Observation of acoustically silent bubble-nucleation events in superheated drop detector

Abstract

A Superheated Droplet Detector (SDD) is a ubiquitous detector consisting of a large collection of micron-sized droplets of superheated liquid held in a gel matrix. An energetic particle while passing through the superheated droplet can induce a sudden liquid-vapor phase transition, which is known to be associated with the emission of an acoustic signal. An earlier experiment indicated that the amplitude of the acoustic signal decreases with increasing ambient pressure indicating that bubble nucleation may become acoustically silent at some point. This study using R1216 (C₃F₆) based SDD reveals that, under certain condition the spontaneous and gamma-ray induced bubble nucleation, and subsequent vaporization of superheated droplet does indeed become acoustically silent, as they are no longer detected by acoustic transducers. Here a method of detecting such acoustically silent nucleation-events is presented. These acoustically silent bubble nucleation events have been detected using an optical detection system developed in our laboratory, by which the pressure pulse created by the sudden gel-displacement during bubble nucleation is detected. A tentative explanation of acoustically silent bubble nucleation is also presented here which roughly outlines the condition for a bubble nucleation to become acoustically silent.