R&D studies for the development of acoustic sensors for dark matter bubble chamber detectors

PICO Dark Matter bubble chamber detectors use piezoelectric sensors in order to detect and discriminate the acoustic signals emitted by the bubbles grown within the superheated fluid from a nuclear recoil produced by a particle interaction. These sensors are attached on the outside walls of the vessel containing the superheated fluid. The acoustic discrimination depends strongly of the properties of the complete sensor and there are constrains as well in the size and radiopurity of the piezoelectric ceramics. With the aim of understanding the complete acoustic process and optimizing the sensor system, a test bench for the characterization of the sensors glued to the vessel has been developed. The sensor response for different piezoelectric materials, geometries, matching layers and experimental designs has been measured and contrasted with FEM simulations and analytical models. The results of these studies and designs lead us to have a design criterion for the construction of specific sensors for next generation of PICO detectors (250 L bubble chambers).