"Mass Attenuation Coefficient" is a measurement of how strongly a chemical species or substance absorbs or scatters light at a given wavelength, per unit mass.
$\mu_m = \frac{\mu}{\rho}$, where $\mu$ is the linear attenuation coefficient and $\rho$ is the mass density of the medium.
Applicable units are those of quantitykind:MassAttenuationCoefficient
Mass Attenuation Coefficient