A Geiger-mode avalanche photodiode detector that can detect ultralow-level light in a variety of applications has been introduced by Hamamatsu Corp. The S10362-11 series multipixel photon counter can be used for positron emission tomography, DNA sequencing, fluorescence measurement and environmental analysis applications. The device has an active area of 1 x 1 mm and is available in 100, 400 and 1600 pixels. Each pixel contains a quenching circuit so that simultaneous photon events can be counted separately and with a high degree of accuracy. Peak sensitivity is 400 nm, and gain is 1 x 106. The instrument is not sensitive to magnetic field effects.