[en] The secondary emission scintillation (SES) counter is a device designed to count the positive ions of charged-particle tracks in gas volumes simulating sites in tissue with diameters of the order of 1 nanometer. A diagram of the current prototype SES counter is shown. A weak electric field in the cylindrical collection region of the device drifts ions from a track to a small region (∼ 1-mm deep) of high electric field where they are accelerated by several kilovolts onto a dynode, producing secondary electrons. The secondary electrons are then accelerated onto a plastic scintillator, and the resulting light is detected by a photomultiplier. The passage of a charged particle is established by a solid state detector, which triggers electronics detecting coincidences and measuring the timing and amplitude of pulses from the photomultiplier. In the prototype, the beam of charged particles comes from an internal collimated alpha-particle source. The SES counter is filled with gas at a uniform pressure of about 0.05 torr or less. At this pressure, 1 cm of (nitrogen) gas in the counter is equivalent to 0.8 nm at unit density, and potentials of several kilovolts can be maintained in the accelerating region without producing electrical breakdown in the gas because most of the ions cross the region without striking a gas molecule