Investigations on the existence of more than one metastable state of superheated liquids

Abstract

Superheated emulsion detector (SED) contains micronsize superheated liquid drops suspended in visco-elastic gel or in soft polymer matrix. When exposed to energetic radiation, superheated drops nucleate to vapor bubbles due to the energy deposited in the liquid by the energetic radiation. SEDs have been used as radiation detectors in various areas, from health physics to high energy physics. The present work is entirely based on visco-elastic gel based SEDs, fabricated in our laboratory. Lately, it has been observed that the nucleation rate data always shows a non monotonic decay when a SED is periodically exposed to neutrons (or photons) with periods of radiation-off regions in between. This and some other observations could not be explained by a single-state decay model, and one has to consider the SED to be comprised of multiple groups or states of nucleating droplets. Here we have formulated and modeled a two-state decay scheme that explains all the observed discrepancies. Using the two-state model we have studied the neutron induced (using 241 Am-Be neutron source) nucleation of R-12 and R-134a basedSEDs; and gamma induced (using 137 Cs gamma source) nucleation of R-404a based SEDs. Fromthe observed data, the radiation induced nucleation efficiency (for R-12 and R-134a)and detection efficiency (for R-404a) for the normal and the second metastable states are obtained as a function of temperature. The interstate transition kinetics for these SEDs has also been studied. To understand the nature of vi the second metastable state we have studied the effect of external additives on the various nucleation parameters. Nucleation of superheated droplet is associated with the emission of an acoustic pulse and with a change in volume, both of which can be detected by active devices. Here in addition to the acoustic-induced active device, a new active volume measurement device has been presented. This has also been used to study the droplet size distribution in SED in situ. We have also studied the spectra and intensity of acoustic pulses emitted during droplet nucleation. Here we have studied the variation of modal frequency and power of the acoustic pulses for spontaneous and radiation induced eventsusing 137 Cs gamma source and 241 Am-Be neutron source.