The threshold of gamma-ray induced bubble nucleation in superheated emulsion

The bubble nucleation in superheated emulsion starts with the formation of critical sized vapour embryo in the liquid. The homogeneous nucleation can be triggered due to the energy deposition by the different kinds of radiations. At lower degree of superheat, higher ionizing particles can trigger the bubble formation. As the degree of superheat increases, lower ionizing particles also become responsible for bubble nucleation. In the present work, the bubble nucleation by the lower ionizing particles, e.g. gamma-rays has been discussed. Experiments were performed with R-134a (C2H2F4, b.p. -26.3 oC) superheated droplets in presence of 137Cs (662 keV, 5 mCi) gamma source. From the GEANT4.10.02 simulation, it is observed that near the threshold temperature of gamma-ray, stopping power of electron having energy corresponding to its highest value at the end of the track is mostly responsible for initiating a bubble nucleation, whereas with increase in temperature of the liquid, electrons of other energies could play role in the bubble nucleation process. The nucleation occurs due to the energy deposition by the produced electrons (secondary and higher order) at the end of their tracks within the liquid.