Final answer:
Photomultiplier tubes are not used with IR radiation because they are not sensitive to its lower-energy photons, and IR detectors require cooling to avoid interference from their own thermal emissions.
Step-by-step explanation:
Photomultiplier tubes cannot be used with IR radiation (infrared radiation) because C) they are not sensitive to IR radiation. Infrared detectors, including those used in telescopes, require special conditions to operate effectively. Since IR radiation is predominantly from thermal agitation, any warm object, including the detector itself, emits infrared energy.
To detect IR radiation accurately, the detectors must be cooled to low temperatures using liquid nitrogen or liquid helium. This ensures that the thermal emissions from the detector do not overpower the very signals they are intended to detect. Therefore, a photomultiplier tube, which is optimized for detecting higher-energy photons such as those in the visible light spectrum, lacks the sensitivity to effectively detect the lower-energy photons of infrared radiation.
Photomultiplier tubes cannot be used with IR radiation because they are not sensitive to IR radiation. Photomultiplier tubes are designed to detect visible light and ultraviolet radiation, but they do not have the ability to detect IR radiation. In order to detect IR radiation, detectors need to be specifically designed and cooled to very low temperatures to reduce interference from heat and background radiation.