Patent ID: 8049182

Claim:
A charged particle filter comprising: a magnetic deflector having a bore along an axis thereof passing through the deflector from a specimen end to a detector end of the deflector and through which charged particles pass when in use, the deflector being formed from one or more magnets positioned around the bore in a Halbach configuration thereby generating a relatively high magnetic field strength within the bore and a relatively low magnetic field strength outside of the deflector, wherein the deflector has a geometry defining an outer surface and an inner surface, wherein each of the outer and inner surfaces of the deflector taper towards the axis as a respective function of distance in the specimen direction along the axis; and, an outer shield of soft magnetic material surrounding the magnet deflector on an outer surface side of the deflector and having a projecting portion which extends in the specimen direction with respect to the magnetic deflector from the specimen end of the deflector.