Pulse shape analysis of signals from a CsI(T1)/photodiode detector

Use of 1 cm2 CsI(T1) scintillators with photodiode readout in the hard X-ray band is currently limited to above ∼ 40 keV by the lack of intrinsic gain in the photodiode and the electronic noise in the readout system. However, below this energy the photodiode itself becomes 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(T1). We have demonstrated that separation of the two types of event is certainly possible, and the use of an appropriate spectral reconstruction algorithm enables us to normalise the signals from the two 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 both simulations and laboratory measurements, the optimisation which can 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.