New data are reported from the operation of a 4.0-kg ${\mathrm{CF}}_3\mathrm{I}$ bubble chamber in the 6800-foot-deep SNOLAB underground laboratory. The effectiveness of ultrasound analysis in discriminating alpha-decay background events from single nuclear recoils has been confirmed, with a lower bound of $>99.3\%$ rejection of alpha-decay events. Twenty single nuclear recoil event candidates and three multiple bubble events were observed during a total exposure of 553 kg-days distributed over three different bubble nucleation thresholds. The effective exposure for single bubble recoil-like events was 437.4 kg-days. A neutron background internal to the apparatus, of known origin, is estimated to account for five single nuclear recoil events and is consistent with the observed rate of multiple bubble events. The remaining excess of single bubble events exhibits characteristics indicating the presence of an additional background. These data provide new direct detection constraints on WIMP-proton spin-dependent scattering for WIMP masses $>20\mathrm{GeV}/{\mathrm{c}}^2$ and demonstrate significant sensitivity for spin-independent interactions.

• Received 11 April 2012

DOI:https://doi.org/10.1103/PhysRevD.86.052001