Non-Proportionality and Energy Resolution of CsI(Tl)

Experimental studies of CsI(Tl) crystals with different amounts of Tl activator are presented. Properties such as non-proportionality, energy resolution and the number of photoelectrons per 1 MeV-gamma were inspected at two different shaping time constants, i.e., 3 and 12 mus. A CsI(Tl) crystal was coupled to the XP5212 PMT with a photocathode blue sensitivity of 12.2 muA/lmF. Six CsI(Tl) crystals of different sizes were tested. Among them, three samples were of known Tl concentration, which amounted roughly 0.05, 0.1 and 0.25 mol%. The data analysis showed a much more proportional character of the crystal response, higher number of photoelectrons/MeV-gamma, and consequently, a better overall energy resolution obtained for 12 mus time constant for all tested samples. The calculated intrinsic resolution improved for longer shaping time constants as well, indicating that, for CsI(Tl) crystals, a more proportional response implies better intrinsic energy resolution of the host crystal. In contrast, the intrinsic resolution at the low energy region (below about 30 keV) deteriorated for the longer shaping time constant. An alpha/gamma ratio was also measured for each tested CsI(Tl) crystal in order to find any indication of an explanation of the intrinsic resolution deterioration for low energy photons at the 12 mus integration time.