FYI:
Theory of Expired Film Life
A silver halide grain in photo film produces an electric charge when it absorbs a photon of light. This can also happen due to random thermal energy. However, there is one peculiar aspect to this: if the grain collects only one electronic charge, the charge is not stable, and decays back to a neutral state at a random time on the order of a few seconds. If the grain receives a second charge before the first decays, the combination is much more stable, and experiments have determined that, practically, four charges are required to make a fully stable latent image.
This requirement for accumulating multiple charges before the initial charge decays has several important results.
1) If the exposure rate is low (no light, low temperature), most charges will decay and the film will not be fogged significantly. Without this effect, it would never have been possible to make film that could be stored and sold to the public, because it would be continuously getting fogged from the moment it was manufactured!
2) Exposure to elevated temperatures over long periods will gradually fog the film, because a few grains from time to time will get more than one charge due to thermal excitation. So, refrigeration is helpful for long-term storage. (Note that elevated temperatures could have other bad effects on the composition of the emulsion and the sensitizing dyes that are used to make panchromatic film, but I claim no knowledge of the details or severity.)
3) As a result of the need for multiple charges within a few seconds, long exposures to low light scenes results in "reciprocity failure." Reciprocity in exposure is the normal inverse relationship between shutter speed and aperture. This fails when the number of photons striking the film per second is too low to make the required four charges in a grain before the first charge decays. This means that some of the light's effect is leaking away before the exposure is complete. So, if you have a film rated for 100 ISO at usual short exposures, it might lose half of the charges in a ten-second exposure, and you would compensate by using a twenty second exposure instead of ten seconds, or open the aperture by one stop; that is, the exposure time is no longer reciprocal to the aperture (the effective ISO has dropped to 50).
In digital sensors, every charge is captured, but all charge that may have accumulated while the sensor was unused is dumped electrically just prior to exposure. This means there will be some small random variation of charge due to thermal excitation during the exposure, but there is no danger of it accumulating slowly over months ahead of time, the way it can with film. This also means that digital sensors do not have reciprocity falure.
