A bubble detector is a radiation sensitive device that uses small droplets of superheated liquid that are suspended in a gel matrix. It uses the principle of a bubble chamber but since only the small droplets can undergo a phase transition at a time, the detector can stay active for much longer periods than a classic bubble chamber. When enough energy is deposited in a droplet by ionizing radiation the superheated droplet undergoes a phase transition and becomes a gas bubble. The PICASSO detectors contain Freon droplets with an average diameter of. The bubble development in the detector is accompanied by an acoustic shock wave that is picked up by piezo-electric sensors. The main advantage of the bubble detector technique is that the detector is almost insensitive to background radiation. The detector sensitivity can be adjusted by changing the temperature of the droplets. Freon-loaded detectors are typically operated at temperatures between. The validity of the bubble detector concept has been shown in several publications. There is another similar experiment using this technique in Europe called SIMPLE.
PICASSO
The PICASSO experiment operated at SNOLAB. It had two science runs: first with 2 and later with 10 detector elements. The final configuration of the detector had 32 detector elements. It found no dark matter signal.
COUPP
The COUPP collaboration operated a bubble chamber in Fermilab 2011-2012. Particles from a particle accelerator beam were fired at the chamber to evaluate the technology for dark matter detection. The COUPP collaboration operated a bubble chamber with 3.5 kg CF3I in the MINOS underground area at Fermilab. The results were published January 2011. The COUPP collaboration also operated the same bubble chamber with 4 kg CF3I in SNOLAB from September 2010 to August 2011. SNOLAB-results were published in 2012. No dark matter was detected.
Results of PICASSO and COUPP
PICASSO reports results for spin-dependent WIMP interactions on 19F. No dark matter signal has been found, but for WIMP masses of new stringent limits have been obtained on the spin-dependent cross section for WIMP scattering on 19F of . This result has been converted into a cross section limit for WIMP interactions on protons of . The obtained limits restrict recent interpretations of the DAMA/LIBRA annual modulation effect in terms of spin dependent interactions. New results were published in May 2012, using 10 detectors with total exposure, to constrain low-mass WIMP interaction on 19F. The best spin-dependent limits were obtained for a 20 GeV/c2 WIMP mass: 0.032 pb for proton cross section. For the Spin-independent near 7 GeV low mass region cross section: upper limit
PICO
The PICO project started when the PICASSO and COUPP groups merged in 2013. The PICO experiment is located in SNOLAB, Canada, in a mine at the depth of 2 km. As of 2018, the PICO collaboration has operated two experiments in SNOLAB: a bubble chamber called PICO-2L and a chamber called PICO-60, where the numbers 2 and 60 refer to the volume of the target material in each chamber. The target material used was C3F8. A larger version of the experiment is being planned as of 2018. The larger version would have a target volume of 250-500 litres. The construction of the PICO-500 experiment is expected to start in 2019. PICO-2L was the first bubble chamber to start operation in October 2013. Its science runs took place October 2013-May 2014 and February 2015-November 2015. PICO-2L was decommissioned in 2016. PICO-60 followed PICO-2L. It started data-taking in 2013 and this first science run continued until May 2014. For this first run, the detector was filled with CF3I and the detector only used about half its capacity in terms if active volume. After this initial run, the detector was refurbished, the target material was switched to C3F8 and the active volume was increased to full capacity. The refurbishing work was completed and the second science run started summer 2016. The second science run ended in the summer of 2017, after which PICO-60 was decommissioned. The PICO-60 experimental station is used for a new bubble chamber called PICO-40L. The PICO-40L is a new type of bubble chamber whose technical structure eliminates the need for a buffer liquid that is a source of measurement noise. The PICO-40L was under construction in 2018, with initial results expected 2019. The PICO collaboration also has a number of bubble chambers used for calibration purposes, like the CIRTE and PICO-0.1 chambers.
