  We demonstrate that by utilizing an over-screened Josephson junction as a
noise detector it is possible to achieve the threshold regime, whereby the
tails of the fluctuating current distribution are measured. This situation is
realized by placing the Josephson junction and mesoscopic conductor in an
external circuit with very low impedance. In the underdamped limit,
over-screening the junction inhibits the energy diffusion in the junction,
effectively creating a tunable activation barrier to the dissipative state. As
a result, the activation rate is qualitatively different from the Arrhenius
form.
