The resolution of CsI(Tl) for Po210alpha particles has been measured as a function of crystal thickness. The best resolution of a ½‐in. diam cylindrical crystal was obtained for a thickness of 0.38 mm, and the effect of thickness on the resolution is discussed. Based on the proposed model, a conical crystal was designed, which yielded a line width of 1.8% for Po210alpha particles with a selected photomultiplier tube.