Study of Argon for WIMP Dark Matter Detectors and Earth Sciences

暗物质存在的证据现在令人信服，但它的组成仍然是一个根本之谜。特别有趣的是暗物质可能由弱相互作用大质量粒子（WIMPs）形式的基本粒子组成。WIMP可以通过它们与原子核的碰撞来检测，但是预期的反冲原子核的低速率和低能量为检测器提出了令人生畏的尺寸和背景要求。液氩是一种优秀的闪烁和电离探测器，其独特的特性使其对WIMP暗物质的探测特别有吸引力。 作者建议有助于发展液氩作为探测器的WIMP。在过去的一年中，普林斯顿小组参与了意大利格兰萨索地下实验室3.2公斤WARP液氩探测器的开发和测试。测试的结果是非常积极的，并证明了一个敏感的WIMP搜索与大规模LAr探测器的可行性。他们建议更多地参与大型液氩探测器的开发和操作，下一步是140公斤探测器。除了本提案中要求的关键电子设备外，140公斤WARP探测器已获得资金，正在建造中。预计将于2007年夏季投入使用。研究计划包括以下三个目标：1）脉冲形状鉴别：进一步发展脉冲形状鉴别与新的0.5-1 GHz瞬态数字化器（1-2 ns数字化）。2)灯光采集：PI建议研究改进光收集系统的方法，包括在检测器底部的液态氩中添加PMT阵列的可能性。3)地下氩气：由于宇宙射线与大气中的氩相互作用，在商业氩中发现的放射性39 Ar在氩探测器中产生显著的β背景。可能从天然气威尔斯井中获得低含量的~（39）Ar。源自地球地幔的气体，例如新墨西哥州东北部的CO2威尔斯气井，应该具有非常低的39 Ar水平。美国地质氩源的开发将对低背景计数产生更广泛的影响，除了WIMP搜索之外，还将成为即将到来的深层地下科学与工程实验室（DUSEL）的主要资产。对地质氩的研究也是DUSEL所涉及的两个学科，物理学和地质学之间科学合作的一个很好的例子。

Evidence for the existence of dark matter is now compelling, but its composition remains a fundamental mystery. Particularly intriguing is the possibility that dark matter consists of elementary particles in the form of Weakly Interacting Massive Particles (WIMPs). WIMPs may be detected by their collisions with nuclei, but the expected low rate and low energy of the recoil nuclei present daunting size and background requirements for the detector. Liquid argon is an excellent scintillation and ionization detector and has unique features that make it particularly attractive for detection of WIMP dark matter. The authors propose to contribute to the development of liquid argon as a detector for WIMPs. During the past year the Princeton group participated in development and tests of the 3.2-kg WARP liquid argon detector at the Gran Sasso underground laboratory in Italy. The results of the tests are very positive and demonstrate the feasibility of a sensitive WIMP search with a large-scale LAr detector. They propose to increase their involvement in the development and operation of larger liquid argon detectors, with the next step being a 140-kg detector. Except for critical electronics that is requested in this proposal, the 140-kg WARP detector is funded and under construction. Commissioning is expected to occur in Summer 2007. The research plan consists of the following three goals: 1) Pulse Shape Discrimination: further development of pulse shape discrimination with new 0.5-1 GHz transient digitizers (1-2 ns digitization). 2) Light Collection: the PIs propose to investigate ways to improve the light collection system, including the possibility of adding an array of PMTs in liquid argon at the bottom of the detector. 3) Underground Argon: the radioactive 39Ar found in commercial argon, which is due to cosmic ray interactions with the argon in the atmosphere, produces a significant beta background in argon detectors. It is likely that argon with a low content of 39Ar could be obtained from natural gas wells. Gas that originates from the Earth's mantle, such as at the CO2 gas wells in Northeast New Mexico, should have a very low level of 39Ar. The development of US sources for geological argon would have many broader impacts applications to low background counting, in addition to WIMP searches, and would be a major asset for the upcoming Deep Underground Science and Engineering Laboratory (DUSEL). The study of geological argon also constitutes a good example of scientific cooperation between two of the disciplines involved in the DUSEL, physics and geology.