Doses to humans during handling and injection of gamma emitting radiotracers may be reduced by passive shielding, e. g. lead. A narrow beam of monoenergetic photons with an incident intensity I0, penetrating a layer of material with mass thickness d and density p, emerges with intensity I given by the exponential attenuation law

1 = exp(- m m d) (28)

10 where /m = //p is the mass attenuation coefficient (cm2/g), / is the linear attenuation coefficient (cm-1) and p the material density (g/cm3). The mass thickness d = xp (g/cm2) where x is the distance or the linear thickness in the material (cm). Introducing the term d = xp into Eq. (28) and solving for x gives:

ln( / )

x = 10­

m m P

Mass attenuation coefficients for lead (p = 11.35 g/cm3) [23] have been reconstructed from the plot given for lead and used to calculate the linear attenuation coefficients for the main gamma energies of the various radionuclides used as radiolabels and treated below.