Wireless Micromachined Gas Discharge-Based Radiation Detectors.

Abstract

Miniature, wireless radiation detector systems are potentially valuable for environmental and security monitoring. These systems can enable rapid deployment and dynamic reconfiguration of sensor networks. This thesis explores the design and manufacturing of wireless micromachined, gas-based radiation detectors, specifically targeting small form-factors. Two core concepts are investigated in this work: (1) leveraging existing micromachining technologies to design and manufacture miniaturized gas-based radiation detectors and (2) leveraging the radiation-induced microdischarges for wireless signaling purposes. Four micromachined detector structures are presented. Two test-structures target beta detection and two devices target beta/gamma detection. The test-structures for beta detection include bulk micromachined silicon/glass stacks with etched cavities, and planar, metal-on-glass structures. During operation, incident beta-particles ionize the fill-gas between the biased electrodes, resulting in avalanche current pulses or microdischarges measured as “counts”. These microdischarges can inherently transmit wideband RF content extending >1 GHz. The impact of discharge gap-spacing, operating pressure, fill-gases, and electrode materials on operating voltage and wireless signaling performance is evaluated. The two detector designs targeting beta/gamma radiation use in-package assembly of stainless-steel electrodes and glass spacers, which leverage commercial processes and industry-standard packages, e.g., a TO-5 header. The first beta/gamma design uses a single anode/cathode pair and is hermetically-sealed with an Ar fill-gas near 760 Torr. The second design uses an arrayed electrode structure to demonstrate a scalable path for increasing detection efficiency. At 30 cm from a 99 micro-Ci Cs-137 source, count rates exceed 1.3 cps. The calculated gamma sensitivity is 3.79 cps/mR/hr, which is comparable to a commercial unit with 30X greater detection volume. When normalized to sensitive volume, the single-stack and arrayed device demonstrate comparable gamma sensitivities. However, for a given form-factor, the arrayed detector outperforms the single-stack by ~6X. Both designs demonstrate low background rates (5-8 cpm). Receiver operating characteristics (ROC) of these detector designs are described. The measured wireless signal spans 1.25 GHz at receiving antenna-to-detector distances >89 cm. Evaluation of deployment scenarios, e.g., integrating with mobile platforms or networked configurations, are presented, along with descriptions of portable powering modules.