The superheated detector technology has been at the forefront of spin-dependent (SD) searches, using various refrigerant targets including CF3I, C4F10 and C2ClF5 , and two primary types of detectors: bubble chambers and droplet detectors.
The PICO Collaboration (formed from the merger of two existing groups, PICASSO and COUPP) uses bubble chambers to search for galactic dark matter. The detectors consist of a fused-silica jar sealed to a flexible, stainless steel bellows, all immersed in a pressure vessel filled with hydraulic fluid. Three lead zirconate (PZT) piezoelectric acoustic transducers epoxied to the exterior of the bell jar record the acoustic emissions from bubble nucleation and two VGA resolution CCD cameras are used to photograph the chamber.
The PICO bubble chambers are insensitive to electromagnetic interactions by tuning the operational thermodynamic parameters, while the alpha decays are discriminated from nuclear recoils by their acoustic signal, making these detectors very powerful tools in the search for dark matter.
PICO is operating two detectors deep underground at SNOLAB: COUPP60, a bubble chamber with 37kg of CF3I that completed a physics run from June 2013 to May 2014, and PICO-2L, another bubble chamber with 2.9 kg of C3F8 that completed an 8-month physics run, from October 2013 to May 2014.
PICO is the leading experiment in direct detection of dark matter for spin-dependent couplings.