A Bubble Chamber for Dark Matter Detection COUPP: Chicagoland Observatory for Underground Particle Physics E. Behnke1, J.I. Collar2, P.S. Cooper3, M. Crisler3, L. Goetzke2, M. Hu3, I. Levine1, E. Neeley1, B. Odom2, E. Ramberg3, N. Riley2, H. Schimmelpfennig2, T.M. Shepherd1, A. Sonnenschein3, M. Szydagis2, R. Tschirhart3 1Indiana University South Bend 2Kavli Institute for Cosmological Physics, Enrico Fermi Institute, University of Chicago 3Fermi National Accelerator Laboratory A Bubble Chamber Suitable for a Dark Matter Search Chamber Development First Dark Matter Run: Operation of the 2 kg CF3I prototype chamber at Fermilab, MINOS Near Gallery, at a depth of ~300 meters water equivalent 2 kg prototype CF3I chamber: Even this small mass sets improved dark matter limits (see right-hand panel). Known radon sources were eliminated, and a second, lower background science run is currently underway. Dark matter particle candidates (WIMPs, Weakly Interacting Massive Particles) are expected to interact with detectors via nuclear scattering. The probability is however very small, calling for massive detectors. Predicted plateau (for a monoenergetic source in a threshold detector) Predicted a onset 20 kg “windowless” chamber instrumented with encapsulated piezoelectric sensors and submerged cameras. This chamber design is inexpensive and eliminates known sources of radon emanation. Bubble chambers can be operated in a mode insensitive to MIP backgrounds (b, g, etc.) while still sensitive to nuclear recoils caused by heavier particles, such as WIMPs. water pipe pressure vessel Scratch along bottom of quartz vessel The detection fluid is superheated and does not boil until a nuclear recoil of sufficient energy occurs. However, wall-roughness must be passivated to avoid the spontaneous surface nucleation observed in standard bubble chambers. Left: x-y projection of bubble positions from several months of live time, reconstructed from stereo photographs. Middle: x-z projection. The wall rate (0.8/cm2/day) is consistent with alpha-emission from the quartz: it displays the correct pressure onset, and it corresponds to the measured radioactive contamination of the quartz. Right: The bulk background is consistent with 100% Rn and progeny a’s, as indicated by a flat rate with a sharp onset at predicted value and by analysis of time correlation between events. 4p water shielding Air bubble for camera calibration, illuminated by bottom lighting, Encapsulated piezoelectric sensor for sharp determination of bubble onset time (to interface with muon veto) 20 kg synthetic fused silica vessel, sealed against exposure to Rn, and expected to yield an improved wall-event rate, due to lower a-emitter contamination. 60 kg chamber mechanical prototype Above left: nuclear recoil calibration Switchable Am/Be source yields 5 +/- 0.5 neutrons / s when on and O (0.2 neutrons / day) when off A blind absolute comparison shows consistency with 100% efficiency Moderated neutrons produces re-coils approximating WIMP spectrum Above: Improved limits are set for spin-dependent proton interactions, even using the 2 kg prototype with no measures taken against Rn contamination. In particular, limits are set in the low-mass region favored by a spin-dependent interpretation of the DAMA signal. The background rate in the next refill, using metal seals and non-thoriated welds, already looks very promising. Right: Although spin-independent sensitivity from the 2 kg prototype is not currently competitive, the 20 and 60 kg chambers have potential to obtain the world’s best sensitivity in this channel as well. Above right: g-rejection, achieved by proper choice of superheat, is measured to be 10-10 at an 11 keV threshold, as compared with ~10-4 of cryogenic experiments. Right: neutron background rejection, based on multiple scattering. May 21 2008 Funding: NSF grant PHYS-0114422 DOE/NNSA DE-SC04-04NA25441 NSF CAREER award 0239812 DOE/FNAL