Charged particle filter

1. 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.

2. A filter according to claim 1 , wherein the minimum diameter of the bore within the deflector is at least 7 millimeters at the detector end of the deflector.

3. A filter according to claim 1 , wherein the ratio of the external diameter of the outer shield to the internal diameter of the inner surface at the detector end of the deflector is less than 2.

4. A filter according to claim 1 , wherein the maximum magnetic field strength outside the deflector is 10% or less that of the magnetic field strength upon the axis within the deflector.

5. A filter according to claim 1 , wherein the magnetic deflector is formed from at least 8 magnets.

6. A filter according to claim 1 , further comprising a non-magnetic break positioned between the specimen end of the deflector and at least part of the projecting portion of the shield which extends from the deflector in the specimen direction.

7. A filter according to claim 1 , wherein the bore passes through the projecting portion and the diameter of the bore within part of the projecting portion is less than or equal to the minimum diameter of the bore at the detector end of the deflector.

8. A filter according to claim 1 , wherein the collimator has an inner surface which acts as an extension to the inner surface of the deflector.

9. A filter according to claim 1 , further comprising a rear shield of soft magnetic material arranged in a substantially planar manner having a plane normal substantially parallel to the axis and having a bore through which the charge particles pass from the bore of the deflector, the rear shield being positioned facing the detector end of the deflector and spaced therefrom by a gap.

10. A fitter according to claim 1 , further comprising an annular chamber positioned at the detector end of the deflector, the chamber having a diameter greater than at least an internal diameter of the bore at the detector end of the deflector.

11. A filter according to claim 1 , wherein the outer shield is provided with a generally frusto-conical external surface which tapers towards the axis as a function of distance in the specimen direction.

12. A filter according to claim 1 , wherein the bore within the deflector is lined with a corrugated surface.

13. A particle analyser comprising:

a charged particle filter according to claim 1 ; and

a particle detector positioned upon the axis facing the detector side of the deflector.