Geiger-mode avalanche-photodiodes as readout for the CALIFA detector

The CALIFA spectrometer, which surrounds the RB target, will be used for detection of gamma-rays and highenergy protons. Of particular interest for this detector the energy resolution and its dependence on different configurations and components has to be studied. Cs(Tl) crystals with photomultiplier tubes (PMT) or avalanchephotodiodes (APD) readout as well as LaBr3(Ce) so far with PMT readout only, show an energy resolution under 5% for 662 keV gamma-rays [1]. But PMTs are sensitive to magnetic fields and, therefore, cannot be used near the GLAD magnet. APDs show a strong dependence on temperature and bias voltage and require additional amplification. Therefore we consider SiPM [2] (Silicon Photomultiplier) to be another option for scintillator readout. Essentially this device consists of a large number of APDs (pixels) connected in parallel with an applied bias voltage above the breakdown value, hence it is operated in Geiger-mode. Each pixel acts like a digital device. In case an incident photon creates an electron-hole pair in its sensitive volume, an avalanche is initiated. The output of the whole device, however, is the sum of the individual APDs signals, thus representing an analog response until the number of incident photons is smaller than the overall number of pixels. Preserving the insensitivity to magnetic fields of APDs, SiPMs have an internal amplification comparable to PMTs and are less sensitive to temperature and bias variations. Drawbacks are a small size of the sensitive area and a dynamic range limited by the number of pixels. Using an array of SiPMs can overcome these drawbacks. SiPMs provide also a better time resolution which can be important for some CALIFA applications.