A Gamma Radiation Detector With Orthogonally Arrayed Micromachined Electrodes

Microfabricated radiation detectors can be used to provide first alert information about presence of harmful radiation. This paper describes a micromachined gamma detector that operates in the Geiger-Muller (G-M) regime. Gamma rays eject photoelectrons from the cathode material through the photoelectric effect and/or Compton scattering, which in turn ionize the Ar fill-gas providing a current pulse. The detector utilizes cathode stacks that are micromachined from stainless steel #304 foil. Micromachined glass fingers with thin-film anode metal traces are positioned transversely through aligned perforations in the stacked cathodes. This orthogonal array of anodes and cathodes effectively distinguishes electron avalanche regions from drift regions despite the miniaturization, thereby reducing the likelihood of spurious discharges. Overall, the detector diameter and height are 9 and 2.5 mm, respectively. Detector performance is characterized using a 99-μCi ¹³⁷Cs source placed at a distance of 3 cm from the detector. In an integration time of 10 min at an applied voltage of 630 V, a source:background ratio of 89:1 is achieved-a fivefold improvement over the previously reported micromachined devices operating in the G-M regime. This architecture also reduces the typical charge per discharge to 6.6 pC, allowing the estimated dead time between detection events to be ~1 μs.