Disclosed herein is a system for drilling in a multilayer printed circuit board. The system includes a source of electromagnetic radiation configured to transmit a measurement pulse in open air to a workpiece, an anode, a resettable electric charge sensor (ECS), operably connected to the anode, and a control unit, configured to receive at least one value indicative of the quantity of at least part of charged molecules received at the anode and determine a second value indicative of the quantity of charged molecules received at the anode that were derivative of emitted electrons responsive to the measurement pulse.