An ionizing radiation imaging detector, for use with a ionizing radiation beam source comprising a pixellated conversion-detection unit, and a substrate supporting the conversion-detection unit. The substrate includes one or more elements having atomic numbers greater than 22, the elements having a total concentration in the substrate of greater than about 1 mole percent relative to the total number of moles of elements having atomic numbers of 22 or less. The substrate has a dimensionless absorption exponent of less than 0.5 for gamma ray emission of Am.sup.241 at about 60 keV;whereAE(Am.sup.241 60 keV)=t*(k.sub.1 e.sub.1 +k.sub.2 e.sub.2 +k.sub.3 e.sub.3 + . . . )and wherein AE(Am.sup.241 60 keV) represents the absorption exponent of said substrate relative to the about 60 keV gamma ray emission of Am.sup.241 ; t represents said thickness of said substrate in the principle direction of propagation of said x-ray beam; e.sub.1, e.sub.2, e.sub.3, . . . represent the concentrations of elements in said substrate; and k.sub.1, k.sub.2, k.sub.3, . . . represent the mass attenuation coefficients of the respective elements.