Extra length and large sized bubble detectors (BD) of Type T-15 have been irradiated with 500 MeV/u 56Fe ions at HIMAC accelerator in the National Institute of Radiological Sciences (NIRS), Chiba, Japan. Linear tracks composed of regular arrays of tiny bubbles are visible by the naked eyes. The total length of each track is longer than 4.2 cm. In each track there are commonly more than 24 bubbles. The gap distance (or called pitch distance) between two neighboring bubbles in a track has been measured and analyzed, which reflects the distance between the two invisible superheated droplets that form the visible bubbles. The gap distance is related to the volume ratio of the superheated liquid (Freon-115) droplets to the whole sensitive volume of the detector and to the sizes of the droplets of the superheated liquid. A technique has been developed to deduce the average diameter of superheated droplets and to derive the number of the droplets in unit volume of the detector. A method to calibrate bubble detectors with high-energy heavy ions at accelerators is advanced, which is similar to that of calibration of solid state nuclear track detectors (SSNTD) at accelerators. Feasible ranges of the percentage volume of superheated liquid being 10-4-10-1, droplet diameter 1-103 μm, and droplet number density 1-1011 cm-3 in bubble detectors are outlined according to the experiences of the authors. These techniques and method will facilitate the development and applications of bubble detectors.
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