Large sized bubble detectors with different sensitivities have been irradiated with high-energy protons at the cyclotron in RIKEN, Japan. Distributions of bubbles induced by the protons in the detectors have been studied. It shows that quite a large number of random bubbles are produced in the detectors in the range traveled by the beam protons. The existence of random bubbles hints that high-energy protons as an important component of primary and secondary cosmic rays at higher altitude and in space may cause significant number of bubbles in the bubble detectors in aircrafts or spacecrafts. The origin of the random bubbles has been investigated. The detection efficiencies of bubble detectors for protons via the random bubbles have been determined, being (0.93 ± 0.27) × 10-2, (4.6 ± 1.5) × 10-2 and (4.7 ± 1.7) × 10-2 (Bubble cm-3)/(Proton cm-2), respectively for T-14, T-24 and T-12 types of bubble detectors. The detection efficiencies for protons are quite high, which are about two orders of magnitude higher than that for 0.3-19 MeV mono-energetic neutrons. The astonishing high efficiencies for protons can only be attributed to Rutherford scattering of protons with nuclei of C, F, and Cl in the compositions of superheated liquid droplets in the detectors. The recoil nuclei of C, F and Cl from Rutherford scattering create the random bubbles. In contrast, in terrestrial calibrations of bubble detectors with mono-energetic neutrons, only nuclear interactions of neutrons with nuclei are involved. Special consideration must be given to the effect of Rutherford scattering of high-energy protons on detection of neutrons with bubble detectors at higher altitude and in space.
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