What factor determines how fast the film is?

The speed of the film is primarily determined by the size of the silver halide salts present in the emulsion layer. Larger silver halide crystals are more sensitive to light and radiation, which allows them to produce images more quickly and with less exposure compared to smaller crystals. This increased sensitivity results in a film that requires less exposure time to achieve the same level of image density.

While factors such as the thickness of the emulsion, the type of radiation used, and the exposure time do have roles in the overall imaging process, they do not directly determine the intrinsic speed of the film. For instance, a thicker emulsion might provide a broader range of exposure, but it doesn't inherently mean the film will be faster. Similarly, different types of radiation can affect the development process but do not alter the inherent sensitivity characteristic defined by the size of the silver halide crystals. Thus, the size of the silver halide salts is the correct factor influencing how fast the film is.