Fundamental properties of superheated drop (bubble) detectors

Superheated drop detectors and bubble damage detectors find increasing applications in ionising radiation dosimetry and spectrometry. These emulsions of overexpanded halocarbon droplets can be manufactured to respond selectively to densely ionising particles, such as neutron recoils, or to all directly and indirectly ionising radiations. It is shown here that the fundamental properties of the detectors can be predicted by semi-empirical expressions based on the thermal spike theory. A new nondimensional quantity, defined as 'reduced superheat', is introduced and shown to permit a unified parametrisation of the properties of superheated emulsions. In particular, utilising the reduced superheat concept, it is possible to predict the neutron detection thresholds of the emulsions, their sensitisation to thermal neutrons and to photons, and their ultimate thermodynamic instability. This unified characterisation finds immediate application in the selection of the halocarbons and of the operating conditions most suitable for specific radiation detection problems. Finally, some data are presented which question a direct proportionality between the particle track length contributing to the vaporisation and a critical bubble diameter derived from spontaneous nucleation models. An effective track length based on experimental observations is introduced to derive the minimum track-averaged LET for bubble nucleation expressed as a function of reduced superheat.