| 摘要 |
PROBLEM TO BE SOLVED: To provide an isochromatic omega-type energy filter with good characteristics by setting the ratio between the distance from a pupil surface to a slit surface and the distance from the outgoing side end surface of the filter to the slit surface to the specific value or more, in order to reduce the energy value difference between the vicinity of the center of an image on the pupil face and a peripheral part. SOLUTION: Form factors determining the basic characteristic of an omega- type energy filter are the radii of rotation R3 , R4 , anglesθ1 ,θ2 ,θ3 ,θ4 , and distance L3 , L4 , L5 , LL. L5 is the distance from the outgoing side end face of a magnet M4 on the slit side to a slit surface, and LL is the distance from an image face to the slit surface. When seven parameters R3 , R4 , L3 , L4 , L5 , LL and deflection angleΦare changed for obtaining an evaluation functionΔXpD/ri <2> to LL/L, the best evaluation function is proved to increase in proportion to LL/L5 , so as to obtain LL/L5 >2 with various conditions taken into account. The isochromatic omega-type energy filter is thereby obtained by selecting a form to be LL/L5 >2.
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