Energy Down-Conversion and Thermalization in Metal Absorbers

Absorbers play an important role in all radiation detectors, providing effective energy absorption, fast relaxation and spatial expansion of the excitation volume. A review of the details of energy down-conversion in metal absorbers, both normal metals and superconductors, is given. We discuss the initial absorption of a quantum of energy and ejection of a single photoelectron, followed by fast energy down-conversion via the electron-electron and electron-phonon interactions. We describe the spatial and spectral evolution of the distributions of electronic excitations and phonons, together with the mean energy loss and fluctuations. We also review some recent results regarding thermalization in superconductors at low temperatures, a poorly understood area but one which is crucial for many low temperature devices.