Toward a carbon nanotube anode gas-filled radiation detector

A prototype gas-filled proportional counter (PC) based on micro-scale tungsten wire and carbon fiber, and nano-scale carbon nanotube (CNT) anodes was built and tested with a 90Sr source. Tungsten anodes of 500 μm down to 4 μm diameter were used to observe the gradual decrease in operating voltage for the proportional region with a decreasing anode diameter. The 40 nm diameter CNTs anodes ranged in length from 35 to 105 μm. The absolute detection efficiency was measured at ∼10−6%. An electrostatic computer model was used to predict the resulting electric field associated with a single CNT in the coaxial configuration. For a single anode coaxial design the model predicted that the electric field was insufficient for secondary ionizations which contributed to a low amplitude signal and that the small volume of the ionization region resulted in the low absolute detection efficiency. To overcome the problems of low absolute detection efficiency and operational issues with the single anode, CNT arrays were investigated. Electrostatic modeling of 100 nm×40 μm long CNTs in an array with a 50 μm pitch conducted for a parallel plate configuration indicated that each anode functioned independently.