A wide aperture permanent magnet structure which is useful in creating a highly uniform longitudinal or solenoidal magnetic field in the working region of the magnet and in which the aperture of the structure can be equal in cross-sectional area to the working region of the magnet. A liquid cocktail mixture comprised of a neutron absorber and a scintillator is housed in a Teflon\xae tube having a mirror at one end of the tube and a windowed portal at the other end of the tube. An insulation layer is disposed on the capacitor electrode.