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.
" we achieved six major milestones towards the development of argon as a target for direct dark matter searches, and published our findings as follows. 1. Improvement of background of AMS for measurement of 39Ar by reduction of potassium background in electron-cyclotron-resonance ion sources. P. Collon et al., "Reducing potassium contamination for AMS detection of 39Ar with an electron-cyclotron-resonance ion source", Nuclear Instruments and Methods B 283, 77 (2012). We reported our efforts to investigate new methods combining low level potassium cleaning techniques with the use of ultra-pure aluminum liners in the plasma chamber of the ion source for the measurement of 39Ar by Accelerator Mass Spectrometry (AMS). We concluded that the combination of low-potassium cleaning techniques and ultra-pure aluminum liners does not provide the necessary low potassium levels required for measurements of 39Ar well below the natural level using a general purpose ECR ion source. The outcome of this project prompted us to use low background counting to determined the activity of 39Ar in underground gas samples. 2. Discovery of sources of underground argon highly depleted in 39Ar. D. Acosta-Kane et al., "Discovery of underground argon with low level of radioactive 39Ar and possible applications to WIMP dark matter detectors", Nuclear Instruments and Methods A 587, 46 (2008). We reported the measurement of 39Ar in argon from underground natural gas reservoirs. The ratio of 39Ar to stable argon was measured to be less than 5% the value in atmospheric argon. Low radioactivity argon, besides WIMP dark matter searches, may be of interest to enable detectors of reactor neutrinos for non-proliferation efforts. 3. Development of the first large scale production of low radioactivity argon from underground sources. H.O. Back et al., "First Large Scale Production of Low Radioactivity Argon From Underground Sources", arXiv:1204.6024 (2012). We reported the construction and operation of the first large-scale production of low radioactivity argon from underground gas wells. We collected argon from a CO2 well in southwestern Colorado, at the Doe Canyon facility owned by Kinder Morgan. We process the gas directly on the premises with a multi-stage Vacuum Pressure Swing Adsorption (VPSA) plant. The argon concentration in the well is in the range 400-600 ppm. The output stream from the VPSA plant is enriched in argon at the level of 30,000-50,000 ppm (3-5%). 4. Development of a cryogenic distillation column for the purification of low radioactivity underground argon. H.O. Back et al., "First Commissioning of a Cryogenic Distillation Column for Low Radioactivity Underground Argon", arXiv:1204.6061 (2012). We reported the commissioning and performance of a cryogenic distillation column for low radioactivity underground argon. The goal for this project was to perform the chemical separation necessary to turn the stream of gas collected in Doe Canyon, with an argon concentration in the range 3-5%, into pure argon. During the first commissioning run, we ran the distillation column in reaching an argon purity of 99.95% or better, with 500 ppm or less of nitrogen remaining. 5. Development and operation of a low background detector for the measurement of trace contamination of 39Ar. J. Xu et al., "A Study of the Residual 39Ar Content in Argon from Underground Sources", arXiv:1204.6011 (2012). We reported on the design and operation of a low background detector with a single phase liquid argon target that was built to study the 39Ar content of the underground argon. We determined that gas from the Doe Canyon wells has less than 0.65% of the 39Ar activity in atmospheric argon. We also reported interesting results on the measurement of other argon isotopes. Atmospheric argon has a ratio of 40Ar/36Ar of 296, whereas the upper mantle argon has a 40Ar/36Ar value of 41,000. Results support the hypothesis that a large fraction of the Cortez gas is derived from the mantle. 6. Development and operation of DarkSide-10, a prototype two-phase argon TPC, which achieved a record value for the light yield. D. Akimov et al., "Light Yield in DarkSide-10: a Prototype Two-phase Liquid Argon TPC for Dark Matter Searches", arXiv:1204.6218 (2012). We reported results from the detector DarkSide-10. The most critical parameter for argon dark matter detectors is scintillation light yield, as photon statistics limits the rejection of electron-recoil backgrounds by pulse shape discrimination. The best zero-field photoelectron yield obtained so far was an astounding 9 photoelectrons per keVee deposited in the LAr.
