Abstract
A superheated droplet detector (SDD) consists of a uniform dispersion of over-expanded, micrometric-sized halocarbon droplets suspended in a hydrogenated gel, each droplet of which functions as a mini-bubble chamber. Energy deposition by irradiation nucleates the phase transition of the superheated droplets, generating millimetric-sized bubbles that are recorded acoustically. A simple pulse shape validation routine was developed in which each pulse is first amplitude demodulated and the decay constant then determined through an exponential fit. Despite this, low amplitude (< 3 mV) events embedded at naked eye in the noise level are not counted for calibration purposes with neutron and alpha sources. The solution found was to filter the data with a low band-pass filter in the region that the bubbles nucleate (typically from 450 to 750 Hz). After this, a peak finding algorithm to count all the events was implemented. The performance demonstrates better than a factor 40 reduction in noise and an extra factor 10 reduction with the filtering application. The lowering of noise and discovery of low signal amplitudes by the acoustic instrumentation and acoustic analysis permits a capability of discriminating nucleation events from acoustic backgrounds and radiation sources and, having a 95% confidence level on identifying and counting events in substantial data sets like in calibrations.
Highlights
A superheated droplet detector (SDD) [1] is a generic denomination for a class of commonly employed systems for the neutron threshold spectrometry [2]
This has given rise to two different approaches for the measurement of the energy spectrum: the passive system which consists of six bubble detectors with energy thresholds ranging from 10 keV to 10 MeV, and the active system which uses superheated droplet detectors, usually one or two, which are operated at different temperatures to generate nested threshold responses in the 0,01-10 MeV range [6]
The final objective of these studies are not dealt with in this short paper, but the new discoveries that generate to a great extent smaller amplitude nucleation events which, if not accounted for, will harm the final results of the SDDs behavior
Summary
A superheated droplet detector (SDD) [1] is a generic denomination for a class of commonly employed systems for the neutron threshold spectrometry [2] These consist of suspensions of metastable droplets which vaporize into bubbles when they are nucleated by radiation [3,4]. The various superheated emulsions share the same principle for neutron spectrometry but have different manufacturing procedures and constitution (either polymeric or aqueous gels) and require different instrumentation for the bubble nucleation detection or counting This has given rise to two different approaches for the measurement of the energy spectrum: the passive system which consists of six bubble (damage) detectors with energy thresholds ranging from 10 keV to 10 MeV, and the active system which uses superheated droplet detectors, usually one or two, which are operated at different temperatures to generate nested threshold responses in the 0,01-10 MeV range [6]. The emulsion was cooled by cold water circulation at 5°C for 12 hours, the pressure slowly released and the SDD extracted for use
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