  We report temporal measurements of the electronic temperature and the
electron-phonon thermal relaxation rate in a micron-scale metal island, with a
heat capacity of order 1 fJ/K . We employed a superconductor-insulator-normal
metal tunnel junction, embedded in a radio-frequency resonator, as a fast (~ 20
MHz) thermometer. A resistive heater coupled to the island allowed us to drive
the electronic temperature well above the phonon temperature. Using this
device, we have directly measured the thermal relaxation of the hot electron
population, with a measured rate consistent with the theory for dynamic
electron-phonon cooling.
