A near UV envelope detector in the prebreakdown regime based on photoionization of excited gas atoms

A novel near ultraviolet (UV) gas-filled detector which is operated in the prebreakdown regime is described. The detection mechanism is photoionization of the excited state. Modulated continuous UV light was used to study experimentally the detector responsivity and the spectral response for typical electrode materials such as Mo, Ti, W and Ta at around 200 nm wavelength. Of the materials investigated, molybdenum yielded the highest relative responsivity. This is attributed to the secondary electron emission and, in particular, to the initial electron energy after secondary emission, which contributes to internal signal gain and to improved quantum efficiency through increased excitation. For molybdenum electrodes, the detector performance was studied with various electrode geometries. Of the electrode configurations considered, two straight parallel wires which are located parallel to the incoming UV photon direction yields the best responsivity. This is attributed to maximum electric field nonuniformity and photon interaction depth between the electrodes. Effective quantum efficiency, including detector internal gain, is found to be about 5570% per photon at 200 nm wavelength. This leads to an effective yield of 9 A W^-1, including internal gain, at 200 nm wavelength. The noise equivalent power NEP is estimated to be ~10^-15 W Hz^-1/2.