surface 142 of collimator 106 substantially parallel and in alignment with septa 120 and may interact with collimator 106 by absorption or scattering a third portion 148 of the photons may arrive at incident surface 142 of collimator 106 at an angle 150 with respect to a longitudinal axis 152 of aperture 122 if angle 150 is greater than an angle determined by length 124 and diameter 126 a photon entering aperture 122 will interact with collimator 106 before exiting aperture 122 if angle 150 is less than the angle determined by length 124 and diameter 126 the photon may exit aperture 122 so as to interact with collimating mask 134 covering region of variable response 119 accordingly collimating mask 134 facilitates preventing photons that would otherwise interact with region of variable response 119 from doing so second surface 112 may be substantially covered by a single cathode electrode 154 first surface 110 has a rectangular array of small for example between about one millimeters squared mm 2 and about ten mm 2 generally square pixel electrodes 108 configured as anodes a voltage difference applied between pixel electrodes 108 and cathode 154 during operation generates an electric field detector field in substrate 104 the detector field may be for example about one kilovolts.