Research @ KICP
August 20, 2004
A COUPP in the making
by Juan I. Collar
A new effort to search for particle dark matter, the Chicagoland Observatory for Underground Particle Physics (COUPP), will debut at the end of this summer. A bubble chamber sensitive to Weakly Interacting Massive Particles (WIMPs) has been developed by KICP Assistant Professor Juan Collar, postdoctoral fellow Dr. Andrew Sonnenschein and their graduate students and undergraduates.
The design of the COUPP detectors employs new techniques to improve stability, making a WIMP search with a bubble chamber feasible for the first time. The chamber contains two kilograms of CF<sub>3</sub>I, a fire-extinguishing liquid that can be superheated to respond to very low energy nuclear recoils, like those expected from WIMPs, while being totally insensitive to minimum ionizing environmental radiation, such as X, gamma and beta rays and muons. Bubbles produced by nuclear recoils are automatically photographed by CCD cameras. The resulting images (see Figure 1) can help further distinguish WIMPs from residual backgrounds such as neutron-induced recoils. During fall of 2004, the detector will be moved to the MINOS near detector gallery at Fermilab, where it will profit from 300 ft of rock overburden to reduce cosmic ray neutron backgrounds and the convenience of a nearby site during the testing period. COUPP collaborators at Fermilab bring in a long history of expertise in large bubble chamber development. After a few months of preliminary tests there, the chamber will be transported to the Soudan iron mine in Tower, Minnesota, where it will be installed 2400 feet underground. At this location, this first device is expected to be the most sensitive in the world for WIMPs interacting by spin-dependent scattering and to be competitive with other leading projects for detecting the spin-independent scattering mode. Since bubble chambers are easily scalable, a successful measurement at Soudan would lead naturally to the construction of much larger, more sensitive devices. In parallel with this effort, the use of small chambers as the most sensitive fast neutron detectors is to be explored in collaboration with Pacific Northwest National Laboratory.