Beam sensing

A particle beam sensor comprising:

• a scattering surface configured to intercept obliquely a path of an incident particle beam thereby to permit scattering of some particles from the incident particle beam by the scattering surface,
• a sensor responsive to receipt of one or more scattered particles to generate a sensor signal;
• and
• an aperture mask arranged between the scattering surface and the sensor and presenting to the scattering surface a screen opaque to said scattered particles and having an aperture through which an unobstructed view of the scattering surface is provided to the sensor, the aperture thereby being configured to permit selection of particles scattered by the scattering surface which are to be used to form at the sensor an image representative of at least a part of a foot print cast by the particle beam upon the scattering surface,
• wherein:
• a separation (D) between the aperture mask and the scattering surface viewable by the sensor through the aperture differs from a separation (L) between the aperture mask and the sensor, such that an area at the sensor able to be illuminated by the scattered particles differs from an area of the scattering surface able to illuminate the sensor with scattered particles;
• the sensor comprises an array of pixel sensors arranged relative to the aperture mask to provide a sensing surface upon which the image is formable, wherein the aperture mask is configured to provide selected particles through the aperture to a plurality of pixel sensors of the array of pixel sensors, each pixel sensor being responsive to receipt of a scattered particle to generate a pixel sensor signal;
• and
• the sensing surface is substantially planar and adjustable to adjust an obliquity with which the sensing surface is presented to the aperture mask thereby to adjust a size of the image formable on the sensing surface.