= Solution
For <photons>, the total <mass attenuation coefficient> sums contributions from <photoelectric absorption>, <Compton scattering>, coherent <Rayleigh scattering>, and <pair production> in nuclear or electronic fields when above their thresholds. At sufficiently high <photon energies>, <photonuclear reactions> can also contribute. Attenuation counts removal from the incident narrow beam, including scattering without complete local deposition of the original energy.
In water at both specified energies, <Compton scattering> dominates. At $140\,\mathrm{keV}$, <photoelectric absorption> and coherent <Rayleigh scattering> make smaller contributions; at $511\,\mathrm{keV}$ these are still less important. Nuclear-field <pair production> is impossible below $2m_ec^2=1.022\,\mathrm{MeV}$, so it occurs in neither example. The positrons used to produce the fluorine-18 annihilation radiation do not change that threshold for interactions of its $511\,\mathrm{keV}$ <photons> in water.
Strictly, $\mu$ is a <linear attenuation coefficient>, whereas $\mu/\rho$ is a <mass attenuation coefficient>; the source's wording blurs these units. The <mass energy-absorption coefficient> $\mu_{\rm en}/\rho$ weights the fraction of incident <photon energy> transferred to charged particles and retained after radiative losses, rather than simply counting all removals. Coherent scattering contributes attenuation with negligible energy transfer. A <photon> deflected by <Compton scattering> can carry much of the original energy away. One often writes $\mu_{\rm en}=(1-g)\mu_{\rm tr}$, with $g$ the fraction of secondary charged-particle kinetic energy lost radiatively. Under <charged-particle equilibrium>, multiplying <photon energy fluence> by $\mu_{\rm en}/\rho$ gives the <absorbed dose>; that equilibrium condition matters near interfaces and in buildup regions.<Tables of X-Ray Mass Attenuation Coefficients and Mass Energy-Absorption Coefficients by John H. Hubbell and Stephen M. Seltzer>[§§2–3, definitions of total attenuation, energy transfer and energy absorption]{ref}
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