Chicagoland Observatory for Underground Particle Physics

Chicagoland Observatory for Underground Particle Physics (COUPP) is an experimental program to use bubble chambers to search for Weakly Interacting Massive Particles (WIMPs) which could make up a majority of the dark matter. The technique involves mildly superheating freons, typically trifluoroiodomethane, such that the nuclear recoils expected from dark matter create bubbles, but electromagnetic interactions, such as those from gamma-rays, do not nucleate bubbles in the chamber.^[1]

The COUPP project has fielded a number of bubble chambers. The first chamber involved a test tube in the basement of the University of Chicago Kavli Institute for Cosmological Physics. This chamber was a demonstration device to show that a mildly superheated bubble chamber could be used to search for low-energy nuclear recoils.

The main scientific workhorse for the COUPP project has been a one liter bubble chamber commissioned in 2006. This chamber lead to the first limits on dark matter from a bubble chamber.^[2]

The one liter bubble chamber was upgraded and the inner vessel was changed to a two liter synthetic fused silica. The previous one liter vessel was made from natural quartz and there was a rate of bubbles nucleating on the quartz walls. The glass boiling rate was two orders of magnitude lower with the synthetic fused silica vessel showing that residual radioactivity in the natural quartz was the cause of the inner vessel wall boiling. The second improvement to the chamber was the addition of PZT acoustic transducers. With these transducers, the COUPP Collaboration was able to confirm the discovery by the PICASSO Collaboration that single nuclear recoils can be distinguished from alpha-decays through the acoustic emission of the bubble.^[3] The latest results from the two liter chamber demonstrates that the probability for an alpha-decay in CF₃I to acoustically mimic a nuclear recoil is < 0.3% at 34^oC and 30 PSIA.

The COUPP Collaboration is currently running a two liter bubble chamber in SNOLAB and a 30 liter bubble chamber is in the commissioning stages at Fermilab. The 30 liter chamber is planned to move to SNOLAB in 2012.

References

1. ^ Bolte, W.J.; et al. (11July2011). "Development of Bubble Chambers with Enhanced Stability and Sensitivity to Low-Energy Nuclear Recoils". Nuclear Instruments and Methods in Physics Research. A 577 (3): 569–573. arXiv:astro-ph/0503398. doi:10.1016/j.nima.2007.04.149. 
2. ^ Behnke, E.; et al. (15Feb2008). "Improved Spin-Dependent Limits from a Bubble Chamber". Science 319 (5865): 933–936. arXiv:0804.2886. doi:10.1126/science.1149999. 
3. ^ Behnke, E.; et al. (10Jan2011). "Improved Limits on Spin-Dependent WIMP-Proton Interactions from a Two Liter CF₃I Bubble Chamber". Physical Review Letters 106 (021303). arXiv:1008.3518. doi:10.1103/PhysRevLett.106.021303.