Electron detector

Electron detector apparatus for detecting secondary electrons released from a body;

• magnetic lens means having an axis of symmetry;
• means for supporting said body at a region of maximum field strength of said magnetic lens means;
• means for directing an electron beam through said magnetic lens means to bombard said body and release secondary electrons;
• means for accelerating secondary electrons released from said body to travel in an axial direction through said magnetic lens in a direction opposite from that of the electron beam;
• electron retarding means disposed in the path of secondary electrons after acceleration by the accelerating means;
• said magnetic lens means for producing a magnetic field that progressively weakens along said axis from the accelerating means toward said retarding means to cause said secondary electrons to travel in progressively elongated helical paths to approach the retarding means along paths substantially parallel to said axis.

Electron detector apparatus for detecting secondary electrons released from a body bombarded with an electron beam;

• magnetic lens means having an axis of symmetry;
• means for supporting said body at a region of high field strength of said magnetic lens means;
• means for accelerating secondary electrons released from said body to travel in an axial direction through said magnetic lens;
• planar electron retarding means disposed normal to said axis of symmetry in the path of secondary electrons accelerated by said accelerating means;
• said retarding means located between said accelerating means and means for producing a localized region of substantially constant field strength, considerably less than said high field strength, in which the retarding means is located;
• means for producing an electrostatic field for retarding said secondary elections during travel between the accelerating means and said retarding means;
• said magnetic lens means for producing a magnetic field that progressively weakens along said axis from the accelerating means toward said retarding means to cause said secondary elections to travel in progressively elongated helical paths to approach the planar retarding means along paths that are substantially parallel to said axis.

Electron detection apparatus for detecting secondary electrons released from a body;

• comprising magnetic lens means having an axis of symmetry;
• means for supporting said body at a region of high field strength of said magnetic lens means;
• a tube disposed coaxially with said axis of symmetry;
• electron accelerating means adjacent one end of the tube for accelerating secondary electrons released from the body to travel inside the tube in an axial direction through said magnetic lens;
• planar electron retarding means disposed across the tube toward the end of opposite said one end, normal to said axis of symmetry;
• said planar retarding means located between said accelerating means and an energizaton coil surrounding the tube for producing a localized region of substantially constant field strength, considerably weaker than said high field strength, in which said planar retarding means is located;
• a resistive coating on the inner surface of said tube for providing an electrostatic field for retarding said secondary electrons during travel between the accelerating means and the planar retarding means;
• said magnetic lens means for producing a magnetic field that progressively weakens along said axis from the accelerating means towards the planar retarding means to cause said secondary electrons to travel in progressively elongated helical paths to approach the planar retarding means at approach angles to the direction of the axis of symmetry sufficiently small that there is substantial equality between their approach speeds along the direction of the axis of symmetry and their actual speeds.

Electron detector apparatus for detecting secondary electrons released from a body;

• cylindrical magnetic lens means;
• means for supporting said body at one end of said cylindrical lens means, said magnetic lens means for focussing said secondary electrons to a small region on the axis of the magnetic lens means opposite the end at which said supporting means is located;
• an annular electrode at said opposite end centered on said region and a resistive coating on the surface of said annular electrode more remote from said supporting means;
• means for biasing said annular electrode to a positive potential;
• a secondary electron retarding and collector assembly including a part spherical electron retarding electrode having a center located at said small region such that secondary electrons focussed on said small region thereafter travel along radial paths to approach said part spherical retarding means at speeds which are substantially equal to their actual speeds;
• and means for collecting secondary electrons that pass said retarding electrode.