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

当这些国家科学基金会的研究人员说他们正在“回到盐矿”的路上时，他们是认真的。西方学院和新墨西哥大学的教授是一项国际合作的首席科学家，该合作在英国一个盐矿的地下深处进行独特的天体物理测量。这项工作的主要资金是由国家科学基金会和英国粒子物理和天体物理研究委员会提供的。这项名为DRIFT（追踪方向反冲识别）的实验旨在寻找暗物质。天体物理学家认为，暗物质比我们星系中所有的恒星、气体和尘埃都要重近十倍。自从加州理工学院的天文学家弗里茨·兹威基在20世纪30年代首次测量到它的引力效应，并发现它在任何望远镜中都看不见以来，它就被称为暗物质。从那时起，望远镜和天文测量的巨大进步为暗物质的存在奠定了坚实的基础，但并没有解决暗物质是什么这个谜团。在过去的75年里，为数不多的几个暗物质候选者中，有一个经受住了严格审查的是弱相互作用大质量粒子（WIMP）。DRIFT使用了一种新型的粒子探测器（“负离子时间投影室”[NITPC]），这种探测器是DRIFT团队在1999年发明的，专门用于搜索和确定wimp。wimp很少与物质相互作用。当它们这样做时，它们会导致原子核反冲，就像两个台球互相撞击一样。DRIFT探测器能够测量低能量反冲穿过气体体积所留下的极短的电离轨迹。探测器的尺寸为1立方米（约35立方英尺），可以测量在其内部任何地方形成的长度为1毫米（1/25英寸）的轨迹。它是一个智能暗物质探测器。地球在暗物质光晕中的运动产生了反冲的原子核，这些原子核“指向”与我们运动方向相反的方向。因此，DRIFT测量暗物质产生的反冲方向的能力为它提供了这种暗物质的独特特征。与其他探测器不同，这使得DRIFT可以自信地说，它探测到的反冲是暗物质，而不是更普遍存在的背景中子。此外，根据设计，DRIFT几乎完全不受x射线和伽马射线产生的背景的影响，而x射线和伽马射线使其他探测器烦恼。减少背景也是DRIFT必须在地下深处的盐矿中运行的原因，在风景如画的北约克郡荒野海岸线下方1公里（3300英尺）处。地球沐浴在来自外太空的高能辐射（“宇宙射线”）的持续流中。如果没有3300英尺厚的土壤、盐和岩石提供的屏蔽，这种辐射将对像DRIFT这样敏感的探测器造成严重破坏。有几个致力于粒子天体物理学的地下实验室分散在世界各地（南达科他州，日本，意大利，俄罗斯），美国实验室在早期规划阶段进行了重大升级。美国国家科学基金会的DRIFT研究人员与一个英国组织的成员有长期的联系，该组织在博尔比矿经营了一个地下实验室十多年。这是欧洲最深的矿井。自2002年以来，DRIFT小组在该矿安装并运行了两个探测器DRIFT- i和DRIFT- ii。他们的工作包括几名研究生积极参与研究，包括数据采集和分析，以及为DRIFT开发新的探测器。

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.