It is clear that for an efficient detection of g-rays a material with a high density and high Z is required (see above). Inorganic scintillation crystals meet the requirements of stopping power and optical transparency, their densities ranging from roughly 3 to 9 g/cm3 makes them very suitable to absorb penetrating radiation (g-rays). Materials with high Z-values are used for g-ray spectroscopy at high energies