Investigating Effect of Space Radiation Environment on Piezoelectric Sensors: Cobalt-60 Irradiation Experiment

Piezoelectric sensors are used in many structural health monitoring (SHM) methods to interrogate the condition of the structure to which the sensors are affixed or embedded. Among SHM methods utilizing thin wafer piezoelectric sensors, embedded ultrasonics is seen as a promising approach to assess condition of space structures. If SHM is to be implemented in space vehicles, it is imperative to determine the effects of the extreme space environment on piezoelectric sensors in order to discern between actual structural damage and environmental effects. The near-Earth space environment comprises extreme temperatures, vacuum, atomic oxygen, microgravity, micrometeoroids and debris, and significant amounts of radiation. Gamma radiation can be used to emulate the space radiation environment. In this contribution, the effects of gamma radiation on piezoelectric ceramic sensors are investigated for equivalent gamma radiation exposure of more than a year on low Earth orbit (LEO). Two experiments were conducted in which cobalt-60 was utilized as the source of radiation. Freely supported piezoelectric sensors were exposed to increasing levels of gamma radiation. Impedance data were collected for the sensors after each radiation exposure. The results show that piezoelectric ceramic material is affected by gamma radiation. Over the course of increasing exposure levels to cobalt-60, the impedance frequencies of the free sensors increased with each absorbed dose. The authors propose that the mechanism causing these impedance changes is due to gamma rays affecting piezoelectric, electric, and elastic constants of the piezoelectric ceramic. A theoretical model describing observed effects is presented.