its favorable characteristics as scintillator when excited by gamma radiation. This crystal has good mechanical strength and it is not hygroscopic. In the present work the CsI(Tl) crystal was growed in the Nuclear Energy Research Institute (IPEN/CNEN/SP) by Brigdman technique in different sizes. The scintillator response was studied through gamma radiation from 99mTc source with the energy of 140 keV. The crystals were coupled to a photomultiplier tube using 0.5 McStokes viscosity silicone grease as the optical interface. All electronics for signal measurements were developed at IPEN. Measurements of luminescence and gamma spectrometry of a 99mTc source were performed. The energy resolution of the crystals was determined by the spectrum photopeak considering its full width at half maximum (FWHM).