Non-proportionality and Energy Resolution of CsI(Tl)

Experimental studies of CsI(Tl) crystals with different amount of Tl activator are presented. Such properties like non-proportionality, energy resolution and 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 size 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 much more proportional character of the crystal response, higher number of photoelectrons/MeV-gamma, and what follows better overall energy resolution obtained for 12 mus time constant for all tested samples. Calculated intrinsic resolution improved at longer shaping time constant as well, indicating that for CsI(Tl) crystals more proportional response implies better intrinsic energy resolution of the host crystal. In contrast, intrinsic resolution at 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 to an explanation of the intrinsic resolution deterioration for low energy photons at the 12 mus integration time.