Ambient-Background Radiation (effects on raw stock)
Ambient gamma radiation is composed of two sources: a low-energy component which arises from the decay of radionuclides and a high-energy component which is the product of the interaction of cosmic rays with the earth’s upper atmosphere. The radionuclides responsible for the low-energy photons exist in soil and rock and are carried into earth-derived building materials, such as concrete. Upon exposure to ambient-background radiation, photo- graphic negative materials can exhibit an increase in minimum density, a loss in contrast and speed in the dark areas, and an increase in granularity. The changes in film performance are determined by several factors, such as the film speed and length of time exposed to the radiation before the film is processed. A film with an exposure index of 500 can exhibit about three times the change in performance as a film with an index of 125. While this effect on film raw stock is not immediate, it is one reason why we suggest exposing and processing film as soon as possible after purchase. We recommend a period of no more than six months from the time of film purchase before processing, provided it has been kept under spec- ified conditions. Extended periods beyond six months may affect faster speed films as noted above, even if kept frozen. The only way to determine the specific effect of ambient-background radiation is with actual testing or measurements and placing a detector in the locations where the film was stored. The most obvious clue is the observance of increased granularity, especially in the light areas of the scene.
Protection from Physical Damage
Keep films away from heating pipes and direct sunlight, even if the room is air-conditioned. Maintain uniform room temperature throughout the storage area by means of adequate air circulation. If the building is not fireproof, install an automatic fire-extinguisher system. Design storage rooms for motion picture raw stock so that film is at least 6 in. (15 cm) off the floor.