A tissue-equivalent proportional counter apparatus comprising: a) a first endwall, a second endwall spaced apart from the first endwall and sidewalls extending therebetween to provide an enclosed chamber interior containing a tissue-equivalent counting gas, the first endwall being a cathode and formed from a tissue-equivalent material that is operable to produce secondary charged particles within the chamber when exposed to ionizing radiation, the secondary charged particles producing electrons within the tissue-equivalent counting gas; b) a micromesh avalanching structure provided within the chamber and positioned between the first endwall and the second endwall; c) a plurality of anode pads provided within the chamber interior whereby secondary charged particles can travel within the chamber interior from a volume overlying one of the anode pads to a volume overlying an adjacent one of the anode pads without travelling through an intervening solid structure, the plurality of anode pads being spaced apart from the micromesh avalanching structure and arranged in a two-dimensional array positioned between the micromesh avalanching structure and the second endwall, each anode pad configured to collect at least a portion of the electrons produced in the chamber interior and passing through the micromesh avalanching structure and generate a corresponding anode output signal; d) a power source configured to apply a voltage between the cathode and the plurality of anodes; and e) a data acquisition system configured to detect the anode output signals and to determine an amount of ionizing radiation corresponding to each anode output signal. a) a first endwall, a second endwall spaced apart from the first endwall and sidewalls extending therebetween to provide an enclosed chamber interior containing a tissue-equivalent counting gas, the first endwall being a cathode and formed from a tissue-equivalent material that is operable to produce secondary charged particles within the chamber when exposed to ionizing radiation, the secondary charged particles producing electrons within the tissue-equivalent counting gas; b) a micromesh avalanching structure provided within the chamber and positioned between the first endwall and the second endwall; c) a plurality of anode pads provided within the chamber interior whereby secondary charged particles can travel within the chamber interior from a volume overlying one of the anode pads to a volume overlying an adjacent one of the anode pads without travelling through an intervening solid structure, the plurality of anode pads being spaced apart from the micromesh avalanching structure and arranged in a two-dimensional array positioned between the micromesh avalanching structure and the second endwall, each anode pad configured to collect at least a portion of the electrons produced in the chamber interior and passing through the micromesh avalanching structure and generate a corresponding anode output signal; d) a power source configured to apply a voltage between the cathode and the plurality of anodes; and e) a data acquisition system configured to detect the anode output signals and to determine an amount of ionizing radiation corresponding to each anode output signal.