[en] An ionization chamber is peripherally bounded by a stack of four coaxial annular frames with rabbeted confronting faces forming three annular recesses on the peripheral chamber wall, the middle recess receiving a transverse partition whereas the other two recesses form seats for peripheral zones of two dielectric disks with discontinuous metallic coatings constituting mutually insulated segmental electrode elements. The segmental elements terminate short of the peripheral chamber wall, except for a tab extending from each element onto the peripheral disk zone received in the corresponding recess within which that tab is connected to an external lead. The tabs of the two disks are angularly offset from one another, as are the segmental boundaries of their conductive elements. 11 claims, 4 figures