Energy- and angle-differential neutron fluence measurements with superheated drop (bubble) detectors

One of the latest additions to the field of neutron spectrometry is based on the active control of the response functions of superheated emulsions. By varying the superheat of the detectors, either changing their operating temperature or applied pressure, it is possible to generate a matrix consisting of nested responses suitable for few-channel energy spectrometry. In the device presented here, a detector is embedded in a recess milled on the surface of a moderating sphere. This sphere has the purpose of introducing an angular dependence in the otherwise nearly isotropic response of the detector. The device relies on the acoustical detection of bubbles and on temperature stepping to vary the superheated emulsion thresholds. In correspondence to each temperature/threshold, measurements are sequentially performed at different angular orientations of the sphere. The response matrix of the system to monoenergetic neutrons was determined as a function of angular position by means of Monte Carlo neutron transport simulations. The directional spectrometer was tested by means of irradiations with a californium neutron source. Energy- and angle-differential unfolding of the detector readings was performed by means of a maximum-entropy technique which does not require a-priori information. The spectrometer operates well with large energy-angle groups, and produces accurate integral values of total fluence, which can be used to derive quantities such as ambient dose equivalent H*(10) or directional dose equivalent H′(10). However, the device presents limitations in unfolding spectra over a finer group structure, and will require the future developments outlined in the conclusions.