Rupa Sarkar

Characterization of R-134a superheated droplet detector for neutron detection

R-134a (C2H2F4) is a low cost, easily available and chlorine free refrigerant, which in its superheated state can be used as an efficient neutron detector. Due to its high solubility in water the R-134a based superheated droplet detectors (SDD) are usually very unstable unless the detector is fabricated using a suitable additive, which stabilizes the detector. The SDD is known to have superheated droplets distributed in a short-lived and in a relatively long-lived metastable states. We have studied the detector response to neutrons using a (241)AmBe neutron source and obtained the temperature variation of the nucleation parameters and the interstate kinetics of these droplets using a two-state model.

A new method for measurement of droplet size distribution in superheated emulsions

Superheated emulsion contains micron-sized superheated liquid droplets suspended in a host liquid medium. The superheated droplets nucleate to vapor bubbles when energetic radiation deposits a sufficient amount of energy within a very small localized region of the active liquid. Droplet nucleation is associated with a change in volume and also emission of an acoustic pulse, both of which can be detected electronically. When exposed to energetic radiation, the metastable superheated droplets nucleate independently of each other, and the number of drops nucleated and volume of vapor formed due to nucleation of these droplets are both acquired simultaneously as a function of time. These two data are then fitted simultaneously to obtain the droplet size distribution of the emulsion. The size distributions of R-114 (C2Cl2F4)- and R-12 (CCl2F2)-based emulsions are obtained by this method and compared with direct optical measurements.

Study of acoustic emission due to vaporisation of superheated droplets at higher pressure

Abstract Bubble nucleation in superheated liquids can be controlled by adjusting the ambient pressure and temperature. At higher pressure the threshold energy for bubble nucleation increases, and we have observed that the amplitude of the acoustic emission during vaporisation of superheated droplet decreases with increase in pressure at any given temperature. Other acoustic parameters such as the primary harmonic frequency and the decay time constant of the acoustic signal also decrease with increase in pressure. This behavior is independent of the type of superheated liquid. The decrease in signal amplitude limits the detection of bubble nucleation at higher pressure. This effect is explained by the emission of shockwave generated during the supersonic growth of the microbubble in superheated liquids.

The gamma ray detection threshold temperature of different superheated droplet detectors

The response of superheated droplet detectors (SDD) loaded with four different active liquids, R-12 (CCl2F2), R-114 (C2Cl2F4), R-134a (C2H2F4) and R-610 (C4F10) have been studied to obtain the gamma ray detection threshold temperature (Tγ) of the respective detectors. A 137Cs gamma ray source is used for this study. To obtain Tγ from the experimental data a phenomenological model has been used. Result indicates that Tγ maintains a linear relationship with the limit of superheat (Tlim) of different active liquids used in SDD. It indicates that the limit of superheat of a liquid can be used for the prediction of its gamma ray detection threshold temperature.

Note: A new optical method for the detection of bubble nucleation in superheated droplet detector

A superheated droplet detector (SDD) consists of a large number of micron-sized superheated liquid drops suspended in a gel medium. The vaporization of a superheated drop is associated with the emission of an acoustic signal. A novel optical method is developed for the detection of this acoustic signal. In this method, a probe-bubble picks up the acoustic signal, and the oscillation of the probe-bubble is detected by employing a laser and phototransistor. The method can detect vaporization of an individual superheated drop in real-time and can be used for studying the response of SDDs to ionizing radiations.