What is the significance of Density and atomic number (Z)?

To detect y-rays efficiently, a material with a high density and high effective Z (number of protons per atom) is required. 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 (γ-rays). Materials with high Z-values are used for γ-ray spectroscopy at high energies (> 1 MeV).