In general, the required size of the scintillation crystal determines the possibility to use this readout technique. Small crystals perform best with good energy resolution and noise levels around 50 keV or slightly less. Compact size, good energy resolution and immunity to magnetic fields are pros. A totally different application is the measurement of high intensity X-ray beams. Low afterglow scintillation crystals like CdWO 4 are used in arrays coupled to photodiodes.
Advantages are the good signal reproducibility, the absence of gain drift and the compact size.
Examples of photodiode detectors are shown in section 7 . The term low background in itself needs to be specified in detail. Sources of background from within the detector are the photomultiplier tube, the detector housing and the crystal. Special PMTs can be selected with a ultra-low K content and all other materials can be pretested prior to assembly. Plastics should be avoided because these often contain K. Aluminum has a larger U and Th content than steel so for low background applications, steel housings are the best choice.