Learn About the CDMS Experiment

Location

The CDMS experiment uses super-sensitive devices to measure very rare events . WIMP interactions are collisions on a subatomic scale that produce signals with magnitudes much more faint than the noise of the sans-shielding background. Background signals can feign these events, and many of these signals occur much more frequently than the desired WIMP collisions do. To shield the experiment as much as possible from cosmic rays, gamma radiation, and other background particles, such as neutrons and electrons, CDMS II utilizes the earth itself: because WIMPs react so much more infrequently with ordinary matter than the unwanted background, "hiding" the detectors underground reduces the degree to which the signals can be feigned by a factor of 100,000! The CDMS II experiment takes place almost half a mile (2341 feet) underground in the Soudan mine in northern Minnesota. The facility, which was a working iron mine for almost a century until 1963, has housed physics research since 1979. CDMS II currently shares the mine with MINOS (Main Injector Neutrino Oscillation Search), a project that seeks to find the mass of neutrinos.

The Fridge

The CDMSII project and associated test facilities employ helium3-helium4 dilution refrigerator techniques which, with the appropriate cryostat apparatus, are able to achieve detector base temperatures as low as 10mK.

Detection

CDMS II uses five towers of silicon and germanium detectors, called ZIPs, to sense the WIMPs that stream through space. The ZIPs are so sensitive that they operate at 20 mK - twenty thousandths of a degree above absolute zero. Each tower consists of six detectors, four of germanium and two of silicon. The former distinguish WIMP events from photon and electron events and the latter distinguish WIMP events from neutron events.

An incoming WIMP collides with the nucleus of a germanium atom (nuclear recoil) somewhere within the detector's crystal. Neutrons, electrons, photons, etc. also collide with our detectors. Phonons produced in the collisions transfer their energy to transition edge sensors (TESs) that cover one surface of our detectors. TESs are created with superconducting tungsten thin films. The extra energy imparted from the produced phonons vibrating through the crystal raise the tungsten's temperature above its critical temperature, resulting in a drastic change in resistance. This change in resistance, which can represent very small changes in temperature, is measured by an external electrical circuit.

At the same time, on the other side of the detector, charge collection plates measure the amount of charge displaced within the detector by the incident particle. This measurement helps to distinguish WIMPs and neutrons, which cause almost no charge movement, from electromagnetically-responsive background particles such as gamma rays and beta electrons.

Results

CDMS II represents the most sensitive WIMP detection experiment and has produced the most accurate results of any project, having surpassed the sensitivity attained at EDELWEISS, an underground facility near Grenoble, France, by a factor of four. Further, over the next few years, the project's sensitivity is expected to improve by a further factor of twenty or more! Even the project's present results have given scientists a significantly improved estimate on the upper limit on the quantity of WIMPs in the universe. The interaction rate of WIMPs with baryonic matter is now known to be less than 4 x 10-43 cm2, or less than one interaction per twenty-five days per kilogram of germanium. This puts new limits on possible dark matter models and even calls into question previous results found by a team in Italy, the DAMA collaboration.

The Future

CDMS II Run 120, which is expected to improve the project's results by at least another factor of four, will conclude at the end of 2005, upon which CDMS II will be complete. The Soudan facility will continue to house the project through 2006 during the experiment's transition to the next level, SuperCDMS. SuperCDMS will take place at a deeper underground site with almost three times the shielding of CDMS-II. SNOlab in Sudbury, Ontaria, is a likely location. This increase in background shielding will improve results significantly, as will the increase in detector fabrication and design: the SuperCDMS project will use germanium ZIPs almost five times as massive as CDMS-II does(1.13 kg compared to .250 kg).

Learn More!

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