White Paper: Noise In Silicon Photomultipliers And Vacuum Photomultiplier Tubes

By SensL

The ability to detect single photons represents the ultimate sensitivity in optical detection. To achieive such sensitivity a number of technologies have been developed and re ned to suit particular applications. These include Photomultiplier Tubes (PMTs), linear and Geiger mode Avalanche Photo Diodes (APDs), Charge Coupled Detectors (CCDs), Visible Light Photon Counting (VLPC) detectors and Hybrid Photon Detectors (HPD). The need for ever more sensitive, compact, rugged and inexpensive optical sensors in the visible region of the spectrum continues today and is particularly accute in the fields of the biological sciences, medicine, astronomy and high energy physics. Applications such as fluorescence and luminescence photometry, absorption spectroscopy, scintillator readout, LIDAR and quantum cryptography require extremely sensitive optical sensors often in adverse environments, such as high magnetic fields, and where space is limited.

In many of these applications the PMT has become the detector of choice. However, since its inception in the 1980's, the so-called silicon photomultiplier (SPM) has begun to rival the PMT in many of its parameters such as gain, photon detection efficiency and timing. In addition the SPM has all the additional benefits of silicon technology such as compactness and high volume, low cost production. One of the key parameters in determining the most suitable detector for a particular application is the signal to noise ratio. This is defined as the ratio of the signal current (photocurrent) or voltage to the inherent noise produced by the detector. In this paper, the signal to noise ratio of PMTs and SPMs is examined for different types of signal. The signal to noise ratio is determined for a continuous wave analogue measurement, a gated pulse measurement and a photon counting measurement for a high-end PMT and SensL's SPMMini and SPMMicro product. A full derivation of the mathematical equations used is also given.