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Photographic Plate

Photographic plates were important in early high-energy physics, as they are exposed by ionizing radiation. X – Rays were discovered by accident in this way and cosmic radiation was investigated through the traces it left on stacks of photographic plates, left on high mountains or carried on balloons.

Subatomic particles may leave tracks on the plate, but this is comparatively unusually, because to do that they must move in the plane of the plate, which is unlikely on the basis of pure chance, so that many particles must be allowed to pass before any tracks are seen in the film

The diagram illustrates tracks on a photographic plate made by alpha particles emitted from an alpha source.

It is also the case that only tracks from more energetic particles are discernible.