Experimental Verification of a Superheated Liquid Droplet (Bubble) Neutron Detector Theoretical Model

Abstract Superheated liquid droplet ('bubble') neutron detectors utilise thousands of microscopic droplets of freon-based compounds suspended in a viscous matrix material. Neutrons can interact with the atoms of the superheated liquid droplets, resulting in the formation of energetic charged recoil ions. These ions transfer their energy to the liquid in the droplets, sometimes resulting in the droplets vaporising and producing visible bubbles. The basis of bubble detector operation is identical to that of bubble chambers, which have been well characterised by researchers such as Wilson, Glaser, Seitz and others since the 1950s. Each of the microscopic superheated liquid droplets behaves like an independent bubble chamber. This paper presents a theoretical model that considers the three principal aspects of detector operation: nuclear reactions, charged particle energy deposition, and thermodynamic bubble formation. All possible nuclear reactions were examined and those which could reasonably result in recoil ions sufficiently energetic to vaporise a droplet were analysed in detail. Feasible interactions having adequate cross sections include elastic and inelastic scattering, n-proton, and n-alpha reactions. Ziegler's transport of ions in matter (TRIM) code was used to calculate the ions' stopping powers in various compounds based on the ionic energies predicted by standard scattering distributions. If the ions deposit enough energy in a small enough volume then the bubble of vapour that forms will grow to 'critical' size, and spontaneously vaporise the rest of the entire droplet without further energy input. Various theories as to the vaporisation of droplets by ionising radiation were studied and a novel method of predicting the critical (minimum) energy was developed. This method can be used to calculate the minimum required stopping power for the ion, from which the threshold neutron energy is obtainable. Experimental verification of the model was accomplished by measuring the response of two different types of bubble detectors to monoenergetic thermal neutrons, as well as to neutrons from the polyenergetic spectra of bare and moderated californium spontaneous fission sources. The model's predicted response compared favourably with the experimental data.