Effective sensitivity of a
photodetector of an
image sensor is controlled by partitioning
signal charge from incident photons, thus producing a manageable yield and a consequently higher,
photon shot noise limited,
signal to
noise ratio than in the prior art, when imaging
high flux rates of energetic photons or particles, such as produced by x-
ray generators. The invention may be applied, for example, to an
image sensor with a photosensitive layer coupled to a charge collection / readout structure, e.g. photoconductor or
scintillator on
CMOS array, or to an intrinsically sensitive charge collection / readout structure, e.g. deep
active layer CMOS. A
radiation sensor pixel structure 10 for use in the invention includes a
photodetector 11, a transfer gate 131 for controlling charge collection from the
photodetector and a dump drain 12 controlled by a dump gate 121, arranged for selectively dumping charge to the dump drain means and collecting charge from the photodetector means, in a
duty cycle 31, for varying effective sensitivity of the pixel structure. An
image sensor containing such pixel structures may selectively be operated in an integration mode or a
photon counting mode. Preferably the image sensor has imaging pixels and control circuitry arranged on a same
semiconductor die, such as a
CMOS semiconductor die.