The Silicon Photomultiplier (SiPM) is operated using a few Ortec Nuclear Instrument Modules in room B309. Specifically, The model 113 preamplifier, the 570 amplifier, and the 927 Aspec MCA that has a USB port in the back of it for data collection. There are 3 SMA ports that are available to use in the front of the box (Figure 1). Currently the third SMA port is not being used but can be in the future and is okay to be left alone.
SMA port 1 is used to apply bias to the SiPM. This is to be negatively biased only. The power supply used is a BK Precision 9110. This was specifically used because it allows control of how much current the SiPM will get. In the event the cover of the box is removed, the exposed SiPM won’t be fried. The second and third SMA ports are used for signal out of the SiPM. Specifically, it is wired to read signal out of SMA port 2.
One of the unique aspects of the SiPM is the avalanche effect that occurs in Geiger-mode. The SiPM will essentially operate as an insulator under reverse bias until a voltage threshold is met. The internal electric field will increase with increasing voltage and eventually it will cause the SiPM to breakdown and act as a conductor and allow for the avalanche process to take over. Past that voltage threshold is when the SiPM is said to be operating in Geiger-mode and that is when current will flow.
SensL has made this array to have a threshold at 24.5 volts. This is only the voltage where the onset of avalanche will cause current to flow, not necessarily enough current to get useful information. Further increasing the voltage, increases the electric field inside the depletion region of the p-n junction. This electric field is high enough that electron-hole pairs created in the depletion region have enough energy to cause impact ionization and create more electron-hole pairs and those pairs create more pairs under acceleration by the electric field. This is the avalanche effect that occurs in Geiger-mode. SiPMs operate above this voltage threshold, called overvoltage. Recommended overvoltage by SensL is 1 to 6 volts. The spectra taken below was at 29.25 volts.