  We have used normal metal-insulator-superconductor (NIS) tunnel junction
pairs, known as SINIS structures, for ultrasensitive thermometry at sub-Kelvin
temperatures. With the help of these thermometers, we have developed an
ac-technique to measure the electron-phonon (e-p) scattering rate directly,
without any other material or geometry dependent parameters, based on
overheating the electron gas. The technique is based on Joule heating the
electrons in the frequency range DC-10 MHz, and measuring the electron
temperature in DC. Because of the nonlinearity of the electron-phonon coupling
with respect to temperature, even the DC response will be affected, when the
heating frequency reaches the natural cut-off determined by the e-p scattering
rate. Results on thin Cu films show a $T^{4}$ behavior for the scattering rate,
in agreement with indirect measurement of similar samples and numerical
modeling of the non-linear response.
