Neutron spectrometry in mixed fields: superheated drop (bubble) detectors

The BINS neutron threshold spectrometer permits the analysis of the main features of a neutron field for radiation protection purposes. The system offers a virtually complete photon discrimination and nested threshold responses to neutrons, which allow the use of very effective 'few-channel' unfolding procedures. To date, the practical operating energy range of a BINS is 0.1-10 MeV, over which a resolving power of 20-30% can be expected when the deconvolution is performed without explicit pre-information. Spectrum unfolding results in relatively high uncertainties on the differential fluence distributions, but due to negative correlations in adjacent energy groups the uncertainties on integral quantities such as dose equivalent are small and of the order of 5% to 10%, similar to the results of other active spectrometers. In comparison with most radiation detectors, the BINS is an extremely slow system due to the intrinsic duration of a bubble pulse and to the time associated with pulse analysis. For example, the maximum sustainable fluence rate of 1 MeV neutrons is about 10(4) cm(-2) s(-1), which is low for many neutron physics experiments. However, this rate corresponds to an ambient dose equivalent rate of about 1 mSv h(-1), making the active device adequate for radiation protection applications in the workplaces described in Section 1. There are ample margins for improvement of the spectrometer. In particular, in the low-energy region a thermal-epithermal neutron group may be added by using chlorine-bearing emulsions stabilised at suitable temperatures. In fact, the latest version of the system achieves this goal by using a single superheated emulsion of dichlorotetrafluoroethane (R-114) operated at temperatures up to 55 degrees C. This extends the range of the spectrometer and at the same time removes the undue enhancement of the UNFANA output in the low energy region. Above 10 MeV, the resolution can be improved by adding more thresholds, e.g. by starting from a lower initial temperature and using finer temperature increments. Based on neutron kinematics, the theoretical upper energy threshold which can be generated with superheated emulsions is greater than 100 MeV. However, this would most likely require refrigerating the detectors, while the current simpler approach is to operate the detectors at incremental temperature steps starting from the ambient temperature. A range that should be easily achieved in practice is from thermal energies to 20 MeV.