Direct Measurement of the Bubble Nucleation Energy Threshold in a CF3I
  Bubble Chamber
  We have directly measured the energy threshold and efficiency for bubble
nucleation from iodine 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.
