How to set the global material as empty or air? And how the dectors work?

I have some questions about the global settings.The first is how to set the global world materials as empty or air because my phamton’s voxels are small, about i build a small voxel phamton, it will take 25G to 100G to set the global world. The second is the compton scatter will disappear if the energy below the threshold?
I have question about the ct system, the detector’s function. The threshold can be set zero? if we set zero, the detector will detect energy of the all photons? Then we can calculate the rate of the energy that the detectors actually get?

And other process between detector and output file? The pixels of the file are just the theorically physical value about the photon energy?

the threshold is for the single photon ? or for the integrated value of all photon energies