PULSE-SHAPE ANALYSIS OF SIGNALS FROM A CSI(TL) PHOTODIODE DETECTOR

Use of 1 cm(2) CsI(TI) scintillators with photodiode readout in the ha rd X-ray band is currently limited to above similar to 40 keV by the l ack of intrinsic gain in the photodiode and the electronic noise in th e readout system. However, below this energy the photodiode itself bec omes a practical X-ray detector. We report on the use of a pulse-shape analysis technique in an attempt to separate the signals from direct X-ray interactions in the photodiode and from scintillation events in the CsI(TI). We have demonstrated that separation of the two types of event is certainly possible, and the use of an appropriate spectral re construction algorithm enables us to normalise the signals from the tw o detectors. In this way we have constructed a hybrid detector capable of operating in the energy range 10 keV to 1 MeV. We discuss, using b oth simulations and laboratory measurements, the optimisation which ca n be carried out by changing the thickness of the silicon photodiode. We present details of the pulse-shape analysis system, demonstrate the identification and separation of the two types of signal, and present energy spectra for a range of energies.