Among other things, a detector unit for a radiation detector array is provided. The detector unit includes a radiation detection sub-assembly including a scintillator and a photodetector array. A first routing layer is coupled to the photodetector array of the radiation detection sub-assembly at a first surface of the routing layer. An electronics assembly includes an analog-to-digital converter that converts an analog signal to a digital signal. A second routing layer is disposed between the A/D converter and the first routing layer. A shielding element is disposed between the A/D converter and the second routing layer. The shielding element shields the A/D converter from the radiation photons. The second routing layer couples the electronics sub-assembly to the first routing layer. A first coupling element couples the A/D converter to the second routing layer.