Direct measurement of the bubble-nucleation energy threshold in a CF 3 I bubble chamber

We have directly measured the energy threshold and efficiency for bubble nucleation fromiodine recoils in a CF3I bubble chamber in the energy range of interest for a dark matter search. These interactions cannot be probed by standard neutron calibration methods, so we develop a new technique by observing the elastic scattering of 12 GeV/c negative pions. The pions are tracked with a silicon pixel telescope and the reconstructed scattering angle provides a measure of the nuclear recoil kinetic energy. The bubble chamber was operated with a nominal threshold of (13.6 +/- 0.6) keV. Interpretation of the results depends on the response to fluorine and carbon recoils, but in general we find agreement with the predictions of the classical bubble-nucleation theory. This measurement confirms the applicability of CF3I as a target for spin-independent dark matter interactions and represents a novel technique for calibration of superheated fluid detectors.