Depends on the location. An old salt mine would be ideal. However some minerals like granite are radioactive. I would never have granite counter tops in my house. They will peg the needle of a Geiger counter on its lowest setting.
BTW cosmic rays are not the real problem it is the normal background radiation that comes from many sources. Cosmic rays pass through matter and usually do not interact. One has to look at the number of milliSieverts encountered. This is the amount of ionizing radiation that the film would suffer. Ionizing radiation is what does the damage.
But the main cause of fog is caused by heat. Hence keeping film refrigerated or better frozen. Remember if you are not going to develop exposed film in a reasonable amount of time it too should be refrigerated. This is particularly true for Ilford Pan F Plus. There is something about this film that causes the latent image to degrade faster than other films.
Well, my commentary was based upon research by NASA 20 years ago:
"Space radiation is very penetrating and difficult to shield against. Protons, alpha particles (A in figure 3), beta particles (B in figure 3) and some high Z particles make up the majority of the radiation encountered during Shuttle missions. Shielding for beta particles is easier to construct than for the heavier particles. A beta particle colliding with an element such as 8 hydrogen (the smallest element) is analogous to a BB shot at a bowling ball. Most shielding elements are at least 12 times as massive as hydrogen and should have no problem stopping beta particles. The more massive radiation particles are more difficult to control. Shielding, which does not stop these particles, will reduce their energy and momentum. Since this type of radiation has a high LET, these particles are more likely to release energy....The shielding bag does not afford much protection in its present configuration. The bagged sample and sample 5 were compared to evaluate the bag's usefulness. The bag helped 5030 film slightly with a 4 percent reduction in minimum density increase. For most films, the bag afforded no protection. The bag was designed to shield against softer and less penetrating x-rays. However, the film was exposed to high energy particles which passed through the bag quite easily"
...and by additional commentary from physicists with whom I have worked during the past 15 years
i asked if they knew anything about light sensitive photographic film and cosmic rays' affects on film, cause fogging, even when stored in a freezer. I was told that it isn't the cosmic rays per se, but the muons that are created when cosmic rays come into contact with other particles, which fogs the film. And as the muons are an inherent offshoot resulting from the cosmic radiation, the Technical Committee ISO/TC 42, responsible for the International Standard ISO 18928 doesn't differentiate muons from the cosmic radiation.
Finally, another comment from Kodak, about ambient radiation:
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 earths 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, photographic 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 specified 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.