Exploration in nuclear physics may require extreme conditions, such as temperatures down to a few Kelvin, high
magnetic fields of several Tesla, or the small physical dimensions of a few centimeters. As a standard technique for
radiation detection using scintillation materials, it is desirable to develop photodetectors that can operate under these
harsh conditions. Though photomultiplier tubes (PMTs) have been used for most applications for readout of scintillation
materials, they are bulky, highly susceptible to magnetic fields, and present a large heat load in cryogenic environments.
Avalanche photodiodes are a reasonable alternative to PMTs in that they are extremely compact and less susceptible to
magnetic fields. Avalanche photodiodes have been developed in a commercial CMOS process for operation at
temperatures below 100 Kelvin. Here we present the overall operation of the photodiodes at 5 Kelvin. The diodes show a
quantum efficiency of at least 30% at 532 nm at 5 Kelvin. At about 30 Kelvin, the diodes exhibit an internal resistive
term, which generates a second breakdown point. The prototype diode shows a proportional response to the intensity of
light pulses down to 150 detected photons with a hole to electron ionization ratio, k, of 2.3x10-13 at 5 Kelvin. The
properties of the photodiodes and the readout electronics will be discussed for general photon detection below 100 K.