I'm applying airborne gamma ray spectroscopy to the mapping of wetlands and hydric soils. In terms of gamma rays emanating from the upper 300 mm - 1000 mm of the earth's surface the electron density of earth materials and water are noted as the primary control over attenuation rate. Water has a greater electron density than most earth materials and therefore is a stronger gamma ray attenuator. The higher electron density of water relative to earth materials is attributed to the lack of a neutron in the hydrogen atom.

I'm interested to know how the electron density is calculated and whether or not there is a periodic table of electron densities or electron density values for common and important molecular species such as water. Can anyone help?