The heavy-ion fluence required to induce SingleEvent Gate Rupture (SEGR) in power MOSFET's is measured as a function of the drain bias, VDS, and as a function of the gate bias, VGS. These experiments reveal the abrupt nature of the SEGR-voltage threshold. In addition, the concepts of crosssection, threshold, and saturation in the SEGR phenomenon are introduced. This experimental technique provides a convenient method to quantify heavy-ion effects in power MOSFET's.