Microdosimetry in the nanometer region

The experiment designed to yield information on the deposition of energy by charged particles in regions of the order of a nanometer, is being continued. A particular geometrical configuration for the avalanche grid and shield wires has been investigated. It consists of four stainless steel wires each 10 mils in diameter. One wire, the avalanche grid, is separated by 15 mils from the scintillator and then there are three shield wires between the grid and sensitive volume of the chamber. The actual configuration is shown. The purpose of the shield wires is to keep the high electric field at the grid wire from penetrating into the sensitive volume of the chamber, so as to keep the field there as uniform as possible for positive ion collection. A test chamber was designed for the purpose of investigating the best possible separation of the grid and shield wires from the grounded surface, which represents the scintillator. Measurements made with the chamber indicated that in order to maintain any reasonable voltage on the avalanche grid wire (i.e. -3000 v) the pressure must be 0.1mm Hg or below. Breakdown voltages for various separations ranging from 0.5 mil to 15 mils were measured. Data were obtained for shield voltages of -400 v, -540 v and -5 volts. Highest grid voltages were obtainable with the shield wires at -5 v, which would seem to indicate that the voltages on the grid and shield combine to cause breakdown to the grounded surace. Other configurations of shield and grid wires will be investigated along with the possibility of using smaller diameter wires for the shield