Digital Silicon Photomultiplier Array Readout Integrated Circuits

Silicon photomultipliers (SiPMs) have recently gained considerable interest as replacements for photomultiplier tubes. Like photomultiplier tubes, they are capable of measuring extremely low light levels, to the point of being able to detect single photons. However, compared to photomultiplier tubes, SiPMs offer the further solidstate advantages of lower operating voltages, ruggedness, smaller physical size, lighter weight and excellent immunity to magnetic fields; SiPMs can also achieve GHz count rates. Nevertheless, SiPM detector subelements are tied to one another in parallel, putting considerable constraints on the array size, pixel resolution, and dark count performance. In these arrays, passive quenching of the avalanche events does little to mediate the occurrence of after pulse events. These factors all limit the signaltonoise performance of SiPM detectors. Furthermore, the design of current SiPM devices also limits the integration of onfocalplane digital processing. The lowpower CMOS electronics necessary to control and operate the detectors of a silicon Geiger mode avalanche photodiode (GmAPD) array will be developed. The digital readout integrated circuit (ROIC) consists of inpixel circuits that allow each photon detection event to be converted directly into an ultrahighspeed digital pulse that can be directly counted by onchip timing circuitry. When integrated with the GmAPD array, the digital ROIC will allow the silicon photomultiplier to be operated as an all digital device (digitalin/digitalout). As a result, faster and more accurate photon counts are achievable with extremely well defined timing of the photon detection events. Through the programmable digital command register, pixels can be adjusted for optimal performance, thereby improving detector performance and reliability, and decreasing cost.