Temperature dependence of cesium iodine(thallium) gamma ray excited scintillation characteristics.

Abstract

The gamma-ray excited, temperature dependent scintillation characteristics of CsI(T1) are reported over the temperature range of $-$100 to +50$\sp\circ$C. The modified Bollinger-Thomas and shaped square wave methods were used to measure the rise and decay times. Emission spectra were measured using a monochromator and corrected for monochromator and photocathode spectral efficiencies. The shaped square wave method was also used to determine the scintillation yield as was a current mode method. The thermoluminescence emission of CsI(T1) were measured using the same current mode method. At room temperature, CsI(T1) was found to have two primary decay components with decay time constants of $\tau\sb1$ = 679 $\pm$ 9 ns (63.7%) and $\tau\sb2$ = 3.34 $\pm$ 0.10 $\mu$s (36.1%), an absolute scintillation yield of 65,500 $\pm$ 4,100 photons/MeV, and emission bands at about 400 and 560 nm. The $\tau\sb1$ luminescent state was observed to be populated by an exponential process with a resulting rise time constant of 19.6 $\pm$ 1.9 ns at room temperature. An ultra-fast decay component with a $<$0.5 ns decay time was found to emit about 0.2% of the total scintillation light (about 100 photons/MeV). Except for the ultra-fast decay time, the rise and decay time constants were observed to increase exponentially with inverse temperature. At $-$80$\sp\circ$C, $\tau\sb1$ and $\tau\sb2$ were determined to be 2.22 $\pm$ 0.31 $\mu$s and 18.0 $\pm$ 1.44 $\mu$s, respectively, while the 400 nm emission band was not observed below $-$50$\sp\circ$C. At +50$\sp\circ$C, the decay constants were found to be 628 $\pm$ 3 ns (70.5%) and 2.63 $\pm$ 0.03 $\mu$s (29.3%) and both emission bands are present. The scintillation yield of CsI(T1) was observed to be only slightly temperature dependent between $-$40 and +50$\sp\circ$C, peaking at about $-$30$\sp\circ$C (about 6% above the room temperature yield). Four different commercially-available CsI(T1) crystals were used. Minimal variations in the measured scintillation characteristics were observed among these four crystals. Thermoluminescence emissions were observed to have peak yields at $-$90$\sp\circ$C, $-$65$\sp\circ$C, $-$40$\sp\circ$C, +20$\sp\circ$C, and possibly $-$55$\sp\circ$C. The relative magnitudes and number of thermoluminescence peaks were found to vary from crystal to crystal.