Data from the operation of a bubble chamber filled with 3.5 kg of CF3I
in a shallow underground site are reported. An analysis of ultrasound signals
accompanying bubble nucleations confirms that alpha decays generate a
significantly louder acoustic emission than single nuclear recoils, leading to
an efficient background discrimination. Three dark matter candidate events were
observed during an effective exposure of 28.1 kg-day, consistent with a neutron
background. This observation provides the strongest direct detection constraint
to date on WIMP-proton spin-dependent scattering for WIMP masses >20
GeV/c2.Comment: 4 pages, 4 figures V2 submitted to match journal versio