Caesium iodide detector

The Imager consists of three layers, where the bottom two layers each consist of 2880 thallium doped caesium iodide (CsI(Tl)) scintillation crystals. Each of these 3 cm deep crystals is viewed by a photodiode. The CsI(Tl) crystals are hexagonal in cross section and have a cross sectional area of Icm, forming a pixelated array of independent detectors. 

Using this pulse-shape spectrum, the cross-over point between the two detectors was set at a rise-time of 3/ s, shown by the dotted line in the figure. Events with faster rise-times were analysed as coming from the photodiode and were essentially all due to 60 keV events all others were assumed to come from the caesium iodide scintillator. The two types of event were analysed individually in order to obtain raw energy spectra from each detector (also shown in Figure 2). The peaks in the energy spectra were then used to calibrate the energy scales for the two systems. 

---