A particle beam sensor (1) comprising: scattering means (2) providing a surface for intercepting obliquely a path of a particle beam (3) thereby to permit a scattering of particles from the particle beam by the scattering means (2); sensor means (8) responsive to receipt of one or more said scattered particles to generate a sensor signal; aperture mask means (6) arranged between the scattering means (2) and the sensor means (8) to present to the scattering means a screen opaque to said scattered particles and having at least one aperture (7) through which an unobstructed view of the scattering means is provided to the sensor means, the aperture (s) thereby permitting selection of all of those particles scattered by the scattering means (2) which may be used to form at the sensor means (8) an image representative of at least a part of a foot print cast by the particle beam upon the scattering means. By scattering particles from a sectional area of a particle beam, scattered beam particles can be used more efficiently compared to existing techniques.
