Which of the following materials can't be used as interspace material inside a radiographic grid?

In a radiographic grid, the interspace material must allow for the passage of radiation while effectively supporting the grid's structure. Lead, while commonly used for the grid strips themselves due to its high atomic number and effectiveness at absorbing scatter radiation, is not suitable as an interspace material. This is primarily because lead is dense and would excessively attenuate the primary x-ray beam, reducing image quality and negating the purpose of the grid, which is to improve contrast by absorbing scattered radiation.

In contrast, materials like aluminum, plastic, and carbon fiber are lightweight and possess the necessary properties to allow x-rays to pass through while maintaining structural integrity. These materials help in effectively reducing scatter while ensuring sufficient primary radiation reaches the film or detector. Thus, selecting lead as an interspace material would be counterproductive and decrease the overall effectiveness of the grid in a radiographic setup.