The present invention provides an apparatus of charged-particle beam e.g. an electron microscope comprising an in-column plasma generator for selectively cleaning BSE detector and BF/DF detector. In various embodiments, the plasma generator is located between a lower pole piece of objective lens and the BF/DF detectors, but outside trajectory area of the charged-particles from the sample stage to the BF/DF detector. Cleaning decomposed biological samples or contaminants on the surface of the detectors frequently and selectively with in-situ generated plasma can prevent the detectors from performance deterioration such as losing resolution and contrast in imaging at high levels of magnification.