A radiation detector has an electron emitter that includes a coated nanostructure on a support. The nanostructure can include a plurality of nanoneedles. A nanoneedle is a shaft tapering from a base portion toward a tip portion. The tip portion has a diameter between about 1 nm to about 50 nm and the base portion has a diameter between about 20 nm to about 300 nm. Each shaft has a length between about 100 nm to about 3,000 nm and an aspect ratio larger than 10. A coating covers at least the tip portions of the shafts. The coating exhibits negative electron affinity and is capable of emitting secondary electrons upon being irradiated by radiation. The nanostructure can also include carbon nanotubes (CNTs) coated with a material selected from the group of aluminum nitride (AlN), gallium nitride (GaN), and zinc oxide (ZnO). The detector further includes an electron collector positioned to collect electrons emitted from the electron emitter and to produce a signal indicative of the amount of electrons collected, and a signal processor operatively connected to the electron collector for processing the signal to determine a characteristic of the radiation. The detector can be used to detect radiations of changed particles or light such as X-ray.