DRIFT-II: R&D and Operations for Dark Matter Detection

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When these NSF researchers say they're on their way "back to the salt mine", they mean it. The Professors of Occidental College and the University of New Mexico are lead scientists on an international collaboration which works deep underground in a salt mine in England performing unique astrophysics measurements. Major funding for the work is provided by the National Science Foundation and the British Particle Physics and Astrophysics Research Council. The experiment, dubbed DRIFT (Directional Recoil Identification from Tracking) is a search for dark matter. Astrophysicists believe that the dark matter outweighs all the stars, gas and dust in our galaxy by nearly a factor of ten. It has been called dark matter since the Caltech astronomer Fritz Zwicky first measured its gravitational effects in the 1930's while finding it invisible in any telescope. Tremendous advances in telescopes and astronomical measurements since then have put the existence of dark matter on a firm footing but have not solved the mystery of what dark matter is. One of the few candidates for dark matter to have withstood the intense scrutiny over the last 75 years is the Weakly Interacting Massive Particle (WIMP). DRIFT uses a new type of particle detector (the "Negative Ion Time Projection Chamber" [NITPC]) invented in 1999 by the DRIFT team specifically to search for and positively identify WIMPs. WIMPs interact very rarely with matter. When they do they cause a nucleus to recoil much like two pool balls hitting each other. The DRIFT detector is capable of measuring the very short tracks of ionization left behind by low energy recoils traveling through the volume of gas. The detector is 1 cubic meter (about 35 cubic feet) in size and can measure tracks ~1 mm (1/25 inch) in length formed anywhere in its interior. It is a smart dark matter detector. The earth's motion through the halo of dark matter produces recoiling nuclei that "point" in a direction opposite our motion. DRIFT's ability to measure the direction of the recoils produced by dark matter therefore provides it with a unique signature of this dark matter. This allows DRIFT, unlike any other detector, to say with confidence that the recoils it detects are dark matter and not the more ubiquitous background neutrons. Also, by design, DRIFT is almost completely immune to the ever present backgrounds created by X-rays and gamma rays which plague other detectors. Reducing the background is also the reason DRIFT must be run deep underground in a salt mine, 1 kilometer (3300 feet) down, below the picturesque coastline of the North Yorkshire Moors. The Earth is bathed in a constant flux of high energy radiation from outer space (the "cosmic rays"). Without the shielding provided by 3300 ft of soil, salt, and rock, this radiation would wreak havoc with a detector as sensitive as DRIFT. There are several underground laboratories dedicated to particle astrophysics scattered around the world (South Dakota, Japan, Italy, Russia), and a major upgrade for a U.S. lab in the early planning stages. The NSF researchers on DRIFT had a long association with members of a British group who for more than a decade have operated an underground lab in the Boulby Mine. This is the deepest mine in Europe. The DRIFT team has installed and run two detectors DRIFT-I and DRIFT-II in the mine since 2002. Their work involves several graduate students as active participants in the research, both in data taking and analysis, and in development of new detectors for DRIFT.