What material is designed to stop ionizing radiation?

The correct choice is absorber, as this term refers to materials specifically engineered to attenuate or stop ionizing radiation. In medical imaging and radiation therapy, absorbers are used to shield patients and health care workers from harmful radiation exposure. These materials can be composed of various substances, including lead, concrete, or specialized plastics that are dense enough to absorb the energy of ionizing radiation, thereby reducing its intensity.

The other options do not relate accurately to radiation shielding. A sponge is primarily a porous material used for cleaning and doesn't possess properties effective for radiation absorption. The term detainer does not specifically refer to a material used in radiation protection but could imply a device used to hold objects in place. Capturer is an ambiguous term that may suggest something that collects or gathers but does not effectively convey the concept of stopping radiation. Therefore, absorber is the most appropriate choice when referring to materials designed to mitigate the effects of ionizing radiation.