The Operation of Solid-State Alpha Counters in Chemical Process Streams

The alpha activity in a chemical process stream may be monitored by placing an etched N-type silicon wafer in the stream to operate as a surface barrier alpha counter. The counters are constructed by the Blankenship technique with the gold cover omitted. The bare silicon surface is immersed in the sample stream. Since the liquid of the stream forms the front contact to the silicon, the stream conductivity must be high. The detectors were operated in solutions of the nitrate salts of thorium, uranium, neptunium, plutonium, and americium. The effect of solution conductivity and acidity was investigated. The effects of aluminum and fluoride ions were studied also. The detector will cease to operate if the oxidation of the silicon surface is allowed to run to completion. The silicon dioxide layer will insulate the P'' inversion layer on the surface of the silicon from the liquid contact. If the bias is operated in the forward direction for short periods, the silicon dioxide insulator will be punctured and contact between the liquid and P ' inversion layer will be restored. A similar result can be achieved by illuminating the silicon surface for short periods to induce a photo-etch process. Both of thesemore » methods were used to maintain detectors in a counting condition for periods of up to nine months. (auth)« less