Patent ID: 8592761

Claim:
A monochromator, comprising: a first dispersion unit and a second dispersion unit both aligned with a straight optical axis of the monochromator and configured to in sequence deflect a charged particle beam with a normal energy and an energy spread, the charged particle beam propagating along the optical axis and including charged particles with the normal energy passing straight through each dispersion unit and charged particles with energy deviations from the normal energy being deflected by said each dispersion unit in a same direction as a dispersion direction thereof, wherein a deflection angle of each charged particle generated by said each dispersion unit depends on a dispersion power thereof and energy deviation of said each charged particle, and wherein the dispersion directions of the first and second dispersion units are same; a particle-blockage unit between the first and second dispersion units; and a beam adjustment element aligned with the straight optical axis and configured to focus the charged particle beam to form a real crossover at a plane inside the particle-blockage unit, wherein inside the real crossover, each particle with energy deviation has a position shift due to said deflection angle generated by the first dispersion unit, wherein the particle-blockage unit cuts off particles located outside a spatial area of the real crossover, wherein a virtual crossover is formed inside the monochromator after the charged particle beam is dispersed by the second dispersion unit.