Which substance is used in processing X-ray films to help eliminate the excess halide crystals?

The substance used in processing X-ray films to help eliminate excess halide crystals is the fixer. The fixer plays a crucial role after the development stage of the film processing. During the developing phase, the image is made visible by converting the exposed silver halide crystals to metallic silver. However, not all halide crystals are exposed to radiation and need to be removed to prevent fogging and to ensure the clarity and stability of the image.

The fixer contains chemicals, such as ammonium thiosulfate, which dissolve the unexposed silver halide crystals and render the film insensitive to further exposure to light. This step is essential to stop the development of the image and to make it permanent. By removing the excess halide, the fixer ensures that the film can be safely handled and viewed without risk of further change or deterioration.

While the developer is responsible for the initial exposure transformation, the stop bath is used to halt the development process temporarily before fixing, and water serves primarily as a rinsing agent. Each of these substances has its own specific role in the X-ray film processing workflow, but it is the fixer that specifically addresses the elimination of excess halide crystals effectively.