In X-ray diodes (XRDs), X-rays eject electrons from the detector surface.The ejected electron emission is composed of both primary and secondary electrons. Both the primary and secondary quantum yields are proportional to the product of the absorption cross section times the photon energy. A bias is applied to collect all of the ejected electrons. The electric field must be sufficient to prevent Child-Langmuir current saturation. The resulting current is proportional to the X-ray flux.

XRDs are effective in the soft X-ray portion of the spectrum (0.28-10 keV) and make good Flux monitors and Spectrometers. For the 0.28-1.56 keV spectral range Nu-Trek will typically use a carbon XRD and for the 1.56-10 keV spectral range we will use aluminum XRDs.

Spectral response functions of vitreous carbon and aluminum XRDs.