Large area neutron proportional counter and portal monitor detector

A portal neutron monitor detector for detection of neutron source fissionable materials carried by personnel passing through said detector, comprising:

• an array of large area unitary multicell neutron counters, each counter comprising:
• a container for defining an internal space and for confining counting gas therein, said container being formed of a pair of spaced substantially parallel metallic sheets sealed around their periphery and coated internally with a coating enriched with .sup.6 Li for capturing slow neutrons and emitting into said internal space reaction products thereupon produced by the .sup.6 Li(n,.alpha.).sup.3 H reaction,
• a plurality of wires mounted within said internal space, spaced apart from each other and said metallic sheets and substantially parallel to each other and between and substantially parallel to said metallic sheets and electrically insulated from said metallic sheets, and
• a gas permeable dividing structure disposed within said internal space for dividing the internal space into discrete cells and absorbing electrons incident upon such structure, each cell having a width defined by the spacing between the coatings on said metallic sheets, with at least one wire passing through each cell, and
• counting gas disposed within said container,
• the linear dimensions of each cell being so small that a reaction product from said .sup.6 Li(n,.alpha.).sup.3 H reaction will with substantial probability stop in the counting gas whereas the energy deposition in the counting gas from a single electron resulting from a photon entering a cell will nearly always be small compared with the energy deposition from said reaction,
• whereby ionizing events occurring within the counting gas caused by the .sup.6 Li(n,.alpha.).sup.3 H reaction may be discriminated from events causing lesser ionization;
• moderating means associated with said array for moderating neutrons from said neutron source materials so that a substantial fraction of such neutrons entering said array are slow;
• recirculation means for circulating counting gas through the internal spaces of all said containers;
• purifying means associated with said recirculation means for filtering and purifying said counting gas as the gas is circulated by said recirculation means;
• electrical means for applying voltage to enable the said metallic sheets, wires, and counting gas to function at rated voltage in the proportional region in the collection of charged particles occasioned by ionizing events to produce electrical pulses, the wires being anodes and the metallic sheets being cathodes, and the counting gas providing a medium in which the ionizing events occur;
• electronic means for discriminating electrical pulses within said array caused by ionizing events resulting from the .sup.6 Li(n,.alpha.).sup.3 H reactions occurring within said .sup.6 Li enriched coatings;
• and
• alarm means associated with said electronic means for indicating when the rate at which such pulses occur substantially exceeds a background rate.