Contents lists available at ScienceDirect Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima Response of Superheated Droplet Detector (SDD) and Bubble Detector (BD) to interrupted irradiations Prasanna Kumar Mondal ⁎ , Rupa Sarkar, Barun Kumar Chatterjee Department of Physics, Bose Institute, 93/1, A P C Road, Kolkata 700009, India ARTICLE INFO Keywords: Superheated droplet detector Bubble detector Bubble nucleation Neutron detection ABSTRACT Superheated droplet detectors (SDD) and bubble detectors (BD) are suspensions of micron-sized superheated liquid droplets in inert medium. The metastable droplets can vaporise upon interaction with ionising radiation generating visible bubbles. In this work, we investigated the response of SDD and BD to interrupted neutron irradiations. We observed that the droplet vaporisation rates for SDD and BD are different in nature. The unusual increase in droplet vaporisation rate observed when the SDD is exposed to neutrons after few minutes of radiation-off period is absent for BD. 1. Introduction A superheated emulsion detector (SED) consists of micron-sized metastable superheated liquid halocarbon/hydrocarbon droplets dis- persed in an inert viscoelastic gel (superheated droplet detector, SDD) [1] or in a polymer (bubble detector, BD) [2] medium. The SDD was invented by R.E. Apfel at the Yale University in late 1970's and later in early 1980's the popularity of the technology was increased with the development of BDs by H. Ing. In SED the ionising radiation can trigger the vaporisation of superheated drops. During the vaporisation of a superheated liquid drop an acoustic pulse is produced, which one can detect by an acoustic sensor [3]. Here the nucleation of vapor bubble requires a minimum amount of energy to be deposited within a critical length [1,4]. The threshold energy for bubble nucleation can be varied by changing the operating temperature and/or pressure. This makes SEDs useful in the detection of high ionising radiations (neutrons, heavy ions, etc.) while keeping it insensitive to low ionising radiations (gamma-rays, beta particles etc.) [5,6]. When a SED is irradiated with ionising radiations the superheated drops vaporise randomly. The decay of the superheated drops is expected to be monotonic in nature. However, this was not observed when SDDs were irradiated several times with a no-radiation period between two successive irradiations [7]. In the multiple irradiation experiment it was observed that the droplet vaporisation rate at the start of the irradiation is much higher than the rate at the end of the preceding radiation exposure [7]. This non-monotonic nature of the multiple irradiation data suggests that in SDDs there are two groups of superheated drops, one of which decays in a much faster rate in comparison to other. The droplets that vaporise during one irradiation repopulate during radiation-off period and accounts for the initial high droplet vaporisation rate during next irradiation [7,8]. Here we have studied the response of SDD and BD to neutrons when they were irradiated periodically with some radiation-off time in between two irradiations. In this study we have used SDD and BD containing droplets of superheated R-12 (CCl 2 F 2 ). The discrepancy in droplet vaporisation rate observed previously [7] in SDD is found to be absent in case of BD. Here we observe that the increase in droplet vaporisation rate at the beginning of later irradiations is absent for BD. In case of BD the superheated droplets decay monotonically when exposed to ionising radiation. 2. Theory It is well known that any liquid can be superheated to a temperature much above its boiling point without vaporisation. The metastable superheated liquid consists of a large number of microbubbles and if one of them reaches a certain size (radius, r c ) then it vaporises the entire metastable liquid. This is known as the spontaneous bubble nucleation. Here, after reaching r c the microbubble keeps on increas- ing and vaporises the entire superheated liquid. The bubble nucleation in superheated liquid can also be triggered by ionising radiations. For radiation induced bubble nucleation the energetic radiation needs to deposit a sufficient amount of energy (W) within a certain critical length [1,4]. The threshold energy (W) and the critical radius (r c ) for bubble nucleation are functions of temperature and pressure [1]. In SED, the charged particles generated during neutron interactions move through the superheated liquid and may trigger the bubble nucleation. In SED there is a distribution of droplet size, where the superheated http://dx.doi.org/10.1016/j.nima.2017.03.035 Received 29 December 2016; Received in revised form 21 February 2017; Accepted 19 March 2017 ⁎ Corresponding author. E-mail addresses: prasanna_ind_82@yahoo.com (P.K. Mondal), sarkar_rupa2003@yahoo.com (R. Sarkar), barun_k_chatterjee@yahoo.com (B.K. Chatterjee). Nuclear Instruments and Methods in Physics Research A xxx (xxxx) xxx–xxx 0168-9002/ © 2017 Elsevier B.V. All rights reserved. Please cite this article as: Mondal, P.K., Nuclear Instruments and Methods in Physics Research A (2017), http://dx.doi.org/10.1016/j.nima.2017.03.035