PURPOSE:To improve effective transmittivity to radiations while maintaining mechanical strength by constituting a flat plate electrode of a wire mesh. CONSTITUTION:A wire mesh electrode is used as a high voltage impressing electrode 2 and a current collecting electrode 2'. The electrodes 2, 2' are respectively connected to connecting parts 11, 12 to a high voltage power source and a dose rate measuring circuit which are not shown. The grating density of the wire mesh and the material and size of the wires are selectable according to the nature and energy of output radiations as well as the desired dose distribution on the irradiation plane. The absorption and scattering of the low energy radiations at the detector are thus considerably minimized.
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